computer application and software development for ...

Computers chem. Engng Vol. 20, Suppl., pp. SI601-S1606, 1996

Pergamon

S0098-1354(96)00272-4

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0098-1354/96 $15.00+0.00

C O M P U T E R APPLICATION AND SOFTWARE DEVELOPMENT FOR THE A U T O M A T I O N OF A FLUID CATALYTIC CRACKING PILOT PLANT EXPERIMENTAL RESULTS S.S. VOUTETAKIS, A.A. LAPPAS, D.K. IATRIDIS & I.A. VASALOS Chemical Process Engineering Research Institute, P.O. Box 361, 57001 Thermi, Thessaloniki Abstract - This application deals with the automation and computer aided operation including monitoring and control of a pilot plant scale fluid catalytic cracking unit. The operation and monitoring of the pilot plant is carried out through graphical process flowsheets of the actual process which are presented on the screen of a personal computer. All operational results and all manipulating actions are logged and stored in a time sequential indexed file that can be recalled at any time. Through the use of automation and computer software a complex unit such the FCC unit can be operated unattended in long runs giving useful results used for process modeling and model evaluation, catalyst and process condition evaluation. Real experimental results will show the trends and the good comparison of the unit operation to the FCC theoretical estimations.

INTRODUCTION The fluid catalytic cracking is a process in which heavy oil fractions (i.e. Gas Oil) can be cracked catalytically giving the following main products: gasoline (with b.p. C5 - 210°C), diesel (210 - 340°C) and LPG (C3 + C4). The use of a pilot plant fluid catalytic cracking (FCC) unit in order to test the performance of FCC catalysts towards the production of high octane gasolines is routinely practiced by oil companies. The reliability of the process results obtained by the pilot plant is dependent on the efficiency and satisfactory operation of the control system of the unit. The present study discusses the structure and the operation of the automatic control, the role of the graphics assisted automatic control of the unit. This study also discusses the reliability of the control system by comparing the process results with literature. DESCRIPTION OF THE FCC PILOT PLANT The main parts of the FCC pilot plant unit designed and constructed by the Chemical Process Engineering Research Institute (CPERI) are: the riser type vertical reactor, the regenerator (fluid bed) and the stripper. A simplified process flowchart is given in Figure 1. The unit made of SS-316 consists of a vertical reactor (riser) with 9.5 mm diameter and of a fluid bed reactor with 76.2 mm diameter. a. The riser type reactor

The liquid feed is introduced in the riser reactor inlet where it is vaporized. The reactor is a two phase flow reactor where the one phase is the feed vapours and the other phase is the catalyst phase. The feed injected inside the reactor comes in contact with the catalyst which is controlled through the slide valve SV-101 (Fig. 1). In the riser the reactions take place and subsequently at the reactor exit the mixture is driven to the stripper vessel. In the stripper the separation (stripping) of gases from the solid catalyst occurs. The solids through the slide valve SV-401 and the lift line (Fig. 1) return to the reactor bottom after passing through the regenerator which is described later. The reaction products, from the stripper exit, flow through a heat exchanger and then their temperature is reduced to 20°C, for the condensation of heavier products. Then the gaseous mixture is led to a stabilizer column for the better separation of liquid and gaseous products where the gases are cooled down to -30°C. For the measurement of the total volumetric flow rate the gaseous products flow through a wet test meter. The composition of the gases is also determined by GC's. The liquid products $1601

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are collected and weighed in a special collection vessel (Fig. 1). The specifications of the riser reactor are reported in Table 1. The mixture of gasoline, Light Cycle Oil (LCO) and Heavy Cycle Oil (HCO) is obtained through the bottom of stabilizer. b. The regenerator reactor The fluid bed regenerator reactor is used to bum the coke which is deposited on the catalyst. The coke is produced as a by-product of the cracking process. The fluidization gas is introduced from the base of the reactor and its flow rate is controlled by mass flow controllers. The regenerator reactor exit stream passes through cyclones that remove any entrained solids. The remaining products arc cooled by a heat exchanger and subsequently their total flow is measured by a wet test meter (WTM2) and analyzed by a GC. The specifications of the fluid bed reactor are represented in Table 1.

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V-604 Figure 1: Schematic Diagram of FCC Pilot Plant Table 1: Specifications of FCC Pilot Plant Residence time Reactor temperature Stripper temperature Liquid feed rate Catalyst rate System pressure Regenerator fluid bed temperature Catalyst or solid mass Rate of fluidization gas

>0.5 sec max 590°C max 590°C max 25 8r/min 2 kg~/hr- 30 kgffhr 1.6 - 3.3 aan up to 690°C up 4.5 k8 30 nl/min

GENERAL CHARACTERISTICS OF THE PILOT PLANT Observation of the fluidization state and catalyst movement inside the unit is accomplished by monitoring the differential pressures inside the pilot plant. The flow of solids inside the unit is controlled through two slide valves. For the introduction of liquid feed a gear dosing pump is used. The pilot plant is fully automated using a software and hardware system which will be described later. For monitoring the system on the basis of various measurements the operator uses the main screen which presents a simplified process flowchart as well as a number of more detailed views which are parts of the process, numerical charts and virtual control panels. The operator at any time can also

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execute other parts of the software which provide him with trending charts of process variables values and parameters. STRUCTURE OF THE AUTOMATIC CONTROL The process control system of the unit is based on a special industrial computer control system. This system consists of two processing units lxDCS-6000 by Analog devices and a 386 PC by Compaq. These three micro computers are co-operating in a tight network that performs all the data acquisition and control operations. This system is completed and coordinated with the FIX/DMACS S/W by Intellution. The control system collects the values of the inputs and drives the output signals as well as maintains a digital record of the signals. The process pressure control valves as well as the power to electrical heaters, are controlled by numerous digital PID controllers. The control software is capable of executing concurrently process algorithms for the supervisory control of existent digital controllers as well as new direct acting digital controllers of novel design. The development and execution of algorithms can be carried out either on the same computer that is monitoring the unit or through a local area network on a different computer (PC or VAX). This flexibility of the software that acts in a multitasking environment gives the capability of executing concurrently on-line process models that interact with the real system. This capability can be exploited in various ways such as model tuning and verification of the fundamental process modeling, selection of better operating conditions and control during operation for the compensation of variations on feed composition, application of novel digital control algorithms, feedback on operating conditions based on the analysis of process products or the execution of the on-line process model etc. OPERATION OF THE AUTOMATIC CONTROL The control system allows data insertion by the keyboard while allowing safe storage of data for later recall and manipulation. The personal computer is linked with the special industrial computer control system and allows the communication of the operator with the control system. All commands for the control of the operation of the unit are given by the keyboard. The state of the process of the FCC unit is supervised by the screen which is timely updated. To ease the recognition of the operation a simplified flow sheet of the process is displayed graphically on the screen of PC. The descrete values of inputs and outputs of the process (flows of pumps, positions of solenoid valves and motors) are displayed on the screen with their names and states. The values of measuring and the output values of the control loops are displayed on the screen on the respective position on the schematic diagram of flow sheet. The signals collected for the supervision of the control system and the output signals transmitted from the personal computer to the unit are displayed on Table 2. Table 2: Overview of automatic control signals

Type of signal Analog inputs Analo[~ outputs Digital inputs Digital outputs Frequency inputs

Number 100 60 40 2

Type of apparatus connected Thermocouples, DPT, MFI, Pumps Electrical power, Valves, Mfc, Pumps Supervision of feed Automatic relay, Pumps control, Two position valves Wet Test Meters (WTM)

RELIABLE OPERATION OF CONTROL SYSTEM - COMPARISON OF FCC PILOT PLANT RESULTS WITH LITERATURE The efficient control system of the FCC pilot plant oughts to maintain in steady state the dynamic equilibrium of the system. Through disturbance tests the operating limits concerning the control of the system were established. For example, the maximum air capacity in the regenerator pilot plant is limited to 30 SLM. A such an upper limit air capacity to regenerator leads to a limited feed rate in the reactor due to spent catalyst regeneration. Thus through a thoroughly experimental work the operating limits of the operating variables of pilot plant were established. The performance of the unit after 1 year operation was found to be excellent. The pilot plant has been run in co-operation with Greek refiners (Hellenic Aspropyrgos Refinery. -HAR, and Motor Oil

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Hellas Refinery-MOH). Three different types of catalyst and feedstocks have been used in the unit. In this study some results will be presented which concern the catalyst and type of MOH. The properties of the feedstock used are described in Table 3. The mass balance of a high number of runs with this feedstock is presented in Figure 2 and are satisfactory for all the runs. The selectivity of gasoline and coke is presented in Figures 3, 4 for two temperatures. These results are in accordance with literature (Gary and Handwerk, 1975, Biswas and Maxwell, 1990). The gasoline yield increases with conversion up to a maximum value. After this point the overcraeking reduces gasoline yield. The coke yield increases with the conversion. The effect of temperature on gasoline and coke yield is also in accordance with literature (Creighton, 1984, Lee, Chen and Huang, 1989). For a constant conversion the gasoline and coke yield decreases increasing the temperature. Another approach which can prove the satisfactory operation of the unit is the conversion function. Figure 5 presents this function which according to literature (Wollaston, Haflin and Ford, 1975) must be linear. Table 3: Properties of the feedstock Density, (60°C) Sulphur, (mass %) Viscosity, (50°C, cSt) Carbon residue, (%) Water content, (%) Basic nitrol~en, (%) Refractive index, (70°C)

0.8711

1.820 17.48 0.07 0.023 0.024 1.48299

Distillation (10 mmHg) Volume % 10 30 50 70 90 EP

AET °C 335.5 393.1 425.4 453.1 485.6 509.6

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CONCLUSIONS As shown in the presented results the unit is performing well based on the theoretical expectations and the results are repeatable. The control system helps to assure the effective operation and safety of the unit. The availability of the data through the computer network will be used for on-line simulation models tuning. REFERENCES

Biswas, J. and I.E. Maxwell, 1990, Recent Process- and Catalyst-Related Developments in Fluid Catalytic Cracking, Appl. Cat. 63, 197-258. Creighton, J.E., 1984, Effect of Reactor temperature on FCC products, Davison Chemical Division W.R. Grace and Co Columbia, Maryland. Gary, J.H. and Handwerk, G.E., 1975, Chemical Processing and Engineering Volume 5, (Petroleum Refining: Technology and Economies), Marcel Dekker, Inc., New York. Lee, L.S., Chen, Y.W. and Huang, T.N., 1989, Four-lump Kinetic Model for Fluid Catalytic Cracking Process, Can. 3.. Chem. Eng. 67. Wollaston, E.G., Haflin, W.J. and Ford, W.D., 1975, What Influences Cat Cracking, Hydrocarbon

Processing.