The main objective of the Delayed Coking Unit is to convert low value residual feedstocks 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 (refinery vacuum tower residue) to a high temperature of about 900 DEG F and introducing it into a large drum to provide residence time for the reactions to take place. The creation of lighter products from the feed using high temperatures is referred to as thermal cracking. The production of coke in the drums after heating is referred to as delayed coking.
Heaters and Coke Drums; Fresh feed from battery limits is preheated through a heat exchange system prior to entering the bottom of the Coker Fractionator. The fresh feed mixes with recycle liquid from the Coke Drums in the bottom of the Fractionator and is pumped through the Heaters, where increasing the temperature causes cracking of heavier hydrocarbons. The Heaters have facilities to add steam to the heater coils to provide the proper tube velocity to improve heat transfer and minimize coking within the heater tubes. The effluent from the Heaters then enters the bottom of one of the Coke Drums, where the gaseous products flash overhead, and as the liquid heats in the drum, it cracks into lighter products that also flash overhead. Coke forms a solid of high carbon content as the liquid cracks. The coke agglomerates within the Coke Drum and builds up from the bottom. When the drum becomes nearly full of coke, flow from the Heaters is routed to the standby Coke Drum, while the coke is removed.
Fractionator and Product Strippers; The effluent material (uncracked oil and cracked products) from the Coke Drums goes to the Fractionator, where it is separated in the desired fractions such as gas, light coker gasoline or naphtha, light coker gas oil (LCGO), and heavy coker gas oil (HCGO). A stream of light coker gas oil is used as lean oil in an external absorber, while rich oil is returned to the Fractionator. Heavy coker gas oil also serves as a pumparound fluid for cooling the lower portion of the Fractionator. Two product stripping towers, the LCGO and HCGO Strippers, use steam to strip light gases from the products before they are pumped to tankage. Vapor from the top of the Fractionator is cooled in the Overhead Fin-Fan Condenser, where light naphtha and steam are condensed. Net overhead gas, light naphtha, and sour water, are separated in the Overhead Receiver and sent to respective battery limits. A portion of the light naphtha is refluxed to the Fractionator. Uncracked recycle oil from the Coke Drums is retained in the bottom of the Fractionator and is mixed with the fresh feed entering the unit. Since the unit does not produce net bottoms oil, the material is recycled through the Heaters and the Coke Drums until it is completely cracked and converted into coke and lighter products. The process is described in detail in the following sections.
Coker Feed System
Warm vacuum residue feed from battery limits (FIC-2) is mixed with two cold feeds from storage (FIC-100 and FIC-101) that have been preheated in Feed/LCGO Exchanger, E-100, by cooling product light coker gas oil from the Sponge Oil/LCGO Exchanger, E-2. The mixed feed is preheated in Combined Feed/HCGO Exchanger, E-101, by cooling product heavy coker gas oil from HCGO Product Pump, G-103. The feed then passes through Combined Feed Steam Preheater, E-102, which uses high pressure steam to heat the feed. Normally no steam flows to the Combined Feed Steam Preheater, but the control system will automatically start flow in case of loss of heat transfer in the Combined Feed/HCGO Exchanger and/or Combined Feed/HCGO Pumparound Exchanger, E-103. The feed then passes through HCGO Pumparound/Feed Exchanger before entering the bottom of Coker Fractionator, T-9001, where it mixes with cracked oils and gases returned from the Coke Drums, V-9001/9002. The bottom of the Fractionator serves as the surge volume for the feed.
In addition to the heavy feeds to the unit, recycled LCGO/HCGO (HV-14) can be introduced to the combined feed line before Combined Feed/HCGO Exchanger. HCGO from storage (HV-15) can be introduced directly to the bottom of the Fractionator. These two lines are normally closed. The bottom of the Fractionator can be pumped out to the slop system (HV-25)using Slop Header Pump, P-9002. A line from the Slop Header Pump back to the bottom of the Fractionator (FIC-31) allows the Slop Header Pump to continuously circulate feed and recycle from the Coker Drums through the Slop Header Pump Suction Filter, FIL-9001. This helps remove any small particles of coke and scale from the system.
Coker Feed Heaters
A mixture of feed and heavier cracked oil from the bottom of Fractionator is pumped by Heater Charge Pump, P-9001, to both the Coker Feed Heaters, F-9001/9002. The flow is evenly split between the two heaters (FIC-58 and FIC-87). Velocity steam (HIC-663 and HIC-665) is added before each heater to improve the heat transfer and to reduce coking within the heater tubes. The velocity steam also helps protect the heater tubes from excessive temperatures in case of a loss of oil charge flow. The Coker Feed Heaters are fired, natural-draft heaters that use fuel oil (FIC-444 and FIC-453). The stack dampers and air registers are assumed to operate ideally for the simulation. Charge oil and velocity steam from each heater are raised to the ideal temperature (TIC-67 and TIC-94) for cracking to occur and are then combined and routed to one of the operating Coke Drums. The velocity and residence time of the oil and steam within the heater tubes is such that coking within the tubes is minimal. However, cracking of the feed oil into lighter hydrocarbons and gas will start in the tubes and continue as the outlet streams reach the Coke Drum. High pressure steam from battery limits (FIC-82 and FIC-108) is superheated and boiler feedwater from battery limits (FIC-158 and FIC-159) is heated in each of the convection sections of the Coker Feed Heaters. The flow rates of the steam and boiler feedwater are normally controlled in proportion to the oil flow to the respective heater they serve. The heated fluids are returned to battery limits.
The outlet lines from Coker Feed Heaters combine before entering Coke Drums, V-9001 and V-9002. Normally only one drum is in service (HV-4 or HV-5) and the other is on cold standby. A bypass line and valve (HV-12) is also provided for circulation through the heaters without going to either drum. A logic program allows only one of the three routing valves (HV-4, HV-5, and HV-12) to be open at any given time. The combined stream from the heaters enters at the bottom of the Coke Drum in service. The drums are sized to give a long residence time to permit the coking and cracking actions to occur. As the hot cracked oil passes into and upward through the wide drum at a low velocity, solid coke particles form, settle out, agglomerate, and polymerize. The agglomerated coke is fairly porous, depending on the feedstock and processing conditions, and eventually starts filling the drum. Cracked oils, naphtha, and gases rise to the top of the Coke Drums and are routed to the Fractionator for separation by distillation. After the operating drum becomes nearly full of coke, the feed is switched to a standby drum after warming it up. While the second drum fills with coke, the coke-filled drum is stripped, cooled, and decoked. Steam (FIC-128) is provided for stripping hydrocarbons entrained in the solid coke prior to opening the vessel for physical removal of the coke. Steam-stripped hydrocarbons should be routed to the Blowdown Tower, T-9002 (tower not simulated).
Note: The logic system will prevent steam and/or cooling water from being lined up to a Coke Drum (HV-6 or HV-7) if the respective cracked oil valve (HV-4 or HV-5) is closed.
Coke is cooled completely using direct injection of cooling water (FIC-267). The coke is broken up inside the drum using the cooling water. The Condensate Drum, D-9001, is used to collect any water or liquid hydrocarbons from the drums when they are drained (HV-16). The Condensate Drum separates any gases in these liquids and returns them to the Fractionator (HV-11).
Note: The Condensate Drum, D-9001, and Condensate Pump, P-9007, are assumed to operate ideally in the simulator.
The Coke Drums can also be drained to deck to recover the loosened coke (HV-17). To warm up the Coke Drums after they are decoked, a line (HIC-101) is provided to route a stream of hot effluent from the operating Coke Drum to the standby Coke Drum. Warming up the drum to near-operating temperature prevents thermal shock to the Coke Drum when it is finally switched back into service. At startup, the Coke Drums can be warmed up with Feed Heater effluent by bypassing the drums (HV-12) and routing warm charge oil from the Feed Heaters to the top of the drums (HIC-400, HV-20, and HV-21).
The Coker Fractionator, T-9001, is a distillation tower that consists of 24 trays. The hot stream from the Coker Drums enters the Fractionator below the bottom tray, No. 24. The lower section of the tower is larger in diameter than the upper section because it is designed to condense and remove heavy coker gas oil from the effluent of the Coke Drums. Condensing of HCGO is accomplished with a pumparound system using HCGO itself as the cooling medium (FIC-154). The HCGO product is drawn off to a side stripper for removal of light gases before being pumped to storage. A stream of HCGO (FIC-57) is also directed to the bottom section of the Fractionator below to wash down any heavy oils to the bottom of the tower. This prevents the HCGO system from being polluted with entrained heavy hydrocarbons returning from the Coker Drums. The top section is designed to purify naphtha from light coker gas oil by refluxing top liquid product (FIC-55) back down the Fractionator. LCGO is drawn off from tray No. 9 of the top section and sent to a product stripper. LCGO can also be drawn off from tray No. 14 and sent to the HCGO stripper to adjust its properties. A portion of the cooled LCGO product from the LCGO Stripper, D-2, is used as sponge oil (FIC-216) in an absorber tower (not simulated). The return sponge oil stream is used to cool the LCGO product, which returns to the Fractionator. Naphtha vapors, light gas, and steam rising from the top of the Fractionator are cooled in an air-cooled condenser, before being separated in an overhead receiver. The balance of the naphtha not used as reflux is sent to storage (FI-439). The net gas (FI-206) is routed to a compressor (not simulated in detail). Sour water is also separated and sent to treating facilities. The bottom of the Fractionator serves as the surge volume for the feed to the unit. Any liquids from the Coker Drums and the liquid from the wash section of the Fractionator (tray No. 18 to tray No. 24) are also collected in the bottom of the Fractionator and returned to the Feed Heaters and the Coke Drums.
HCGO Pumparound System
The HCGO Circulating Reflux Pump, P-9004, takes suction from tray No. 17 and pumps hot HCGO to Combined Feed/HCGO Pumparound Exchanger, E-103, and then on to HCGO Steam Generator, E-9002, before being returned to tray No. 16 (FIC-154). A bypass line around the HCGO Steam Generator allows precise control of the HCGO return temperature (TIC-167). The circulating HCGO through trays No. 16 and No. 17 of the Fractionator results in the condensing of most of the heavier hydrocarbons rising from the lower section of the Fractionator. HCGO is also routed to the lower section as wash oil(FIC-57). This stream also serves to partially cool and condense the hot vapors rising from the return line of the Coke Drums below tray No. 24. Product HCGO is withdrawn from the supply line to the HCGO Circulating Reflux Pump and is drawn off to the HCGO Stripper, D-3, by gravity. HCGO is also drawn off to the outlet line of the Coke Drums as quench oil.
HCGO Product System
HCGO is drawn off tray No. 17 of the Fractionator (LIC-44) and sent to the HCGO Stripper by gravity. The HCGO Stripper consists of 5 trays and the HCGO feed is stripped free of light gases using medium pressure steam (FIC-25). Stripped gases and steam from the top of the HCGO Stripper are routed to tray No. 14 of the Fractionator. LCGO drawn by gravity from tray No. 15 of the Fractionator (FIC-22) can also be supplied to the HCGO Stripper in order to adjust the HCGO boiling point properties. Normally, LCGO is not sent to the HCGO Stripper. Stripped HCGO is collected in the bottom of the HCGO Stripper (LIC-14), which supplies the HCGO Product Pump, G-103. Product HCGO from the HCGO Product Pump is sent to the Combined Feed/HCGO Exchanger, E-101, to be cooled and then on to storage (FI-155).
LCGO Product System
LCGO is drawn off tray No. 9 of the Fractionator (HIC-8) and sent to the LCGO Stripper, D-2, by gravity. The LCGO Stripper consists of 5 trays and the LCGO feed is stripped free of light gases using medium pressure steam (FIC-24). Stripped gases and steam from the top of the LCGO Stripper are routed to tray No. 4 of the Fractionator. Stripped LCGO is collected in the bottom of the LCGO Stripper (LIC-41), which supplies the LCGO Circulation Reflux Pump, P-9003. Product LCGO from the LCGO Circulation Reflux Pump is sent to the Sponge Oil/LCGO Exchanger, E-2, and the Feed/LCGO Exchanger, E-100, to be cooled. A portion of the LCGO product is used as sponge oil (FIC-216) in an absorber. The balance of the LCGO is sent to storage (FI-166). The rich sponge oil (HV-13) is returned to Sponge Oil/LCGO Exchanger and is then routed to tray No. 8 of the Fractionator.
Naphtha vapor, steam, and light gas produced from the top of the Fractionator, T-9001, are routed to the Overhead Fin-Fan Condenser, E-9011, which cools and condenses the naphtha and steam. The three-phase mixture is sent to the Overhead Receiver, D-9005, which separates the three phases. The cooled light wet gas (FI-206) from the Overhead Receiver is sent off to battery limits to be compressed. A recycle line from the compression facilities (HIC-221) is simulated. Normally there is no flow in this line. The light gas can also be sent to the flare system in case of an emergency. Naphtha from the Overhead Receiver is pumped as reflux (FIC-55) back to the Fractionator with Naphtha Reflux Pump, G-4. The balance of the naphtha produced in the Overhead Receiver is sent off to tankage (FI-439) via Naphtha Product Pump, P-9009. Produced sour water from the Overhead Receiver is taken off to battery limits (FI-91). The simulator assumes the separation and level control of the sour water is perfect (P-9008 and LIC-16 are not simulated). The wash water line from battery limits to the overhead line from the Fractionator is drawn to show typical facilities in a Fractionator. However, wash water injection is not simulated.
The design capacity of Delayed Coking Unit is 23,000 BPD of hot vacuum residuum (ANS 970 F+) plus 2,125 BPD of recovered oil. An alternate cold feed operation of ANS resid requires blending of either 2,550 BPD of light cycle gas oil (LCGO) from the FCC Unit or 4,115 BPD of light atmospheric gas oil (LAGO) with 17,380 BPD cold vacuum residuum. 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.
Basic Controls: Feed System
Feed Flow; Vacuum residue is pumped from the vacuum unit through several heat train exchangers and into the bottom of the Fractionator, T-9001. The flow rate of coker feed is controlled upstream of the exchangers by flow controller FIC-2. LIC-45 adjusts the setpoint of FIC-2 to keep the Fractionator bottom level at a safe operating value. The rate at which feed from the Fractionator bottom is converted to lighter products and coke, will depend on the circulation rate through the Feed Heaters and their outlet temperatures.
Note: Changes in any other feed flows to the bottom of the Fractionator will cause LIC-45 to compensate by adjusting the setpoint of FIC-2. These flows include cold recovered oil (FIC-101) and cut vacuum residue from storage (FIC-100), which are first preheated in the Feed/LCGO Exchanger using product LCGO and then mixed with warm vacuum residue before the Combined Feed/HCGO Exchanger, E-101. Also, a recycled LCGO/HCGO mixture can be introduced into the feed at this point via HV-14 to lighten it. HCGO from storage can also be directly added to the bottom of the Fractionator with HV-15. These supplemental lines are unmetered.
Basic Controls: Feed Temperature
Coker feed is preheated through three heat exchangers prior to entering the Fractionator. The first exchanger, Combined Feed/HCGO Exchanger, E-101, recovers heat from heavy coker gas oil (HCGO) product. The second exchanger, Combined Feed Steam Preheater, E-102, is a steam heater, and the third exchanger, Combined Feed/HCGO Pumparound Exchanger, E-103, removes heat from HCGO pumparound. The temperature of the feed entering the Fractionator is controlled by TIC-52 which has “A” and “B” valves. The “A” valve controls the amount of HCGO circulating reflux that bypasses Combined Feed/HCGO Pumparound Exchanger. The “B” valve is located on the steam line to Combined Feed Steam Preheater. It is normally closed and opens only when there is insufficient heat supplied by Combined Feed/HCGO Pumparound Exchanger. The use of steam can be avoided by blocking the “B” valve with HV-3. 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.
Basic Controls: Feed Heaters
Heater Feed Flow; Coker heater feed consists of fresh vacuum residue and recycle liquid from the Coke Drums. The flow of this mixture from the bottom of the Fractionator to the Heaters is controlled by FIC-58 and FIC-87. 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. HIC-663 and HIC-665 inject steam into the line just upstream of the heater inlets. Four pressure indicators, PI-63 and PI-90 at the inlet to the heaters and PI-700 and PI-702 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.
Basic Controls: Feed Heater Outlet Temperature
The outlet temperature controller of each Feed Heater (TIC-67 and TIC-94) adjusts the setpoint of its respective fuel flow controller (FIC-444 and FIC-453). Fuel oil is the only fuel used in these heaters. Unexpected changes in rate of fuel consumption can be an indication of heater performance (e.g., tube fouling, excess air, or improper atomization of fuel). The outlet temperature of a heater greatly impacts the conversion rate of the feed. If it is too low, less cracking and coking will take place. This will be reflected by a higher calculated recycle rate and lower conversion (see below). If the temperature is too high, too much gas and coke may be produced. This can lead to operational problems if the wet gas production exceeds the Fractionator Overhead System’s capability to handle it. Additionally, the Coke Drum will fill up faster and the type of coke produced may be of a less saleable quality.
Recycle Rate and Conversion
The recycle rate is an estimation of the volume of liquid returned to the bottom of the Fractionator from the Coke Drums and from any net liquid produced in the wash section of the Fractionator (trays No. 18 through No. 24). It is calculated as follows:
- Recycle rate = Heater flow rates – fresh feed rates Recycle rate = (FIC-58 +FIC-87) – (FIC-2 + FIC-100 + FIC-101)
The total conversion in the Heaters/Coke Drums is estimated as:
- Conversion = fresh feed rates / Heater flow rates * 100% Conversion =(FIC-2 + FIC-100 + FIC-101) / (FIC-58 + FIC-87)
Note: these variables are only meaningful when the unit is steady. If the conversion or recycle rate changes significantly for unknown reasons it indicates some problem with the process or the instrumentation. For example, if a drift of the thermocouple for one of the Feed Heaters occurs it will obscure the real process temperature which will differ from the indicated temperature. This will be reflected by a change of the fuel rate to the affected Feed Heater. Spotting problems early will help avoid costly operating problems and/or equipment damage.
Convection Section Utility Heating
Each Feed Heater employs heating of boiler feedwater and steam superheating from the utility system (not simulated) using hot flue gas from the feed heating section. The flow of these fluids through the coils in the convection section of the heater is regulated by flow controllers (FIC-82 and FIC-158; FIC-108 and FIC-159), which are normally operated in ratio control with the feed flow rate to the respective heater (FIC-58 and FIC-87). It is essential to maintain sufficient flow through these services when fuel is firing to avoid tube damage.
Basic Controls: Coke Drum
Coke Drum Temperature; The Coke Drum temperature is controlled by the heater outlet temperature controllers (TIC-67 and TIC-94). TI-138 and TI-121 at the top of the Coke Drums show the Coke Drum outlet temperatures. Normal temperature drop across the drums is about 100 F. If the Coke Drum outlet temperature 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 will also result in low volatile matter coke, which is difficult to cut from the drum.
Note: on Schematic 9 TI-138 and TI-121 are drawn on the lines from the Coke Drums to the Fractionator. However, they always indicate the conditions in the Coke Drums even if the valves to the Blowdown Tower (HV-8 and HV-9) are opened. In case of high temperature in the return line to the Fractionator, TIC-126 will adjust the setpoint of HCGO flow controller FIC-127.
Coke Drum Level; Nuclear type level indicators attached externally to each Coke Drum indicate the level inside of the drum. LI-113 and LI-130 are used as a guide to the operator to indicate “foam” and when to take a drum offline if full of coke.
Coke Make; The simulated coke make is indicated on FR-300 just upstream of the Coke Drums on the common line from heaters. This is not a true flowmeter; it is provided to give the operator an insight to how the coke make changes with conditions.
Basic Controls: Coke Drum Pressure
The pressure of the Coke Drum in service (HV-4 or HV-5 is open) is controlled by PIC-205 (Fractionator Overhead Receiver pressure controller). If a Coke Drum is out of normal service, its pressure will depend on where it is opened to. HV-8 or HV-9 opens a Coke Drum to Blowdown Tower T-9002, HV-16 opens a Coke Drum to Condensate Drum, D-9001, and HV-17 opens a Coke Drum to pad (i.e., atmosphere). A Coke Drum should be routed to only one of these destinations at any given time. The Blowdown Tower is designed to handle steaming of hydrocarbons from a Coke Drum. The Condensate Drum is designed to handle any hydrocarbons drained from a Coke drum. The pad is used to empty coke from the Coke Drum after it has been broken up with water. This operation is performed on the simulator in lieu of actually opening up the Coke Drums to remove the coke.
Basic Controls: 4-Way Drum Switch Valve System
The 4-Way Drum Switch Valve System is a set of valves and associated logic to line up flow from the Feed Heaters to or around the Coke Drums. The operations involving the 4-Way Drum Switch Valve System are found in Group 11 “Switch Valve Panel”. This panel is normally mounted in the field, but for simulation purposes, the operator has control of this operation from the DCS. The effluent from the Feed Heaters can take one of three paths permitted by the 4-Way Drum Switch Valve System: through HV-4 into Coke Drum, V-9001, through HV-5 into Coke Drum, V-9002, and through HV-12 to bypass both Coke Drums. Only one of these lineups is permitted by the system. Procedure for moving to another valve position:
- Set Timer (0 to 100 minutes)
- Select HV-4, HV-5, or HV-12 to Open.
- Wait for current valve selection to close while the selected valve will begin to open.
Notes: (1) The Timer set to 0 minutes will provide an instantaneous switch (2) One valve (HV-4, HV-5, or HV-12) will always be open. (3) Group 11 indicates the valve position of each of the three valves. The 4-Way Drum Switch Valve System also prevents lining up steam and water to a Coke Drum if it is in service (HV-6, HV-7) and prevents lining up a Coke Drum to the Blowdown Tower if it is in service (HV-8, HV-9).
Blowdown Tower Routing Valves
When a Coke Drum is taken out of service and purged, the Coke Drum can be lined up to the Blowdown Tower, T-9002, using HV-8 or HV-9. When one of these valves is opened, the simulation assumes the outlet line to the Fractionator is closed off. Note that the 4-Way Drum Switch Valve System prevents opening a blowdown valve if its Coke Drum is in service.
Basic Controls: Coke Drum Steam Control
Steam is used to purge hydrocarbons from a Coke Drum taken out of service. The Coke Drum should be lined up to Blowdown Tower at battery limits. The rate of steam used for purging is controlled by flow controller FIC-128. Take the following steps to purge a Coke Drum: Take the Coke Drum out of service (see procedure for 4-Way Drum Switch Valve System above). Open the steam/water block valve to the Coke Drum (HV-6 or HV-7). Gradually introduce steam to the Coke Drum using FIC-128.Steam and purged hydrocarbons will be routed to the Fractionator so they can be recovered. After purging for some time to the Fractionator, open the valve from the Coke Drum to the Blowdown Tower (HV-8 or HV-9). Steam and purged hydrocarbons will be routed to the Blowdown Tower. Monitor the pressure of the line to the Blowdown Tower with PIC-123. If it becomes too high, reduce, or stop the steam flow and investigate. Note that the 4-Way Drum Switch Valve System prevents opening up a steam/water block valve if its Coke Drum is in service.
Steam stripping should be started immediately after a Coke Drum is taken out of service to prevent solidification of unconverted feed within the channels of the coke. Such plugging prevents cooldown of all the coke, thereby forming hot spots within the coke structure. Such hot spots are dangerous to personnel who must remove the coke after opening the drum, since water is used to cut the coke, water that contacts a hot spot may explosively vaporize.
Steam should not be used for warmup of the Coke Drums since it will condense and collect in the drums. When hotter material from the operating Coke Drum or the Feed Heater outlet is introduced, it may cause the steam condensate to explosively vaporize.
Basic Controls: Coke Drum Cooling Water Control
Cooling water is used to quench the coke in a Coke Drum that has been taken out of service and purged with steam. Because there may be hot spots in the Coke Drum after purging with steam, cooling water must be added very gradually to avoid any explosive vaporization of the water, which could damage the drum. Also, quenching the coke too quickly can make the coke very hard; removal of hardened coke is more difficult. Cooling water flow is controlled with FIC-267. Flow will only occur to a Coke Drum if its steam/water block valve (HV-6 or HV-7) is open. Note that the 4-Way Drum Switch Valve System prevents opening up a steam/water block valve if its Coke Drum is in service. If a Coke Drum is lined up to the Blowdown Tower,PIC-123 will indicate the pressure on the line to Blowdown Tower. PIC-123 is an override controller that will close the cooling water flow control valve for FIC-267 in case of high pressure in the line to the Blowdown Tower. The signal to the valve is the lowest of the outputs from PIC-123 and FIC-267.
PIC-123 should always be in automatic mode with a safe setpoint.
When adding cooling water to a drum, always route the top of the drum to the Blowdown Tower (HV-8 or HV-9). Otherwise, water may spill over from the drum to the transfer line to the Fractionator, where it will cause dangerous conditions when water contacts hot effluent from the Coke Drum that is in service.
In real Coke Drums, coke is removed by draining the quench water and then unbolting the heads of the drums, so the coke can be removed by operations personnel. On the simulator, this procedure is simplified using the valve to pad HV-17. To remove coke from a drum, use the following steps:
- Take the Coke Drum out of service, purge it with steam to the Fractionator and then purge it to the Blowdown Tower (see above procedures).
- Make sure PIC-123 on the line to the Blowdown Tower is in automatic mode with a safe setpoint.
- Gradually start cooling water to the Coke Drum with FIC-267.
- Gradually reduce steam flow FIC-128 while continuing to increase the flow of cooling water.
- Stop the steam flow. If PIC-123 is indicating a significant pressure after stopping the steam, cooling water is vaporizing within the Coke Drum. This is normal.
- Keep adding cooling water until the temperature of the Coke Drum (TI-121 orTI-138) reaches below the normal boiling point of water or PIC-123 drops to anear-atmospheric reading.
- Allow the coke to soak for a few minutes to allow water to reach any hot spots. If the pressure shown on PIC-123 begins to rise, continue adding water until it drops.
- Close the steam/water block valve (HV-6 or HV-7).
- Once the pressure and temperature of the Coke Drum are indicating safe conditions, the coke can be dumped to pad by opening HV-17. If the coke has been properly broken up, the level of the drum (LI-113 or LI-130) will begin decreasing. If this does not happen, close HV-17 and add more water(remember to open the steam/water block valve) for a few minutes.
- Close the steam/water block valve, reopen HV-17 and check the level.
- Keep repeating this procedure until the coke level drops.
The simulator assumes that once the mass of water present in the Coke Drum exceeds the mass of accumulated coke, the coke can be drained out via HV-17 or even HV-16 (to the Condensate Drum, D-9001). Repeating the procedure in step 9 above will eventually cause this condition to be met.
Basic Controls: Coke Drum Warmup – Other Drum in Operation
A cold drum must be gradually warmed to a near-operating temperature before it can be switched into service during normal operation of the Delayed Coker Unit. To warmup a cold drum, a tie-line with a control valve (HIC-101) allows routing of hot hydrocarbons from the Coke Drum in service to the cold drum. Follow this procedure to warmup a cold drum:
- On the cold drum, make sure all the lineup valves connected to it are closed.
- Open HV-16 on the line to the Condensate Drum D-9001 Open HV-11 on the line from D-9001 to the Fractionator.
- Gradually open HIC-101 to avoid a large disruption of the unit as some cracked material is transferred into the cold drum. The temperature of the cold Coke Drum should start increasing as hot vapors from the operating drum begin to condense.
- Wait a few minutes, condensate from the cold drum will be pushed to the Condensate Drum. As this happens, the temperature of the line TI-148 will increase.
- Keep opening HIC-101 to obtain a satisfactory heat up rate of the cold drum.
- Once the temperature of the standby drum is reasonably close to the other drum’s temperature (within 100 DEG F), gradually close HIC-101.
- Once HIC-101 is closed, close HV-16 to stop draining condensate to Condensate Drum.
- Ensure all routing valves on the warmed-up drum are closed.
- Open the feed valve (HV-4 or HV-5) to switch operation to the warmed-up drum.
- Adjust operation as needed.
Coke Drum Warmup – No Drum in Operation
If the Delayed Coker Unit is cold, one of the Coke Drums can be gradually warmed up by placing it in service so it heats up along with the Feed Heaters when they are placed in service. This is the easiest and safest way to heat up a Coke Drum. However, if both Coke Drums are cold and out of service but the Feed Heaters have been significantly warmed up, there are startup lines to introduce warm Feed Heater effluent to the top of either Coke Drum. These lines connect downstream of the Coker Drums Bypass Valve HV-12. HIC-400 allows control of the rate of heat up to either drum. HV-20 or HV-21 can be opened to admit hot material from the bypass line. Follow this procedure to warmup a cold drum using warm Feed Heater effluent:
- On the cold drum, make sure all the line up valves connected to it are closed.
- Open HV-16 on the line to the Condensate Drum D-9001.
- Open HV-11 on the line from D-9001 to the Fractionator Open the isolation valve on the heat up line (HV-20 or HV-21).
- Gradually open HIC-400 to avoid a large disruption of the unit as some of the Feed Heater effluent is transferred into the cold drum. The temperature of the cold Coke Drum should start increasing.
- Wait a few minutes, warm liquid and/or condensate from the cold drum will be pushed to the Condensate Drum. As this happens, the temperature of the lineTI-148 will increase.
- Keep opening HIC-400 to obtain a satisfactory heat up rate of the cold drum.
- Once the temperature of the drum is reasonably close to the combined Feed Heater effluent temperature (within 100 DEG F), gradually close HIC-400.
- Once HIC-400 is closed, close HV-16 to stop draining liquid to D-9001.
- Ensure all routing valves on the warmed-up drum are closed.
- Open the feed valve (HV-4 or HV-5) to switch operation to the warmed-up drum.
- Adjust operation as needed.
Basic Controls: Fractionator
The Fractionator, T-9001, separates effluent from the Coke Drums into the fractions that constitute the naphtha and distillate products produced in the unit. Adjustments made in the operation of the Fractionator determine the quantity and quality of each of the products. The control scheme is designed to facilitate these adjustments.
Achieving Separation; Separation of the products in the Fractionator is achieved by: Varying the heat removal using the Overhead Fin-Fan Condenser, E-9011, (HIC-9,) to condense all the naphtha and steam leaving the top of the Fractionator. Varying the top reflux flow rate (FIC-55) to obtain a desired top temperature (TIC-54). This will ensure the naphtha purity by refluxing any cut LCGO components back down the Fractionator. Varying the HCGO pumparound flow rate (FIC-154) and its return temperature (TIC-167) to condense the cut HCGO components on tray No. 17, while leaving naphtha and LCGO components to remain in vapor and travel to the top section of the Fractionator where they are removed. Keeping a desirable flow of HCGO reflux (FIC-57) to the lower wash section of the Fractionator (trays 24 through 18). This prevents pollution of the HCGO section by heavy components from the Coke Drums by washing them back to the bottom of the Fractionator. It also quenches the hot vapors leaving the Coke Drums, keeping Fractionator pressure fairly constant. This is accomplished by the Overhead Receiver pressure controller PIC-205.
Basic Controls: Fractionator Product Rate Control
Control of the product flows from the Fractionator are as follows: Wet gas (FI-206) is produced from the Overhead Receiver, D-9005, by the action of pressure controller PIC-205. Naphtha product (FI-439) is produced in the Overhead Receiver by the action of level controller LIC-197. LCGO product is manually drawn off tray No. 9 of the Fractionator using HIC-8. Net LCGO product flow from the LCGO Stripper, D-2, is indicated on FI-166. Excessive draw of LCGO will result in little or no internal reflux of LCGO to the HCGO section. This will cause heavier HCGO components to reach the LCGO draw tray. No draw of LCGO will cause LCGO components to be refluxed completely to the HCGO section. HCGO is drawn off tray No. 17 by the action of level controller LIC-44. Net HCGO product flow from the HCGO Stripper, D-3, is indicated on FI-155.
Product Quality Control; Product quality is indicated on the boiling point analyzers for naphtha (AR-102), LCGO (AR-103 and AR-104), and HCGO (AR-105 and AR-106). These analyzers indicate the predicted boiling point of the product at the point where a certain volume percentage (i.e., “cut”) of the original sample has been boiled off (either 5% or 95%). Distilled petroleum products contain a mixture of lighter and heavier hydrocarbons. Because the boiling point increases with the molecular weight of a hydrocarbon, a higher boiling point indicates a heavier hydrocarbon composition at that volume of the product and vice-versa. The boiling points of a product directly affect the product performance usage as fuel in automobiles, trucks, boats, ships, and aircrafts. Obtaining optimum product qualities requires adjustment of the following key Fractionator variables: Fractionator top temperature TIC-54 will mainly affect the 95% volume boiling point of the product naphtha AR-102. The LCGO draw rate HIC-8 will mainly affect the 95% volume boiling point of the product LCGO AR-104. The HCGO pumparound flow FIC-154 and, to some extent, temperature TIC-167 9 will mainly affect the 5% volume boiling point of the product HCGO AR-105. The HCGO reflux flow to the wash section FIC-308 will mainly affect the 95% volume boiling point of the product HCGO AR-106. The main challenge of meeting all product quality targets is that a change in one section will also affect an adjacent section to some extent. Another challenge is that feedstock changes will usually change product distributions, thus requiring further adjustments to these operating variables. Therefore, adjustments to operating conditions of the Fractionator should be made slowly and all product qualities and flow rates should be regularly checked to make sure they are maintained within desired targets.
Basic Controls: Fractionator Temperatures
The Fractionator temperature profile and pressure profile, as shown by several temperature and pressure indicators located at crucial points on the Fractionator, give an indication of tower performance. Keep in mind that a tray/draw temperature from the tower is a direct indication of the average product composition at that spot. Thus, higher temperatures reflect higher boiling (i.e., heavier) hydrocarbons and vice-versa. They also give a faster indication of composition trends than do the analyzers on the product streams.
Basic Controls: Fractionator Pressures
An increasing differential pressure across a tower section is indication of imminent operating trouble in that section. High pressure drops are indicative of flooding of that section of the tower. Flooding means liquid is being held up in the tower. It can be caused by the following problems: Excessive tray vapor rates versus design due to insufficient reflux or excessive gas at the source. The high velocity vapor literally pushes liquid upward leading to a “snowball” effect on pressure drop as liquid builds on the trays. Damaged or plugged trays that block liquid and/or vapor flows. Excessively foamy liquid properties. Foamy liquids are more easily blown upward in the tower by vapor. Poor separation is usually accompanied by low pressure drop that results from liquid and vapor bypassing each other due to tray failures. This could be due to dislodged or corroded trays.
Basic Controls: HCGO Pumparound System
HCGO is drawn from tray No.17 of the Fractionator and supplied to the HCGO Stripper (by gravity) and the HCGO Circulating Reflux Pump, P-9004. The flow rate of HCGO taken to the HCGO Stripper is controlled by LIC-44. The HCGO pumparound reflux from the HCGO Circulating Reflux Pump is cooled in HCGO Pumparound/Feed Exchanger, E-103, and HCGO Steam Generator, E-9002, and returned under flow control to tray No. 16 (FIC-154) and tray No.18 (FIC-57). The HCGO draw off temperature is indicated by TI-648 and the return temperature is controlled by TIC-167. TIC-167 adjusts the flow of HCGO bypassing HCGO Steam Generator. The steam produced in the Steam Generator is indicated on FI-152. These temperatures, in conjunction with the pumparound flow rate, give an indication of the heat removal achieved in the pumparound circuit. Varying the flow rate and thus the heat removal in this circuit has an effect on the quantity and quality of distillate products drawn from the Fractionator, as discussed above. Circulating HCGO is also sent to the outlet of the Coke Drums under control of FIC-127 to quench the outlet line in case of high temperatures.
Basic Controls: HCGO Product System
HCGO draw from the Fractionator is sent to the HCGO Stripper, where it is steam-stripped to remove light hydrocarbons and gases before being cooled and stored. Flow controller FIC-25 regulates the steam flow. If steam flow is insufficient, the 5% volume boiling point AR-105 will tend to read low. The 95% volume boiling point of product HCGO is indicated on AR-106. LIC-14 controls the level in the bottom of the HCGO Stripper by adjusting the flow of HCGO to storage. Flow is indicated on FI-155. HCGO can be made lighter by drawing liquid from the LCGO section of the Fractionator using FIC-22.
Basic Controls: LCGO Product System
LCGO draw from the Fractionator is sent to the LCGO Stripper, D-2, by gravity using HIC-8. LCGO is steam-stripped in the LCGO Stripper to remove light hydrocarbons and gases before being cooled and stored. Flow controller FIC-24 regulates the steam flow. If steam flow is insufficient, the 5% volume boiling point AR-103 will tend to read low. The 95% volume boiling point of product LCGO is indicated on AR-104. LIC-41 controls the level in the bottom of the LCGO Stripper by adjusting the flow of LCGO to storage. Flow to storage is indicated on FI-166. Note: The LCGO draw flow from the Fractionator is not metered because of the low gravity differential between the LCGO draw tray and the top of the LCGO Stripper. Therefore, adjustments on the LCGO draw rate via HIC-8 should be made gradually. The draw rate will ultimately be reflected on FI-166 when LIC-41 is steady. If the LCGO draw rate is too high, it will be reflected by a high LCGO draw temperature TI-649. Conversely a too low draw rate will be reflected by a low reading on TI-649. Cooled LCGO is also taken off as lean sponge oil using FIC-216. Rich sponge oil is returned back to the Fractionator via Sponge Oil/LCGO Exchanger, E-2, with HV-13. The rate of returned rich sponge oil is proportional to the lean sponge oil flow.
Basic Controls: Overhead System
TIC-54 controls the top temperature of the Fractionator by adjusting the setpoint of the naphtha reflux flow controller FIC-55. The top temperature, like all temperatures in a distillation tower, is indicative of the composition at the point of measurement. A lower top temperature will result in a lighter, lower-boiling point naphtha product. HIC-9 allows manual adjustment of the air flow through Overhead Fin-Fan Condenser, E-9011. TI-192 indicates the outlet temperature of the Overhead Fin-Fan Condenser. If this temperature becomes too high, some of the lighter compounds in the naphtha cut will be lost as vapor in the wet gas. PIC-205 controls the pressure of Overhead Receiver, D-9005, by adjusting the rate of wet gas flow to the compressor system at battery limits. PIC-198 is provided to automatically vent wet gas to the flare system in case the pressure of the Overhead Receiver becomes too high. The naphtha level controller LIC-197 adjusts the flow rate of naphtha product to storage. The flow is indicated on FI-439. AR-102 indicates the 95% volume boiling point of product naphtha.
There is a 4-Way Drum Switch Valve for lining up effluent from the Feed Heaters to the Coke Drums. This panel is normally mounted in the field, but for simulation purposes the operator has control of this operation from the DCS (Group 11). The configuration includes a timer and all the functionality of the 4-Way Drum Switch Valve. Refer to the section above entitled Basic Controls: Coke Drum for details of operation in conjunction with Coke Drum operations.
A fuel oil isolation valve (XV-451 or XV-460) on a Feed Heater will trip closed when the flow through its respective heater is less than 30%. (When FIC-57 or FIC-58 goes below approximately 158 BPH, the fuel oil valves will trip.) The closed isolation valve must be manually reopened once the feed flow is reestablished above the trip setting.