DELAYED COKER UNIT (DCU) INTRODUCTION Delayed Coker unit is largestest of its type among all IOCL plants in India. This unit uses vacuum residue obtained from fractionation as the feed the main objective of this unit is to separate out middle distillates, and give products such as LPG and Naphtha. Coke obtained from this unit generates maximum amount of profit for this unit. MAIN PRODUCTS Light Coker Gas Oil (LCGO) Heavy Coker Gas Oil (HCGO) Coker Fuel Oil (CFO) Light Naphtha Heavy Naphtha Fuel Oil (FO) Coke FEED CHARACTERISTICS The feed, which comes mainly, comes from fractionating column of units like AVU, DHDT; RFCCU is stored in DCU storage tank this is classified in two types: Hot Feed: which is at the temp. Of 240 C Cold feed: which is at temp. of 140 C There are four feed tanks in this unit. The final feed obtained before loading is the mixture. Its contains are: 1. A mixture of hot and cold RCO (Reduced Crude Oil) 2. The filterate obtained from 25-micron filter. 3. Heavy Cycle Oil from FCCU 4. Refinery Slop Earlier this feed is used for producing TAR but since advent of DCU we have started using this unit to generate coke. PROCESS DESCRIPTION The delayed coking process is essentially a thermal cracking process to minimize refinery yields of residual fuel oil by conversion of the gas,
naphtha, Gas Oil and Coke. The coker feed is vacuum residue from Arab mix crude s or mixed supply (middle east crude). The feed stream is preheated by HCGO product and pumparound exchangers and charged to the bottom section of the coker fractionator where it mixes with Heavy recycle liquid from the bottom tray. The combined coker feed and heavy recycle liq. is pumped to the coker heater where it is rapidly heated to the temp. Above the coking point. Significant coke formation in the heaters is therefore is avoided. The heater effluents are charges to coke drum where further cracking reactions and subsequent polymerization takes place to form coke. The vapour products leave the top of the coke drum where they are immediately quenched with a slip stream of HCGO pumparound. The quenching stops the cracking and polymerization reactions. Quenched coke drum effluents are charged to the coker fractionator where light gas, naphtha, LCGO, HCGO, CFO and heavy recycled liquids are separated. LCGO, HCGO and CFO are steam stripped to meet product specifications before cooling and final routing to battery limits. The vapour from the coker fractionator is partially condensed and collected in the coker fractionator overhead receiver, which separates vapours, hydrocarbon liq. and sour water. Vapours are routed to the vapour recovery section. A portion of the hydrocarbon liq. is pumped to the coker fractionater top tray as reflux, and remaining portion is pumped to the vapour recovery section the part of the sour water goes to the recontact product coolers as awash, and the net production goes to OSBL for treatment. The vapour recovery section separates the light products to debutanised naphtha C3/C4 LPG and Fuel gas. The Wet Gas from coker fractionator overhead receiver is compressed into two stage wet gas compressor vapour discharge is partially condensed and separated in the recontact drum. The vapour effluents are stripped of the bulk of its H2S using lean amine in deethaniser feed gas scrubber. The H2S free vapour is then charged to the absorber section of the deethaniser primary absorber or stripper. The hydrocarbon liq. from the recontact drum is pumped to the stripper section. Mean while the sour water interstage drum and the recontact drum recycles to the coker fractionator over head receiver. The Deethaniser separates the C2 and the lighter from the fractionator from the LPG and the naphtha. Absorption oil, a combination of coker fractionator overhead receiver hydrocarbon liquid and debutanised naphtha liq. recycle feeds the top tray. The deethaniser overhead vapour is contacted with LCGO in the Sponge Absorber to further reduce the loss of Naphtha. The resultant sour fuel gas is scrubbed free of H2S using lean amine in the fuel gas scrubber before routing to the sweet fuel gas to OSBL. The deethanizer bottoms are routed to the debutanizer to
separate LPG from naphtha. The product LPG is scrubbed free of H2S using lean amine in LPG amine scrubber before routing to the OSBL. The product naphtha is cooled and routed to the naphtha splitter. The naphtha splitter separates the debutanised naphtha into light and heavy naphtha. Light naphtha product is routed to the light naphtha amine scrubber to remove H2S and then caustic and water wash prior to being routed to OSBL. The heavy naphtha is caustic and water wash prior to being routed to OSBL. COKER FRACTIONATOR The coker fractionator separates the coke drum effluents vapour into light gas, naphtha, LCGO, HCGO, CFO and a heavy recycle stream. The column is provided with 36 trays and, is divided into two main sections by the HCGO drawpan. The upper section consists of the 25 valve trays; lower section contains 6 bubble trays and 5 baffle trays. The quenched coke drum effluent vapours flows upward through the baffle trays and bubble trays with cooling is accomplished by the contact with the down flowing reflux liquid. Heavy recycle liq. is condensed and over flow bottom baffle trays mixed with the freshcoker feed in the tower bottom as described earlier. Net products flows to the upper tower section to the vapour risers in the HCGO drawpan. This vapour consists of the products, reflux and steam. The heat removal and fractionation is accomplished in the upper section of the coker fractionator. WET GAS COMPRESSOR The Wet Gas Compressor (WGC) controls the System Pressure of DCU. The WGC compresses the wet gases from the Coker Fractionator Overhead Receiver in two stages. The first page of this steam turbine driven compressor discharges vapour at 4.5 kilogram per square centimeter per gram. The vapour is then cooled to 40 degree Celsius in the Compressor Interstage Coolers and enters the Compressor Interstage Drum. Hydrocarbon Vapour, Hydrocarbon Liquid and Condensed water are separated in the drum. The condensed sour water is recycled under pressure back to the inlet of the Coker Fractionator Overhead Condensers as wash water. The Compressor Interstage Liquid Pump pumps the hydrocarbon liquid back to the inlet of the Recontact Product Coolers where it joins the second stage discharge stream from the Wet Gas Compressor. The second stage of the WGC compresses the vapour from the Compressor Interstage Drum upto a pressure of 16.87 kg/cm*cmg where
the vapour combines with the Interstage Drum Hydrocarbon liquid and the Recycled sour water from the Coker Fractionator Overhead Receiver. The combined stream is cooled and condensed in the Recontact Product Coolers and enters the Recontact Drum. Vapour from the drum flows to the bottom of the Deethanizer Feed Gas Scrubber. The Deethanizer Feed Gas Scrubber is a packed Column where counter current contact with Lean Amine removes about 90% of the H2S from the Vapour Stream. The Lean Amine Pumps increase the pressure of the Lean amine from OSBL for all of the users in the Delayed Coker Unit. Lean Amine from these pumps flows under flow control to the top of the Deethanizer Feed Gas Scrubber. The sweetened vapour steam from the Deethanizer Feed Gas Scrubber flows to the Feed Gas Scrubber to remove any entrained amine prior to entering the Deethanizer. Hydrocarbon liquids from the Recontact Drum are pumped on flow control with level reset to the Deethanizer Feed Coalescer to remove any free water prior to entering the Deethanizer. The water recycles to the Coker Fractionator Overhead Receiver Sour water from the Recontact Drum is withdrawn under level control and routed to the inlet of the Compressor Interstage Coolers. DEETHANIZER The deethanizer (Primary Absorber/Stripper) is a column with 60 total trays that is divided by an internal head into 2 sections of 30 trays each that are essentially separate columns. This design utilizes a Recontact drum for additional cooling on the Absorber liquid and Stripper vapour. The feeds, the columns themselves, and the flows to and from the Recontact drum are discussed first; then he flows forward to other items of equipment are discussed. The upper (Absorber) section receives the vapour from the Feed Gas Scrubber KOD at the bottom. It receive on the top tray, Tray 60, the Fractionator overhead liquid from the Recontact Naphtha Pumps and the recycled, cooled Debutanizer bottoms (Lean Naphtha) from Debutanizer Bottoms Pumps. These liquid streams absorb LPG from the gas feed. The liquid from the Tray 54 is pumped through the Deethanizer Intercooler into the Deethanizer water separator. The Hydrocarbon liquid flows from the top of the separator back to the column and enter on tray 53. The water from the bottom of the separator flows under interface level control on its own pressure back to the Coker Fractionator Overhead receiver. The bottoms stream from the column section flows to the Recontact Drums via the Compressor Interstage Coolers.
The lower (Stripping) section of the Deethanizer strips ethane and lighter components from the LPG and Naphtha. It receives the liquid from the Recontact Drum on its top tray, tray 30. The Stipping section is configured with 3 Reboilers. At the bottom of the column, the liquid from the tray 1 flows preferentially to the shell side of the Deethanizer Bottoms reboiler by thermosiphon action. This Reboiler uses LCGO Pumparound as heating medium, and is designed for 90% of the bottoms reboil duty. Liquid from the bottom sump of the column flows to the shell side of the Deethanizer Bottoms Steam Reboiler by thermosiphon action. This Reboiler uses MP steam as the heat source, and is designed for 50% of the required bottom reboil duty although it is expected to do only 10%. The additional capacity provides flexibility to handle start-up and upset conditions. There is a partial draw from Tray 5 liquid to the Deethanizer Side Reboiler under flow control by thermosiphon action again. The hot Debutanizer Bottoms Stream is the heating medium. The vapour from the top section Flows to the Recontact Drum via the Compressor Interstage coolers. SPONGE ABSORBER Deethanizer Overhead Vapour flows from the top of the absorber section to the bottom of the Sponge Absorber a simple absorber with 21 trays. Cooled lean sponge-oil (LCGO) from the LCGO stripper is fed to the top of the Sponge Absorber primarily to reduce the loss of Naphtha in the Deethanizer Overhead Vapour Stream, but also to absorb a little more LPG. The flow of the lean sponge oil was discussed previously with the flow of LCGO Product. The Sponge Absorber Overhead Vapour (Sour Fuel Gas) is routed to the fuel gas Scrubber for H2S removal. Rich Sponge oil from the Sponge absorber bottoms is used to cool the lean sponge oil (Light Coker Gas Oil) in the LCGO Lean/Rich Sponge Oil Exchangers and is returned to the Coker Fractionator. Deethanizer Bottoms liquid flows by its own pressure to the Debutanizer under flow control reset by level control in the bottom of Stripper section of the Deethanizer. DEBUTANIZER The debutanizer column is provided with 36 trays and separates its feed into an overhead product of C3/C4 LPG and bottom product of Debutanized Naphtha. Overhead vapour is totally condensed against cooling water in the Debutanizer Overhead Condensers and is routed to the Debutanizer Overhead receiver.
COKING SECTION Coking is semi batch operation and will be on 24 hr coke drum cycle for the initial operating cases. Each drum will be online for 24 hrs of filling and offline for 24hrs of the decoking. The total cycle for each drum from oil in to oil in will be 48 hrs. for the future operating cases at increase throughput the coke drum cycle may be reduced to 18 hrs. Following completion of the drum filling cycle, coker heater effluents is diverted to the other (empty) coke drum by means of the inlet switch valve. Steam is then injected into the bottom of the coke filled drum to cool it. For the first 30 min. volatile light HC are purged to the coke fractionator. During the next 60 min. steam rate is increased and the resultant vapours (mostly steam) are routed to the bottom of the blow down tower