International Conference on Electrical, Electronics, Computer Science and Mathematics Physical Education & Management, (ICEECMPE), 8th November 2014, New Delhi, India, ISBN: 978-93-84209-63-6

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DESIGNING OF A PROPYLENE FRACTIONATION COLUMN

SHWET KUMAR RINWA

Department of Chemical Engineering, IIT-BHU, Varanasi, Uttar-Pradesh, India E-mail: shwet.krinwa.che11@itbhu.ac.in

Abstract- Designing a Distillation column for the Propylene Plant. This involved the development of flow sheet model using simulation software (Pro/II) for the given feed and product specification as well as to make a process flow diagram of column and generate overall material for the column. It was followed by finding optimum number of stages and feed tray location. It also included estimation of column height and diameter. This was followed by a study of tower internals and safety factors to be considered during tower design. Mc-Cabe Thiele method was used to find the number of stages required to separate Propylene from Propane. The number of stages and the feed tray location were then calculated using Pro II software by doing simulation. Corresponding to number of stages and minimum reflux ratio overall heat duty was calculated. Keywords- Propylene, Optimum, Mc-Cabe, Simulation I. INTRODUCTION Distillation is by far the most important separation process in the petroleum and chemical industries. It is the separation of key components in a mixture by the difference in their relative volatility, or boiling points. It is also known as fractional distillation or fractionation. In most cases, distillation is the most economical separating method for liquid mixtures. However, it can be energy intensive. Distillation can consume more than 50% of a plant’s operating energy cost. There are alternatives to distillation process such as solvent extraction, membrane separation or adsorption process. On the other hand, these processes often have higher investment costs. Therefore, distillation remains the main choice in the industry, especially in large-scale applications.

Feed and Product Specification

Table -1

Data of Relative Volatility and mole fraction of propylene:

Procedure The stepwise procedure is given as follows: 1. Firstly calculate the equilibrium conc. y by

2. Feed line or q line: q value is calculated by

And equation of q line is given by:-

Designing of a Propylene Fractionation Column

International Conference on Electrical, Electronics, Computer Science and Mathematics Physical Education & Management, (ICEECMPE), 8th November 2014, New Delhi, India, ISBN: 978-93-84209-63-6

81

Since the feed is saturated liquid so, q line will be vertical. 3. Now plot graph between y Vs x

III MCCABE-THIELE MODEL The McCabe–Thiele method is the simplest and perhaps most instructive method for the analysis of binary distillation. It uses the fact that the composition at each theoretical tray (or equilibrium stage) is completely determined by the mole fraction of one of the two components and is based on the assumption of constant molar overflow which requires that: the molar heats of vaporization of the feed components are equal for every mole of liquid vaporized, a mole of vapor is condensed heat effects such as heats of solution are negligible For finding the theoretical number of stages by McCabe Thiele Diagram we first need to draw the operating line for Rectifying and Stripping Section for the specified R. Let’s take R=1.25 Rmin.

Therefore, Operating line for Rectifying Section: We know that equation of operating line for Rectifying Section is:

Which gives us a line DC (refer Graph 1) intersecting q line on F”. Operating line for Stripping Section: Now since we already have two points (W and F”) so we simply join them to get the operating line for stripping section. Finally, as exemplified in Fig. 1, steps between operating lines and the equilibrium line are drawn and then counted which turned out roughly around 163. These steps represent the Theoretical plates (or equilibrium stages). Therefore required number of theoretical plates is roughly 163 for the binary distillation depicted in Fig. 1. IV. INTRODUCTION TO PRO II 9.1 PRO II is a comprehensive computer simulation system for process engineers in the chemical, petroleum, natural gas, solids processing, and polymer industries. It contains the data resources of a large chemical component library and extensive thermodynamic property prediction methods with the most advanced and flexible unit operations techniques. It provides the process engineer with the computational facilities to perform all mass and energy balance calculations needed to model most steady-state processes. The simulation for Propylene fractionator was carried out using GUI (Graphical User Interphase). For the given set of data the most suitable

Designing of a Propylene Fractionation Column

International Conference on Electrical, Electronics, Computer Science and Mathematics Physical Education & Management, (ICEECMPE), 8th November 2014, New Delhi, India, ISBN: 978-93-84209-63-6

82

Thermodynamic model is Peng Robinson. Inside-out algorithm was used. Heat, material and design specification was solved in the inner loop on the basis of simple thermodynamic for enthalpy and Vapor-Liquid Equilibrium (VLE). In the outer loop simple thermodynamic models are updated on the basis of the result from inner loop. Using PRO II Propylene Fractionator was simulated for the specified design specification and a case study was done which resulted in R v/s N (i.e. Reflux Ratio v/s Number of stages, refer Fig. 1 and table 5. This also gives the optimum feed tray location for minimum condenser and re-boiler load. Optimum feed tray location is that where the operating cost of the column is minimum, and minimum operating cost means minimum condenser and re-boiler duty. From the graph we find out that for 1.25 Rmin the theoretical number of stages comes out to be 212 and feed tray location being 123rd tray for minimum operating cost.

Table -4

Fig-1

Any reflux ratio between the minimum and infinity will provide the desired separation, with the corresponding number of theoretical trays required varying from infinity to the minimum number, as in Fig. 2. The reflux ratio to be used for design should be the optimum, or the most economical, reflux ratio, for which the cost will be the least. Refer to Fig. 3. At the minimum reflux ratio the column requires an infinite number of trays, and consequently the fixed cost is infinite, but the operating costs (heat for the re-boiler, condenser cooling water, power for reflux pump) are least. As R increases, the number of trays rapidly decreases, but the column diameter increases owing to the larger quantities of recycled liquid and vapor per unit quantity of feed. The condenser, reflux pump, and re-boiler must 'also be

larger. The fixed costs therefore fall through a minimum and rise to infinity again at total reflux. The heating and cooling requirements increase almost directly with reflux ratio, as shown. The total cost, which is the sum of operating and fixed costs, must therefore pass through a minimum at the optimum reflux ratio. This will frequently but not always occur at a reflux ratio near the minimum value (mostly at 1.2Rmin or 1.5 Rmin). I have taken the operating value of R to be 1.25 Rmin (13.80).

Note: Difference between Theoretical Number of stages and Theoretical number of trays is 2, as re-boiler and condenser both are counted while calculating theoretical number of stages. Therefore operating point specifications are:

V. EFFICIENCY Tray efficiency is the fractional approach to an equilibrium stage which is attained by a real tray. Ultimately, we require a measure of approach to equilibrium of all the vapor and liquid from the tray. One method of describing the performance of a tray tower is through the overall tray efficiency, given that overall efficiency is 0.9.

Therefore, Number of real trays required =210/0.9= 234 So, we require 233 number of trays and 1 sump of tower DA-01 which total gives up 234 rea trays for having the specified product specification, with feed at 137th tray. VI. PROCESS DESCRIPTION Now, for the given number of trays and reflux ratio simulation was carried out on PRO II with

Designing of a Propylene Fractionation Column

International Conference on Electrical, Electronics, Computer Science and Mathematics Physical Education & Management, (ICEECMPE), 8th November 2014, New Delhi, India, ISBN: 978-93-84209-63-6

83

Peng-Robinson as the thermodynamic model. The C3 splitter consists of a two-tower fractionation (DA-01 and DA-02) system each containing 127 and 126 trays respectively (refer PFD). Two tower design is considered as single tower design would have resulted in impractical tower height and less energy efficient system. The C3mixed Feed (100) is fed to #127 tray of DA-01. The bottom product from column 1(215) i.e. DA-01 serves as external reflux to column 1 (DA-02) and the top product from column 1(101) i.e. DA-02 serves as a source of vapor to DA-01. The bottom product from DA-02 is 99.0% Propane @69.5°C and 25.31 kg/cm2g. This bottom product is spilt into 2 streams 115 and 111. Stream 115 is then cooled to 40°C for storage by cooling water in Heat Exchanger (HE-02) and then sent (116)to propane outside battery limit. Whereas stream 115 is fed to the thermosyphon Re-boiler (HE-01) with 0.5 and liquid to vapour ratio in the outlet stream (116), which is sent back to DA-02. Low pressure steam at 3.5 kg/cm2g is used as the heating utility. The condensed Water from HE-01 is the flashed into condensate pot (D-02). Overhead vapor from DA-01(201) is split into two stream one is fed to condenser (AC-01), outlet from which is then fed to reflux drum (D-01), and the other goes as equalising stream directly to reflux drum (D-01). Reflux drum (D-01) has one outlet which goes to flare when there is excess of vapour in the drum. This stream is normally closed. The other outlet is again split into the distillate product (210) and the reflux to DA-02(208). The distillate (210) is sent to propylene outside battery limit for storage. VII. DIAMETER CALCULATION The main factor which determines column diameter is the vapor flow rate. Vapor velocity must be below that which would cause excessive liquid entrainment or high ∆P. For hydraulic calculations on a tray, consider the liquid leaving the tray and vapor entering the tray. So, maximum “Vapor To” and “Liquid From” flow rates were find out and were used to decide the tower diameter for proper functioning of tower.

Data used: Tray Type: Valve Tray VG-10 Percentage open area: 13.5 % Number of passes: 4 Tray spacing: 450 mm RESULTS Results of KG Tower obtained are Diameter of Tower = 6100 mm Height of Tower Tray Spacing above Top Tray - The distance from the top tray floor to the upper tangent line is normally controlled by the internals design and often results in a generous margin over the normal tray spacing(usually 1000mm), but in no case should it be less than the tray spacing. At any point in the tower where there is a disturbance between adjacent trays, e.g., an inlet, outlet, transition, etc., the resulting inter tray distance generally exceeds the normal tray spacing (usually taken as 1000mm) since again it is controlled by the dimensions required to accommodate the internals.

Height of DA-01= (114-1)x450 + 1000+1000 =52850 mm

Height of DA-02= (120-1)x450 +1000+1000 =55550 mm

Reflux Drum specification: Reflux Drum Residence Time= 5min Reflux Drum L/D ratio= 4 REFERENCES

[1]. Lummus Technology “Standard Data Book”

[2]. Basic Engineering Package, “Zhenhai 1,000KTA Ethylene Plant, Ningbo, China ”

[3]. Coulson, J.M. and Richardson, J.F. : “Chemical Engineering” , Vol. 2 Fifth edition, 2002

[4]. Coulson, J.M. and Richardson, J.F. : “Chemical Engineering” , Vol. 6 Third edition

[5]. Perry: Chemical Engineers’ Handbook.

[6]. Engineering of Chemical Reactions: Ch7. Catalytic Reactor and Mass Transfer “– Lanny Schmidt.