5/20/2018 CHE451_Chemical Process Design and Economics
1/99
i
KWAME NKRUMAH UNIVERSITY OF SCIENCE AND
TECHNOLOGY, KUMASI
INSTITUTE OF DISTANCE LEARNING
CHE 451
Chemical Process Design & Economics
(Credits: 4)
Benjamin Afotey, PhD
Chemical Engineering Department
August, 2014
5/20/2018 CHE451_Chemical Process Design and Economics
2/99
ii
Course IntroductionDesign is the synthesis of ideas to achieve a desired goal (product). The designer starts with
an idea and proceeds to develop several alternative designs that he evaluates and finallysettles on the one that satisfies his objective (goal).
The search for alternative designs is important to the economic viability of the design project.CHE 451 is a fourth year core course offered in first semester in BSc. Chemical Engineering.
Course OverviewMethodology of the Design Process: Constraints on a Design Problem; Fixed/Rigidconstraints, Less rigid constraints, The design process; Design objectives, Data collection,
Generation of possible designs,Selection, Chemical manufacturing processes, Continuous
and batch processes, Organization of a chemical engineering design project.
Codes and Standards, Design Factors, Variable & Mathematical Representation of DesignProblems: Codes and standards, Design factors, Systems of units, Mathematical
representation of the design problem, Selection of design variables. Optimization and Batch
Production Process: Introduction, Simple models, Multiple variable systems, Methods ofanalysis, Other optimization methods, Batch production process.
Process Synthesis: Introduction, Raw materials and chemical reactions, Summary of process
design heuristics, Heuristics in equipment design. Process Simulation: Introduction, Processsimulator, Types of process simulation, Units operation solvers, Uncertainty and sensitivity
issues. Flow sheeting, Piping and Instrumentation: Introduction, Piping & instrumentation
diagrams, Valve selection, Pumps, Classification of pumps, Factors to consider in pump
selection, Centrifugal pumps, Effective characteristics curves, Design parameters of
centrifugal pumps, Operating point, Choice of rotational speed,. Process Economics: Costestimation, Cash flow for industrial operation, Factors affecting investment and production
costs, Capital investment, Estimation of capital investment, Types of capital cost estimates,cost indices, Methods of estimating capital investment, Turnover ratio, Estimation of total
product cost, Break-even point. Process Economics: Depreciation and profitability analysis,
Service life, Salvage value, Present value, Methods for determining depreciation,Profitability standards, Basis for evaluating project profitability, Mathematical methods for
profitability evaluation, Rate of return on investment, Discounted cash flow, Capitalized cost,
Pay-back time, Sensitivity analysis.
Course ObjectiveThe following is the main course Objective
1. To enable students understand and acknowledge the importance of economic analysis
in chemical process design, by way of efficiently maximizing profit.
5/20/2018 CHE451_Chemical Process Design and Economics
3/99
iii
Course OutlineThe course outline is divided into eight units. Each of the eight units is broken into subtopics.
Each unit addresses one or more of the course objectives.
Unit 1: Methodology of the Design Process
Unit 2: Codes and Standards, Design Factors, Variable &Mathematical Representation ofDesign Problems
Unit 3: Optimization and Batch Production Process
Unit 4: Process Synthesis
Unit 5: Process SimulationUnit 6: Flow Sheeting, Piping and Instrumentation
Unit 7: Process Economics: Cost Estimation
Unit 8: Process Economics: Depreciation and Profitability Analysis
GradingContinuous Assessment: 30%
End of Semester Examination: 70%
Reading List/Recommendation Textbooks/Websites/CDs1. Plant Design and Economics for Chemical Engineers: By Peters and Timmerhaus
2. Coulson & Richardsons Chemical Engineering Design: By Sinnot, Volume 6
Course Writer
Dr. Benjamin Afotey received his BSc. Degree in Chemical Engineering in 2000 at KwameNkrumah University of Science and Technology, Kumasi. He received his MSc. and PhD
Degrees at the University of Texas, Arlington, and U.S.A in 2003 and 2008 respectively.
He worked with the Texas Commission on Environmental Quality, U.S.A between 2008 and
2009 and joined the Chemical Engineering Department in 2010.
AcknowledgementI wish to thank the Almighty God for His guidance throughout the write up. I would also like
to thank my colleagues who provided encouragement of any kind. Finally, I acknowledge the
effort of my Teaching Assistant who contributed in a way to the successful completion of thematerial.
5/20/2018 CHE451_Chemical Process Design and Economics
4/99
iv
TABLE OF CONTENT1.0 METHODOLOGY OF THE DESIGN PROCESS 1
SESSION 1-1 : 1
1-1.1 Introduction 1
1-1.2.Constraint on a Design Problem 11-1.2.1 Fixed/Rigid constraint 1
1-1.2.2 Less rigid constraint 11-1.3 The Design Process 2
1-1.3.1 The design objective 3
1-1.3.2 Data collection 41-1.3.3 Generation of possible designs 4
1-1.3.4 Selection 5
1-1.4 Chemical Manufacturing Processes 5
1-1.5 Continuous and Batch Processes 71-1.5.1 Choice of continuous verses Batch processes 7
1-1.5.2 Organization of a chemical engineering design project 72.0 CODES AND STANDARDS, DESIGN FACTORS, VARIABLE &MATHEMATICAL REPRESENTATION OF DESIGN PROBLEMS 11
SESSION 2-1 11
2-1.1 Codes and Standards 112-1.2 Factors of Safety 12
2-1.3 System of Units 13
2-1.4 Mathematical Representation of the Design Problem 13
2-1.5 Selection of Design Variables 183.0 OPTIMIZATION AND BATCH PRODUCTION PROCESS 20
SEESION 3-1 20
3-1.1 Introduction 203-1.2 Simple Methods 21
3-1.3 Multiple Variable Systems 22
3-1.4 Methods of Analysis 23
3-1.5 Other Optimization Methods 24
3-1.6 Batch Production Process 24
4-0 PROCESS SYNTHESIS 26
SESSION 4-1 26
4-1.1Introduction 264-1.2 Raw Materials and Chemical Reactions 264-1.3 Summary of Process Design Heuristics 34
4-1.4 Heuristics in Equipment Design 345.0 PROCESS SIMULATION 38SESSION 5-1 38
5-1.1 Introduction 38
5-1.2 Process Simulator 395-1.3 Types of Process Simulation 39
5-1.4 Unit Operation Solvers 40
5-1.5 Uncertainty and Sensitivity Issues 40
5/20/2018 CHE451_Chemical Process Design and Economics
5/99
v
6.0 FLOW SHEETING, PIPING AND INSTRUMENTATION 42
SESSION 6-1 426-1.1 Introduction 42
6-1.2 Piping and Instrumentation Diagrams 43
6-1.3 Valve Selection 44
6-1.3.1 Gate valves 456-1.3.2 Globe valves 46
SESSION 6-2 466-2.1 Introduction of Pumps 46
6-2.2 Classification of Pumps 46
6-2.3 Factors to Consider in Pump Selection 476-2.4 Centrifugal Pumps 48
6-2.4.1 Effective characteristics curves 48
6-2.4.2 Design parameters of centrifugal pumps 49
6-2.4.3 Operating point 496-2.4.4 Q/H Curve versus Technical choices 51
6-2.4.5 Choice of rotation speed 526-2.4.6 Suction Conditions: Concept of NPSH 537.0 PROCESS ECONOMICS: COST ESIMATION 57
SESSION 7-1 57
7-1.1 Cost Estimation 577-1.2 Cash Flow for Industrial Operations 58
7-1.3 Factors affecting Investment and Production Costs 59
7-1.4 Capital Investment 59
7-1.5 Estimation of Capital Investment 617-1.5.1 Introduction 61
7-1.5.2 Types of capital cost estimates 61
7-1.5.3 Cost indexes 627-1.5.4 Methods for estimating capital investment 63
7-1.5.4.1 Power factor applied to plant-capacity ratio 63
7-1.5.4.2 Detailed item estimate 667-1.5.4.3 Other Methods for Estimating Equipment or Capital Investment 67
7-1.5.5 Turn-over ratio 68
SESSION 7-2 68
7-2.1 Estimation of Total Product Cost 687-2.2 Break-even Point 70
8.0 PROCESS ECONOMICS: DEPRECIATION AND PROFITABILITY ANALYSIS 75
SESSION 8-1 75
8-1.1 Depreciation 758-1.2 Service Life 76
8-1.3 Salvage Value 768-1.4 Present Value 76
8-1.5 Methods for Determining Depreciation 77
8-1.5.1 Straight-line method 778-1.5.2 Decliningbalance ( or fixed percentage) method 77
8-1.5.3 Other methods for determining depreciation 78
5/20/2018 CHE451_Chemical Process Design and Economics
6/99
vi
SESSION 8-2 82
8-2.1 Profitability Analysis 828-2.2 Profitability Standards 82
8-2.3 Basis for Evaluating Project Profitability 83
8-2.4 Mathematical Methods for Profitability Evaluation 84
8-2.4.1 Rate of return on investment 848-2.4.2 Discounted cash flow 86
8-2.4.3 Capitalized cost 888-2.4.4 Payout period (or Pay-back time) 90
8-2.4.5 Sensitivity Analysis 91
5/20/2018 CHE451_Chemical Process Design and Economics
7/99
vii
LIST OF FIGURESFigure 1-1.1 Design Constraints 2
Figure 1-1.2 The Design Process 3
Figure 1-1.3 Anatomy of a Chemical Process 5
Figure 1-1.4 The Structure of a Chemical Engineering Project 8Figure 1-1.5 Project Organisation 9
Figure 3-1.1 Effect of Constraints on the Optimum of a Function 23Figure 3-1.2 Yield as a Function of Reactor Temperature and Pressure 24
Figure 6-1.1 Flow-sheet of Simplified Nitric Acid Production Process 42
Figure 6-1.2 Polymer Production Diagram 43Figure 6-1.3 Commonly used Valves 45
Figure 6-2.1 Approximate Range of Operation for the Three Main Types of Pump 48
Figure 6-2.2 Basic Curves Characterizing a Centrifugal Pump 49
Figure 6-2.3 Curve Characteristic of the System 50Figure 6-2.4 Variation of Operating Point by means of a Valve 50
Figure 6-2.5 Variation in Specific Speed versus the Type of Impeller used 51Figure 6-2.6 Different Types of Characteristic Curves 52Figure 6-2.7 Variation in the Operating Point versus the Rotation Speed 53
Figure 7-1.1 Cash Flow for an Overall Industrial Operation 58
Figure 7-2.1 Cost Involved in Total Product Cost for a Typical Chemical Process Plant 70Figure 7-2.2 Break-even Chart for Chemical Processing Plant 71
Figure 7-2.3: Project Cash Flow Diagram 72
5/20/2018 CHE451_Chemical Process Design and Economics
8/99
viii
LIST OF TABLES
Table 6-2.1 Main Types of Pumps 47Table 7-1.1: Cost indexes as Annual Averages 63
Table 7-1.2 Typical Exponents for Equipment Cost vs. Capacity 64
5/20/2018 CHE451_Chemical Process Design and Economics
9/99
1
UNIT 1METHODOLOGY OF THE DESIGN PROCESS
IntroductionThis unit discusses the methodology of the design process and its application to thedesign of chemical manufacturing processes.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Explain chemical process design2. Distinguish between fixed and less rigid constraints3. Know what constitutes the design process4. List the basic components of a chemical manufacturing process
SESSION 1-1 In this session we shall discuss what a chemical process design is,
the constraints of the design problem, the components of the chemical manufacturing
process and the difference between a continuous and a batch process.
1-1.1 Introduction: Design is the synthesis of ideas to achieve a desired goal (product).
The designer starts with an idea and proceeds to develop several alternative designs that
he evaluates and finally settles on the one that satisfies his objective (goal).
The search for alternatives: this step becomes necessary because the designer will be
constrained by several factors.
1-1.2 Constraints on a Design Problem
1-1.2.1 Fixed/Rigid constraints: these are constraints the designer must live with outside
his influence. E.g. physical laws, government regulations and standards. The fixed
constraints define the outer boundary of all possible designs.
1-1.2.2 Less rigid constraints: these are constraints the designer can manipulate inorder
to arrive at the best design. E.g. materrials of construction, time. These are interrnal
constraints over which the designer has some control.
In summarry we have the following diagramatic sketch.
5/20/2018 CHE451_Chemical Process Design and Economics
10/99
2
Figure 1-1.1: Design Constraints
1-1.3 The Design Process
The design process can be shown in schematic form in Figure 1-1.2.
5/20/2018 CHE451_Chemical Process Design and Economics
11/99
3
Figure 1-1.2: The Design Process
The diagram shows the design process as an iterative procedure because as the design
proceeds the designer will be looking for information and ideas to refine the design.
1-1.3.1 The Design objective
In the particular case of a chemical process plant, the objective/goal is to satisfy the
public need for a product. In large commercial organizations, this need is identified by
the sales/ marketing department. Before starting to work, the designer should obtain
complete information/background on the need for the product and its application
areas/uses.
5/20/2018 CHE451_Chemical Process Design and Economics
12/99
4
1-1.3.2 Data collection
To proceed with the design, the designer must assemble all the relevant facts and data
required. For process design, the information should include process alternatives,
equipment performances, physical property data. In large design companies, they have
in house manuals containing all the process know how on which the design is based
and preferred methods and data for the frequently used design procedures.
1-1.3.3 Generation of possible designs
At this stage the designer must come up with all possible solutions for analysis,
evaluation and selection. To do this, he must rely on his own experience or that of others,
using tried or tested methods.
Chemical engineering projects can be divided into 3 types:
1. Modification, additions to existing plant often undertaken by the plant design
group.
2. New production capacity to meet growing sales demand, and the sale of
established processes by contractors. Repetition of existing designs, with only
minor design changes.
3. New processes, developed from laboratory research, through pilot plant, to a
commercial process. Here most of the unit operations and process equipment willuse established designs.
5/20/2018 CHE451_Chemical Process Design and Economics
13/99
5
1-1.3.4 Selection
The selection process can follow the following screening stages:
Possible designs (credible)within the external constraints
Plausible designs (feasible)- within the internal constraints
Probable designslikely candidates
Best designs (optimum)judged the best solution to the problem
To select the best design from the probable designs, detailed design work and costing will
be necessary.
1-1.4 Chemical Manufacturing ProcessesThe basic components of a typical chemical process can be shown using the block
diagram below.
Figure 1-1.3: Anatomy of a Chemical Process
Each block represents a stage in the overall process for producing a product from the raw
materials. Each stage is a collection of equipment required to accomplish a defined task.
5/20/2018 CHE451_Chemical Process Design and Economics
14/99
6
Stage 1: Raw Material Storage
Unless the raw materials are supplied as intermediate products from a neighboring plant,
storage space is needed to hold several days or months supply. Types of storage required
will depend on the nature of the raw materials, and the methods of delivery.
Stage 2: Feed Preparation
Some purification of the raw materials will be necessary to render them in a form
required for feed to the reaction stage.
Stage 3: Reactor
In the reactor the raw materials are brought together under conditions that promote the
production of the desired product. However by-products and unwanted compounds/
impurities will also be formed.
Stage 4: Product Separation
After the reactor, the products and by-products are separated from any unreacted
material. If in sufficient quantity, the unreacted material will be recycled to either the
reactor directly or to the feed purification and preparation stage.
Stage 5: Purification
Before sale, the main product is purified to meet product specification. If the by-product
is also produced in sufficiently large quantities, it must also be purified for sale.
Stage 6: Product Storage
Provision for product packaging and transport will be required. Besides some inventory
of finished product must be held to match production with sales
Ancillary Process
In addition to the main process stages/ units, provision will have to be made for the
supply of utility services, process water, cooling water, compressed air, steam. Facilities
are required for maintenance, firefighting, offices, laboratories and accommodation.
5/20/2018 CHE451_Chemical Process Design and Economics
15/99
7
1-1.5 Continuous and Batch Processes
Continuous processes are designed to operate 24 hours a day, 7 days a week, throughout
the year (365 days). However down time is allowed for maintenance and process catalyst
regeneration.
Plant attainment: this is the percentage of the available hours in a year that the plant
operates. This is usually 90-95%.
1-1.5.1 Choice of continuous versus batch production
The choice will not be clear - cut, however one can use as a guide the following rules:
Continuous
1. Production rate greater than 5*106 kg/h (5000tonnes/h)
2. Single product
3. No severe fouling
4. Good catalyst life
5. Proven process design
6. Established market
7. Cost can be reduced
8. Less laborBatch
1. Production rate less than 5*106 kg/h (5000tonnes/h)
2. A range of products or product specifications
3. Severe fouling
4. Short catalyst life
5. New product
6. Uncertain design
1-1.5.2 Organization of a chemical engineering design project
The design work required in the engineering of a chemical manufacturing process plant
can be divided into two broad phases:
5/20/2018 CHE451_Chemical Process Design and Economics
16/99
8
Phase 1.Process Design
This covers the steps from the initial selection of the process to be used, through to the
issuing of the process flow-sheets; and includes the selection, specification, and chemical
engineering design of equipment. In any organization, this phase is handled by the
process design group composed of chemical engineers. The group is also responsible for
the preparation of piping and instrumentation diagrams.
Organization of a project group is shown in Figure 1-1.4 below.
Figure 1-1.4: The Structure of a Chemical Engineering Project
5/20/2018 CHE451_Chemical Process Design and Economics
17/99
9
Phase 2. The detailed mechanical design of equipment; the structural, civil and electrical
design; specification and design of ancillary services. Other specialist groups will be
responsible for cost estimation, and the purchase and procurement of equipment and
materials.
The sequence of steps in the design, construction and start up of a chemical process plant
is shown diagrammatically in Figure 1-1.5.
.
Figure 1-1.5: Project Organization
Project manager; a chemical engineer by training is responsible for the coordination of
the project.
5/20/2018 CHE451_Chemical Process Design and Economics
18/99
10
SELF ASSESSMENT 1-1
(a) Distinguish between Fixed and less rigid constraints
(b) List the components of a basic chemical manufacturing process and in just two
sentences explain the importance of each in the manufacturing process.
(c) In a tabular form, list six differences between a continuous process and a batch
process.
5/20/2018 CHE451_Chemical Process Design and Economics
19/99
11
UNIT 2CODES AND STANDARDS, DESIGN FACTORS, VARIABLE &
MATHEMATICAL REPRESENTATION OF DESIGN PROBLEMS
IntroductionThis unit discusses the codes & standards, design factors and variables and mathematicalrepresentation of the design problems.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Distinguish between codes and standards2. Understand the importance of design factors as a margin of safety
in meeting design specifications3. Appreciate the importance of mathematical representation of the
design problem
SESSION 2-1 In this session we shall study the difference between codes and standards,
design factors and their relation to equipment safety and the mathematical representation
of the design problem.
2-1.1 Codes and Standards
The terms CODE and STANDARD are used interchangeably, though CODE should be
reserved for a code of practices. That is a recommended design or operating procedure.STANDARD on the other hand refers to preferred sizes, eg. pipes, composition etc.
In modern engineering practice we have standards and codes that cover various functions
e.g
1. Materials, properties and composition
2. Testing procedures for performance, composition and quality
3. Preferred sizes, eg tubes, plates, sections
4. Design methods, inspection, fabrication
5. Codes of practice, for plant operation and safety
All developed countries have national organizations responsible for the issue and
maintenance of standards for the manufacturing industries and for the protection of
consumers.
5/20/2018 CHE451_Chemical Process Design and Economics
20/99
12
In the U.K:; British Standards Institution
In the U.S; National Bureau of Standards
They are responsible for coordinating information on standards. Standards are issued by
the Federal State and various commercial organizations. The major ones of interest to
chemical engineers are:
American National Standards Institute (ANSI)
American Petroleum Institute (API)
American Society for Testing Materials (ASTM)
American Society of Mechanical Engineers (ASME)
International Organization for Standardisation (ISO) coordinates the publication of
International Standards.
In Ghana, there is no national standards organization to coordinate local standards for
industries. However there are national standard organizations with standards for the
protection of consumers eg. Standards Boards, Food and Drug Administration (FDA),
and the Environmental Protection Agency (EPA).
Equipment manufactures work together to produce standardized designs and size ranges
for commonly used items; electric motors, pumps, pipes, and pipe fittings.
2-1.2 Factors of Safety (Design Factors)
Design is an inexact act; errors and uncertainties can arise in the design data available
and in the approximations necessary in the design calculations. To meet design
specifications, factors are included to give a margin of safety in the design so that the
equipment will not fail to perform satisfactorily, and that it will operate safely.
e.g. in a mechanical and structural design, the magnitude of design factors used to allow
for uncertainties in material properties, design methods fabrication and operating loads
are well established. In process design, design factors are used to give tolerances in the
design e.g. process stream average flows calculated from material balances are often
increased by a factor of 10%, to give some flexibility in process operation. This factor
then sets the maximum flows for equipment, instrumentation and piping design.
5/20/2018 CHE451_Chemical Process Design and Economics
21/99
13
2-1.3 System of Units
Chemical engineering uses a diversity of units from American and British engineering
Systems, CGS (grain, centimeter, second)
MKS( kilogram, meter, seconds)
English and Americanpound mass (lb), foot, second or hours, pound force.
If working in S. I units is preferred, data expressed in the American and British
engineering systems can be converted to S.I units. Conversions factors are available in
the literature.
2-1.4 Mathematical Representation of the Design Problem
A process unit e.g. distillation unit in a chemical process plant performs some operation
on the inlet material stream to produce the desired outlet stream.
Inlet stream Outlet stream
In the design of such a unit, the design calculations model the operation of the unit. Thus
the flow of materials is replaced by flow of information into the unit and flow of derivedinformation out of the unit.
Input Output
Information Information
Process
Unit
Calculationmethod
5/20/2018 CHE451_Chemical Process Design and Economics
22/99
14
Information flows are the values of the variables which are involved in the design.
Example:
Input stream Output stream
variables variables
Flow rate same as the input variables.
Composition
Temperature
Pressure
Enthalpy
Composition, temperature, pressure are called intensive variables, i.e. independent
of quantity of material flow (flow rate)(Nv).
The constraints on the design will place restrictions on the possible values that
these variables can take; for example the values of some of the variables will be
fixed directly by process specification. The values of other variables will be
determined by design relationships arising from constraints.
Some of the design relationships will be in the form of explicit mathematicalequations (design equations): such as those arising from material and energy
balances, thermodynamic relationships, and equipment performance parameters.
Other relationships will be less precise such as those arising from the use of
standards and preferred sizes and safety considerations (Nr).
The difference between the number of variables in the design and the number of
design relationships is called the number of degrees of freedom.
If Nvthe number of possible variables in a design problem
Nrthe number of design relationships
Nd = NvNr
Nd = number of degrees of freedom
5/20/2018 CHE451_Chemical Process Design and Economics
23/99
15
Case 1.
If Nv=Nr implies Nd = 0; this implies that there is only one unique solution to the
problem. The problem is not a true design problem and no optimization is possible.
Case 2
Nv is less than Nr, Nd is less than 0; this implies that the problem is over defined, and
only the trivial solution is possible.
Case 3
Nv >Nr, Nd > 0; implies there is an infinite number of possible solutions. However for a
practical problem, there will be only a limited number of feasible solutions. Nd represents
the number of variables which the designer must assign values to solve the problem.
EXAMPLE 2-1.1: Consider a single phase stream (liquid/vapour) containing C
components.
Input OutputProcess Unit
5/20/2018 CHE451_Chemical Process Design and Economics
24/99
16
Note
(1) The sum of the mass/mol fractions equal 1.
(2) The enthalpy is a function of stream composition, temperature and pressure.
Therefore, Degrees of Freedom, Nd = NvNr =(C+4)(2) = C+2
Specifying C+2 variables completely defines the stream.
EXAMPLE 2-1.2: Flash distillation / Equilibrium distillation
In this process unit, a feed is passed into a still (fractionating column), where part is
vaporized, and the vapour remaining in contact with the liquid. The mixture of vapor and
liquid leaves the still and is separated so that the vapour is in equilibrium with the liquid.
Where, F= Stream flow rate, P= pressure, T=temperature, xi= concentration of
component i, q = heat input.
Surfixes, 1= inlet; 2=outlet vapor; 3 =outlet liquid
5/20/2018 CHE451_Chemical Process Design and Economics
25/99
17
Note: 1. given the temperature and pressure, the concentration of any component in the
vapor phase can be obtained from the concentration in the liquid phase ( v-l-e data)
2. An equilibrium separation implies that the outlet streams and the still are the
same pressure and temperature.
This implies that P2 = P (1) T2 = T (3)
P3 = P (2) T3 = T (4)
Gives 4 equations.Degrees of freedomNd(No of degrees of freedom)=Nv - Nr= (3C + 9) - (2C + 5) = C + 4
Though the total degrees of freedom calculated is (C+4), some of the variables will be
fixed by the process conditions and will not be free for the designer to select as design
variables. For example: the flash distillation unit will normally be one unit in a process
system and the feed composition and feed conditions will be fixed by the upstream
process. Hence defining the feed, fixes (C+2) variables and the designer is left with,
(C+4)-(C+2) = 2
5/20/2018 CHE451_Chemical Process Design and Economics
26/99
18
2-1.5 Selection of Design Variables
To solve a design problem, the designer has to decide which variables are to be chosen as
design variables, i.e the ones he can manipulate to produce the best design. This choice is
crucial to enable the simplification of the calculations.
Example: Flash distillation problem in the previous example.
For a binary mixture, C=2; this implies Nd=C+4=6
If the feed stream flow, composition, temperature, pressure are fixed by upstream
conditions, then the number of design variables is Nd=6-(C+2)=6-4=2. This implies, the
designer is free to select 2 variables from the remaining variables to proceed with the
calculation of the outlet stream composition and flows.
Scenerio 1
Suppose you select the still pressure implies for a binary system, vapour-liquid
equilibrium (V-L-e) relationship is determined, and one outlet stream flow rate, then the
outlet compositions can be calculated by the simultaneous solution of mass-balance and
(v-l-e) relations.
5/20/2018 CHE451_Chemical Process Design and Economics
27/99
19
Scenerio 2
If you select the still pressure and the liquid outlet stream composition, then the
simultaneous solution of the mass balance and the v-l-e relationship will not be
necessary. Following the procedure below, one can calculate the stream compositions.
1. Specify P determines the v-l-e curve from experimental data
2. Knowing the outlet liquid composition, the outlet vapor composition can be
calculated from the v-l-e.
3. Knowing the feed and outlet compositions, and the feed flow rate, the outlet
stream flows can be calculated from a material balance.
4. An enthalpy balance gives the heat input required.
SELF ASSESSMENT 2-1
(a) List 3 American organizations responsible for coordinating information on standards.
(b) List 3 national standard organizations with standards for the protection of consumers.
5/20/2018 CHE451_Chemical Process Design and Economics
28/99
20
UNIT 3OPTIMIZATION AND BATCH PRODUCTION PROCESS
Introduction
This unit discusses batch production processes and the importance of optimization.
Learning ObjectivesAfter reading this unit, you should:
1. Appreciate the importance of optimization in plant design2. Understand the process of batch production
SESSION 3-1 In this session we shall discuss the steps involved in optimizing the design
of a chemical process plant. Further, we shall
3-1.1 Introduction
Optimizing the design of a chemical process plant is a foreboding task. This can be
achieved by subdividing the plant into subunits and optimizing each subunit. However
this does not result in the optimal design of the whole plant because the optimization of
each subunit is at the expense of the other.
The general procedure for optimizing process units and equipment design:
Step1: the first step is to clearly define the objective. i.e the criteria to be used for
measuring the performance of the system. For a chemical process plant, the overall
objective is tomaximize profit.
This overall objective can be broken down into sub objectives such as; minimize
operating cost, minimize capital investment, maximize yield of the product, reduce labour
requirements, reduce maintenance, operate safely.
Step 2: the second step is to determine the objective function; the system of equations
and other relationships, which relate the objectives with the variables to be manipulated
to optimize the function.
Step 3: this step is to find values of the variables that give the optimum value of objective
function i.e maximum or minimum. The best technique to be used will depend on the
complexity of the system and the type of mathematical model used to represent it.
5/20/2018 CHE451_Chemical Process Design and Economics
29/99
21
3-1.2 Simple Models
If the objective function can be expressed as a function of one variable (single degree of
freedom), the function can be differentiated or plotted to find the maximum or minimum.
This situation arises only for exceptional cases. In most practical situations, the numbers
of variables exceed the number of relationships.
Example of a simple model: Determine the optimum proportions for a closed cylindrical
container.
D
L
The surface area A in terms of the dimensions is:
( )
VV4VLabove(2)inDngsubstituti
VD
VDD
D
4V
zerotoequationabovethesetweDoptimumthefinding
(3)DD
4V
(D)fatingdifferenti
2
D
D
4Vf(D)
haveweD,variableoneoftermsinfunctionobjectivethegres
(2)D
4VLL
4
DV
have,we(V)volumegivenafor
(1)2
DLDLD,fisfunctionobjectivethe
DDLDDLA
3
2
2
2
2
31
32
31
'
22
44
440
,
;
sinexp
242
=
=
=
==+
+
=
+
=
=
=
+=
+=
+=
5/20/2018 CHE451_Chemical Process Design and Economics
30/99
22
For a cylindrical container, the minimum surface area to enclose a given volume is
obtained when the length (height) is made equal to the diameter.
In practice, when the cost is the objective function; L=2D; this is because the cost must
include that of forming the vessel, making the joints in addition to the cost of the
material.
3-1.3 Multiple Variable Systems
The general optimization problem can be represented mathematically as:
variablesthearevvvvandfunctionobjectivefwhere
vvvvff
n
n
,.......,,
),.......,,(
321
321
=
In a design situation, there will be constraints on the possible values of the objective
function due to constraints on the variables.
- Equality constraints are expressed by equations of the form
0),.......,,( 321 == nmm vvvv
- Inequality constraints are expressed by equations of the form
pnpp Pvvvv = ),.......,,( 321
Optimization of the problem involves finding values for the variables nvvvv ,.......,, 321
that will optimize the objective function ( i.e give maximum or minimum values within
the constraints).
3-1.4 Methods of Analysis:
a. Analytical method: objective functions can be expressed as a mathematical function;
use the methods of calculations to find maximum or minimum values.
- For practical situations where the values of the variables are subject to constraints,
the optimum of the constrained objective function will not necessarily occur
where the partial derivatives of the objective function are zero.
e.g.
5/20/2018 CHE451_Chemical Process Design and Economics
31/99
23
Figure 3-1.1: Effect of Constraints on the Optimum of a Function
- The method of Lagranges undetermined multipliers is a useful analytical
technique for dealing with problems that have equality constraints (fixed designvalues).
b. Search Methods: Relationships between variables and constraints that arise in
practical design problems are such that analytical methods are not feasible; hence the use
of search methods.
For single variable problems where the objective function is unimodal, the simplest
approach is to calculate the value of the objective function at uniformly spaced values of
the variable until a maximum or minimum is obtained.
Figure 3-1.2 Yield as a Function of Reactor Temperature and Pressure
5/20/2018 CHE451_Chemical Process Design and Economics
32/99
24
3-1.5 Other Optimization Methods
- Linear programming; a technique used when the objective function and constraints can
be expressed as a linear function of the variables.
- Dynamic programming; used for the optimization of large systems.
3-1.6 Batch Production Process
- Productive period: this is the period when product is being produced
- Nonproductive period; this is the period when the product is discharged and
equipment prepared for the next batch.
- Total batch time: productive period + nonproductive period
- The rate of production is determined by the total batch time as follows:
Batches per year = 8760 x Plant attainment
Total batch time (batch cycle time)
Annual production rate = (quantities produced per batch)x(batches per year)
Cost per unit of production= annual cost of production
Annual production rate
SELF ASSESSMENT 3-1
A rectangular tank with a square base is constructed from 5 mm steel plates. If thecapacity required is eight cubic meters. Determine the optimum dimensions if the tank
has a closed top.
5/20/2018 CHE451_Chemical Process Design and Economics
33/99
25
UNIT 4PROCESS SYNTHESIS
IntroductionThis unit discusses the synthesis of chemical processes.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Explain process synthesis2. Define and explain heuristics
SESSION 4-1 In this session we shall discuss process synthesis and the importance of
heuristics in chemical process optimization.
4-1.1 Introduction: process synthesis aims at the optimization of the logical structure of
a chemical process; specifically the sequence of steps; reaction, separation (distillation,
extraction etc), the source and destination of recycle streams.
The logical structure of a chemical process: Given the following;
- Raw materials, required products, allowed byproducts, a set of unit operations for
consideration, cost factors for materials and unit operations required to generate
and rank in order of preference and feasible chemical plant flow sheets.
Approach 1. Combinatorial algorithms are used to find all feasible flow sheets contained
in a toolkit of raw materials and operating steps. The flow sheets are then reviewed and
optimized based on performance, economic and safety criteria.
Approach 2. Heuristic rules based on experiences that are used for the selection and
positioning of processing operations as flow sheets are assembled.
4-1.2 Raw Materials and Chemical Reactions
Heuristic 1: select raw materials and chemical reactions to avoid or reduce the
handling and storage ofhazardous and toxic chemicals.e.x. Manufacture of ethylene glycol
(2)OHCHHOCHOHOHC
(1)OHCOHC
++
++
22242
42242 21
5/20/2018 CHE451_Chemical Process Design and Economics
34/99
26
- Both reactions are extremely exothermic, therefore they need to be controlled
carefully. Such processes are designed with two reaction steps with storage of the
intermediate, to enable continuous production, even when maintenance problems
shut down the first reaction operation.
Alternative to the 2-step example process:
1. Use chlorine and caustic in a single reaction step, to avoid the intermediate:
NaClOHCHHOCHaqNaOHClCHCH 2)(2 22222 +++=
2. Use the 2-step reaction with the following modifications:
- As ethylene-oxide is formed, react it with carbon dioxide to form ethylene
carbonate, a much less active intermediate that can be stored safely. This can then
be hydrolysed to form the required ethylene glycol product
2222343
343242
42242 21
COOHCHHOCHOHOHC
OHCCOOHC
OHCOHC
++
+
++
Heuristic 2: Distribution of Chemicals
Use an excess of one chemical reactant in a reaction to completely consume a second
valuable, toxic or hazardous reactant.
e.x. use an excess of ethylene in the production of Dichloroethane.
C2H4
Cl C2H4Cl2 +C2H4
C2H4
5/20/2018 CHE451_Chemical Process Design and Economics
35/99
27
Heuristic 3: when nearly pure products are required, eliminate the inert species before
the reaction operations, when the separations are easily accomplished, or when the
catalyst is adversely affected by the inert.
Heuristic 4: introduce liquid or vapour purge streams to provide exit for species that;
- Enter the process as impurities in the feed
- Produced by irreversible side reactions
e.x. Ammonia, NH3 synthesis loop
5/20/2018 CHE451_Chemical Process Design and Economics
36/99
28
Heuristic 5: Do not purge valuable species or species that are toxic and hazardous, even
in small concentrations
- Add separators to recover valuable species
- Add reactors to eliminate toxic and hazardous species
e.g. catalytic converter in car exhaust
Heuristic 6: For competing series or parallel reactions, adjust temperature, pressure, and
catalyst to obtain high yields of desired products. In the initial distribution of chemicals,
assume that these conditions can be satisfied; obtain kinetic data, and check this
assumption before developing a base-case design.
e.x. Manufacture of alkyl-chloride
HClClCHCHCHClClCHClCHCH
kk
Cl
HClClCHCHCHClCHCHCH
2
k
+=
++=+=
223
3
2
222321
Dichloropropane dichloropropene
This is a series/parallel reaction;
- for each reaction, obtainR
EKH oR ,,
5/20/2018 CHE451_Chemical Process Design and Economics
37/99
29
For each reaction, obtain kinetic data and examine the dependency of reaction rate on
temperature; implies RTE
oekk
=
Since for multiple reactions, high temperature favors the reaction of higher activation
energy and vice versa.
Heuristic 7: for reversible reactions, consider conducting them in a separation device
capable of removing products, and hence driving the reaction to the right.
e.g. manufacture of ethyl-acetate (ethyl ethanoate) using reactive distillation
Conventionally, this will call for the reaction:
EtOH + HOAc EtOAc +H2O
followed by separation of products using a sequence of separation of towers, using areactive distillation:
5/20/2018 CHE451_Chemical Process Design and Economics
38/99
30
Heuristic 8: Separations : separate liquid mixtures using distillation, stripping towers
and liquid-liquid extractors.
Heuristic 9: attempt to condense vapour mixtures with cooling water, then use heuristic
8.
5/20/2018 CHE451_Chemical Process Design and Economics
39/99
31
Heuristic 10: Heat Transfer in reactors: to remove highly exothermic heat of reaction,
consider the use of excess reactant, an inert diliuent.
Heuristic 11: For less exothermic heat of reaction, circulate reactor fluid to an external
cooler, or use a jacketed vessel or cooling coils.
5/20/2018 CHE451_Chemical Process Design and Economics
40/99
32
Heuristic 12: Pumping and Compression: To increase the pressure of a stream, pump a
liquid rather than compress a gas: i.e condense a vapor as long as refrigeration ( and
compression) is not needed before pumping.
Instead of:
Compressors have large capital cost and consume a lot of power.
5/20/2018 CHE451_Chemical Process Design and Economics
41/99
33
4-1.3 Summary of Process Design Heuristics
The discussion focused on the following:
- understanding the importance of selecting reaction paths that do not involve toxic
or hazardous chemicals.
- Be able to distribute the chemicals in a process flowsheet, to account for the
presence of inert species, to purge species that would otherwise build up to
unacceptable concentrations, to achieve a high selectivity of the desired products.
- Apply heuristics in in selecting separation processes to separate liquids, vapours,
and vapour-liquid mixtures.
- Understand the advantages of pumping a liquid rather than compressing a vapor.
4-1.4 Heuristics in Equipment Design
1. Equipment Size
Need information on the required throughput to determine vessel size
- General guidelines for vessel size
o Height: 2-10 m
o L/D: 2-5m
- Towers/ Columns
o
Height: 2-50mo L/D: 2-30m
Note: Do not specify units outside these ranges
2. Heat Exchangers
Several kinds are used
- Area: 10-1000m2
- For shell and tube (tubular)
o Tube diameter: 1-2 cm
o Tube length: 2-6m
o Shell diameter:0.3-1m
5/20/2018 CHE451_Chemical Process Design and Economics
42/99
34
- Plate and frame
Newer technology- thin, gasketed plates separate hot and cold fluids.
Advantage: more compact and very efficient
3. Heat transfer considerations
- Minimum temperature approach: for fluids 10oC and for refrigerants, 5
oC
- Cooling water in: 30oC; Cooling water out: 45
oC
- Equipment heat transfer(overall) coefficient in decreasing order of magnitude
Reboiler>Condenser>Liquid-to-Liquid>gas-to-gas
- Heat Exchangers:tlm < 100oC
4. Towers/ Columns
Tower operating pressure is usually determined by the temperature of condensing
medium or maximum allowable reboiler temperature
Sequencing multiple towers: typically
- Do easy separation first and leave difficult ones for last
- If relative volatilities of all species are close , remove one-by-one from overhead
- When volatilities are close but feed concentrations vary, remove high
concentrations first. Distillation operating conditions
- Economically reflux ratio is typically 1.2-1.5 times Lmin
- Optimum number of theoretical trays trays is typically 2 times Nmin
- Find Nmin from Fenske-Underwood equation
Tower design
- Tray spacing are typically 20-24
- Pressure drop typically 0.1psi per tray
- Tray efficiencies: 60-90% for gas absorption and stripping
5/20/2018 CHE451_Chemical Process Design and Economics
43/99
35
5. Process Conditions: General Guidelines
(a) High Pressure
To achieve a high pressure in a process:
- For a liquid use a pump; for a vapour first condense it and pump it into an
evaporatior
- For a gas compression; P/Po
5/20/2018 CHE451_Chemical Process Design and Economics
44/99
36
(c) Low Temperature
Operate at low temperature to achieve the following:
o Slow reaction rate
o Prevent thermal sensitive species from degradation
o Change thermodynamics
Achieve a high temperature in a process by the following means:
o Using a heat exchanger with the following fluids( cold steam)
Cooling tower water
Chilled water
Active refrigeration
SELF ASSESMENT 4-1
In the manufacture of ethylene glycol, a two step reaction process is used; the first step isthe formation of the intermediate ethylene oxide which can be stored, followed by itshydrolysis to form the ethylene glycol. Since both reactions are extremely exothermicand explosive, show with balanced chemical equations a safer two step approach after theinitial production of ethylene oxide.
5/20/2018 CHE451_Chemical Process Design and Economics
45/99
37
UNIT 5PROCESS SIMULATION
IntroductionThis unit discusses the simulation of a chemical process.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Explain what process simulation is.2. Identify the types of process simulations
SESSION 5-1 In this session we shall discuss the importance of process simulations, the
various types of simulations available and the importance of mathematical models in
process simulation.
5-1.1 Introduction
- Process simulation is the act of representing some aspects of the real world by
numbers or symbols that may be easily manipulated to facilitate their study.
- The important step in process simulation is the representation of that aspect of the
real world to be studied in terms of a mathematical model.
- With respect to chemical engineering, the real world is a chemical process
described by a process flow sheet. Therefore process simulation is used to solve
problems related to the process flow sheet, i.e process design, process analysis,
process control etc.
- The mathematical model that simulates an aspect of the process needs an
appropriate method of solution.
- A process simulation is a computer program developed to solve the model and
study its behavior by manipulating the model parameters.
5/20/2018 CHE451_Chemical Process Design and Economics
46/99
38
5-1.2 Process Simulator
Computational package that enables predictions of the process behavior
Inputs for the package are:
Basic engineering relationships such as:
- Mass and energy balances
- Phase and chemical equilibrium
- Flowsheet topography
Uses are:
- For design of new plants
- Increase profitability in existing plants
- Run many possible cases
- Sensitivity studies, and optimization
Accuracy depends on the use of
- Reliable thermodynamic and property data
- Realistic operating conditions
- Rigorous equipment models
5-1.3 Types of Process Simulation
There are several commercial packages. E.g Aspen Plus, Chemcad, Hysis Pro/II.Common features of all these are:
1. Database of hundredsthousands( a large number) of compounds
2. A parameter library to compute /estimate properties of these compounds
3. Flowsheet builder- graphical interface that enables units to be defined and
connected.
4. Thermo Solver; computes phase equilibrium and thermodynamic properties given
the database
5. Unit operations Solver: short cut and rigorous solvers for equipment
6. Overall flowsheet solver; mathematical convergence of the flowsheet
5/20/2018 CHE451_Chemical Process Design and Economics
47/99
39
5-1.4 Unit Operations Solvers; e.g Reactors
- specifies the reaction stoichiometry, temperature, pressure and conversion
- when the reaction is equilibrium limited, specifies the stoichiometry , the approach to
equilibrium and equilibrium constants
- for the reaction involved, specifies the kinetic expression, reactor type and enables the
reactor size calculation.
- distillation columns
- Uses the Feuske, Underwood methods to provide a good initial guess for subsequent
calculations
- for rigorous plate-by-plate approach, it solves simultaneously, material and energy
balance equations, and VLE relations for each plate.
5-1.5. Uncertainty and Sensitivity Issues
It is important to be able to quantify the uncertainty of results:
- Determine the probability of accuracy of results
- Determine what part of results obtained is most likely to be incorrect, and
estimate error range
- Sensitivity issues; in cost estimation and profitability studies, estimate the
sensitivity of the results to variations in capital cost, operating cost etc.Other causes of uncertainty of results using a simulator.
- Thermo model used
- Physical properties data
- Convergence tolerance
- Simulation method ( simulator)
Because there are disturbing variations between different process simulators using
the same models.
5/20/2018 CHE451_Chemical Process Design and Economics
48/99
40
- What can we do?
o Estimate uncertainties by performing simulations using a range of
parameters, different models.
o Determine the sensitivity of results
o Use statistical methods to design experiments over ranges of parameters
and the results will provide confidence limits
o Ultimately, final designs should be tested on pilot studies.
5/20/2018 CHE451_Chemical Process Design and Economics
49/99
41
UNIT 6FLOW SHEETING, PIPING AND INSTRUMENTATION
IntroductionThis unit discusses flow sheeting, piping and instrumentation in chemical process design.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Appreciate the importance of flow sheet in chemical process design2. Understand why instrumentation in plant design is critical3. Acknowledge the processes involved in valves and pumps selection
SESSION 6-1 In this session we shall study the essence of flow sheets in process design.
We shall also look at piping and instrumentation diagram and the types of valves used in
a process plant design.
6-1.1 Introduction
Industrial equipment are always arranged and interconnected in a certain fashion. The
process flow sheet gives a pictorial representation of the various equipment selected to
carry out the process. The process flow sheet always convey information on the operating
conditions, stream flow-rates and composition. The information included in a typical
flow sheet is either essential or optional. An example flow sheet is shown in Figure 6-1.1
Figure 6-1.1: Flow-sheet of Simplified Nitric Acid Production Process
5/20/2018 CHE451_Chemical Process Design and Economics
50/99
42
The essential information included in the flow-sheet are;
1. Stream composition, either:
The flow-rate of each individual component, kg/h, which is preferred, or
The stream composition as a weight fraction.
2. Total stream flow-rate, kg/h.
3. Stream temperature, degrees Celsius preferred.
4. Nominal operating pressure (the required operating pressure).
Optional information
1. Molar percentages composition.
2. Physical property data, mean values for the stream, such as:
Density
Viscosity
3. Stream name, a brief, one or two-word, description of the nature of the stream
4. Stream enthalpy, kJ/h.
Figure 1 below shows the information included in a polymer production process.
6-1.2 Piping and Instrumentation Diagram (P&ID)The piping and instrumentation diagram shows the engineering details of the equipment,
instruments, piping, valves and fittings; and their arrangement.
Figure 6-1.2: Polymer Production Diagram
5/20/2018 CHE451_Chemical Process Design and Economics
51/99
43
In the polymer production flow sheet above, all the following information should be
included in the P&ID:
1. All process equipment identified by an equipment number. The equipment should be
drawn roughly in proportion, and the location of nozzles shown.
2. All pipes, identified by a line number. The pipe size and material of construction
should be shown. The material may be included as part of the line identification number.
3. All valves control and block valves, with an identification number. The type and size
should be shown. The type may be shown by the symbol used for the valve or included in
the code used for the valve number.
4. Ancillary fittings that are part of the piping system, such as inline sight-glasses,
strainers and steam traps; with an identification number.
5. Pumps, identified by a suitable code number.
6. All control loops and instruments, with an identification number.
For simple processes, the utility (service) lines can be shown on the P and I diagram.
For complex processes, separate diagrams should be used to show the service lines, so the
information can be shown clearly, without cluttering up the diagram. The service
connections to each unit should, however, be shown on the P and I diagram.
6-1.3 Valve Selection
Control of flow in lines and provision for isolation of equipment when needed are
accomplished with valves. Depending on primary function, valves are classified as:
1. Shut-off valves (block valves), whose purpose is to close off the flow.
2. Control valves, both manual and automatic, used to regulate flow.
Figure 6-3 shows the valves commonly encountered in industry.
5/20/2018 CHE451_Chemical Process Design and Economics
52/99
44
Figure 6-1.3: Commonly used Valves
The main types of valves used are:
Gate Figure 6-1.3a
Plug Figure 6-1.3b
Ball Figure 6-1.3c
Globe Figure 6-1.3d
Diaphragm Figure 6-1.3e
6-1.3.1 Gate valves
In gate valves, the flow is straight through and is regulated by raising or lowering the
gate. The majority of valves in the plant are of this type. In the wide open position they
cause little pressure drop.
5/20/2018 CHE451_Chemical Process Design and Economics
53/99
45
6-1.3.2 Globe valves
In globe valves, the flow changes direction and results in appreciable friction even in the
wide open position. This kind of valve is essential when tight shutoff is needed,
particularly of gas flow.
The ball and plug valves are also frequently used for the purpose of flow shutoff.
Butterfly valves are often used for the control of gas and vapor flows. Automatic control
valves are basically globe valves with special trim design.
SESSION 6-2: In this session we shall discuss the classification of pumps and factors to
consider in pump selection.
6-2.1 Introduction of Pumps
They are mechanical devices used to increase the energy of a liquid stream flowing in a
closed conduit or pipe. This energy may be used to increase the velocity (move the fluid),
the pressure or the elevation of the fluid.
6-2.2 Classification of PumpsPumps are generally classified into two main categories namely:
Centrifugal pumps
Positive displacement pumps
5/20/2018 CHE451_Chemical Process Design and Economics
54/99
46
Table 6-2.1: Main Types of Pumps
Category Types Structure
Centrifugal Single-stage
Multi-stage
Volute
Diffuser
Regenerative
Vertical
Hellico-centrifugal
Axial flow
Positive
displacement
Rotary Gear
Screw
Vane
Reciprocating Piston
Diaphragm
Plunger
There are many subgroups of pumps as indicated in Table 6-2.1.
6-2.3 Factors to Consider in Pump Selection
Capacity (flow rate in m3
/h)
The pressure head that is generated by the pump
The type of liquid pumped (its viscosity and vapour pressure under inlet
conditions)
An initial selection is generally made based on the first two criteria mentioned, i.e. the
capacity and the pressure generated.
The centrifugal pump is often the only possible choice for high capacities whereas
positive displacement pumps are better suited to generating high pressure heads.
Other criteria such as the viscosity of the liquid can modify this initial choice. A positive
displacement pump is generally recommended to pump liquids with a viscosity higher
than 2000 cP. Figure 6-2.1shows the approximate range of operation generally covered
by the main types of pump mentioned above.
5/20/2018 CHE451_Chemical Process Design and Economics
55/99
47
Figure 6-2.1: Approximate Range of Operation for the Three Main Types of Pump
6-2.4 Centrifugal Pumps
Basically consists of an impeller equipped with radial vanes rotating inside a shell called
the pump casing. It works by the transfer of centrifugal force of the rotating impeller into
kinetic energy of the liquid. This energy is then converted into pressure when the fluid
velocity decreases.
6-2.4.1 Effective characteristic curves
A centrifugal pump is characterized by 4 basic curves, all of which are expressed versus
the flow rate as shown in Figure 6-2.2.
A. The head generated
B. The mechanical/hydraulic conversion efficiency
C. The mechanical power input consumed at the shaft
D. The pump suction capacity or NPSH.
5/20/2018 CHE451_Chemical Process Design and Economics
56/99
48
Figure 6-2.2: Basic Curves Characterizing a Centrifugal Pump.
6-2.4.2 Design parameters of centrifugal pumps
- The rotation speed
- The number of impellers
- The impeller diameter
- The impeller design
6-2.4.3 Operating point
The pressure head HA required by the installation is represented by the system curve
versus flow rate, Q. It is the sum of the static and dynamic heads of the installation as
shown in Figure 6-2.3. The static heads are independent of the flow rate and include
differences in height and pressure between the unit inlet and outlet. The dynamic heads
correspond to pressure drops and are proportional to the square of the flow rate.
5/20/2018 CHE451_Chemical Process Design and Economics
57/99
49
Figure 6-2.3: Curve Characteristic of the System.
A centrifugal pump adjusts itself on an operating point B, corresponding to the
intersection between the Q/H curve of the pump and the HA curve of the system (Figure
6-2.4). A variation in the operating point (and therefore in the flow rate and the head) can
be obtained by a physical modification in the pump, but also by modifying its speed or
the system curve, usually by means of a valve.
Figure 6-2.4: Variation of Operating Point by means of a Valve.
5/20/2018 CHE451_Chemical Process Design and Economics
58/99
50
6-2.4.4 Q/H Curve versus Technical Choices
1. Basic choices; concept of specific speed
The number Nq, called the specific speed, allows all centrifugal pumps to be compared
with one another. It is calculated from the following expression:
43
60H
QNNq =
With:
N = rotational speed in rpm
Q= flow rate the best efficiency in m3/h (through one eye if double-suction impeller)
H= head in m generated at the best efficiency (for one stage)
For the same specific speed, the hydraulic designs will be similar on varying scales. The
choice of Nq is a major parameter in impeller hydraulic design (Figure 6-2.5). The
specific speed also considerably influences the best efficiency achievable by a pump. If
high pressure is required, a compromise will have to be found between a reasonable
number of stages and an acceptable number efficiency.
Figure 6-2.5: Variation in Specific Speed versus the Type of Impeller used.
2. Choice of Design: Concept of Q/H Curve Slope
For a given hydraulic choice, the different design parameters, in particular the number of
vanes can modify the curve shape downward, flat or bell-shaped (Figure 6-2.6). These
curves should be considered according to the requirements of the resisting network. Bell-
shaped curves in particular, should be avoided for pumps that have to work in parallel
(risk of instability).
5/20/2018 CHE451_Chemical Process Design and Economics
59/99
51
Figure 6-2.6: Different Types of Characteristic Curves
6-2.4.5 Choice of Rotation Speed
The rotation speed is a dominant parameter for the characteristic curve of a centrifugal
pump. Figure 6-2.7 shows how the operating point varies with speed. The flow rate varies
linearly with speed:
1
212N
NQQ =
The head generated varies with the square of the speed:
2
1
212
=
N
NHH
As a result, the power absorbed varies with the cube of the speed
3
1
212
=
N
NPP
5/20/2018 CHE451_Chemical Process Design and Economics
60/99
52
Figure 6-2.7: Variation in the Operating Point versus the Rotation Speed.
6-2.4.6 Suction Conditions: Concept of NPSHVapour pressure
For a given temperature, each liquid has a specific boiling pressure, called the vapour
pressure Tv. if the pressure at one point in the liquid becomes less than T v, the liquid
vaporizes instantly.
Cavitation
The lowest static pressure inside a centrifugal pump is located at the impeller inlet. If
vaporization begins at this point, the liquid will be repressurized nearby downstream. The
bubbles formed condense by collapsing suddenly, most often near a wall. This very noisy
phenomenon is called cavitation. The head generated by the pump and the absorbed
power then drop, the vibrations and noise increase and erosion can be observed, mainly in
the impeller, in the form of characteristic pits. If the pump is kept working under these
conditions, permanent damage may occur.
Required NPSH
To prevent cavitation, the total liquid pressure at the inlet must be such that no
vaporization can occur. The minimum value depends on the pump design
Available NPSH
It is the pressure available to force a given flow through the suction piping into the pump.
This is a function of the system. It is easily calculated with the formula below.
5/20/2018 CHE451_Chemical Process Design and Economics
61/99
53
=
+ ( )
EXAMPLE 6-1.1: Pressure drop calculation
A pipeline connecting two tanks contains four standard elbows, a plug valve that is fully
open and a gate valve that is half open. The line is commercial steel pipe, 25mm internal
diameter, length120m. The properties of the fluid are: viscosity 0.99mNm-2s, density
998kg/m3.Calculate the total pressure drop due to friction when the flow rate is 3500
kg/h.
SOLUTION 6-1.1
Cross-sectional area of pipe = ( ) 2323 10491.010254
m =
Fluid velocity, u = sm/98.1998
1
10491.0
1
3600
35003
=
Reynolds number, Re = ( )
4
3
109.9
102598.1998
4105900,49 ==
Absolute roughness commercial steel pipe = 0.046mm
Relative roughness = 0018.01025
046.03=
, round to 0.002
From friction factor chart, f = 0.0032
Fitting/valve Number of velocity
heads, K
Equivalent
pipe diameters
Entry
Elbows
Globe valve, open
Gate valve, open
Exit
0.5
(0.84)
6.0
4.0
1.0
25
(404)
300
200
50
Total 14.7 735
5/20/2018 CHE451_Chemical Process Design and Economics
62/99
54
Method 1, velocity heads
A velocity head mg
u20.0
8.92
98.1
2
22
=
= of liquid,
Head loss = 0.20 14.7 = 2.94m
As pressure = 2.94 998 9.8 = 28,754 N/m2
Friction loss in pipe,2
98.1998
1025
1200032.08
2
3
= fP
= 240,388 N/m2
Total pressure = 28,754 + 240,388 = 269,142 N/m2 = 270 kN/m2
Method 2, equivalent pipe diameters
Extra length of pipe to allow for miscellaneous losses
= 735 25 10-3
= 18.4 m
So total length for P calculation = 120 + 18.4 m
2
98.1998
1025
1380032.08
2
3
= fP
= 277 kN/m2
Note: the two methods will not give the same results. The method of velocity heads is the
more fundamentally correct approach. But the use of equivalent diameters is easier to
apply and sufficiently accurate for use in design calculations.
5/20/2018 CHE451_Chemical Process Design and Economics
63/99
55
SELF ASSESSMENT 6-1
A process fluid is pumped from the bottom of one distillation column to another, using acentrifugal pump. The line is standard commercial steel pipe 75 mm internal diameter.From the column to the pump inlet the line is 25 m long and contains six standard elbowsand a fully open gate valve. From the pump outlet to the second column the line is 250 m
long and contains ten standard elbows, four gate valves (operated fully open) and a flow-control valve. The fluid level in the first column is 4 m above the pump inlet. The feedpoint of the second column is 6 m above the pump inlet. The operating pressure in thefirst column is 1.05 bara and that of the second column 0.3 barg. Determine the operatingpoint on the pump characteristic curve when the flow is such that the pressure drop acrossthe control valve is 35 kN/m
2. The physical properties of the fluid are: density 875 kg/m
3,
viscosity 1.46 mNm-2s.Pump characteristicFlow-rate, m3/h 0.0 18.2 27.3 36.3 45.4 54.5 63.6Head, m of liquid 32.0 31.4 30.8 29.0 26.5 23.2 18.3
5/20/2018 CHE451_Chemical Process Design and Economics
64/99
56
UNIT 7PROCESS ECONOMICS: COST ESIMATION
Introduction
This unit discusses the estimation of capital investment, total product cost and thedetermination of breakeven point.
Learning ObjectivesAfter reading this unit, you should be able to:
1. Define total capital investment2. Apply the Marshall & Swift cost index and capacity ratio to estimate
equipment cost3. Distinguish between fixed capital and working capital4. Define turnover ratio
SESSION 7-1 In this session we shall discuss the estimation of capital investment.
Further we shall discuss fixed capital investment, working capital, cost indices and turn-
over ratio.
7-1.1 Cost Estimation
An acceptable plant design must present a process that is capable of operating under
conditions which will yield a profit. Since net profit equals total income minus all
expenses, it is essential that the chemical engineer be aware of the many different types
of cost involved in manufacturing process. Capital must be allocated for direct plant
expenses, such as those for raw materials, labor and equipment. Besides direct expenses,
many other indirect expenses are incurred and these must be included if a complete
analysis of the total cost is to be obtained. Examples of these indirect expenses are
administrative salaries, product-distribution cost, and cost for interplant communications.
A capital investment is required for any industrial process, and determination of the
necessary investment is an important part of a plant-design project. The total investmentfor any process consist offixed-capital investment for physical equipment and facilities
in the plant plus working capital which must be available to pay salaries, keep raw
materials and products on hand and handle other special items requiring a direct cash
outlay.
5/20/2018 CHE451_Chemical Process Design and Economics
65/99
57
Thus in an analysis of cost in industrial processes, capital investment costs,
manufacturing costs, and general expenses including income taxes must be taken into
consideration.
7-1.2 Cash Flow for Industrial Operations
Figure 7-1.1 shows the concept of cash flow for an overall industrial operation.
Figure 7-1.1: Cash Flow for an Overall Industrial Operation.
5/20/2018 CHE451_Chemical Process Design and Economics
66/99
58
7-1.3 Factors affecting Investment and Production Costs
Sources of equipment
Price fluctuations
Company policies
Operating time and rate of production
Governmental policies
7-1.4 Capital Investment
Before an industrial plant can be put into operation, a large sum of money must be
supplied to purchase and install the necessary machinery and equipment. Land and
service facilities must be obtained, and the plant must be erected complete with all
piping, controls, and services. In addition it is necessary to have money available for the
payment of expenses involved in the plant operation.
- Fixed capital investment
The capital needed to supply the needed manufacturing and plant facilities.
- Working capital
The capital necessary for the operation of the plant
- Total capital investment = Fixed capital investment + Working capital
- Fixed capital investment= Manufacturing fixed capital investment + Non-
manufacturing fixed capital Investment.
- Manufacturing fixed capital investment: the fixed capital necessary for the
installed process equipment with all auxiliaries that are needed for complete
process operation. These include expenses for piping, instruments, insulations,
foundations, and site preparation.
5/20/2018 CHE451_Chemical Process Design and Economics
67/99
59
- Non- manufacturing fixed capital investment: the fixed capital required for
construction overhead and for all plant components that are not directly related to
the process operation. These plant components include the land, processing
buildings, administrative and other offices, warehouses, laboratories,
transportation, shipping and receiving facilities, utilities and waste disposal
facilities, shops and other permanent parts of the plant.
- Construction overhead cost: consists of field-office and supervision expenses,
home-office expenses, engineering expenses, miscellaneous construction costs,
contactors fees and contingencies. In most cases, construction overhead is
proportioned between manufacturing and non-manufacturing fixed-capital
investment.
- Working capital: the working capital for an industrial plant consist of the total
amount of money invested in
o Raw materials and supplies carried in stock
o Finished products in stock and semi-finished products in process of being
manufactured
o Accounts receivable
o Cash kept on hand for monthly payment of operating expenses , such as
salaries, wages, and raw materials purchases
o Accounts payables
o Taxes payables
The ratio of working capital to total capital investment varies with different
companies. Most chemical plants use an initial working capital of 10 to 20 percent
of the total capital investment. It is 50% or more for companies producing
products of seasonal demand because of the large inventories which must be
maintained for appreciable period of time.
5/20/2018 CHE451_Chemical Process Design and Economics
68/99
60
7-1.5 Estimation of Capital Investment
7-1.5.1 Introduction
Of the many factors which contribute to poor estimates of capital investments, the most
significant one is usually traceable to sizeable omissions of equipment, services, or
auxiliary facilities rather than to gross errors in costing. A check list of items covering a
new facility is an invaluable aid in making a complete estimation of the fixed capital
investment. A typical list of items for estimating fixed capital investment is:
Direct cost
1. Purchased equipment; all equipments listed on a flow sheet
2. Purchased-equipment installation
3. Instrumentation and controls
4. Piping
5. Electrical equipment and materials
6. Buildings(including services)
7. Yard improvements
8. Service facilities
9. land
Indirect cost1. Engineering and supervision
2. Construction expenses
3. Contractors fees
4. Contingencies
7-1.5.2 Types of capital cost estimates
An estimate of the capital investment for a process may vary from a predesign estimate
based on little information except the size of the proposed project to a detailed estimate
prepared from complete drawings and specifications. The following five categories
represent the accuracy range and designation normally used for design purposes:
1. Order-of-magnitude estimate(ration estimate) based on similar previous cost data:
probable accuracy of estimate over30 percent
5/20/2018 CHE451_Chemical Process Design and Economics
69/99
61
2. Study estimate (factored estimate) based on knowledge of major items of
equipment; probable accuracy of estimate up to30 percent
3. Preliminary estimate (budget authorization estimate; scope estimate) based on
sufficient data to permit the estimate to be budgeted; probable accuracy of
estimate within20 percent.
4. Definitive estimate(project control estimate) based on almost complete data but
before completion of drawings and specifications; probable accuracy of estimate
within10 percent
5. Detailed estimate (contractors estimate) based on complete engineering
drawings, specifications and site surveys; probable accuracy of estimate within
5 percent.
7-1.5.3 Cost indexes
Most cost data which are available for immediate use in a preliminary or predesign
estimate are based on conditions at some time in the past. Because prices may change
considerably with time due to changes in economic conditions, some methods must
be used for updating cost data applicable at a past date to costs that are representative
of conditions at a later time. This is done using a cost index.
Acost index is hence an index value for a given point in time showing the cost ofanequipment or plant at that time relative to a certain base time.
=
obtainedwascostoriginaltimeatvalueindex
timepresentatvalueindexcostOriginalcostesentPr
Cost indexes can be used to give a general estimate, but no index can take into
account all factors, such as special technological advancements or local conditions.
The common indexes permit fairly accurate estimates if the time period involved is
less than 10 years.
5/20/2018 CHE451_Chemical Process Design and Economics
70/99
Some indexes are used fo
labor, construction, mate
The most common of the
process-industry equipm
the Nelson refinery const
index. This is shown in T
Table
7-1.5.4 Methods for Estima
7-1.5.4.1 Power factor appl
( )nn RCFeC =
Where:
R = size (or capacity) ratio o
62
r estimating equipment cost, others apply speci
ials etc.
se indexes are theMarshall and Swift all-indust
nt indexes, the Engineering New-Record const
ruction index and the Chemical Engineering pl
ble 7-1.1.
7-1.1: Cost indexes as Annual Averages
ing Capital Investment
ed to plant-capacity ratio:
equipment=plaorequipmentoldofcapacity
plaorequipmentnewofcapacity
ically to
ry and
uction index,
nt cost
t
t
5/20/2018 CHE451_Chemical Process Design and Economics
71/99
Fe = equipment cost index ra
n = equipment size (or capac
C = purchased cost of old eq
Cn = purchased cost of new e
Table 7-1.2 shows typical ex
Table 7-1.2 Typi
63
tio=obwascostoriginaltimetheatvalueindex
timepresentinvalueindex
ity) exponent
ipment
quipment
ponents for equipment cost vs. capacity
al Exponents for Equipment Cost vs. Capaci
tained
ty
5/20/2018 CHE451_Chemical Process Design and Economics
72/99
64
EXAMPLE 7-1.1
If a process plant was erected in Kumasi at a fixed capital investment of $436,000 in
1970, determine what the capital investment will be in 1975 for a similar process plant
located near Dansoman in Accra with twice the process capacity but with an equal
number of process equipment.
SOLUTION 7-1.1
( )nn RCFeC =
Where:
R = size ( or capacity) ratio of equipment
Fe = equipment cost index ratio
n = equipment size exponent
C = purchased cost of equipment in 1970
Cn = cost of equipment in 1975
From Table 7-1.1
M & S index, 1970 =303,
M & S index 1975 = 444,
From Table 7-1.2
Equipment size exponent, n = 0.6
( ) 378,968$2303
444000,436
6.0 ==nC
5/20/2018 CHE451_Chemical Process Design and Economics
73/99
65
7-1.5.4.2 Detailed item estimate
EXAMPLE 7-1.2
Initial design work was done for a chemical plant to revamp the process in order to
recover valuable product from an effluent gas stream. The gas will be scrubbed with a
solvent in a packed column and the recovered product and solvent separated by
distillation. The solvent will then be cooled and recycled. The major items of equipment
required and their purchased costs in cedis are:
Absorption column
Column purchased cost = 19,800
Cost of column packing = 6,786
Recovery column
Column purchased cost = 45,000
Cost of 30 sieve trays = 8,670
Reboiler
Cost of reboiler = 7,600
Condenser
Purchased cost = 4,800
Recycle solvent cooler
Cost of cooler = 2,550Storage tank
Cost of tank = 7,790
The relevant component factors for this processing plant are;
f1: equipment erection = 0.40
f2: piping = 0.70
f3: Instrumentation = 0.2
f4: electrical = 0.10
f10: design and engineering = 0.30
f12: contingencies = 0.20
Estimate the total capital investment for the project, if the working capital can be taken as
10 % of the fixed capital cost.
5/20/2018 CHE451_Chemical Process Design and Economics
74/99
66
SOLUTION 7-1.2
Total capital investment, TCI = Fixed capital investment, FCI Working capital, WC
Total purchased cost of equipment, PCE
Absorption column = 19,800 + 6,786 = 26,586
Recovery column = 45,000 + 8,670 = 53,670
Reboiler = 7,600
Condenser = 4,800
Recycle solvent cooler = 2,550
Solvent and product storage tank = 7,790
Total purchased cost of major equipment = 102,996
Equipment erection = 102,996 (0.4) = 41,198
Piping cost = 103,996 (0.70) = 72,097
Instrumentation cost = 102,996 (0.2) = 20,599
Electrical = 102,996 (0.10) = 10,300
Total physical plant cost (Direct cost) = 10,300 + 20,599 + 72,097 + 41,198 + 102,996
= 247,190
Indirect cost
Design and engineering, f10 = 0.30 (247,190) = 74,157
Contingencies = 0.20 (247,190) = 49,436Total indirect cost = 123,595
Fixed capital investment (Direct cost + Indirect cost) = 247,190 + 123,595 = 370,785
Working capital = 0.10 (370,785) = 37,079
Total capital investment = 370,785 + 37,079 = 407,864
7-1.5.4.3 Other Methods for Estimating Equipment or Capital Investment
(i) Unit cost Estimate
(ii) Percentage of delivered-equipment Cost
(iii) Lang factors for approximation of capital Investment
5/20/2018 CHE451_Chemical Process Design and Economics
75/99
67
7-1.5.5.Turnover Ratios:
A rapid evaluation method suitable for order-of-magnitude estimates is known as the
turnover ratio method.
Turnover ratio is defined as the ratio of gross annual sales to the fixed capital
investment
tinvestimencapitalfixed
salesannualgrosstioTurnoverra =
Where the product of the annual production rate and the average selling price of the
commodities produced is the gross annual sales figure.
ratioTurnoverratioinvestmentorratioCapital
1=
Turnover ratio ranges between 0.2 to 5. For chemical industries, as a very rough rule of
thumb, the ratio can be approximated to 1.
SESSION 7-2: In this session we shall study the estimation of total product cost and
determination of breakeven point.
7-2.1 Estimation of Total Product Cost
Manufacturing cost: all expenses directly connected with the manufacturing
operation or the physical equipment of a process plant itself.
There are 3 classifications of these expenses:
Direct production cost: expenses directly associated with the
manufacturing operation. E.g. expenses for raw materials ( including
transportation, unloading)
5/20/2018 CHE451_Chemical Process Design and Economics
76/99
68
Fixed charges: expenses that remains practically constant from year to
year. Do not vary widely with changes in production rate. E.g. property
taxes, insurance.
Plant overhead cost: are for medical and hospital services; general plant
maintenance and overhead; safety services; social security
General Expenses:
Administrative expenses
Distribution and marketing expenses
Research and development
Financing expenses: extra costs involved in procuring the money
necessary for the capital investment.
Gross earnings expenses
Total Product Cost= Manufacturing cost + General Expenses.
Gross earnings (Gross Profit)= The total incomethe total product cost.
Net annual earnings = Gross annual earningsincome taxes.
Figure 7-2.1 shows the components constituting the estimation of total product cost
5/20/2018 CHE451_Chemical Process Design and Economics
77/99
69
Figure 7-2.1: Cost Involved in Total Product Cost for a Typical Chemical
Process Plant
7-2.2 Break-even Point
(a) Break-even point occurs (or is the percentage of plant capacity) when the total
annual product cost equals the total annual sales.
i.e: Total Annual Product Co
Top Related