|Click to view schematic display A||Click to view schematic display B||Click to view schematic display C||Click to view schematic display D|
|Click to view schematic display E||Click to view schematic display F||Click to view schematic display G||Click to view schematic display H|
The main objective of the delayed coking unit is to convert low value residual products to lighter products of higher value and to produce a coke product, whose value will depend on its properties such as sulfur, metals, etc. The conversion is accomplished by heating the feed material to a high temperature of about 900oF and introducing it into a large drum to provide soaking or residence time for the reactions to take place.
Process Flow - Fresh feed is preheated through a heat exchange system prior to entering the bottom of the coker fractionating tower. The fresh feed, mixed with recycle (about 20%) from the unit, is then pumped through two fired heaters to bring the mixture up to temperature. The heaters have facilities to add steam to the heater coils to provide the proper tube velocity and minimize coking in the heater tubes. The effluent from the heaters then enters the bottom of one of the coking drums where the gaseous products pass out the top and the liquid soaks in the drum until it cracks into lighter products that will exit the top of the drum or forms coke that stays in the drum and builds up from the bottom of the drum.
The material from the drum goes to a fractionating tower where it is separated in the desired fractions such as gas, light coker gasoline or naphtha, light coker gas oil, heavy coker gas oil and a heavy recycle oil that is mixed with the fresh feed entering the unit. A steam of light coker gas oil is used as lean oil in an external absorber. The rich oil is returned to the fractionator.
This unit has two coke drums that are operated batchwise. When one drum is filled with coke, the feed is switched to the other drum. The full drum is then prepared for removing the coke. After drum has all coke removed, then the drum can be brought back on line after pressure testing and warm-up. After a drum has been pressure tested, it is ready to be preheated for return to service. This is done by introducing steam and then small amounts of the heater effluent until the drum is within 100/150oF of normal operation. The drum is then ready to bring on line and the other drum is made ready for coke cutting.
Vacuum residue is pumped from the vacuum unit through several heat train exchangers and into the fractionator. The flow rate of coker feed is controlled upstream of the exchangers by the flow controller DCF127C in the line leading to the fractionating tower.
Coker feed is preheated through three heat exchangers prior to entering the fractionator. The first exchanger (E-101) is the vacuum residue vs. heavy coker gas oil. The second exchanger (E-102) is a steam heater, and the third (E-103) is a vacuum residue vs. bottom pump-around.
The temperature of the feed entering the tower is controlled by DCT52I which has "A" and "B" valves. The "A" valve controls the amount of bottom circulating reflux that by-passes E-103. The "B" valve is normally closed. It opens only when there is insufficient heat supplied by E-103.
Temperature indicators (TI-103 and TI-106) provide data on the heat pick-up in each of the exchangers. Monitoring these intermediate temperatures will give an indication of the extent of fouling in each of the feed preheat exchangers.
Coker heater feed consists of fresh vacuum residue and recycle. The flow of this mixture to the heaters is controlled by DCF58C and DCF85C. Monitoring the tower level controller (DCL45C) will let the operator know whether he is maintaining the heater flow at the proper rate.
Control of heater tube velocity is important to minimize heater tube fouling. In order to maintain acceptable tube velocities for varying feed rates injection steam is used to increase tube velocity. DCH663V and DCH665V injection of steam into the line just upstream of the heater inlets.
Four pressure indicators, DCP63I and DCP90I at the inlet to the heaters and DCP700I and DCP702I at the heater outlets, show the heater pressure drop. Normally it should be about 220 PSIG. Monitoring these readings over a period of time will show the amount of fouling and coking occurring in the heaters.
Fuel oil is the only fuel used in these heaters. Fuel oil supply to the heaters is controlled by temperature controllers (DCT67C and DCT94T) at the heater outlets. The quantity of fuel consumed in the heaters is shown by DCF444C and DCF453T. Monitoring these flows and noting any changes it their rates can be an indication of heater performance. Increases in flow that are not caused by changes in feed rate or feed temperature may be the result of heater tube fouling, excess air, or improper atomization of fuel.
Coker heater feed is composed of vacuum residue plus recycle. The rate of vacuum residue is controlled by DCL45C cascaded to DCF02C and the rate of recycle is determined by the temperature of tray 17. The temperature of tray 17 is controlled by DCT43I.
Recycle rate can be calculated by subtracting DCF02C (vac residue) from DCF58C and DCF85C (heater flow rates).
The coke drum temperature is controlled by the heater outlet temperature controls (DCT67C and DCT94C). DCT138I and DCT121I top of the coke drums show the coke drum outlet temperatures. Normal temperature drop across the drums is about 100F.
If the coke drum outlet is too low, insufficient coke will be formed, and it will be mushy (excess volatile matter). If the coke drum outlet temperature is too high, it will cause excessive cracking which results in heater tube fouling and also will result in low volatile matter coke which is difficult to cut from the drum.