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458.401 Process & Product Design

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458.401 Process & Product Design

13

Jong Min Lee

School of Chemical and Biological Engineering Seoul National University

Synthesis of a Process Using 
 a Simulator and 


Simulator Troubleshooting

Simulator Troubleshooting 13

Introduction

• A process simulator typically handles batch, semi-batch, and continuous processes

• Blind use of simulators can be potentially dangerous

• This lecture emphasizes the general approach to setting up processes and

• Highlights some of the common problems that process

simulator users encounter and offers solutions to these

problems

(2)

3

Select Thermodynamic Model

Select Units and Select Feed Stream Properties

Select Equipment Parameters

Select Convergence Criteria and Run Sim.

Select Output Display Options

Component Database Thermodynamic

Model Solver Flowsheet


Builder Unit Operation


Block Solver Data Output
 Generator

Flowsheet
 Solver

Select Chemical Components

Input Topology of Flowsheet Basic Computational Elements

in Process Simulator Simulation Problem

Simulator Troubleshooting 13

Sequential Modular Approach

• Groups the equations describing the performance of equipment units and solves them in modules

• Most widely used, e.g., Aspen Plus

• Each piece of equipment is solved in sequence

• The output from a given piece of equipment, along with

specific information on the equipment, becomes the input to the next piece of equipment

• For a process with recycle streams, tearing of recycle

streams and iterations are involved

(3)

5

A B C D E F

1 2 3 4 5 6 7

8 9 r

A B C D E F

1 2 3 4 5 6 7

8 9

r2 r1

Tear Recycle Streams

Simulator Troubleshooting 13

Input Data: Chemical Components

• Simulators will have a databank of many components

• All components (inerts, reactants, products, by-products, utilities, and waste chemicals) should be identified

• Chemicals unavailable in the databank can be added (user-

added components)

(4)

• Simulators use both pure component and mixture properties

- viscosity, thermal conductivity, diffusivity, enthalpy, fugacity, K-factors, critical constants, density, M.W., surface tension, etc.

• Expert systems to help the user select the appropriate model

- But, not recommended

• A moderately complex simulation will use at least two

different thermodynamic packages for different parts of the flowsheet

7

Simulator Troubleshooting 13

Example 13.1 HCl absorber (T-602)

13

12 11

10

lps E-607

10a

T-602

Propylene, HCl H 2 O

H + , Cl - , H 2 O

Phase Component Flows (kmol/h)

SRK Model PPAQ Model

Stream 12 Liquid

Stream 13 Vapor

Stream 12 Liquid

Stream 13 Vapor

Propylene 0.05 57.48 — 57.53

Ally Chloride 0.01 — 0.01 —

Hydrogen Chloride 0.91 18.78 19.11 0.58

Water 81.37 0.63 81.88 0.12

Total 82.34 76.89 101.00 58.23

(5)

Input Data: Topology and Feed Stream

• Selection and Input of Flowsheet Topology

- Every time a stream splits or several streams combine, a simulator equipment module (splitter or mixer) must be included

- Care must be taken when connecting batch and continuous unit operations

• Feed Stream Properties

- Feed with n components, 1 or 2 phases: n + 2 specifications

- If the feed is a single component and contains 2 phases

‣ T and P are not independent

‣ Vapor fraction (vf) and T or P are specified

- Vf can be used to specify a two-phase multicomponent system, but not recommended

- Use the vf to define feed stream only for saturated vapor (vf = 1), saturated liquid (vf = 0), and two-phase, single-component (0 < vf < 1) streams; All other streams should be specified using the T and P

9

Simulator Troubleshooting 13

Input Data: Equipment Parameters

• Level 1 Simulation

- Minimum data are supplied to solve material and energy balances

• Level 2 Simulation

- Do as many of the design calculations as possible; more input data than Level 1

Heat Exchanger

Process Stream In Process Stream Out

Outlet Condition of Process Stream

Duty

Heat Exchanger

Process Stream In Process Stream Out

Outlet Condition of Process Stream Utility Conditions

Heat Transfer Coefficients Exchanger Effectiveness Factor

Duty

Heat Exchanger Area

Utility Flowrate

(6)

Pumps

- Desired P of the fluid leaving the pump or desired +△P

• Compressors and Turbines

- Desired P of the fluid leaving the pump or desired +△P

- Mode of compression or expansion: adiabatic, isothermal, or polytropic

• Heat Exchangers

- Single process stream, Fired Heaters

‣ Exit process stream (exit P and T - single phase exit condition or exit P and vf - two phase exit condition)

- Two or multiple process streams (e.g., heat integration)

‣ Exit conditions for both streams

‣ Be aware of the possibility of temperature crosses: check the T profiles

11

Simulator Troubleshooting 13

Input Data: Equipment Parameters

• Mixers: simple tees in pipes

- Outlet pressure or pressure drop at the mixing point (very small)

- If feed streams enter the mixer at different pressures, the outlet stream is assumed to be at the lowest pressure of the feed streams (little error in MEBs) — correct analysis in Section 22.4

Splitters

- Outlet pressure or pressure drop across the device

- Relative flows of the output streams

Valves

- Either the outlet pressure or pressure drop

(7)

Input Data: Equipment Parameters

Reactors

- Stoichiometric Reactor

‣ # and stoichiometry of the reactions, T, P, conversion of the limiting reactant

‣ Reactor configuration (PFR, CSTR) is not required because no estimate of reactor volume is made. Only basic MEBs are performed

- Kinetic (Plug Flow and CSTR) Reactor

‣ Kinetic expressions are known

‣ # and stoichiometry of the reactions,

‣ Kinetic constants (Arrhenius rate constants, Langmuir-Hinshelwood, etc.), and

‣ Reactor configuration (PFR, CSTR) is required

‣ Cooling and heating of reactants may be available to generate T profiles in the reactor

13

Simulator Troubleshooting 13

Input Data: Equipment Parameters

Reactors

- Equilibrium Reactor

‣ # and stoichiometry of the reactions and

‣ Fractional approach to equilibrium are required

‣ Equilibrium constants as f(T) may be required for each reaction or may be calculated directly from information in the DB

- Minimum Gibbs Free Energy Reactor

‣ Another common form of the equilibrium reactor

‣ Outlet stream composition is calculated by a free energy minimization technique

‣ Usually data are available from the DB

‣ The only input data: list of components that one anticipates in the output from the reactor

‣ Equilibrium conversion: infinite residence time

(8)

Reactors

- Batch Reactor

‣ Similar to kinetic reactor (requires the same kinetic input)

‣ The volume of the reactor is specified

‣ The feeds, outlet flows, and reactor T (or heat duty) are scheduled (i.e., specified as time series)

• Which reactor to choose?

- Start with the least complicated reactor that will allow the heat and material balance to be established

- The reactor module can always be substituted later with a more complex one

- Make sure you choose a correct ________ reactant when specifying a desired conversion

15

Simulator Troubleshooting 13

Input Data: Equipment Parameters

• Flash Units: single-stage VLE calculation

- In order to specify completely condition of the two output streams (liquid and vapor), two parameters must be input

- Many possible combinations

‣ T and P, T and heat load, or P and mole ratio of vapor to liquid in exit streams

- The flash module is a combination of two pieces of physical equipment: phase separator and heat exchanger

- Can serve as a surge or storage vessel for batch operation

(9)

Input Data: Equipment Parameters

• Distillation Column

- Rigorous methods: ________________ calculations

- Shortcut methods: Fenske and Underwood relationships using key components

‣ DSTWU (Aspen Plus)

• Winn method for N min

• Underwood method for R min

• Gilliland to relate actual # of stages and RR

- Use shortcut first to get preliminary design calculations

- In both methods, duties of the reboiler and condenser are carried out. Detailed design of these heat exchangers (area calculations) often cannot be carried out during the column simulation

17

Simulator Troubleshooting 13

Input Data: Equipment Parameters

• Distillation Column: Shortcut (e.g., DSTWU in Aspen Plus)

- Inputs

‣ Identification of the key components to be separated

‣ Fractional recoveries of each key component in the overhead product

‣ Column pressure and pressure drop

‣ The ratio of actual to minimum reflux ratio

- The simulator will estimate the number of theoretical plates

required, the exit stream conditions (bottom and overhead

products), optimum feed location, and the reboiler and

condenser duties

(10)

• Distillation Column: Rigorous (e.g., RADFRAC in Aspen Plus)

- Inputs: total # of specs = # of products (top, bottom, and side streams)

‣ # of theoretical plates

‣ Condenser and reboiler type

‣ Column pressure and pressure drop

‣ Feed tray location and side product locations (if needed)

- Tray-to-tray equilibrium calculations can be handled for several hundred stages

- This can be used to simulate accurately other equilibrium staged devices, for example, absorbers and strippers

• Batch distillation: similar to the rigorous module, except that feeds and product draws are on a schedule

19

Simulator Troubleshooting 13

12

11 10

19

mps

P-102 A/B E-104

42 20 1

V-102 cw

13 T-101

15

E-106

Benzene 99.6 mol%

Component Stream 10 Stream 15 Stream 19 Stream 11

Hydrogen 0.02 — 0.02 —

Methane 0.88 — 0.88 —

Benzene 106.3 105.2 — 1.1

Toluene 35.0 0.4 — 34.6

Example 13.2 Benzene Recovery Column

(11)

21

Specification 1

Key components: Benzene and Toluene Top composition: Benzene 99.6 mol%

Recovery of toluene in the bottoms is 0.98

Specification 2

Top composition: Benzene 99.6 mol%

Recovery of benzene in the bottoms is 0.01

Violates the MB

Do not violate the MB

Make sure your specifications do not violate the MB

Simulator Troubleshooting 13

Input Data: Equipment Parameters

• Absorbers and Strippers

- Usually simulated using the rigorous distillation module

- Condensers and reboilers are not normally used

- Two feeds

• Liquid-Liquid Extractors

- A rigorous tray-by-tray module is used to simulate this multistage equipment

- Thermodynamic model for two liquid phases (appropriate

liquid-phase activity coefficients)

(12)

• Ill-posed problem. This normally means that an equipment specification has been given incorrectly .

• Too tight tolerance

• The number of iterations is not sufficient for convergence.

This occurs most often with many recycle streams.

- Remove as many recycle streams as possible and add the recycle streams back, one by one, using the results from the preceding simulation as the starting point for the new one.

(Read 13.3)

23

Simulator Troubleshooting 13

Physical Properties Estimation - Pure Component Properties

• 3 Most Fundamental Fixed Properties

- Molecular weight

- Standard liquid density

- Normal boiling point

• Benzene, Toluene

(13)

Pure Component Properties - Critical Point

25

What is a critical point?

A maximum point at which vapor and liquid phases coexist with each other

Gas vs. Vapor

Gas: H2, O2, N2, CO, CO2, or CH4

Vapor (증기 - 증발된 기체): Water, Benzene, Toluene, Xylenes, …

액상이 base에 있어야 함

(임계점이 매우 낮은 온도, 일반 온도에서 대부분 초임계 상태. 기상-액상 공존 X)

Z c = v real

v ideal = p c v c RT c

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임계압축인자

Simulator Troubleshooting 13

Critical Properties of CO 2

Tc: 304.21 K

Pc: 7,383 kPa

Vc: 0.094 m 3 /kmol

Zc: 0.274

(14)

27

Estimated overhead vapor product compositions at column top are: CH 4

65 mol%, C 2 H 6 15 mol%, C 3 H 8 15 mol%, and i-C 4 H 10 5 mol%

If reflux drum operating temperature is 45℃, estimate reflux drum

operating pressure. Use Peng-Robinson EOS model for your simulation

If you cannot get a converged solution, explain the reason

Estimate pseudo critical temperature for the above 4 component mixture

Example-01

Simulator Troubleshooting 13

Peng-Robinson

(15)

NRTL

29

Simulator Troubleshooting 13

Components Tc [℃] Mole Fraction

C1 -82.59 0.65

C2 32.17 0.15

C3 96.68 0.15

iC4 134.99 0.05

Tcm -27.61

Pseudo Critical Temperature = -27.6039 ℃

Pseudo Critical Pressure = 45.3975 bar

(16)

31

Simulator Troubleshooting 13

Pure Component Properties

Standard heat of reaction from heat of formation Heats of formation

Enthalpy of formation: Standard heat of formation Gibbs free energy of formation

From the standard heat of formation of each component, estimate standard heat of reaction

CH 4 + 2O 2 = CO 2 + 2H 2 O

(17)

Heat of Formation

33

Simulator Troubleshooting 13

Heat of Reaction

(18)

35

Component 𝛥H f [kJ/kmol]

CH 4 -74,520

O 2 0

CO 2 -393,510

H 2 O -241,810

𝛥H 0R (25℃) CH 4 + 2O 2 = CO 2 + 2H 2 O -802,610

Simulator Troubleshooting 13

Physical Properties Estimation - Pure Component Properties

• Temperature-Dependent Properties (열역학적 물성)

- Vapor pressure (liquid phase property)

- Enthalpy (열함량) for gas, liquid, and solid

- Idea gas heat capacity (열용량, 비열) (function of temperature)

- Heat of vaporization (증발 잠열)

- Densities for gas, liquid, and solid

• Transport Properties (전달 물성)

- Viscosity, thermal conductivity, diffusion coefficient, surface

tension (역시 온도 의존)

(19)

Vapor Pressure (액체의 증기압)

• Method 1: Clausius-Clapeyron Equation



 


• Method 2: Equation of state - ⍵ (accentric factor) or Twu’s Alpha Function 이용

• Method 3: Antoine Equation (매개변수 개수가 simulator에 따라 다를 수 있음)

37

ln

✓ P vap P 0

= H vap R

✓ 1 T

1 T b

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_______________

Simulator Troubleshooting 13

Choosing Thermodynamic Model

• If Tc and Pc are known for each pure component, the parameters for simple, cubic equations of state can be estimated.

• Pure-Component Properties

- Density, viscosity, thermal conductivity, and heat capacities are generally available in simulator.

- GCM is reasonably good for estimation

Enthalpy

- Although the pure-component heat capacities are calculated with acceptable accuracy, the enthalpies of phase changes often are not.

- If △Hvap is an important part of a calculation, simple EOS should not be used. In fact, “latent heat” or “ideal” options would be better.

- If the substance is above or near its Tc, EOS must be used. However,

the user must beware, especially if polar substances such as water are

present

(20)

39

of 500 kg/h of water entering at 70℉. Assume atmospheric pressure

H2 22.72 kg/h

N2 272.24

CO 268.40

HCl 26.84

Perform a simulation to determine the final temperature of the cooled gas stream with the default thermodynamic model and with the ideal model.

PR or SRK gives an outlet temp of 480℉. The ideal model gives an outlet temp of 348℉. The value calculated by the ideal model is closer to reality in this case because the EOS do a poor job of estimating the enthalpy of vaporization of water, which is the most important property for the energy balance.

The ideal model uses the experimentally determined value of this enthalpy of vaporization (from the DB)

Simulator Troubleshooting 13

Phase Equilibria

• PE is sometimes called the fugacity coefficient, K-factor, or fluid model.

• Whenever possible, phase-equilibrium data should be used to regress the

parameters in the model, and the deviation between the model predictions and the experimental data should be studied.

• EOS: an algebraic equation

- Pressure of mixture = f (composition, volume, temperature)

- Through standard thermodynamic relationships, the fugacity, enthalpy, etc. for the mixture can be determined

- These properties can be calculated for any density; therefore, both liquid and vapor properties, as well as supercritical phenomena, can be determined

f i v = v i y i P

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f l i = l i x i P

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(21)

Phase Equilibria

• Activity-coefficient models: can only be used to calculate liquid-state nugacities and enthalpies of mixing

- 𝛾 i = f (composition, temperature)

- Cannot be used for supercritical or non condensable

components because it is a correction factor for ideal-solution model (Raoult’s law)

41

f i l = i x i f i 0

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Simulator Troubleshooting 13

Equations of State

• Recommended for simple systems (nonpolar, small molecules) and in regions where activity-coeff. models are inappropriate.

• Default fugacity model: Soave-Redlich-Kwong (SRK) or Peng- Robinson (PR)

- Normally use three pure-component parameters per

substance and one binary-interaction parameter per binary pair

- Give qualitatively correct results even in the supercritical region

- Poor predictors of enthalpy changes except for light hydrocarbons and they are not quantitatively accurate for phase equilibria

- Good for systems containing hydrocarbons and light gases

(22)

43

The simplest EOS is an ideal gas law, a combination of Boyle and Charle’s law

P v = RT

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P = RT /v

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(pressure-explicit form)

Why pressure-explicit form?

ln ˆ v i = 1 RT

Z v v 1

"✓

@P

@n i

T,V,n j 6=i

RT V

#

dV ln Z v

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ln ˆ l i = 1 RT

Z v l 1

"✓

@P

@n i

T,V,n j6=i

RT V

#

dV ln Z l

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Simulator Troubleshooting 13

Ideal Gas Law

• Assumptions

- 1st assumption

‣ Molecules have no size

‣ As T approaches zero or P approaches infinity, V equals zero

- 2nd assumption

‣ Molecules do not interact with each other

‣ They never condense to liquefy

(23)

van der Waals EOS

45

https://ko.wikipedia.org/wiki/요하너스_디데릭_판데르발스

The van der Waals theory (1873) assumes

molecules have finite limiting volume b and attract one another according to a/v 2

• Molecules have finite size

• ‘b’: Liquid molar volume

• Molecules to interact with each other

‘a/V 2 ’: attractive pressure (a: energy parameter) (b: size parameter)

1910 노벨상 수상

Simulator Troubleshooting 13

Step 1

Real gas has finite size since it liquefies whereas ideal gas has no size when the pressure approaches infinity or temperature goes to

absolute zero point

v real = v ideal + b = RT P + b

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P = RT v b

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(24)

47

Pressure exerted by real gas is less than P = RT/(v-b) by a/v 2 due to the existence of intermolecular forces

P = RT

v b C 1 ⇥ C 2

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공간 내 분자 밀도와 관련

/ n 1

v ⇥ n 2

v = a v 2

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논문 제목: On the continuity of vapor phase into condensed liquid phase with a same tool; equation of state

증기상, 액상, 증기+액상 공존을 모두 ___________으로!

Simulator Troubleshooting 13

Vapor Pressure of CH 3 Cl at 322K with vdw EOS

Terms Value

Tc (K) 416.3

Pc (atm) 65.9

P vap (322K) 10.49

R 0.08206

T (k) 322

a = 27

64 ⇥ R 2 T c 2 P c

= 27

64 ⇥ (0.08206) 2 (416.3) 2

65.9 = 7.4709

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b = RT c

8P c

= (0.08206)(416.3)

(8)(65.9) = 0.0648

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P = RT v b

a

v 2 = (0.08206)(322) v 0.0648

7.4709

v 2 = 26.42 v 0.0648

7.4709 v 2

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(25)

49

Pressure

Volume

v vap

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v

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liq

P vap = 21.9 atm > 10.4 atm

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21.9

Simulator Troubleshooting 13

Equations of State

Peng-Robinson EOS

P = RT V b

a

V (V + b) + b(V b)

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with the van der Waals mixing rules,

a = X

i

X

j

z i z j p a i a j (1 k ij )

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b = X

i

x i b i

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BIP (Binary Interaction Parameters)

fugacity for phase equilibrium dG = V dP

<latexit sha1_base64="iX/euxC4u9/SBdt9TyAiLB/bqk8=">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</latexit>

ln f 2 f 1 =

Z p 2

p 1

V RT dP

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e.g.) @ constant T

There are usually six to ten EOS choices, and a few mixing-rule choices

(26)

51

Temperature: 225K, Pressure: 60.78 bar Feed flowrates:

Carbon dioxide 6 kmol/h Hydrogen sulfide 24 kmol/h

Methane 66 kmol/h Ethane 3 kmol/h Propane 1 kmol/h

DB BIPs Zero BIPs

Peng-Robinson 52.0 kmol/h 32.5 kmol/h

SRK 60.1 kmol/h 50.5 kmol/h

Simulator Troubleshooting 13

Liquid-State Activity-Coefficient Models

If the conditions are far from the critical region of the mixture or that of the major component, and if experimental data are available for the VLE or LLE, a liquid-state activity-coefficient model is a reasonable choice

ˆ v

i y i P = P i x i i i exp 1 RT

Z P

P i

v l i dP

!

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The most frequently-used activity coefficient models are the Wilson, NRTL, and UNIQUAC

i = ˆ a i x i 6= 1

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Liquid-State Activity-Coefficient Models

53

Model Features

Wilson Equation

Uses 2 adjustable parameters to model binary interactions between molecules. Can be extended to multicomponent systems using only binary parameters. Cannot predict formation of a second liquid phase.

NRTL (Non-Random Two-Liquid)

Equation

Uses 3 parameters for each binary pair, where two are energies of interaction and the third is a randomness factor. Can predict liquid- liquid or vapor-liquid equilibrium.

UNIQUAC (Universal Quasi- Chemical) Equation

Mathematically more complex than NRTL, but uses fewer adjustable parameters. Can predict liquid-liquid and vapor-liquid equilibrium. In the absence of experimental data, the parameters can be predicted by the UNIFAC method. Probably most widely-used model (?)

Towler and Sinnott, 2nd Ed.

Simulator Troubleshooting 13

Others

• Sour-water Systems

- Water containing carbon dioxide, hydrogen sulfide, and

ammonia. Special correlations have been developed to handle the VLE of such systems, and these are incorporated in most simulators.

• Electrolyte Systems

- When water and salts are present in a mixture then the salts can dissociate into ions in aqueous solution. The phase-equilibrium model must account for dissociation and the presence of long- range interactions between charges on ions as well as VLE or LLE. Special electrolyte models and databases such as the OLI model have been developed. These models are available in the commercial simulators, but with additional fee.

Towler and Sinnott, 2nd Ed.

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