Rigorous industrial dynamic simulation of a crude distillation unit considered valve tray rating parameters

Pergamon PII:

Compurers them. Engng Vol. 22, Suppl., pp. S863-S866, 1998 0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0098-1354/98 $19.00 + 0.00 SOO98-1354(98)00167-7

Rigorous Industrial Dynamic Simulation of a Crude Distillation Unit Considered Valve Tray Rating Parameters Byoungmu Chang, Seunghoon Lee, Hyeoktae Kwon* and 11Moon Department

of Chemical Engineering.

Shinchon-dong

134, Seodaemun-gu,

*Process Department,

Yonsei University

Seoul, 120-749. Korea

LG Engineering

LG Mapo Bldg., 275, Kongdok-dong,

Mapo-gu,

Co., Ltd Seoul, 12 I-72

I, Korea

Abstract

A rigorous dynamic simulation trays and 65,000 nonlinear limit

is performed

barrel per day capacity.

equations

capturing

and downcomer

the fluid behavior

backup.

Discrete

Hundreds of thousand differential a result. the dynamic Published

behavior

by Elsevier Kqwords

to find optimal operating

unit with 56

behavior

of pump and heater on/off,

algebraic equations(DAEs)

load, jet flooding,

valve

open/close

are set up and solved by gPROMS

of the unit during the starting and shutdown

procedure

is fully

but also

hole velocity

are also included. simultaneously.

understood.

As

0 1998

Science Ltd. All rights reserved. : Rigorous dynamic

and simulation

simulation,

of distillation

Valve tray rating parameters. Dynamic

columns

complex,

on the nature of assumptions (Gani

Lj_l,‘ii_l-1’ HL,j.l

vary from simple to quite (1987)).

1 at the tray j arc

models

used for distillation

et al.( 1986), Choe and Luyben

behavior

and energy balances for component shown in equation (1) and (2).

columns are not

a brand new research area. The mathematical depending

of a crude distillation

in the column such as downcomer

Previous Studies on Distillation Unit Modeling

conditions

The model includes not only mass and energy balance equations

According

VI, Yijs HV,j

to

research (1995), more complex model results in a better agreement with experimental data as compared with a simple model than simpler one based on linear tray hydraulics and constant molar flows. To achieve more accurate knowledge of material and energy flows of the process systems, it is needless to say that more accurate model Wittgens

should

and

be

Skogestad’s

developed.

Recently,

al.( 1997) suggested a rigorous

Ingunn

Olsen

et

model of a high degree

of flexibility, however, the various fluid dynamics on each tray are not included. This research focuses on the development of a rigorous model for distillation column considered the heat and material balances on each tray and the valve tray rating parameters such as system factor, downcomer load, jet flood, hole velocity limits and downcomer backup. The main distinction of this study is the consideration of several design parameters used in industrial valve tray design and the dynamic behavior of pumps and valves. The tray model of the distillation column is refined with the equations based on an enormous number of tables, graphs and various relations used in industrial field, mainly through data regression technique.

Mathematical

Model for the Traies

includes material, energy balance, equilibrium relations and hydraulics. Figure 1 shows the tlow of material and energy in the tray. The material Modeling

$1

\ Yij+l

HV,j+l

L,,xij.

Figure 1. Material

p1 L

and Energy Flows in the Tray.

Material balance :

d/W

zdt =v,+,Y,,,, + L/-,X,,-,+ E/+,x,,+,

f

+Fiz,,-(L/,+W,V)~,,-(L,iW,‘+E,j~;, Energy Balance :

dHt,t , = 1/,+,H,“,,+ L,_,H;_, + E,+,H;+, + F.H)’ (7, dt

-(V,

iP,“)H,‘-(L,

Murphree efficiency Equilibrium : yii

of a tray

-

Y~+~ ( I-

The Liquid

S863

flow

+P/i

+E,)H,(

is applied.

vi

1 =

kiiq7ix,l

rate due to the pressure

(.3 drop

IS

S864

European Symposium on Computer Aided Process Engmeering--X

calculated by the equation (4). ilydraulics : L,

=

Ahole ('j+l

- p

-

PPLOhlevel )

I

(4)

a(Cx,MWX/,V The SRK for the vapor phase and GS for the liquid phase are used to compute physical properties. The with distillation column is composed of square matrix differential algebraic equations and can be calculated by numerical technique such as method of lines and Newton-Rapson method.

:

,

Figure 2. Procedures of the Valve Tray Rating.

Modeling of Valve Tray Rating Parameters example, is divided by six steps ; commtsstoning. Valve tray rating parameters supports the information of bringing the column to its normal pressure. column tray performance using the velocities and densities of heating and cooling, introducing feed, startup heating liquid and vapor. The valve tray rating parameters and cooling sources, bring the column to the desired considered in this study are : (1) System factors ; They operating rates. are used in four of the rating correlations to account for The above tray and valve tray rating model was applied system effect on hydraulic capacity limits. System to an industrial plant which is currently being operated. effects include both foaming effects and high vapor The target of this simulation is a crude oil refinery plant. density. (2) Downcomer load ; This is the flow rate The function of this unit is to handle a combination of coming down to the lower tray. Designs up to 100% of Thai Condensate and Arabian Light Crude Oil and the maximum downcomer load are safe for atmospheric separate the feed material into series of fractions, and pressure services and conservative for vacuum characterized by their boiling point range. In the process services, (3) Jet flood ; It occurs when the vapor energy scheme the combined feed is preheated, desalted, is sufficient to displace a significant amount of the partially vaporized and distilled. The distillation flowing liquid from one tray to the tray above. The jet products are then fractionated and stripped and the heat flood equation checks the tray efficiency drop and the they contain is partially recovered by heat exchanging sharp pressure drop increase. (4) Hole velocity limits ; with the fresh feed. The distillation column has 56 The valve velocities must be limited to a maximum hole fractionation trays. The feed stream is lumped by 48 velocity factor in order to avoid the entrainment and pseudo components. possible premature flood. (5) Weep point ; It is that In this research, we use a general purpose dynamic point at which the vapor energy is not sufficient to simulator, called gPROMS, developed by Imperial overcome the liquid head on the tray deck and liquid College. Differential-algebraic equations(DAEs) for the begins to flow downward through the valve orifice. The above target plant including RKS and GS equations for weep point is determined by the relation between the physical properties have been modeled. The DAEs for design vapor energy and liquid head. (6) Tray pressure this research have continuous and discrete variables. drop ; It has the relation with the hole velocity factor. The prerequisites for valve tray rating is the tray (7) Downcomer backup ; This is the phenomena of geometry and is shown in Table I. liquid transportation to the upper tray. Actually, the refinement of the tray model is to check the Table 1. Geometry of Tray performance of the trays rigorously during the simulation of the column. If the feed inlet flow to the column is increased, for example, by 30%, the status of tray operation can be easily checked. Figure 2 ~~~~ represents the procedures of valve tray rating during the simulation. The notation * means the use of industrial data. Effective Weir Length

(

49.2(in.)

Simulation of the Distillation Unit The dynamic simulation plays an effective role in tinding optimal operating conditions and procedures. The optimality of operating conditions and procedures will contribute to the safety, productivity and energy consuming and environmental constraints. The industrial manuals offers the basis of the modularization of the operating procedures. The start-up sequence, for

Figure 3. represents the liquid flow rate at some trays and the flow rate tends to increase exponentially. Figure 4. shows the vapor flow rates and the rat& slowly increase and reach the fixed values.

S865

European Symposium on Computer Aided Process Engineering-8

.

. _

_ _ _ _ _ _

-

.-

Figure 3. Liquid Flow Rstes. Figure 6. Vapor Densities. The results of jet flood, hole velocity backup and downcomer Figure

fg ..,

7, 8, 9 and

performances

,, ._._-.--

limit, downcomer

load on each tray is shown at IO. All

trays

show

the

because the value of parameters

stable change

below unity.

I

4i

-..^

.._.

^

..... .--

rn.

.

.

.

.

Figure 4. Vapor Flow Rates. The densities

of liquid

has the steady state value from

640 kg/m’ to 650 kg/ m3 and for the vapor from 28.3kgl m’ to 28.6kg/ behavior

ml.

Figure

Figure 7. The Ratio of Jet Flood and Maximum Value on each Tray.

5 and 6 show the dynamic

of liquid and vapor densities.

..

..

.

.

. . I . .

-!

\

‘\ i

\

.-.--

‘.

.

.

.

_

..-.

~ _

.

_-a_

--~sacJs _yuI

\

.

_

Figure 5. Liquid Densities.

Figure 8. The Ratio of Hole Velocity Limit and Maximum Value on each Tray.

European Symposium on Computer Aided Process Engineering-8

S866

H- Molar enthalpy(kJ/kmol) M= Molar holdup(kmol) MW= Molecular weight(kg/kmol) L= Liquid flow rate (kmol/s) P= Pressure(bar) t= Time(s) V= Vapor flow rate (kmol/s) v = Specific volume(m’/kg) W = Product flow rate(kmol/s) x= Liquid mole fraction (-) y= Vapor mole fraction (-) z= Feed mole fraction(-) Greek letters

Figure 9. The Ratio of Downcomer Backup and

a = hydraulic

Maximum Value on each Tray.

coefficient

/? = hydraulic coefficient 7 = Murphree efficiency(-) p =density(kg/m3) Subscripts i = Component j = Tray number tot = total hole = Tray hole Superscript L= Liquid

Figure 10. The Ratio of Downcomer Load and Maximum Value on each Tray.

V= Vapor

ACKNOWLEDGEMENT The authors acknowledge Science and Technology

Conclusion ‘The valve tray

rating

parameters

describe

the fluid

behavior in the column such as downcomer flooding,

hole velocity

load, jet

Policy Institute(Project

number EG-09-0 I-O I ) and LG

Engineering for financial supports.

limit and downcomer backup.

Thus in this study, enormous quantities of industrial

References

data. graphs and relations are applied to the dynamic

Oh, M. and Pantelides, CC.

simulation

the

simulation

language

physical behavior of the process and checking the tray

distributed

parameter

of

a crude

distillation

unit to find

Engng., 20,6l

performance. To

find

optimal

mathematical

operating

model

conditions

a

rigorous

of a crude distillation

column

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R.,

for

(1996)

A modelling

combined

systems,

lumped

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and and

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l-633

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CA.

and Cameron,

I.T.

(1986)

A

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, IO, 18 1-I98

and a dynamic simulation is performed for an industrial

Choe,

and

plant. The target of the dynamic simulation is the crude

dynamic models of distillation columns, Ind. Eng. Chem.

oil refinery plant which has a distillation column with

Res. 26,21%X-2161

considered valve tray rating parameters is

56 trays and can treat 65,00OBD(barrel

developed

per day) of

crude oil composed of 48 pseudo components.

The

Y.-S.

Luyben,

Wittgens, B. and Skogestad, S. (1995) on initial response, IFAC

algebraic equations(DAEs)

Denmark

discrete events. This paper shows the rigorous model of

Olsen,

a distillation

rigorous and efficient

column considered the valve tray rating

I.,

Endrest01,

Symposium

G.O.

and Sira,

distillation

parameters and the application to a crude distillation

engineering and training simulator,

onit which is currently being operated.

Engng, Vol.21, Suppl., $l93-$198

Notation A= Area

E= Entrainment flow rate (kmol/s) F= Feed flow rate (kmobs) g = gravitational acceleration

(1987)

Rigorous

Evaluation

of

dynamic models of distillation columns with emphasis

whole system of the simulations involves differentialincluding a large number of

W.L.

DYCORD+

‘95

T.

(1997)

A

column

model

for

Computers

C’hem.