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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