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8/10/2019 Subject 6. Analysis of Processes. Material and Heat Balance OCW (1)
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http://creativecommons.org/licenses/by-nc-sa/3.0/es/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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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.