Operation and Control of Crude Oil Refining Process using Atmospheric Distillation Column with PLC

Transcription

1 43 Journal of Electrical Engineering The Institution of Engineers, Bangladesh Vol. EE xx, No. xx, June/December xxxx Operation and Control of Crude Oil Refining Process using Atmospheric Distillation Column with PLC P.K. Bhowmik 1, J. A. Shamim 2, and S. k. Dhar 3 1 Department of Instrumentation and Communication, Eastern Refinery Limited, North Potenga, Chittagong, Bangladesh 1 Department of Inspection, Eastern Refinery Limited, North Potenga, Chittagong, Bangladesh 1 Member IEEE, Chittagong, Bangladesh pkbcuet@yahoo.com 1 jubair.buet.bd@gmail.com 2, sagor378@gmail.com 3 Abstract The operation & control of crude oil distillation process, which is a very sophisticated system involving modern technologies, is of utmost importance in any crude oil refinery. In this paper, prominent features of a PLC based automatic operation and control of crude oil refining process in an atmospheric distillation column has been outlined. The complete crude oil distillation process has been ramified into four consecutive phases viz. initial start-up & cold circulation, furnace firing & selection of appropriate fuel, hot circulation & commissioning of reflux system & finally storage of the refined products. In first phase, if all the physical systems are in good condition, then all respective valves will be opened so that the crude may be circulated through the associated circuit from crude oil storage tank by the action of gravity. After that the main feed pump will be started to continue the flow of crude until 50% bottom level of the distillation column if filled up & this circulation of crude is termed as cold circulation. In the second phase, the furnace will be fired after completion of furnace pre-purging, steam commissioning in the super-heated circuit & appropriate fuel (NG/RG/FO) selection. In this stage, the temperature of crude oil is also gradually increased at furnace outlet and maintained by throttling the fuel supply valves. In the third phase, a temperature gradient is maintained in the distillation column by commissioning the reflux (top, mid & bottom) systems and thus the fractional distillation is facilitated. Finally, the purified products are stored in the respective storage tanks from the distillation column. If any aberrant situation arises, owing to which the operation may be unstable & unsafe, the feed pump, furnace fuel supply valves and respective safety shutoff valves will be tripped and thus the entire cycle is accomplished. Index Terms PLC, Crude oil refining process, Distillation column, Sucosoft V5.02 Moeller PLC software T I. INTRODUCTION HE demand for higher quality, greater efficiency & automated control in the arena of process plant, chemical industry and energy sector has been aggrandized to a great extent [4]. To cope with this advancement & also to construe the advantages of using PLC based automation in crude oil distillation process, an attempt has been taken in this paper. Every crude oil refinery requires continuous monitoring & spasmodic inspection of its various process parameters in order to ensure safe and smooth operation. There are possibilities of errors in human operation. In order to automate the operation & control of crude oil distillation process and subsiding human intervention, there is a need for developing PLC based control system. Typical applications of PLC based control systems are in the industries e.g. power plants, oil refineries, ship building industries etc. The process system of Eastern Refinery Limited (ERL), Chittagong, Bangladesh is taken as a reference for this study. II. PLC MODULE SETUP PLC is a 'digital operating system' designed especially for use in an industrial environment, which uses a programmable memory for its internal operation of user-orientated instructions and for implementing specific function such as logic, sequencing, timing, counting and arithmetic [2-3]. The Sucosoft V5.02 PLC software is used in this study [2-3]. III. PROCESS DESCRIPTION The feed (crude oil) is pumped directly from the crude oil storage tank by the feed pump P1101A/B. The flow rate of the feed is controlled by FRCV. Feed (crude) in furnace is heated to 360 C and so the crude is partially vaporized. This partially vaporized feed is then sent to the atmospheric distillation column C1101, which is equipped with 33 trays. In this column the crude is fractionated to give the following products: TG, KI, KII, LGO, HGO, and Residue. TG is condensed in the aero condenser EM-1123, then in the water cooler E1110AB. The pressure of the reflux drum B1102 is controlled by departure of gas to the flare through PICV In this reflux drum, gasoline and water are separated by decantation; part of the gasoline is taken out by pump P1109A/B to be refluxed under temperature control valve TRCV1102 at the top of the column. The other part is taken out by pump P1110A/B and sent as feed for the stabilization column under the level control valve LCV1103 of B1102. Water is evacuated by gravity to the sewer under the level control valve LCV1104. KI, KII, LGO and HGO are withdrawn separately from their respective trays which are flow by gravity into the stripper column under the respective LCV. After that thus products are taken out by pump cooled by exchanging heat through exchanger and cooler and stored in the storage tank. The bottom product (residue) is stripped by superheated steam and is taken out by pump P1107 then cooled by heat

2 exchanger and cooler and stored in the storage tank. Its flow rate is controlled by LCV. The column temperature gradient is maintained to get the required products by commissioning the reflux (top, mid and bottom) system [1]. IV. PROCESS SCHEMATIC DIAGRAM The process schematic diagram of crude oil distillation process is as follows-. Start up of furnace firing is performed by the following steps- Commissioning the furnace by snuffing steam to remove gases to avoid explosion during furnace firing. Superheated steam vent and low points open to allow the flow of steam in superheated coils at 3.0~3.5 bar. Check and maintain the negative draft pr. of the furnace 10~12 mm of H 2 O. Select the required fuel system. Pilot valve open and pilot burner fired by spark plug. Then main gas valve open to fired main burner. For FO (Furnace Oil) both FO valve and atomizing steam valve have open simultaneously with maintaining 1.6 bar higher pressure at steam than FO pressure. After main burner fired the pilot valve closed to off the pilot burner. C. Hot circulation & commissioning of Reflux and product storage system Furnace outlet crude temperature increased gradually and maintained near 360 C by TRCV1101A/B, or TRCV1101 and PDCV1101. In the C1101 the crude is fractionated to TG, KI, KII, LGO, HGO, and Residue. The refluxed drum B1102 is commission to get the TG by maintaining pressure and level of the balloon. A part of TG is taken out by pump P1109A/B to be refluxed under temperature control valve TRCV1102 at the top of the column. The stripping column, C1102 is commission to get the fractionated products KI, KII, LGO and HGO under the respective LCV and FRCV to store in the tank. The circulating reflux is commission to maintain C1101 mid zone temperature gradient. The bottom product of C1102 is taken out by pump P1107 and flow is controlled under the LCV1102 and finally stored to the tank. Fig. 1 Simplified schematic diagram of the crude oil refining process [1] D. Process Flow Diagram V. PROCESS OPERATION DESCRIPTION Successful operation of a process system depends upon careful installation & initial start up. Before start up there should be a thorough check of the system, like valves, control trip, safety device, and all the mechanical and electrical system. The crude oil distillation process is categorized in the subsequent phases: [1]. A. Initial Start up and Cold Circulation At the initial start up the respective valves to ensure the flow of crude by gravity from the crude tank to the crude circuit. Start Feed pump that will force crude to circulate though the full crude circuit. The flow of crude is controlled by FRCV and circulation continues to the circuit up to 50% bottom level build up in C1101. B. Furnace Firing and Fuel Selection In ERL NG, RG and FO are used as fuel of the furnaces through their consequent fuel system network. Start Furnace purging & steam commissioning completed Start Feed pump fill the C1101 bottom 50% by crude Required fuel system NG/ RG/NG+RG/FO selected Furnace purging & steam commissioning completed Main burner fired No Yes Pilot valve closed & pilot burner off Pilot valve opened and header pressure maintained Pilot burner fired by spark plug ignition A 44

3 A NG/RG/NG+RG Fuel selection Main gas valve open Start FO pump to maintain Main burner fired FO header FO Steam valve open to maintain header pr. E. Program Development Moeller PLC is programmed and simulate in Sucosoft V5.02 programming software. The Ladder Diagram (LD) PLC programming language used easy to understand. LD is based on graphical representations with contacts, coils and boxes, as per the circuit diagrams [2-3]. Pilot burner off Maintain pr. Diff. TRCV Opening Furnace outlet temp TRCV Closing B1102 commissioning & start respective pumps PICV1101 Opening B1102 pr. (than set) PICV1101 Closing B1102 level (than require) LCV1103 Opening LCV1103 Closing LCV1104 Opening B1102 water level C1101 top temp LCV1104 Closing TRCV1102 Closing C1102 commissioning & start respective pumps C1101 distillate level LCV1105~8 Opening LCV1105~8 Closing Circulating reflux commissioning TICV1101 & FRCV1108 Opening C1101 Mid zone temp Bottom Residue pump start TICV1101 & FRCV1108 Closing Fig. 4 Process variable declaration for PLC system C1101 bottom level LCV1102 Opening LCV1102 Closing Return Fig. 3 Crude oil refining process flow diagram 45

4 Fig. 5 LD program for crude refining process 46

5 F. Program Execution The ladder logic program execution by Sucosoft V5.02 software requires the following steps [2-3]: Program development in pou editor, Topology configuration, Program code generation, Program transfer from computer to PLC, Test and commissioning, Program run. Then the PLC executes the program successfully after pressing the start switch. As the number of input & output address is restricted in the software used in this study, so few false address is used in the program. PLC always scan its input address and give an output signal according the instruction of the program. VI. RESULT AND DISCUSSION The modification of many single loop control loops by a single PLC will provide more flexibility of the system in case of operation, control, troubleshooting, commissioning, testing and modification. The proposed system will also be beneficiary to the economical aspect. Though the practical process control system have used several PID (proportional derivative and integral) control, due to the limitation of PLC software used in this study all the control loops are simplified for analysis that can be easily improved by using higher version of the software. It has also ensured the option of further advancement in system modification by advance PLC that is SCADA system. VII. CONCLUSION To cope with the technological advancement, to ensure higher efficiency and effectiveness and reliability in a more economical way this proposed system will be a great scope. It will also provide better flexibility in customization of operation and control with minimum effort. The paper has furnished itself to study the integral parts of the entire process involved, their implementation and the problems that may show up have also been given their due importance. ACKNOWLEDGMENT The authors would like to thank Mr. Sharafat Ali, Assistant General Manager, Department of Instrumentation & Communication, Eastern Refinery Limited, Bangladesh for his valuable guidelines and supervision to complete the task. Cordial thanks also to Md. Ali Zulquarnain, Member, Planning, Bangladesh Atomic Energy Commission (BAEC) for his valuable review and comments. REFERENCES [1] Crude Oil Refining Process Operation Guideline, Eastern Refinery Limited, Chittagong [2] GUNTE, Equipment for Engineering Education,(04/02/2012) [3] Sucosoft S40 V5.02, Moeller PLC Software [4] P. K. Bhowmik and M. Dey, Protection and Control of Steam Turbine Generator using PLC First International Conference on Mechanical Engineering and Renewable Energy 2011 (ICMERE2011), Chittagong, Bangladesh, December 22-24, [5] S. K.Dhar, P.K. Bhowmik and S. Chakraborty, Operation and Control of Nuclear Reactor with PLC International Conference on Information and Communication Technology, volume 3, 11C-25, Yantai, CHINA August 27-28,