Subject 6. Analysis of Processes. Material and Heat Balance OCW (1)

8/10/2019 Subject 6. Analysis of Processes. Material and Heat Balance OCW (1)

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INDEX

1.- Analysis of the inputs effect on the outputs.

2.- Methodology for process analysis: Material and energy balances.

3.- Basic Ideas to develop LINEAR MASS BALANCES (LMB) models.

4.- Develop of LMBs.

5.- Case Study: Application of LMB algorithm and setting pressure andtemperature levels in flowsheet.

6.- Heat Balances.

7.- Further Reading and References.


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1.- Analysis of the inputs effect on the outputs

Two categories of relationships

1.- Relationships independent of the equipment

Independent of the equipment specificationsBM / BE / Equilibrium / Kinetic

2.- Relationships dependent of the equipment

Design equations with equipment specifications:

- Heat transference equation (with Area value)- Frictional pressure relationships (with D, Le)


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2.- Methodology for process analysis: Material and energybalances.

Analysis methodology to apply on the synthesizedflowsheets

- Simples- Fast

- Useful to the preliminary design

Mass balances Systematic modeling withEnergy balances different degree of rigorousness


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2.- Methodology for process analysis: Material and energybalances.

Once we synthesize flowsheet, we must do mass and energybalances to analyze its PERFORMANCE and to SIZE equipmentfor economic evaluation

2.1.- Equation OrientedWrite al equations that describe the process and solve themsimultaneously.Equations: Material balances

Equilibrium relationsKinetic expressionsEnthalpy balances, etc.

Need solve 100s equations with Newtons

Software: gPROMS, GAMS, EXCEL, ASPEN equations


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2.2.- Sequential Modular

Solve for input streams in each unit at the flowsheetfollowing the information flow. For each module, computeoutlet from given inlet.

1.Calc. COMP 22.Guess stream 63.Calc. MIX 34.Cal. RXN 45.Cal. FLASH 5, 66.If stream 6 is similar 6 STOP, otherwise return to step 2

More robust, but less flexibleSoftware: ASPENPLUS, PRO-II, HYSYS, UniSIM

COMP MX RXN FLASH1 2 3

66

5

4


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3.- Linear Mass Balances (LMB) models

Basic Ideas to develop LMB Fix levels at P, T in flowsheet to derivelinear approximations at mass balances in each unit.

1.- Fix P and T levels in flowsheet . Specify recoveries, split fractions keycomponents, conversion per pass, recycle ratios (Degrees Freedom-d.f.)

2.- Approximate each unit with (fracciones divididas) (Split Fractions)(e.g. , in flash unit; , , , , in absorber) to relate linearly output molar flowwith inputs

3.- Set up linear equations and solve for molar flows at each component

4.- Recalculate P, T in flowsheet with equilibrium equations* If there are not big changes go to the step 5.* If there are big changes go to step 2.* If process does not meet specifications (e.g. equil. of DEE), change the

values of the d.f. or modify flowsheet structure returning to step 2.

5.- Perform heat balances (heating + cooling utilities). Perform Heatintegration at this stage. Idea Decouple mass and heat balances


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8SIZING COSTS

Specify splitfractions of the key

components

Global massbalance

)(inout

=

P , T specify

P and T

Determine coefficientsfor linear models in

each unit: k/n, , ,

Specifications and/orrestrictions

Fenske, Kremser, Antoineequations

Equilibrium equations(Bubble point, dew point)

T, P restrictions

LMB algorithm


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3.- Linear Mass Balances (LMB) models

ASSUMPTIONS- L and V streams with ideal equilibrium relationships- Saturated streams- Linear equations simple to solve tearing- Convergence in few interactions (2)- Use basis at 100 gmol/s feed and calculate scale up factor to meetrequired production

Approximations for following modules

1) Mixer Not real or physically equipments. Could be junctions.2) Splitter3) Reactor4) Flash Linear in terms of split fractions5) Distillation6) Absorption and Stripper Associated (MEA Process)

There are many other equipments as Adsorption, pervaporation, fermenters, etc .


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Simplifications and approximations Shortcut methods smaller problemMAIN CHARACTERISTICS:

- Ideal solutions. Saturated streams. Ideal thermodynamic behaviour- Process units calculations Linear equations

====l

k j i

k Mixer l l ,

k

ji 11 ,

k ji 22 ,

k M M

k in j

k j Splitter ====,

k i

NS

j j

k NS Spitter )1(

1

1,

========

k in

k S 1 k S 2

S

k IN k R R

k IN

k F 1

F

k F 2

ink k

F =1

inkk 2F )1( =

====++++==== NR

r

r l inr k r

k in

k actor

1

)(,Re

Mixer

Splitter

Reactor

Flash

Linear coefficients

Split fraction

Split fractions

Conversion

Stoichiometriccoefficients

------

4.- Develop of (LMB) models


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Distillation

Absorption

Rdew Rbubcdewcbub T T T T ,,,,

Hk Lk

Hk

Hk

Lk

Lk

m N /ln/1ln =

+=

Hk

Nm

Hk k

Hk

Nm

Hk k

k

)1(1 /

/

ABS

VN

L0 V1

LN

)ln(/)1()(

ln10

10 n

E n

N

nn

E

n

n

N

n

E

nn

Avr Al v Ar l

N

+=+

+

k

ink

k

D =

k

k

N

k

N k

N k

N

k l vv 01

11

1

+

+ +=

k k k

N

k

N vl vl

101+= +

k

ink

k

B )1( =

)(1)(1 1k

E

N k E k

N A=

+

)(1

)(11 k

E

N k

E k

N

A

=

4.- Develop of LMB models


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5.- CASE STUDY: Application of LMB algoritm and settingpressure and temperature levels in flowsheetOBTENTION OF ETHANOL

Establish levels of P, T in PFD

Specify recoveries, splits key components. Fixed the recycle rate (specify d.g).

Determine coefficients linear models

Calculate k/n , k, AE, , etc. Based on Antoine equation.

Set-up linear equations and solve for flowrates of each component. Solvingequations of PFD by sequential approach

Calculate P, T through flowsheet

If the guesses different step 1 return to step 2

If process does not meet specifications either change recoveries (e.g. therecycle rate is too low) or modify flowsheet (e.g. need a 2 nd flash unit to obtainthe NH3 purity need)

Heat Balances (Heating steam- and Cooling water- utilities)

Memo 2 is to calculate a mass and energy balance for a specified process flowsheet


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6.- Heat Balances

ENTLAPHIES: H (P,T ,z k) [J/gmol]

- Reference state is necessary for the calculations.Ideal Gas, P 0=1atm, T 0=298 K

- Assume elemental species- Assume ideal behaviour reflect effects of P and mixing (speciallyin the liquid phase)

GAS MIXTURES

LIQUID MIXTURES

)(),(0

00 dT C f H y yT H T

pk k

k v k +=

[ ])(),(0

00 T H dT C f H x xT H k vT

pk k

k L k +=

by Watson correlation in functionof Tb, Tc, and Hvap(Tb)

Cpk as function of T by heatcapacity coefficients Ak,Bk,Ck, Dk

Constants for each k Handbookvalues

(Perry,2008;

Poling et al.,2000)


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7.- Further Reading and References

Biegler, L., Grossmann, I., Westerberg , A., 1997, Systematic Methods of ChemicalProcess Design, Prentice Hall.

Green, D., Perry, R., 2008, Perry's Chemical Engineers' Handbook. 8 th edition.McGraw-Hill.

Kent, 1992, Riegels Handbook of Industrial Chemistry.

Lide, D., Ed., 1997, CRC Handbook of Chemistry and Physics. CRC Press.

McKetta, J., Ed. 1993, Chemical Processing Handbook. Marcel Dekker.

Poling, B., Prausnitz, J., O'connell, J., 2000, The properties of gases and liquids. 5 thedition. McGraw-Hill.

Treybal, R., 1980, Mass Transfer Operations. 2 nd Ed. McGraw Hill.

Woods, D., 1995, Data for Process Design and Engineering Practice. Prentice-Hall.

Subject 6. Analysis of Processes. Material and Heat Balance OCW (1)

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