ECUST PROII Advanced Training

September 19, 2017 | Author: bakhtyar21 | Category: Distillation, Mathematical Optimization, Chemical Reactor, Physical Chemistry, Chemistry
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PRO/II Training

ECUST PRO/II Advanced Training Copyright © 2004 SIMSCI-ESSCOR™ All Rights Reserved

PRO/II Training

What is the Power of Simulation3? ¾Who we are –

trusted results

Provider of software and services to the Hydrocarbon and Power industries that allow for plant start-up, operation and efficient: training of operators with

plants and processes – steady state & dynamic simulation (PRO/II, HEXTRAN, VISUAL FLOW,INPLANT PIPEPHASE, DYNSIM, TACITE, iFEED Suite/COMOS)

modeling, dynamic simulation & control emulation

design

operate increased profitability

optimize

(PRO/II, DATACON, OTS, DYNSIM, FSIM)

advisory and closed loop of their processes (ROMeo, ARPM, Connoisseur, PRO/II, NETOPT)

focused on simulation 2

PRO/II Training

SimSci-Esscor’s Vision ¾ Be the leading provider of software and solutions for

– – – – –

Simulation & Modeling Performance Monitoring Optimization Operator Training − Data Reconciliation Integrated Process Engineering − Collaborative Engineering ¾ Use the Best-in Class Technology & Expertise − Control Checkout − Reactors – Thermodynamics − Intuitive Engineering GUI – Separations – Heat Exchange Applied Simulation Every Engineer’s Desktop Deliver Performance for our Customers – Improved Fluid Flow Asseton – Optimization 3

PRO/II Training

products

sample applications verticals

sim4me SIM4ME - Delivering on our Vision Hydrocarbon Power Pulp & Paper

Process Design

High Fidelity OTS

MRA & ROMeo

Flare System Design

Decision Support

PowRx

Well Design/ Nodal Analysis

Engineering Studies

Oil/Gas Crude FCCU Ethylene

Debottlenecking

PRO/II HEXTRAN

Oil/Gas Crude FCCU

PIPEPHASE NETOPT

DYNSIM OTS FSIM TACITE

design

operate

VISUAL FLOW

Plant

Lifecycle

ATI/Hyprotech CANNOT do this easily with their current architecture!

Ethylene Crude FCCU Gas Lift Optimization

ROMeo ARPM MRA Connoisseur

optimize Management

4

PRO/II Training

Application During Plant Lifecycle SIM4ME

Basic Design Concept

Revamp

tate S y d Stea

Engin eering Dbs D

On line Op tim Ad iz a van tio ced n S im C on u la tro tio n& l Pla

nn ing

Detailed Design yn am

ic

Plant Design

Si m ul

at io n

ol r t n Co

m ste y S

Controls

Operator Training

Operation Commissioning

Construction 5

PRO/II Training

Process Engineering Suite PRO/II®

Process Flowsheet Simulator for Design, Operational Analysis, and Optimization

HEXTRAN®

Heat Exchanger Network Simulator for Design, Operational Analysis, and Optimization

DATACON™

Data Reconciliation Program for Heat/Mass/Composition balance on plant data

INPLANT™

Plant Piping and Utility Systems Flow Simulator

VISUAL FLOW™

Flare Network and Regulatory Compliance Simulator 6

PRO/II Training

PES Features

¾ Enhances productivity in the plant life cycle Basic Design Concept

Revamp

PLANT LIFE CYCLE

Detailed Design Plant Design

Controls Operation & Troubleshooting

Commissioning

Construction 7

PRO/II Training

Integration within PES Hextran MS Office

Datacon

PRO/II

Inplant

Complete Could be done The Plant

Visual Flow 8

PRO/II Training

Introduction

History of PRO/II ¾

First Generation: 1974

– ¾

Second Generation: 1979

– ¾

PRO/II with Provision 4.x PRO/II with Provision 5.x version 5.61 – March of 2002

Sixth Generation: 2003

– ¾

Version 3.30 - Spring of 1993

Fifth Generation: 1997

– – ¾

PRO/II Simulation Program

Forth Generation: 1995

– ¾

Process Simulation Program

Third Generation: 1988

– – ¾

SSI/100 Simulation Program

Over 40 new features

Seven Generation: 2004



Just released in August 2004

9

PRO/II Training

PES Solution Client Benefits ¾ ¾ ¾ ¾

Reduced process engineering time & cost Reduced plant capital cost Reduced plant lifecycle costs Increased plant operating profits

– – – –

higher product rates improved product quality lower operating costs more feed flexibility

¾ A valuable tool for experienced process manager and engineers 10

PRO/II Training

Flowsheets Features ¾ PRO/II is much better for larger flowsheets

– No over-specify flowsheet – Recycles estimates not required – Recycle block not required – More option to define sequence – Easier diagnosis of problems since each specification in linked to a particular unit operation and color indicates status.

11

PRO/II Training

Distillation Features ¾ Multiple column algorithms to model complex columns – IO, Sure, Chemdist, Liquid-Liquid, Electrolytes, Enhanced IO ¾ Multiple methods for generating initial estimated values – Simple, Conventional, Refinery, Chemdist, Electrolytes ¾ Reactive distillation – robust algorithm – derivative data not required ¾ Tray Hydraulic for rating and design – Volve, Sieve, and Cap structured tray – Sulzer structured packing – Norton random packing 12

PRO/II Training

Reaction Option ¾ Enter reactions in the reaction data section ¾ In Reactor units, select which reactions to use First Create a Library of Reaction Data

Then Select Reactions for Each Unit

13

PRO/II Training

Reactor Types ¾ General: (no reactor geometry required) – CONVERSION REACTOR (multiple reactions) – EQUILIBRIUM REACTOR (multiple reactions)

¾ Kinetic: (reactor geometry required) – PLUG FLOW REACTOR (PFR) – CONTINUOUS STIRRED TANK REACTOR (CSTR)

¾ GIBBS: (stoichiometry optional) – Free energy minimization – Kinetics not considered

14

PRO/II Training

Optimizer Features ¾ Optimize based on an objective function ¾ Utilizing tag data values ¾ No needs to have a dynamic calculation ¾ Automatic identification of the best design or operating conditions from a collection of alternatives ¾ Frees user from evaluating all possible cases

15

PRO/II Training

User-Added Program Features ¾ UAS/PDTS enhancements

– – – – –

additional function calls additional subroutines additional simulation database access full documentation supported in PROVISION

16

PRO/II Training

Tag Data Features ¾ Directly access plant historical data ¾ Read tag data from a file ¾ Read tag data from server

– PI – ODBC – @aGlance/IT – AIM ¾ Write tag data back to a file

17

PRO/II Training

OLE Features ¾OLE/COM Automation Layer – documented access to simulation database for most data – two way link y simulation data out, design/plant data in

– any OLE compliant application y e.g. MS Office can use VB or VBA

– used for Zyqad or Icarus interface – examples available at www.simsci.com

18

PRO/II Training

Spreadsheet Tools

19

PRO/II Training

OLE Automation

20

PRO/II Training 11

Operator Interface

TURBOEXPANDER PLANT

C1 100 10

8 E1 2

1

E2

2 3X 3

4 7 5

6 6 7

8

4

9

Feed Flowrate Pressure Temperature Composition N2 C1 C2 C3 IC4 NC4 IC5 NC5 NC6 NC7

100 1016700.0000 FT3/HR 587.0000 PSIG 120.0000 F

Range

Products Flowrate Pressure Temperature Composition N2 C1 C2 C3 IC4 NC4 IC5 NC5 NC6 NC7

7.9100 73.0500 7.6800 5.6900 0.9900 2.4400 0.6900 0.8200 0.4200 0.3100

9 483.7454 125.0000 24.0874 0.0000 0.0086 0.3633 0.3141 0.0548 0.1351 0.0382 0.0454 0.0233 0.0172

3

11 D1 2195.4231 LB-MOL/HR 161.2292 PSIG 5 157.5738 F 0.0965 0.8896 0.0137 0.0002 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

X1

T1

10 9

V1

Units X1 adiabatic efficiency X1 outlet pressure C1 adiabatic efficiency C1 work from X1 T1 top pressure T1 C1:C2 ratio bottom E1 HICO Stream 3 temperature

80.0000 125.0000 75.0000 0.9000 125.0000 0.0150 10.0000 -83.9990

% PSIG %

0-100 0-100 0-1

PSIG 0-1 F F

Units Reboiler duty E1 duty E2 duty X1 actual work C1 actual work

2.2889 5.2814 4.9133 392.2247 353.0023

MM BTU/HR MM BTU/HR MM BTU/HR HP HP

Run Simulation

21

Introduction

PRO/II Training

Simulation in Seven Steps 2

1

Check Units of Measur 3 e

Build Flowshe et

4

6

Select Therm o 5

Define Compone nts

Suppl y Strea m Data

Provide Process Condition s 7 Run & View Results

22

PRO/II Training

Defining the Components

PRO/II Training

Defining the Components

Component Types ¾ Library component ¾ Petroleum component ¾ User-defined component ¾ Solid component ¾ Polymer component ¾ Ionic component

24

PRO/II Training

Defining the Components

Component Selection

25

PRO/II Training

Defining the Components

PRO/II Component Library ¾ Composite of several databanks ¾ Databank search order

– – –

PROCESS (default) SIMSCI (default) DIPPR or OLI available as an optional PRO/II add-on

¾ Component selection by list or access name ¾ Pure component data

– –

Fixed properties Temperature-dependent properties 26

PRO/II Training

Defining the Components

Adding Library Components

27

Defining the Components

PRO/II Training

Component Data Printout ¾ Component name

¾ Phase type

¾ Component type

¾ Nine fixed properties

PROJECT TRAINING PRO/II INPUT PROBLEM COMPONENTS COMPONENT DATA ============================================================================ NO. --1 2 3 4

COMPONENT NAME -------------N2 C1 C2 C3

COMP. TYPE PHASE MOL. WEIGHT SPGR ----------- ----------- ----------- ---------LIBRARY VAP/LIQ 28.013 0.80811 LIBRARY VAP/LIQ 16.043 0.30000 LIBRARY VAP/LIQ 30.070 0.35640 LIBRARY VAP/LIQ 44.097 0.50770

NO. COMPONENT NAME

NBP CRIT. TEMP. CRIT. PRES. CRIT. VOLM. F F PSIG GAL/LB-MOL ----------- ----------- ----------- -----------320.440 -232.420 477.619 10.7963 -258.682 -116.680 652.499 11.8628 -127.534 90.140 693.648 17.7343 -43.726 206.006 601.652 24.3247

--1 2 3 4

-------------N2 C1 C2 C3

NO. COMPONENT NAME --1 2 3 4

-------------N2 C1 C2 C3

ACEN. FACT. HEAT FORM. G FORM. BTU/LB-MOL BTU/LB-MOL ----------- ----------- ----------0.04500 0.00 0.00 0.01040 -32066.21 -21726.14 0.09860 -36120.21 -13810.45 0.15290 -44650.04 -10139.64

28

PRO/II Training

Defining the Components

Using DATAPREP ¾ Menu-driven DOS interface ¾ Total access to PRO/II component database ¾ Additional information:

– – – –

Fixed properties Data source Data accuracy Plots and tables

29

Defining the Components

PRO/II Training

Enthalpy Curve for Water from DATAPREP (10E+ 2) 320 Ideal Gas Curve

ENTHALPY BTU/lbmol

240

Sat urat ed Vapor Curve

160 Heat of Vaporizat ion at NBP

80 Solid Curve

Heat of Fusion at NMP

Crit ical Point

0

-80 -500

Sat urat ed Liquid Curve

-250

0

250

500

750

Temperat ure F

30

PRO/II Training

Defining the Components

Petroleum Components ¾ Normal Boiling Point ¾ Gravity ¾ Molecular Weight At least two of three required

31

PRO/II Training

Defining the Components

User-defined Components ¾ Component Name ¾ Component Properties

32

PRO/II Training

Defining the Components

Component Properties ¾ Fixed properties ¾ Temperature-dependent properties ¾ User Defined and Refinery Inspection properties ¾ Solid properties ¾ Polymer properties ¾ Structure data

33

PRO/II Training

Defining the Components

Component Property Window

34

PRO/II Training

Selecting the Thermodynamics

PRO/II Training

Selecting the Thermodynamics

Example: Propane-Propylene Splitter ¾ Choice of Thermo strongly effects results!

Thermodynamic Condenser Reflux/Feed System Duty Ratio Peng-Robinson

-59.6

13.1

Grayson-Streed

-37.3

8.2

36

PRO/II Training

Selecting the Thermodynamics

Thermodynamic Data ¾ Required for all flowsheets ¾ Thermodynamic Property Methods ¾ Transport Property Methods



Required for certain units: y Column

y Dissolver

y Rigorous heat exchanger

y Depressuring unit

y Pipe

y Output tables

37

PRO/II Training

Selecting the Thermodynamics

Thermodynamic Properties ¾ K-Values (Mass Balances)

¾ Enthalpies (Heat Balances)

¾ Entropies ¾ Densities

38

PRO/II Training

Selecting the Thermodynamics

K-Value Calculation Methods ¾ Ideal ¾ Equation of State ¾ Liquid Activity ¾ Generalized Correlations ¾ Special Packages ¾ Electrolytes ¾ Polymers

39

PRO/II Training

Selecting the Thermodynamics

Selecting the Thermodynamic Method

40

PRO/II Training

Selecting the Thermodynamics

Enabling VLLE Calculations

Default

41

PRO/II Training

Selecting the Thermodynamics

Modifications ¾ Very important to choose the correct thermodynamic method ¾ Even more important to insure that binary interaction parameters are available

42

PRO/II Training

Selecting the Thermodynamics

Modifications (Cont.) ¾ Advanced Equations of State

– – – –

Model hydrocarbon behavior Advanced Alpha forms Advanced mixing rules Databank of regressed binary interaction coefficient

43

PRO/II Training

Selecting the Thermodynamics

Modifications (Cont.) ¾ Liquid Activity Coefficient methods

– – – –

Model non-ideal behavior Databank of regressed binaries Databank of azeotropes Fill options for binaries

44

PRO/II Training

Selecting the Thermodynamics

Modifications (Cont.) ¾ Generalized Correlation

– –

Typically designed for a specific application Do a good job for heavier hydrocarbons

45

PRO/II Training

Selecting the Thermodynamics

Modifications (Cont.) ¾ Enthalpy, Entropy and Density

– – – –

Library correlation for enthalpy No Library correlation for entropy Library correlation for density Rackett parameters in Library

46

PRO/II Training

Selecting the Thermodynamics

Transport Properties ¾ Viscosities ¾ Thermal conductivities ¾ Surface tension ¾ Liquid diffusivity ¾ 4 methods: Pure, Petroleum, Trapp, User-defined

47

Selecting the Thermodynamics

PRO/II Training

Calculation with Two Liquid Phases ¾ Water decant option

¾ Rigorous VLLE calculations

V

V

L = HC + W

L1 = HC + W

W = pure water

L2 = W + HC 48

Selecting the Thermodynamics

PRO/II Training

Water Decant Option Vapor VLE K-values

Liquid

Water Vapor Pressure

Pure Water Solubility Water

49

PRO/II Training

Selecting the Thermodynamics

Rigorous VLLE Calculations Vapor VLE K-values

Liquid 1

VLE K-values

LLE K-values

Liquid 2

Must enable two-liquid phase calculations. 50

PRO/II Training

Selecting the Thermodynamics

Hydrocarbon Systems ¾ Refining Processes:



Grayson-Streed: Hydrogen rich systems, Crude tower, Vacuum unit, Coker fractionator, FCC main fractionator



SRK and PR: Light ends columns, Splitters, Gas recovery plants, Hydrogen rich systems (SRKM)

– –

SOUR, GPSWATER: Sour water systems SRKK, SRKM, SRKS, IGS: Use if H/C solubility in liquid water (VLLE) is important.

51

PRO/II Training

Selecting the Thermodynamics

Hydrocarbon Systems ¾ Gas Processing:



SRK and PR: All types of processing plants, cryogenic systems



SRKM, PRM, and SRKS: Systems with water, methanol, and other polar components



GLYCOL: Dehydration with TEG. Improved for aromatic emissions. Based on SRKM.

– –

AMINE: Natural gas sweetening. SRKK, IGS, SRKM, SRKS: Use if light gas solubility in water (VLLE) is important. 52

PRO/II Training

Selecting the Thermodynamics

Online Thermodynamic Help ¾ Reference Manual



Detailed technical reference

¾ Application Guidelines



When to use each method

53

PRO/II Training

Selecting the Thermodynamics

Chemical Systems: Activity coefficient methods ¾ Non-ideal components

¾ Low to medium pressures ¾ Rely on binary interaction parameters (if missing will be close to Ideal!) ¾ Missing parameters estimated from structures, azeotrope composition, mutual solubilities etc. ¾ Used with Henry’s Law for non-condensibles ¾ VLLE with some methods 54

Selecting the Thermodynamics

PRO/II Training

Chemical Systems: Activity coefficient methods

NRTL UNIQUAC WILSON UNIFAC

Two Liquids?

Binary parameters in databank?

Yes

Yes

Yes

Yes

No

No

Yes

Estimates non-ideality from structure

¾ Other methods - see Reference Manual 55

PRO/II Training

Selecting the Thermodynamics

Chemical Systems: Equations of State ¾ SRK-SIMSCI, SRKM, and PRM for polar mixtures ¾ SRK-Hexamer for mixtures involving HF ¾ Can model high-pressures ¾ Also rely on binary interaction parameters ¾ Some binary parameters in databanks for above methods

56

PRO/II Training

Multicomponent Distillation

57

Multicomponent Distillation

PRO/II Training

Tray Model _

Vj yj _

Lj , Vj Liquid, vapor flowrate _

Lj-1 xj-1

VDj

Fj

Feed flowrate

Qj

Heater/cooler duty

_ _

Fj X F

xj , yj Liquid, vapor mole frac

_

Tj Pj

XF

Qj

Feed mole fractions

hj , Hj Liquid, vapor enthalpies LDj _

Vj+1 yj+1

_

Lj xj

Subscript denotes tray number

Tj , Pj Temperature, pressure LDj

Liquid Draw rate

VDj

Vapor Draw rate

Overbar denotes _ component vectors: e.g., x = (x1, x2, ...xNC)

58

PRO/II Training

Multicomponent Distillation

Tray Numbering ¾ Normally use Theoretical Trays (Stages) ¾ Numbered from top down ¾ Condenser is Stage 1



Even for subcooled condenser

¾ Reboiler is last stage



Thermosiphon adds 2 stages

¾ Convert packing to stages



Rule of Thumb: 2 to 3 feet of packing per stage

59

Multicomponent Distillation

PRO/II Training

Tray Efficiency ¾ Murphree Efficiency = 75% z

yA 100% efficient: step to equilibrium curve

z

xA

75% efficient: step 3/4 to equilibrium curve

xA 60

PRO/II Training

Multicomponent Distillation

Other Tray Efficiency Models ¾ Vaporization

yi = ciKixi

¾ Equilibrium

K’s adjusted towards 1.0

¾ Vapor leaving stage not at dew point ¾ Can lead to Mixed Phase Condenser product ¾ Better to use Overall Efficiencies

– – –

Theoretical / Actual trays to carry out separation Use different values in different column zones Tune from experimental data if possible

61

Multicomponent Distillation

PRO/II Training

Overall Efficiencies ¾ Efficiency increases as components decrease ¾ Efficiency increases as reflux increases

Reflux

¾ Results can be very sensitive to number of trays High reflux: number of stages strongly affects results

Low reflux: number of stages is less important

Number of Stages 62

Multicomponent Distillation

PRO/II Training

Typical Overall Tray Efficiencies SERVICE Simple Absorbers/Strippers Reboiled Absorbers/Strippers Deethanizers Depropanizers Debutanizers Deisobutanizers (Refluxed) Splitters C2, C2C3, C3C4抯or C5抯

PERCENT 20-30 40-50 60-65 65-75 80-90 85-95 85-95 95-100 90-100

Notes: 1) Assume 65-75% for most columns with reboilers and condensers. 2) At low reflux, split insensitive to number of trays in the model. 3) Pumparounds usually modeled as 2 stages.

63

PRO/II Training

Multicomponent Distillation

All Column Algorithms are Iterative ¾ Want to solve f(x) = 0 ¾ Generate a sequence of estimates of solution: x0, x1, x2, ... xN ¾ Equations are satisfied when x stops changing: | xN - xN-1 | < 0.00001 ¾ xN is regarded as the solution

64

Multicomponent Distillation

PRO/II Training

Convergence of Newton’s method ... Good initial guess leads to solution

f(X)

x

n +1

⎡ ∂f = x −⎢ ⎣ ∂x n

−1

⎤ n f x ( ) ⎥ xn ⎦

Solution

0 x0 x1

x2

x*

X 65

PRO/II Training

Multicomponent Distillation

Convergence is not guaranteed! f(x)

0 x*

X 66

Multicomponent Distillation

PRO/II Training

Convergence is not guaranteed! f(x)

Periodic

0 x*

X 67

PRO/II Training

Multicomponent Distillation

Convergence is not guaranteed! f(x)

Bad guess converges...

But better guess fails!

0 x*

X 68

PRO/II Training

Multicomponent Distillation

Available Distillation Algorithms in PRO/II ¾ Inside Out (I/O) ¾ Chemdist ¾ Sure ¾ Liquid-liquid ¾ Enhanced I/O

69

PRO/II Training

Multicomponent Distillation

Inside Out (I/O) Algorithm – – – – – – –

Relatively ideal thermodynamics including hydrocarbon with water decant Incorporates sidestrippers into column -- No recycle! Thermosiphon reboilers Flash zone model Very forgiving of bad initial estimates Fast! No VLLE

70

Multicomponent Distillation

PRO/II Training

I/O Column Features

1 2 Heater/Cooler

2 phase condenser + water decant

Side Streams

Heat Source/Sink

Multiple Feeds

Side Columns

Pumparounds N-1

Kettle and Thermosiphon N Reboilers

71

PRO/II Training

Multicomponent Distillation

I/O Algorithm Uses Nested Loops ¾ Inner Loop

– – –

Simple thermo model (Fast) Approximate Matrix Inversion (Fast) Converge enthalpy balance and performance specs

¾ Outer Loop

– – –

Updates, checks thermo using rigorous model (Slow) Checks Bubble Point Criteria If thermo changing or not bubble point, goto Inner Loop

72

Multicomponent Distillation

PRO/II Training

I/O Algorithm

Outer Loop

Prepare approximate thermo models for K*(K) and H*L(HL), and H*V(HV).

Approx. Thermo. Model

Iteratively solve the column equations using approximate thermo, K*(T,P) and H*(T,P).

Inner Loop

x, T, L, V, Q ... 1) Calculate rigorous K(x,T,P), H(x,T,P). 2) If K and H differ significantly from previous iterate, repeat from Done, solution is: x, T, L, V, beginning. Q ...

Convergence Check

73

Multicomponent Distillation

PRO/II Training

Initial Estimate Generator (IEG) ¾ Generates “good” initial estimates for all column variables P1 P2 PN LN Column Spec’s

IEG

You supply column specs and guesses for a few variables...

x0 y0 T0 P V0 L0 Q0R Q0C

Solver

IEG calculates initial estimates for all column variables...

x* y* T* P V* L* Q*R Q*C

Solver (I/O, Chemdist) converges on solution

74

PRO/II Training

Multicomponent Distillation

Four Types of IEG ¾ SIMPLE (default): Simple columns – Only choice for liquid-liquid extraction ¾ CONVENTIONAL: Works well with most columns – Based on shortcut methods – Strongly dependent on your product rate estimates ¾ REFINING: Complex refinery columns (e.g., Crude, Vacuum, FCC main fractionator, Coker) ¾ CHEMICAL: Nonideal thermodynamics (e.g., azeotropic and extractive distillation). Can be slow.

75

PRO/II Training

Multicomponent Distillation

Specifications and Variables ¾ Specifications are constraints to be met by the column ¾ Variables are calculated to meet specifications. ¾ Column always balances equations and unknowns ¾ To impose a specification, you must add a variable, otherwise equations and unknowns don’t balance. ¾ Example: Impose two product specifications by declaring reboiler & condenser duties as variables.

76

Multicomponent Distillation

PRO/II Training

Column Status at Initialization ¾ Fixed quantities remain at their current values unless you declare them as variables. ¾ If no specs/variables provided, default status used: QUANTITY Overhead and Bottoms Rates Side Draw Rates Duties Feed Rates Tray Temperatures Tray Pressures Vapor and Liquid Rates Product Properties (e.g. Viscosity) Tray Vapor or Liquid Properties

STATUS Calculated Fixed Fixed Fixed Calculated Fixed Calculated Calculated Calculated

77

PRO/II Training

Multicomponent Distillation

Improper Specifications ¾ 0% methane in crude column bottoms



Infinitely many solutions

¾ 300 lb-mole/hr propylene in overhead



No solutions if column feed only 250 mol/hr propylene

¾ 98% ethanol product



No solutions if Water-Ethanol Azeotrope present

78

Multicomponent Distillation

PRO/II Training

What is alpha (α)? ¾ Length of correction:

Xn+1 = Xn + αn δn

0< |α| < 1

¾ Decrease α if full step increases error α1 = .7 Unknown 1

α2 = 1

X1 X2

δ3 X3 Solution

Reject: full step increases error

Unknown 2 79

PRO/II Training

Multicomponent Distillation

You Can Help I/O by Using Damping ¾ Damping reduces iteration step and suppresses oscillation ¾ Conventional columns:

DAMP = 1.0 (default)

¾ Columns with steam:

DAMP = 0.6 - 0.8



Crude, Vacuum, FCC Main Fractionator

¾ Less-ideal:

– –

DAMP = 0.2 - 0.6

Increase number of allowed iterations If oscillations persist, use Chemdist

80

PRO/II Training

Multicomponent Distillation

Reboiler Models ¾ Most reboilers can be simulated as:

– – –

Kettle Thermosiphon with Baffles Thermosiphon without Baffles

81

Multicomponent Distillation

PRO/II Training

Kettle Reboilers LN-1

Vapor in Equilibrium with Bottoms VN-1

VN

LN-2

N-1 Bottom Tray VN BTMS BTMS

LN-1

N Reboiler

Q

82

Multicomponent Distillation

PRO/II Training

Single Pass (Once Through) Thermosiphon

¾ Equivalent to a Kettle Reboiler because Bottoms is in Equilibrium with VN LN-1

LN-1 VN VN

Bottom Sump BTMS

Bottom Sump Reboiler Sump BTMS

Baffle 83

Multicomponent Distillation

PRO/II Training

Circulating Thermosiphon Adds 2 Stages ¾ Simulate as TS without Baffles

N-2 Bottom Tray

LN-2

VN-1

VN-1

RV BTMS

RL

Combined Sump BTMS

N-1 Combined Sump RL

RF

LN-2 RV

RF

N Reboiler

Q

84

Multicomponent Distillation

PRO/II Training

Circulating Thermosiphon ä Simulate as TS without baffle N-2 Bottom Tray

LN-2 VN-1

RV

VN-1

RL BTMS Bottom Sump

BTMS

Reboiler Sump

N-1 Reboiler Sump RL

RF

LN-2 RV

RF

N Reboiler

Q

85

Multicomponent Distillation

PRO/II Training

Preferential Thermosiphon ä Simulate as TS with baffle LN-2

N-2 Bottom Tray

RV

VN-1 RL

VN-1 LO

Bottom Sump Lo

Bottom Sump Reboiler Sump RF

RV N-1 Reboiler Sump RF

BTMS R L BTMS

LN-2

N Reboiler 86

Q

PRO/II Training

Multicomponent Distillation

Tips... ¾ Start simple

– – – –

Converge water decant thermo before trying VLLE Converge side draws before trying sidestrippers Test pumparound duties with side coolers Remove coolers from pumparounds so all cooling is taken at condenser. Then add duties to pumparound.

¾ Recovery usually safer than composition specs ¾ Spec reflux ratio and product rate ¾ Spec reflux rate and component recovery

87

PRO/II Training

Multicomponent Distillation

Tips... ¾ If Water condenses in column:

– –

Increase temperature estimates to keep water in vapor Reduce steam flow: y 0.1 lb/Gallon bottoms in main column y 0.1--0.2 lb/Gallon sidestripper product

¾ Pumparounds solve best when you:



Fix rate and duty, calculate return temperature

88

PRO/II Training

Multicomponent Distillation

Tips... ¾ Excess cooling cause drying above PA return



Specify Tray Liquid rate

Remedy: Specify liquid flow above return tray and calculate pumparound duty

Declare Duty as a Variable

¾ Eliminate loops whenever possible

– –

Break thermal recycles with reference stream Simulate furnace as column tray heater

FZ 89

PRO/II Training

Multicomponent Distillation

Tips... ¾ Don’t believe your answers until you:

– – – – –

Verify thermodynamic method with expert Rerun with tighter column and loop tolerances Rerun with more pseudocomponents Rerun with different assay characterization method Check sensitivity to estimated parameters, i.e., number of trays y Example: Add a stage to column. If results change drastically, then model is very sensitive to this parameter. y Assess if this is physical reality or model defect.

90

Multicomponent Distillation

PRO/II Training

Distillation Algorithm Selection Inside/Out (I/O)

CHEMDIST

SURE

Unique Features

• Side and main columns solved simultaneously

• Reactive distillation • VLLE on any tray

• Total pumparounds • VLWE on any tray • Water draw any tray

Strengths

• Very fast • Insensitive to initial estimates

• Highly non-ideal systems

• Generality: complex column and thermo

• Thermo non-ideality • NO VLLE capability (VLWE at condenser)

• No pumparounds • Side columns solved as recycles

• Slow • Sensitive to initial guesses

• Non-ideal systems • Mechanically simple columns • VLLE within column

• Free water or water draw on trays other than condenser • Total pumparounds or vapor bypass

Limitations

• Hydrocarbons Applicability • EOS & slightly nonideal LACT thermo • Interlinked columns

91

Multicomponent Distillation

PRO/II Training

Distillation Algorithm Selection Liquid-Liquid Unique Features

Enhanced I/O

• LLE on each stage

• Total draws and water decants off trays

Strengths

• Perform liquid-liquid extraction

• Converges when zero flowrates on trays

Limitations

• Thermo must be a liquid activity method

• IEG does not work for all cases

• Liquid-liquid extraction columns

• Same as I/O

Applicability

92

PRO/II Training

Flowsheet Optimization

93

PRO/II Training

Flowsheet Optimization

Optimization allows... ¾ Automatic identification of the best design or operating conditions from a collection of alternatives ¾ Frees you from evaluating all possible cases

94

PRO/II Training

Flowsheet Optimization

Setting Up the Optimizer ¾ Objective function

– – – –

A result calculated in PRO/II (duty, product recovery,...) Usually evaluated with a calculator unit operation Minimize or maximize this value (i.e., maximize profit) Include all relevant costs

¾ Optimization variables



A fixed input parameter with defined MIN, MAX values

95

PRO/II Training

Flowsheet Optimization

Setting Up the Optimizer ¾ Process constraints (inequality)

– –

Limits on flowsheet values which cannot be violated Physical limitations on equipment y Constrain compressor operation to prevent surging y Constrain column tray flows to prevent flooding

¾ Process specifications (equality)



Additional criteria imposed on optimum solution y Total cooling water flowrate = 100 y Kerosene product rate = 10000

96

Flowsheet Optimization

PRO/II Training

One Variable Optimization ¾ Value of OVHD [$/lb-mole] is proportional to the square of its mole fraction C1 and C2 ¾ What temperature maximizes profit from OVHD? ¾ Objective: maximize

[ OVHD (YC1 + YC2)2 ] OVHD

H2O; C1-C6 -60ºF 900 psia

T=? 30 psia

97

Flowsheet Optimization

PRO/II Training

One Variable Optimization 1000

Objective Function Flowrate of C1 and C2 times Purity of C1 and C2

800 600

Optimal Temperature

400 200 0 -150

-110

-50

10

70

110

Flash Temperature

Optimization Variable

98

Flowsheet Optimization

PRO/II Training

Multivariable Optimization ¾ What temperature and pressure maximize profit from OVHD? ¾ Objective: maximize

[ OVHD (YC1 + YC2)2 ] OVHD

H2O; C1-C6 -60ºF 900 psia

T=? P=?

99

Flowsheet Optimization

PRO/II Training

Multivariable Optimization Maximum 1400 1200 1000 800 600 400

35

200

25

0

Pressure

90

Tempera ture

50

10

5 -30

-70

15 -110

-150

Objective Function

100

Flowsheet Optimization

PRO/II Training

Optimization with Constraints ¾ Vary temperature and pressure to maximize flowrate of C1 and C2 in OVHD ¾ The OVHD purity must be at least 90% OVHD H2O; C1-C6 -60ºF 900 psia

YC1 + YC2 > 0.9

T=? P=?

101

Flowsheet Optimization

PRO/II Training

Optimization with Constraints 35

0 360 25

Constraint

650

Flash Pressure

Optimization Variable

860

15

1147

Optimum (1411) 5

-150

-110

-70

-30

10

50

90

Flash Temperature

Optimization Variable 102

PRO/II Training

Flowsheet Optimization

Analyzing your Results: Shadow Prices ¾ Indicates the potential benefit of relaxing a limit, specification, or constraint



Positive:

Increasing the value increases the objective function



Negative:

Increasing the value decreases the objective function



Zero:

Constraints and/or limits on optimization variables (MINI, MAXI) are not active

103

PRO/II Training

Flowsheet Optimization

Reading the Optimizer Summary ¾ Best results

– –

Objective Function Values of Variables

¾ Optimizer history at each cycle

– – –

Values for objective function and variables Derivatives (Objective Function/Variable) Shadow Prices

¾ Convergence plots in output report

104

Flowsheet Optimization

PRO/II Training

Optimizer Output ** BEST OBJECTIVE FUNCTION = 1.41158E+03 AT CYCLE NUMBER 6 VARY INDEX ----1 2

--------- VARIABLE ---------INITIAL VALUE OPTIMUM VALUE ------------------------1.00000E+01 -4.12426E+01 3.00000E+01 5.00000E+00

- SHADOW PRICES ---CYCLE 1 ---------- ----------VARY 1 0.0000E+00 VARY 2 0.0000E+00 CNSTR 1 -2.4824E+03

5 ----------0.0000E+00 -3.1750E+01 -1.4458E+03

BEST - 6 ----------0.0000E+00 -3.1430E+01 -1.4146E+03

7 ----------0.0000E+00 -3.1428E+01 -1.4149E+03

8 ----------0.0000E+00 -3.1116E+01 -1.4338E+03

---- VALUES ---CYCLE 1 ---------- ----------VARY 1 1.0000E+01 VARY 2 3.0000E+01 CNSTR 1 9.2355E-01 REL ERR 0.00E+00 SUM SQ ERR 0.0000E+00 OBJECTIVE 9.5967E+02

5 -----------3.9238E+01 5.0000E+00 8.9303E-01 -7.74E-03 5.9977E-05 1.4210E+03

BEST - 6 -----------4.1243E+01 5.0000E+00 8.9935E-01 0.00E+00 0.0000E+00 1.4116E+03

7 -----------4.1439E+01 5.0000E+00 8.9995E-01 0.00E+00 0.0000E+00 1.4106E+03

8 -----------4.1454E+01 5.0000E+00 9.0000E-01 0.00E+00 0.0000E+00 1.4106E+03

105

Flowsheet Optimization

PRO/II Training

Shadow Price Examples 1) Where should you send any extra steam? Stm1

Process

Stm2

Profit

Stm3

2) Which heat exchanger should you clean first? Gas T=200ºF

1

2

3

4

Gas T=10ºF

106

Flowsheet Optimization

PRO/II Training

Solution Technique: Successive Quadratic Programming (SQP) Initialization Second order method (with derivatives) to determine search direction

Quadratic Programming Sub problem

First order method (no derivatives) to check progress towards the solution

Line Search

Convergence

107

PRO/II Training

Flowsheet Optimization

Convergence ¾ Converging loops requires more intervention ¾ Derivative step sizes are very important ¾ Tolerances of units in loops should be lowered

108

Flowsheet Optimization

PRO/II Training

Convergence ¾ Specifications and constraints are satisfied and

– – –

-7

Scaled error below tolerance (10 ) or Variables stop changing (tol=0.1%) or Objective function stops changing (tol=0.5%)

Objective Function Warning: Optimum is T=100, but any guess between 50 and 150 satisfies objective test

1005 1000 Tn 995

50

T (ºC)

100

150

109

Flowsheet Optimization

PRO/II Training

Convergence: Relative Tolerances ¾ Example: want L = 0.99 F ¾ Which specification should you use?

– –

Form 1: Form 2:

F L

L/F = 0.99 V/F = 0.01

¾ PRO/II converges this to a relative tolerance ε ¾ Form 1:

– –

V

Converges when: | (L/F - 0.99) / 0.99 | < ε so L = 0.99F ± 0.99F ε

110

Flowsheet Optimization

PRO/II Training

Convergence: Relative Tolerances ¾ Form 2:

– – –

Converges when: | (V/F - 0.01) / 0.01 | < ε But, V=F-L : | (1-L/F - 0.01) / 0.01 | < ε so L = 0.99F ± 0.01F ε

¾ If ε=0.01 (the default) and F=1000

– –

Form 1: Form 2:

L=990 ± 9.9 L=990 ± 0.1

¾ Form 2 is much more accurate! ¾ To use Form 2, tighten relative tolerance 111

Flowsheet Optimization

PRO/II Training

Convergence: Compounding of Errors ¾ ¾ ¾ ¾ ¾

Example: specify each flash as Ln = 1/2 Ln-1 Exact solution: LN = (1/2)N L0 If each flash specification relative tolerance = ε Then worst case LN relative error ~ Nε Example: N=5, ε=1%, then L5 = L0/32 ± 5%

L0

1

L1

2

L2

3

L3

LN-1

N

LN

112

PRO/II Training

Flowsheet Optimization

Flowsheet Tolerances ¾ Optimization requires flowsheets to be solved more accurately than for simulation

– – –

Tighten tolerances (columns, recycle loops, controllers) Choose appropriate finite difference steps Tighter tolerances allow smaller finite difference steps to be used which is more efficient

¾ Inaccurate flowsheet information may cause optimizer to fail or converge prematurely

113

Flowsheet Optimization

PRO/II Training

Finite Difference Derivative dF(xn)/dx = [F(xn +Δx) - F(xn)] / Δx Largest slope

F(x)

Smallest slope Error Bar xn

xn+Δx 114

Flowsheet Optimization

PRO/II Training

Finite Difference Derivative ¾ Smaller step size can worsen derivatives F(x)

Calculated slope can be negative!

xn

xn+Δx 115

Flowsheet Optimization

PRO/II Training

Finite Difference Derivative ¾ Smaller error bars improve derivative calculations F(x)

xn

xn+Δx 116

PRO/II Training

Flowsheet Optimization

Recommendations ¾ Solve base case separately - Check results ¾ Tighten flowsheet tolerances for improved accuracy ¾ Carefully select bounds and constraints to ensure physically well-defined flowsheet ¾ Select appropriate convergence criteria

117

PRO/II Training

Questions

Getting Started

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