MODELING AND CONTROL OF ACETYLENE HYDROGENATION PROCESS

7/27/2019 MODELING AND CONTROL OF ACETYLENE HYDROGENATION PROCESS

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Salam Al-Dawery, Haider Dakhil

12 Emirates Journal for Engineering Research, Vol. 17, No.1, 2012

A linear low order model was needed. Whilst the

dynamic model have been subjected to some of

linearization following by model order reduction, the

resultant linear model would contain all the

uncertainty of the full model plus the additional errors

due to linearization and model order reduction.

Thus, the energy linearized model for the

converter can be represented by the following transfer

function:

(1)

This model predicted the outlet temperature (Tout)

as a function of the inlet temperature (Tin), flow rate

(F) and carbon monoxide concentration (CO). There

was a gain (K) associated with each input. The same

second- order lag term was used for all inputs; inaddition the effect of a change in the inlet temperature

is delayed by dead time D.

Composition dynamics are assumed to be similar

to that of temperature dynamics. Therefore the

Acetylene concentration model can be described by

the following equation:

(2)

The numerical values for parameters of the above

two equations can be determined based on the

followings:1-assuming temperature rise across the converter

bed equal to 20oC

2- the value of the input temperature gain KTin equalto 2.3

oC/

oC as calculated from Figure 2.

3-the ratio between gains (KF/KTin) was -0.14 0C/(tone/hr), then KF=- 0.322.

4-The carbon monoxide concentration gain isconstant and equal to -0.015 oC/ppm

5-The input temperature gain for acetyleneconcentration model KCin which represents the

relationship between outlet acetylene temperature

and inlet acetylene concentration at study state

condition and equal to 0.001 oC/ppm6-The time constant is fixed at 1.4 minutes.7-Assuming the feed flow rate of gases mixture is

equal to 52630 kg/hr.

8-Time delay can be found 2.5 minutes as calculatedfrom Figure 3.

By substituting all numerical values regarding

parameters in equations 1 and 2, the converter

material and energy balances can be modeled by the

following matrix:

Where,

The dynamic responses of the converter (outlet

temperature and concentration) to step changes in

feed temperature, feed flow rate and carbonmonoxide concentration are shown in Figures 4 to 9.

Figure 2. Inlet to outlet temperature gain versus

temperature rise[7]

Figure 3. Time delay versus inverse of flow[7]

COs

KF

s

KT

s

eKT COFin

S

Tin

out

D

222

)(

)1()1()1( ++

++

+

=

COs

KF

s

KT

s

eKC COFin

S

TinHC

D

out 222

)(

22)1()1()1( +

+

++

+

=


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Modeling and Control of Acetylene Hydrogenation Process

Emirates Journal for Engineering Research, Vol. 17, No. 1, 2012 13

Figure 4 Temperature response of acetylene converter

to a step change in inlet temperature

Figure 5 Acetylene concentration response to a step

change in the inlet temperature


to a step change in the feed flow rate

Figure 7 Acetylene concentration response to a stepchange in the feed flow rate


to a step change in carbon monoxide concentration

Figure 9 Acetylene concentration response of

acetylene converter to a step change in carbon

monoxide concentration

2.2 Steam Heater Dynamic Model

The mathematical model for the shell and tubeexchanger (see Figure 1) was derived based on

unsteady-state energy balance and represented by the

following transfer function:

(3)


as a function of the inlet temperature (Tin), steam

flow rate (Fs) and feed flow rate (F). There was a

gain (K) associated with each input. The steam heater

responses to different step changes are shown in

Figures 10 to 12.

2.3 Intercooler and Aftercooler Dynamic

Model

The mathematical model for this shell and tube

exchanger type (see Figure 1) was derived based on

unsteady-state energy balance and represented by the

following transfer function:

)()1098.0(

0043.0)(

)1098.0(

0032.0)( S

SSinS

Sout FTT +++=

(4))(

)1098.0(

997.0)(

)1098.0(

417.1SWS

SWSTF

++

+


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Figure 10 Temperature response of steam heater to a

step change in inlet temperature


step change in steam feed flow rate


step change in feed flow rate


as a function of the inlet temperature (Tin), cooling

water flow rate (Fw), cooling water temperature (Tw)

and feed flow rate (F). There was a gain (K)

associated with each input. The cooler responses to

different step changes are shown in Figures 13 to 16.

Figure 13 Temperature response of intercooler to a

step change in inlet temperature


step change in feed flow rate


step change in cooling water flow rate


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Modeling and Control of Acetylene Hydrogenation Process

Emirates Journal for Engineering Research, Vol. 17, No. 1, 2012 15


step change in cooling water temperature

Figure 17 Closed loop block diagram for Acetylene

converter

3. ACETYLENE PLANT CONTROLTo study the dynamic behaviors of the acetylene

hydrogenation plant under control, a conventional

PID controller was used to control each units of the

plant based on their derived models. The parameters

of all controllers were determined by using the Cohenand Coon method.

The construction of block diagram for temperature

control of the converter is shown Figure 17, in which

inlet temperature was chosen as manipulated

variables and outlet temperature is the controlled

variable. The best controller settings are presented in

the Table below,

Controller Parameters

Kc T1 TD

PID -2.5 2.5 0.8

The responses of outlet temperature and acetylene

concentration of the converter under PID controller

are shown in Figure 18.

Similarly, for the case of steam heater and

intercooler, the closed loop responses of outlet

temperatures under PID controller are shown in

Figures 19 and 20 respectively.

4. DISCUSSION AND CONCLUSIONSFrom the mathematical model of the converter in

acetylene process, it appears that the real process was

highly non-linear, and thus, a linearized model has

been derived and represented by second order lag

with dead time. To test the acceptability of the model,

many dynamic responses (both outlet temperature and

acetylene concentration) were obtained and all gave a

reliable and acceptable level of accuracy. Also, the

implementation of control system was shows a

reliable and stable responses which agreed to that ofprocess behavior.

Also, The dynamic behaviors of steam heater and

intercooler and aftercooler exchangers were both

described by first order lag model. The temperature

dynamic responses of both systems were at an

acceptable level, and similarly for responses under

control systems.

From above, it can be recommended that the

derived model for the Acetylene plant can be

considered for applying advanced control strategies

such as feedforward and or plant wide control for the

overall plant control improvements.

Figure 18 Responses of outlet temperature and acetylene

of the converter under PID controller.


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Figure 19 Temperature response of Steam Heater to a

negative 100% step change in feed flow rate

Figure 20 Temperature response of the Intercooler to

a negative 100% step change in feed flow rate

REFERENCES

1. Peacock, A. J., (2000) "Handbook of Polyethylene",Marcel Dekker, Inc., Basel, New York.

2. Schbib, N. S., Garcia, M. A., Gigola, C. E. and Errazu,G. A., "(1996) Kinetics of Front-End AcetyleneHydrogenation in Ethylene Production",Ind. Eng.

Chem. Res. vol.35 pp. 1496-1505.3. Mostoufi, N., Ghoorchian, A. and Sotudeh-Gharebagh,

R., (2005), Hydrogenation of Acetylene: KineticStudies and Reactor Modeling", Int. J. Chem. React.Eng. Vol.3.

4. Cider, L., Schoon, N., (1993) "Hydrogenation ofAcetylene at Transient Conditions in the Presence of

Olefins and Carbon Monoxide overPalladium/Alumina", Ind. Eng. Chem. Res. vol. 30 , pp.

1437-1443.5. Gobbo, R., Soares, R. P., Mandarin, M. A., Secchi, A.

R. and Ferreira, J. M. P. , (2004) "Modeling,

simulation, and optimization of afront-end system foracetylene hydrogenation reactors" ,Braz. J. Chem. Eng.

vol.21 no.4 Oct./Dec.6. Dakhil, H.M., (2007) "Integrated multi-unit controller

design for chemical process plant", M.Sc. Thesis,

University of Baghdad. Iraq.7. Weiss, G. H., (1996) "Modeling and control of an

acetylene converter",J.Proc. Cont. Vol. 6, No. 1, pp. 7-15.

8. Hobbs, J. W., (1979) "Computer control of AcetyleneHydrogenation Process", Industrial Process Control,AIChE Workshop, 7.

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MODELING AND CONTROL OF ACETYLENE HYDROGENATION PROCESS