The Delayed Coking Process converts vacuum residuum into wet gas, naphtha, light and heavy gas oils and coke, with the objective of maximizing the yield of liquid product and minimizing the yields of wet gas and coke.
Delayed Coking is a thermal process which has two major reactions – thermal cracking and polymerization. Thermal cracking is the mechanism through which molecules of high molecular weight in the feed stock are decomposed into smaller, lighter molecules that are fractionated into the products noted above. The reaction is highly endothermic (consumes heat). The coker heaters supply the heat necessary to initiate the cracking reaction. Heater temperature and residence time are strictly controlled so that coking in the heaters is minimized. The rate of reaction is very sensitive to the temperature; the rate of reaction almost doubles for every 18ºF increase in temperature.
Delayed Coking Process Overview | Energy Efficiency Guidelines for Oil Refining
Polymerization is a reaction through which many small hydrocarbon molecules are combined to form a single large molecule of high molecular weight. The ultimate result of this reaction is the formation of coke. Polymerization reactions require long reaction time and the Coke Drums provide the necessary residence time for these reactions to proceed to completion. A typical coke has 100 to 200 carbon molecules.
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 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 stream 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 batch wise. 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.
The coke product in the coke drum is removed batch wise from the drums after cooling by means of hydraulic cutting tools which cut the coke into pieces that can be handled by the conveying systems that are used to move the coke product from the process area.
The liquid and gaseous products resulting from the thermal cracking are separated into the desired products by fractionation in a distillation tower. Fractionation of the coke drum effluent is accomplished by separating the components into desired boiling ranges. The effluent enters the fractionator just below the bottom tray. A temperature profile is maintained in the tower by means of reflux returned to the tower at various points.
The temperature profile determines the boiling ranges of the products withdrawn from the tower. A portion of the overhead vapor is cooled and returned to the fractionator as reflux, which controls the overhead temperature.
The heavier components are separated by using light, intermediate and heavy draw off streams. Trays inside the fractionator provide the necessary contact between liquid and vapor to accomplish the fractionation.
Product quality is controlled by throttling the amount of reflux pumped back to the tower at various control points. Stripping steam is used in the HCGO/LCGO strippers to remove lighter components from the products and control their initial boiling points and flashpoints.
Again the first step in targeting for best energy consumption in the plant is to draw a boundary around the plant as shown in the graph below.
Heat in and out, directly, and/or indirectly through steam consumption and/or generation, water production and/or generation, power consumption and/or generation are among the plant’s boundary are all identified.
To understand the Delayed Coking Process in oil refinery you have to read following article thoroughly.