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Application of Compensation Control in FCC Main Fractionator.

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Application of Compensation Control in
FCC Main Fractionator
S.H. Yang, X.Z. Wang and C. McGreavy*
Department of Chemical Engineering,
The University of Leeds, Leeds LS2 9 f l , UK
and
X.X. Sen and F.Z. Mao
FUJIAN Refinery, People’s Republic of China
A compensation conlrol system is developed and applied to the main fractionator of a
fluid catalytic cracking unit. A compensation model is used for the outlet temperature
of light cycle oil (LCO)accounting for the top pressure change of the column. A
corrected temperature is used as a indication of the pour point of LCO,and the top
pressure of the column is used as a feedforward signal to maintain the pour point by
means of a predictive feedforward control. The control strategy has proved successful
in the FUJIAN refinery of the China Petrochemical Inc. which is used as an example
of
the system.
Introduction
Fluid catalytic cracking (FCC) is used to convert gas oil into a range of hydrocarbon
products, of which gasoline is the most valuable. It consists of a reactor-regenerator
section, the main fractionator, and gas processing facilities. The output from the
reactor section comprises both hydrocarbon vapour and coke, the latter causes
deactivation of the catalyst and has to be removed by burning off from the spent
catalyst in a regenerator. This resulting energy released by regeneration is an
important part of the energy recovery and has a crucial influence on operability. The
vapour of hydrocarbons leaving the reactor is separated into various products of
*
Author for correspondence (Email: chebcm@un.leeds.ac. uk).
61
S.H.Yang, el al.
different boiling points in the main fractionator. The overhead vapour from the
fractionator is then compressed by the wet gas compressor before passing to the gas
processing plant where butanes, dry gas and gasoline are separated. The LCO stream
is drawn from the side of the main fractionator. Control of the quality of this stream in
terms of the pour point, is determined by the outlet temperature of the stream.
However, the outlet temperature is only a good indication of the LCO pour point if the
column pressure is constant. T h s paper describes a procedure for correction of the
measured temperature and to allow for pressure changes in the column. A predictive
feedforward control algorithm is then used, and this has made it possible to obtain
consistently hgh quality product compared with the conventional temperature control
scheme.
Compensation Model
The upper part of the FCC main fractionator in the FUJIAN refinery is shown in
Figure 1, with the LCO stream being drawn from the side of the main fractionator. The
quality is measured by the pour point and is open-loop controlled by the outlet
temperature of the stream, this temperature is maintained at the setpoint by changing
the heat drawn out from fvst pump-around of the column using a three-way
manipulated valve. This temperature is often used to characterize the LCO pour point,
I
Heat
i
Temperature
n
I
I
@-- - - - - - - -
Main
Fractionator
Setpoint
I
Light Cycle Oil
First Pump-around
.
T T - Temperature Transmitter
Figure 1. Standard temperature control of LCO
62
Application of Compensation Control in FCC Main Fractionator
but it is only valid if the column pressure is constant. Therefore, it is necessary to
correct the outlet temperature to allow pressure changes in the column to achieve
closed control of the LCO pour point.
The Antoine equation is commonly used to represent vapour pressure in terms of
temperature for both pure substances and mixtures. It takes the general form:
In P = A - [B/(T+c)]
(1)
Extensive plant testing has enabled pressure and temperature pairs of LCO for the
same desired pour point to be obtained, three data points (Pi.T,) i=l, 2 , 3 , are enough
to determine the three Antoine coefficients A, B and C in Equation (1):
A = In
r; + [ B / ( z i+ c)]
(2)
B = W4/G)/[1/Ui + c)- 1/(5 + c>l
(3)
r,
C =[T2(T3 - )-DT3(T, - )I/[D(T2 - T )-(T3 - T )I
where D = In( P3/ P , )/ln( P2 /P, )
(4)
(5)
Therefore, the desired operating pressure and temperature pair of P6 and T,-f
satisfy the Antoine equation with the coefficients A, B and C together with a
correction term
5 which is used to compensate the error introduced by practical data
points (P,,T,) i=l, 2 , 3 . Thls is given by:
InPd = A - [ B / ( T f l +C)]+C
(6)
Suppose that the pour point corresponding to a measured pressure and temperature
pair of P, and T, is the desired pour point . Therefore it satisfies the same h t o i n e
equation as given by:
lnP, = A - [ B / ( T , +C)]+5
(7)
Combining Equation (6) and (7) , the desired temperature Tfi for the desired operating
pressure P,-fis represented as a function of the measured pressure and temperature pair
(P,, T,) as follows:
T<
= (Tm + c?/{l-[(Tm + C)/Bl x In(&/&))
-C
(8)
Since the measured pressure and temperature pair of P, and T, will change, the right
hand side of Equation (8) will predict a different temperature from T,-fas given by :
T, = (T, + C)/{1 -[<Tm + C)/BI x In (Pr
-C+q
(9)
63
S.H. Yang,et al.
In order to compensate the error introduced by practical data points (Pi, Ti) i=l, 2, 3,
the correction term q is introduced for T, in Equation (9).
If (P,,T,)
in Equation (9) are controlled so that T, has the same value as T,.f, the
pour point of the LCO will remain at the desired value. Any difference between T,
and T6 will give rise to a difference in the real pour point and the desired value. In
this sense T, can be considered as the control objective for the pour point, and
Equation (9) as the compensation model of the outlet temperature, T,.
Compensation Control System
The structure of the control system is shown in Figure 2, with the compensation model
given by Equation (9). It includes a temperature controller and a compensation
controller, only one of which is able to be selected by the operators at any time. The
predictive control algorithm with a steady-state feedforward compensation is used in
the compensation controller because the effectiveness of the predictive control has
been proven in the process industries, and the top pressure of the column is an
important source of disturbances. The use of feedforward in the predictive controller
is similar to the traditional situation in the PID controller.
Temperature Controller
I
I
I
I
!
1
I
Compensation Mode
Light Cycle Oil
First Pump-around
PT - Pressure Transmitter
TT - Temperature Transmitter
Figure 2, Schematic structure of the compensation control
64
Application of Compensation Control in FCC Main Fractionator
The control algorithm has a DMC structure[1,4] except that it includes steady-state
feedforward control as given by:
Where ufi) and ufi-1) indicate the values of a manipulated variable at instant k and k1.
Auk is its adjustment
only from DMC. The third term in Equation (10) is
feedforward control action. Auk is calculated by:
bU=(AUk
AUk+l
... A U k + L ) T
d k = T ck - T k
P
,..
The manipulated variable of the system is the same as the conventional
temperature control system for LCO as shown in Figure 1, i.e. the heat drawn out from
the first pump-around of the column. The setpoint is the desired outlet temperature of
the LCO (i.e. T ~ for
) the desired top pressure of the column
(Pd
). The setpoint is
constant until a different desired LCO pour point is required. For this reason, it is
easier to operate the column compared to the use of a conventionaI temperature
controller, where operators have to change the setpoint depending on the value of the
pressure at the top of the column.
Application of the Control System
In Table 1 , several pressure and temperature pairs of LCO which correspond to one
pour point are shown for the FCC main fractionator of the FUJIAN refinery. Based on
Equations (2) to (5), the Antoine coefficients A, B and C corresponding to Table 1
were found to be:
A4.824, B4.948, C=- 196.262
65
S.H. Yang,et al.
Table I . Relationship between Outlet Temperatureand Top Pressure (LCO Pour
Pointo -Pc
Outlet Temperature
Top Pressure
220
101
111.1
240
For a main fractionator design pressure of 108 H a ( i.e. Pd), the correction term is
q = 0.1 in Equation (9). The unit matrix is used as the suppression factor matrix Q,
and 0.001 is adopted for the feedforward gain in Equation (10). Using Equations (9)
and (lo), the resulting control is shown in Figure 2 and implemented based on a PM
(Processing Module) and a AM (Application Module) of the TDC-3000 control
system. Figure 3 shows the compensation control system for the TDC-3000.The
calculation module is used for the compensation model, and control module 1 is for
temperature control. Control module 2 is a CL (control language) program in AM for
the compensation control. The switch module is used to change the control modes
between compensation control and temperature control for safety reasons. The
calculation, switch and control modules are standard features of PM.
T,
Control Module 2
Open to
Three-
-
Valve
Switch
Module
I
A
Temperature
Tm
.
Control Module 1
~
Figure 3. Compensationcontrol system structure in TDC-3000.
66
Application of Compensation Control in FCC Main Fractionator
This system has been used in the FUJIAN refinery of the China Petrochemical Inc.
for more than one year. Typical results are given in Figures 4 and 5 which show that
using the compensation control system has made it possible to obtain consistently
better quality of the LCO compared with conventional temperature control. Curve 1
(in Figures 4 and 5 ) is the recording of the pressure at the top of the main fractionator,
ranging from 105 to 120 P a , curve 2 is the pour point of LCO from -6 to 2 OC, curve
3 is the outlet temperature of LCO from 220 to 230 'C. The pour point of LCO in
curve 2 (Figures 4 and 5 ) was measured by the pour point analyzer which is expensive
to maintain and also unreliable, and is not able to be used directly in the quality
control.
curvel: Top Pressure 105--12OKPa
curve2: Pour Point Temperature -6-24:
lee
curve3: Outlet Temperature 220--230%
1
r
75
5)
2s
Figure 4. Application results with general temperature control.
curvel: Top Pressure 105--1ZO KPa
curve2: Pour Point Temperature -6--Z0C
curve): Outlet Temperature 220--2309C
128
185
30
75
60
45
07/10/94
1 8 : 1 2 : 4 2 flIN
Figure 5. Application results with compensation control.
67
S.H. Yang,et al.
Conclusions
Compensation control has enabled a corrected temperature to be used to give better
control of LCO quality. It uses the Antoine equation to allow for pressure changes in
the main fiactionator. When used in combination with predictive control and a
feedforward algorithm, there is a significant improvement in handling disturbances,
and in achieving good temperature control of the main fractionator. The results
suggest that there is a possible application of such control schemes for endpoint
control of naphtha in the main fractionator.
Acknowledgments
This work was supported by the China Petrochemical Inc., Science and Technology
Committee of China, and the FUJIAN Refinery.
Nomenctature:
A
Dynamic matrix obtained from step responses of the system.
A, B, C
Antoine coefficients.
Predictive error.
Error with output correction at instant i.
Feedforward gain.
Sampling instant.
Number of manipulated variable adjustments in the future.
Predictive horizon.
Absolute pressure(kPa).
Measured pressure at instant k.
Desired pressure.
Suppression factor matrix.
Absolute temperature(k).
Outlet temperature of LCO compensated by Equation (9).
Measured temperature compensated by Equation (9) at instant k.
68
Application of Compensation Control in FCC Main Fractionator
Ti
Predicted effect of the past control actions on the outlet temperature of LCO
T~
at instant i.
Desired outlet temperature of LCO.
u(k)
Auk
Manipulated variable at instant k.
Adjustment of manipulated variable at instant k before feedforward control
is applied.
Superscript
T
Transposition of matrix.
References
1. Cutlet, C.R. and Ramaker, B.L., 1980. Dynamic matrix control- a computer control algorithm. Proc.
Joint Automatic Control Conf., San Francisco, California, WP5-B.
2. Grosdidier, P., Mason, A,, Aitolahti. A., Heinonen, P. and Vanhamaki, V. 1993. FCC unit reactorregenerator control. Comput. Chem. Eng., 17(2), 165-179.
3.
Muske, K.. Young, J.. Grosdidier, P. and Tani, S. 1991. Crude unit product quality control. Comput.
Chem. Eng., 15(9), 629-638.
4.
Richalet, J. 1993, Industrial application of model based predictive control. Automatic& 29(5), 12511274.
Submitted: 10 July 1995: Accepted after revision: 18 December 1995.
69
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