Episode 57 : Simulation for Design and Analysis

SAJJAD KHUDHUR ABBASCeo , Founder & Head of SHacademyChemical Engineering , Al-Muthanna University, IraqOil & Gas Safety and Health Professional – OSHACADEMYTrainer of Trainers (TOT) - Canadian Center of Human Development

Episode 57 : Simulation for Design and Analysis

Problem Description

Methyl-acetate is produced by esterification of methanol and acetic acid which produces water as by-product.

CH3OH + CH3COOH � CH3COOCH3 + H2OThe mixture to be separated is an outlet stream from a reactor (assumed conditions: P=1 atm, T=450 K) and the methyl-acetate product specification is 99 %.

Problem statement: Given the identity and composition of the components in the mixture to be separated, together with the product specifications, identify a physically feasible flowsheet for separation of the mixture into the specified products. Determine the estimates of operating conditions and identify a MSA where required. Verify the generated process flowsheet with process simulation, together with the alternative separation techniques for its separation task.

Tasks to perform ?1. Analyze mixture

2. Compute binary ratio matrix

3. Identify separation techniques

4. Screen alternatives

5. Compute separation factor

6. Select the first separation task

7. Generate process flowsheet

8. Analyze decisions

9. Verify flowsheet


Nature of mixture

State of components at a reference condition

Number of binary pairs

Analysis of azeotropes

Liquid miscibility, solubility & solvents


State of components at reference conditions (T = 298 K, P = 1 atm): All components are liquids at reference conditions, since their boiling points are: MeAc (330.1 K), MeOH (337.7 K), H2O (373.2 K), HOAc (391.0 K).

Number of binary pairs: There are 6 binary pairs (MeAc/MeOH, MeAc/H2O, MeAc/HOAc, MeOH/H2O, MeOH/HOAc, and HOAc/H2O).

Identification of azeotropes with pressure dependence: The following pairs form azeotropes at P = 1 atm, based on vle data (see table below).

Table I: Azeotropes at P = 1 atm for methyl-acetate exampleBinary pair Azeotrope T (K) , x (mole %)MeAc/H2O 330.1 K , 92.8 % MeAcMeAc/MeOH 328.1 K , 67.4 % MeAcH2O/HOAc 371.8 K , 15.7 % HOAc


Table II: Mutual solubilities for methyl-acetate example

Table III: Solubility parameters in methyl-acetate example

Binary pair Mutual solubilities (gr/cm3)

Binary pair Mutual solubilities (gr/cm3)

MeAc in MeOH 0.652 MeOH in MeAc 0.131MeAc in HOAc 1.146 HOAc in MeAc 0.385MeAc in H2O 0.162 H2O in MeAc 0.027MeOH in HOAc 0.342 HOAc in MeOH 1.947MeOH in H2O 0.988 H2O in MeOH 0.280HOAc in H2O 0.917 H2O in HOAc 0.124

Component Solubility parameter (Mpa1/2 ) (ProPred)

Solubility parameter (Mpa1/2 ) (databank)

MeAc 19.35 19.35MeOH 27.46 29.6HOAc 29.73 19H2O 47.8 47.8


Table IV: Multi-phase flash calculation for MeAc/MeOH/CO2 system

Table V: Multi-phase flash calculation for MeAc/H2O/CO2 system

Feed stream(kmol/hr)(molefractions)

T = 420 K , P = 75 atmPhase 1 (molefractions) Phase 2 ( molefractions )

MeAc 67.4 0.337 0.176 0.484MeOH 32.6 0.163 0.098 0.222CO2 100 0.500 0.726 0.294

Fraction of feed 0.476 0.524

Feed stream(kmol/hr)(molefractions)

T = 420 K , P = 75 atmPhase 1 (molefractions) Phase 2 ( molefractions )

MeAc 92.8 0.464 0.232 0.639H2O 7.2 0.036 0.024 0.045CO2 100 0.500 0.744 0.316

Fraction of feed 0.430 0.570

Tasks to perform ?2. Binary ratio matrix : rk = pk / pk

ij i j

where i and j are component indices and, pk

i > pk

j

Tasks to perform ?2. Binary ratio matrix : rk = pk / pk

ij i j

where i and j are component indices and, pk

i > pk

jTable VIII: Computed binary ratio matrix for methyl-acetate exampleRatio of properties for binary pairs

Property MeOH/ HOAc

MeOH/ MeAc

MeOH/ H2O

HOAc/ MeAc

HOAc/ H2O

MeAc/ H2O

Molecular weight 1.87 2.31 1.78 1.23 3.33 4.11Critical temperature 1.15 1.01 1.26 1.17 1.09 1.28

Boiling point 1.16 1.02 1.10 1.18 1.05 1.13Dipolemoment 1.02 1.01 1.09 1.04 1.06 1.10

Radius of gyration 1.68 1.93 2.52 1.15 4.24 4.87Melting point 1.65 1.00 1.56 1.66 1.06 1.56Triple point temperature

1.65 1.00 1.56 1.66 1.06 1.56

Triple point pressure 11455.3 9.12 5487.96 1255.5 2.09 601.5Molar volume 1.42 1.97 2.25 1.39 3.19 4.42

Solubility parameter 1.56 1.53 1.62 1.02 2.52 2.47Van der Waals

volume1.53 1.96 1.76 1.28 2.69 3.44

Vapour pressure 8.11 1.70 5.32 13.78 1.52 9.04Binary azeotrope at

P = 1 atmNo Yes No No Yes Yes

Tasks to perform ?3. Identify separation techniques


Compare values of binary ratio matrix against a similar matrix where the columns are

separation techniques. If the binary ratio value is within the specified range for the same

property in the separation matrix, the select the corresponding binary pair as the

separation task and the separation technique as an alternative unit operation

3. Identify separation techniques & 4. Screen alternativesTable IX: Identified feasible separation techniques for methyl-acetate exampleTask Separation techniqueMeOH/HOAc Distillation, liquid membranes, stripping, pervaporation,

adsorption, absorption, crystallisation, cryogenic distillation, desublimation, gas separation membranes, sublimation, ultrafiltration

MeOH/MeAc Liquid membranes, pervaporation, adsorption, distillation, extractive distillation, absorption, stripping, azeotropic distillation, supercritical extraction (with CO2), pressure swing distillation, pressure distillation, liquid liquid extraction, microfiltration, ultrafiltration

MeOH/H2O Distillation, liquid membranes, pervaporation, adsorption, stripping, absorption, crystallisation, extractive distillation, cryogenic distillation, desublimation, gas separation membranes, sublimation, ultrafiltration

HOAc/MeAc Distillation, crystallisation, liquid membranes, pervaporation, adsorption, stripping, cryogenic distillation, desublimation, gas separation membranes, sublimation

HOAc/H2O Liquid membranes, pervaporation, adsorption, absorption, distillation, extractive distillation, stripping, azeotropic distillation, supercritical extraction, pressure swing distillation, liquid liquid extraction, microfiltration, ultrafiltration

MeAc/H2O Distillation, liquid membranes, pervaporation, adsorption, absorption, crystallisation, stripping, liquid-liquid extraction, distillation+decanter, pressure swing distillation, pressure distillation/P=0.3 atm, cryogenic distillation, desublimation, gas separation membranes, microfiltration, sublimation, ultrafiltration

Tasks to perform ?

1. Analyze mixture








9. Verify flowsheet

split factors

easiest separation first ?

repeat steps 6,7 & 8

Tasks to perform ?

1. Analyze mixture








9. Verify flowsheet

distillation

find solvents

determine design parameters

Tasks to perform ?9. Verify flowsheet

Description of a sample feasible flowsheet

T1

Reactor

FEED-T1

FEED

Molecular Sieve

TOP-T1

H20-Purge

Mixer

TOP_NO_H2O

T2 T3

FEED-T2

TOP-T2

FEED-T3

ValveTOP-T3

RECYCLE-T3

BOT-T3MeAc 99.9%

BOT-T2MeOH 98%

T4

T5 BOT-T5 HOAc 99.9%

BOT-T4 HOAc 99.9%

BOT-T1

TOP-T4 TOP-T5

P=6atm

P=1atm

P=6atmP=1atm

P=1atm

Tasks to perform ? 9. Verify flowsheetVerification (by simulation) of the flowsheet

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