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Applying Advanced Process Control to Fccu in a Petroleum Refinery

Xiaoming Jin, Department of Control Science and Engineering, Zhejiang University, 388 Yuhangtang Road, Hangzhou, China and Maying Yang, College of Information Engineering, Zhejiang University of Technology, 18 Chaowang Rd, Hangzhou, China.

Fluid catalytic cracking (FCC) is one of the most important units in the petroleum refining industry for the conversion of heavy gas oil to gasoline and light hydrocarbons. In any refinery, the throughput of the FCC unit usually boasts a capacity that is one-third of the crude unit¡¯s. Since the performance of the FCC units plays a major role on the overall economics of a refinery, there is a strong demand for advanced process control and real-time optimization with higher control performance to meet the challenges imposed by the growing technological and market competition. The FCCU at Zhenhai Refinery, PRC, which was designed to convert low value mixtures of heavy distillate oil such as wax distillate and residual oil into high value light products, e.g. gasoline, light diesel, etc. by cracking the oil at high temperature in the presence of a catalyst. The unit consists of multiple chemical reaction and physical change, the dynamics are complex. It should be operated to produce the maximum amount of light products subject to mechanical and economic constraints. The incremental benefits from increasing production in the unit are quite considerate. The FCCU at Zhenhai Refinery processes approximately 5,400 tonnes of feed per day. It was revamped by applying the innovative techniques of MIP and CGP for producing clean gasoline. The catalyzed cracking is occurred in the two-stage riser with its goal to maximize propylene. Reaction products and spent catalyst are discharged in the reactor to disengage the catalyst particles from the vapor product through a battery of cyclones. Catalyst regeneration is achieved by burning off the coke deposit in fluidized bed inside the two cascaded regenerators. Steam turbine driven air blowers supply the oxygen to burn the coke deposit. The main fractionator separates the reactor products into bottoms slurry oil, heavy and light cycle oil, heavy naphtha, and wet gas stream, the latter is separated in a gas recovery section including absorber, stripper, and stabilizer. The production of FCCU is to maintain safe and reliable operation, to produce the cracked light products with cost optimization, i.e. improving the process operating stability, maximizing the process throughput and desirable products yield, improving the product quality, and minimizing energy consumption which in turns maximize the profit. From process control point of view, FCCU can be considered as a highly nonlinear, interacted, complex system. Furthermore, the immeasurable, uncontrollable factors (such as feedstock and catalyst properties) and process constraints continuously affect the unit, making process control and optimization difficult. The objective of the reactor-regenerator control is to attain the best possible operation and gain most profits at large throughputs of residual oil and low energy consumption. In the maximum gasoline mode, the major independent reaction variables in the FCCU are: reactor temperature, combined feed rate, combined feed (feed preheat) temperature, reactor pressure, catalyst activity. The dependent variables, on the other hand, include: the catalyst circulation rate, the catalyst/oil ratio, the regenerator temperature, conversion. The purpose of the main fractionator is to de-superheat and recover vapor. The hot-product vapors from the reactor flow into the main fractionator; these vapors enter the column near the base. The main function of the fractionator is to condense and separate the reaction products. The operation of the main column is similar to a crude tower but has two differences. First, the effluent vapors must be cooled before any fractionation begins. Second, large quantities of gases will go overhead with the un-stabilized gasoline for further separation. Aside from slurry oil product and wet gas, the main fractionator has three side cuts: raw gasoline, light diesel and cycle oil. The recovered heat from the main fractionator is used to preheat the fresh feed, generate steam, and serve as a heating medium for re-boilers of the gas recovery section. The control objective in the gas recovery section is to maintain distillate composition at set point in the presence of disturbances. These disturbances may be characterized as due to (i) loads, (ii) changes in cooling and heating medium supply condition and (iii) equipment fouling. The distillation control system should be designed to cope with the types of disturbances in the feed and supply conditions of the heat exchangers. In short, there are more than ten operating variables to be controlled for the performance of the FCCU. The main variables corresponding to product qualities, desirable products yield, energy consumption of the three sections should be carefully controlled. Consequently, the FCCU needs to be maintained close to optimal operating conditions lay at some constraints. Nevertheless, regulatory controls of the unit are difficult to achieve favorable control performance all the time for moving the unit to its optimal operating point and rejecting disturbances on the controlled variables. Therefore, for the control of the FCCU affected by many constraints, model predictive control (MPC) can be used to improve control performance characterized by a reduction in the variability of the controlled variables through information gathering, process analysis, and constrained multivariable optimization. This proposal introduces the industrial application of commercial software of MPC in the FCCU at Zhenhai Refinery. The MPC system is developed to deal with the constrained multivariable control problem on-line of the reactor-regenerator, main fractionator, and gas recovery section. Several soft sensors are developed to implement direct control of product quality. After the implementation of the MPC system, the deviation of the main process variables became one half of that before implementation, as shown in Fig 1 (TI521, LIC401, LI405, AND TI405 are temperature of 2nd regenerator, level of separator, level of stripper bottom, and temperature of stripper bottom respectively). As a result, The overall economical merit from this implementation is approximately more than four million RMB yuan as a result of increase of the production of light product, improvement of product quality, and minimization of operating cost. Industrial application results show that the MPC software can maintain the best operation for a long time and realize ultimate operating potential of the FCCU by reducing the consumption of energy, improving product quality, and minimizing operating cost. (a) before MPC implementation (b) after MPC implementation Fig 1 Comparison of pressure control performance under MPC and classical control