Delayed Coking Unit (DCU)

Process Description

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.

Process Specifications

The design capacity of Delayed Coking Unit is 23,000 BPD of hot vacuum residuum (ANS 970oF+) 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 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.

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 about doubles for every 18oF increase in temperature.

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 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.

The coke product in the coke drum is removed batchwise 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. Coke cutting and handling are not part of this program.

Equipments Specifications

Major pieces of equipment include T-9001 Coker Fractionator, V-9001 and V-9002 Coke Drums, F-9001 and F-9002 Heaters, D-2 LCGO Side Stripper, and D-3 HCGO Side Stripper. The Fractionator CVOerhead has D-9005 Overhead Recovery Drum and E-9001 Overhead Fin Fan. Heat Exchangers include E-100 and E-101, E-102 and E-103, E-2, and E-9002.

Instrumentation

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.

Advance Control

There is a Neles-Jamesbury 4-Way Drum Switch Valve. This panel is normally mounted in the field, but for simulation purposes the operator has control of this operation from the DCS. The configuration includes a Timer and all the functionality of the Neles-Jamesbury 4-Way Drum Switch Valve.

Faults

All faults can be failed high or low to any degree with any of 8 fault function generators (step change, square wave, staircase, stairs, ramp, saw tooth, slope, or sine wave). Faults can be programmed to start and/or stop at various times during a simulation exercise.

Fault 1: Fractionator Bottoms Pump

Fault 2: Heavy Gas Oil Circulating Reflux Pump

Fault 3: Heavy Gas Oil Product Pump

Fault 4: Light Gas Oil Product Pump

Fault 5: Naphtha Product Pump

Fault 6: Fractionator Overhead Condenser Fan

Fault 7: Fractionator Tower Top Reflux Pump

Fault 8: Fuel Failure to Heater

Fault 9: BFW Failure to Exchanger

Fault 10: Sponge Oil Return Flow to Fractionator

Fault 11: Pressure Controller Failure

Fault 12: Heat Exchanger Fouling

Fault 13: Circulating Reflux Temperature Transmitter

Fault 14: Vacuum Resid Feed Temperature Controller

Fault 15: Steam to Coker Feed Heater

Fault 16: Steam Failure to LCGO Stripper

Fault 17: Coker Heater Outlet Temperature Control

Fault 18: Level Controller for Fractionator Tray 17

Fault 19: Steam Failure to HCGO Stripper

Fault 20: Fractionator Top Reflux Control Valve

Fault 21: Fractionator Top Temperature Controller

Fault 22: Coking of Feed Heater

Fault 23: Fractionator Bottoms Pump

Fault 24: Steam to Coker Feed Heater

Fault 25: Coking of Fired Heater

Fault 26: Coker Heater Outlet Temperature Controller

Petroleum Coke Glossary

A Anode grade coke

A grade of pet coke from a delayed coker (see same) low in metals such as vanadium, nickel and iron that is suitable for making graphite anodes for the aluminum smelting industry. If the metals content does not qualify as anode grade coke, then the coke is generally known as fuel grade (see same). The choice of whether a coke may qualify as anode or fuel grade is driven solely by the crude slate of the crude unit upstream of the coker and the metals contained therein.

Generally presence of any shot coke (see same) and Volatility (see same)> 10% are unacceptable. Sometimes anode grade is also referred to as calcineable coke (see same)

A Ash The residue remaining when all of the coke is burned off. It is mostly metals and silica.

B    C Calcineable

cokePetroleum coke that qualifies to be calcined, i.e., generally non shot and low in metals that will qualify for anode grade. Calcineable coke is generally referred to as the available supply to calciners. Also known a s green coke (see same).

C Calcined coke

Petroleum coke or green coke (see same) that has been processed in a calciner (see same)

C Calciner A large rotary kiln (similar to a cement kiln) that drives off the moisture and volatility of green or calcineable coke (see each) so that the coke can be used for aluminum anodes, or titanium dioxide applications. The kiln receives the coke at the higher end where the coke flows downhill as the kiln rotates. At the same time heated air counterflows uphill from the lower end, driving off the moisture and volatile material

C Carbon rejection

Crude oil contains a wide variety of hydrocarbon molecules, ranging from a single carbon atom (methane) to very long chain molecules. The lighter molecules that make up gasoline, jet and diesel contain a lower ratio of carbon to hydrogen than the heavier molecules. In order to convert the heavier molecules to lighter products, the heavier molecules must not only be cracked, but the excess carbon must be removed to reduce the carbon to hydrogen ratio of the cracked material.

  Circulating Fluidized Bed (CFB) Boiler

A type of boiler where the solid fuel is fluidized in a vertical furnace. The advantage for the pet coke industry is that 100% very high sulfur coke can be burned. The sulfur is removed by addition of limestone in the bed which forms an easily removable calcium sulfate ash.

C Coker A refinery processing unit that converts the residual oil from the crude unit vacuum or atmospheric column into gas oil that can be made into light products (gasoline, jet and diesel), weak (i.e., low energy content) fuel gas and pet coke. There are 3 types: Delayed, Fluid, and Flexi (see each). These are all basically carbon rejection schemes (see same).

D Delayed coker

A type of coker whose process consists of heating the residual oil feed to its thermal cracking temperature in a multi parallel pass furnace. This

cracks the long chain heavy carbon and hydrogen molecules of the residual oil into coker gas oil and pet coke. Both are in a liquefied form in the mixture as it leaves the furnace and enters the coke drum.

 In the drum, the coker gas oil vaporizes and separates from the mixture. It is directed to a fractionation column where it is separated into the desirable boiling point fractions. The liquid coke solidifies in the drum as it cools and the velocity slows down.

 After the drum is full of the solidified coke, the hot mixture from the furnace is switched to a second drum. While the second drum is filling, the full drum is steamed to further reduce hydrocarbon content of the pet coke, and then water quenched to cool it. The top and bottom heads of the full coke drum are removed, and the solid pet coke is then cut from the coke drum with a high pressure water nozzle, where it falls into a pit for reclamation to storage.

 Larger cokers have several pairs of tandem drums.D Demurrage Contracts for rail cars, trucks and marine ships always include a

specified time limit that the respective vehicle or ship will take to unload or load. The penalty for delaying beyond the time allowed is also specified in each contract. Upon completion of the actual load or unload, the vehicle or ship owner presents the customer with a demurrage bill specifying the time delay beyond the allowance, reasons for it and the $ penalty.

D Dry metric or short ton

A unit of measure for billing pet coke weight, which refers to the gross coke weight adjusted for excluding the contained moisture content. To get this measure, the coke must be tested for moisture content (see same). Calcineable coke (See same) is frequently measured in these units. Also see Wet metric ton.

E Energy content

The energy content of coal and coke is usually specified in either BTU/lb or Kcal/kg. It is measured in a calorimeter that literally ignites the test sample and measures the temperature rise of the water bath surrounding it.

F Flexi coker The flexicoker adds a third vessel, a gasifier, to the fluid coker to gasify the purge coke into a weak fuel gas. Coke is made in 3 areas: purge coke from the heater, and both larger and smaller recovered coke fines from the weak gas scrubbers. It is a “flexible” coker in that the gasifier can be run to make either more coke or more weak fuel gas.

F Fluid Coker

A fluid coker produces more light product yield and less coke but the coke is higher in metals and harder than a delayed coker. The process is a continuous fluidized bed consisting of a reactor and a burner vessel. Feed is sprayed onto seed pellets of coke in the reactor where it is coked. Purge coke is drawn off the burner vessel.

F Fuel grade coke

Pet coke that competes with steam coal as fuel for a furnace, boiler or cement kiln. If the coke contains low metals, it may qualify as anode

grade (see same). The choice of whether a coker makes fuel grade vs anode grade is dictated solely by the crude slate and the metals content therein.

G Green coke Usually refers to calcineable coke, i.e, potential anode quality coke that still contains moisture and volatile material, i.e, before calcining.

H Hargrove Grindability Index (HGI)

A measure of the relative hardness of the coke in terms of resistance to grinding. An HGI below 35 usually means a very hard coke requiring considerable grinding before it can be properly sized to meet burner nozzle specifications for the pulverized feed to the furnace where the coke will be burned. An HGI above 65 is usually recognized as a soft coke that will grind very easily.

I    J    K    L Loss on

Ignition (LOI)

Coke that is added to coal in a furnace originally designed for coal, may not fully burn in the furnace. Some coke particles escape the flame envelope and are caught by the precipitators or bag houses. This LOI results in reduced furnace efficiency and some carbon contamination of the flue gas ash recovered.

M Metals content

The distillation processes of the atmospheric and vacuum columns result in almost all of the metals content of the crude to concentrate in the residual oil (the coker feed), and then further concentrated in the coker. Generally metals are not a problem to fuel grade coke buyers, other than vanadium to some. However anode grade coke buyers prefer low metals as the metals tend to reduce the efficiency of the anode in the aluminum smelting process

M Moisture content

Pet coke as cut from a coker contains roughly from 7-10% moisture which the coke picked up from the steaming and quench operations. It is measured by heating a 100g sample of ground coke, until the moisture is gone, and then re weighing. See Wet metric tons and Dry Metric tons.

N Needle Coke

A special quality coke produced from aromatic feed stocks. It has a crystalline structure with more unidirectional pores. It is used for high quality graphite anodes such as for electric arc furnaces in the steel industry.

O    P    Q    R    S Shot coke Due to mechanisms not well understood, the coke from some crudes

forms into small, tight, non attached clusters that look like pellets, marbles or BB’s. It usually is a very hard coke , i.e., low HGI (see same). Such coke is less desirable to the end users because of difficulties in handling and grinding. It is also less desirable for calcineable coke because the shot tends to “pop” in the kiln reducing the thermal stability.

S Sponge Most delayed coke is in a form that resembles a sponge, i.e, sort of

coke bubbly looking.S Sulfur

contentSulfur in crude naturally distributes itself throughout the range of the crude’s hydrocarbon molecules. However, the residual oil and coke usually attract a disproportionately higher Sulfur percent. Also, as most low sulfur crudes become extinct, refineries will run higher sulfur crudes thereby increasing the coke sulfur.

T    U    V Volatility

or VCMPet coke from a coker contains a small amount (<10%) of light hydrocarbons trapped in the pores of the coke. The amount of such volatility is generally related to the how hard the coker furnace is driven: hotter furnace outlet temperatures result in more hydrocarbons driven off and therefore lower volatility.

W Wet metric ton

Fuel grade coke is typically sold in these billing units which is at a standard 8% moisture. If the coke tests for a higher %, the billing weight is adjusted accordingly.

X    Y    Z    

Hazards of Delayed Coker Unit (DCU) Operations

The Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) are jointly issuing this Chemical Safety Alert/Safety and Health Information Bulletin (CSA/SHIB) as part of ongoing efforts to protect human health and the environment by preventing chemical accidents. We are striving to better understand the causes and contributing factors associated with chemical accidents, to prevent their recurrence, and to provide information about occupational hazards and noteworthy, innovative, or specialized procedures, practices, and research that relate to occupational safety and health and environmental protection. Major chemical accidents cannot be prevented solely through regulatory requirements. Rather, understanding the fundamental root causes, widely disseminating the lessons learned, and integrating these lessons into safe operations are also required. EPA and OSHA jointly publish this CSA/SHIB to increase awareness of possible hazards. This joint document supplements active industry efforts to exchange fire and safety technology and to increase awareness of environmental and occupational hazards associated with DCU operations. It is important that facilities, State Emergency Response Commissions (SERCs), Local Emergency Planning Committees (LEPCs), emergency responders, and others review this information and take appropriate steps to minimize risk. This document does not substitute for EPA or OSHA regulations, nor is it a regulation itself. It cannot and does not impose legally binding requirements on EPA, OSHA, states, or the regulated community, and the measures it describes may not apply to a particular situation based upon the circumstances. This guidance does not represent final agency action and may change in the future, as appropriate.

Purpose

The batch portion of DCU operations (drum switching and coke cutting) creates unique hazards, resulting in relatively frequent and serious accidents.

The increasingly limited supply of higher quality crude oils has resulted in greater reliance on

more intensive refining techniques. Current crude oils tend to have more long chain molecules, known as “heavy ends” or “bottom of the barrel” than the lighter crude oils that were more readily available in the past. These heavy ends can be extracted and sold as a relatively low value industrial fuel or as a feedstock for asphalt-based products, such as roofing tile, or they may be further processed to yield higher value products. One of the most popular processes for upgrading heavy ends is the DCU, a severe form of thermal cracking requiring high temperatures for an extended period of time.

This process yields higher value liquid products and creates a solid carbonaceous residue called “coke.” As the supply of lighter crude oils has diminished, refiners have relied increasingly on DCUs.

Unlike other petroleum refinery operations, the DCU is a semi-batch operation, involving both batch and continuous stages. The batch stage of the operation (drum switching and coke cutting) presents unique hazards and is responsible for most of the serious accidents attributed to DCUs. The continuous stage (drum charge, heating, and fractionation) is generally similar to other refinery operations and is not further discussed in this document. About 53 DCUs were in operation in the United States in 2003, in about one third of the refineries.

In recent years, DCU operations have resulted in a number of serious accidents despite efforts among many refiners to share information regarding best practices for DCU safety and reliability. EPA and OSHA believe that addressing the hazards of DCU operations is necessary given the increasing importance of DCUs in meeting energy demands, the array of hazards associated with DCU operations, and the frequency and severity of serious incidents involving DCUs.

Understanding the Hazards

Safe DCU operations require an understanding of the situations and conditions that are most prone to frequent or serious accidents.

Process Description

Each DCU module contains a fired heater, two (in some cases three) coking drums, and a fractionation tower.

This document focuses on the coke drums, which are large cylindrical metal vessels that can be up to 120 feet tall and 29 feet in diameter.

In delayed coking, the feed material is typically the residuum from vacuum distillation towers and frequently includes other heavy oils. The feed is heated by a fired heater (furnace) as it is sent to one of the coke drums. The feed arrives at the coke drum with a temperature ranging from 870 to 910°F. Typical drum overhead pressure ranges from 15 to 35 psig. Under these conditions, cracking proceeds and lighter fractions produced are sent to a fractionation tower where they are separated into gas, gasoline, and other higher value liquid products. A solid residuum of coke is also produced and remains within the drum.

After the coke has reached a predetermined level within the “on oil” drum, the feed is diverted to the second coke drum. This use of multiple coke drums enables the refinery to operate the fired heater and fractionation tower continuously. Once the feed has been diverted, the original drum is isolated from the process flow and is referred to as the “off oil” drum. Steam is introduced to strip out any remaining oil, and the drum is cooled (quenched) with water, drained, and opened (unheaded) in preparation for decoking. Decoking involves using high pressure water jets from a rotating cutter to fracture the coke bed and allow it to fall into the receiving area below. Once it is decoked, the “off oil” drum is closed (re-headed), purged of air, leak tested, warmed-up, and placed on stand-by, ready to repeat the cycle. Drum switching frequency ranges from 10 to 24 hours. DCU filling and decoking operations are illustrated in Figure 1. Equipment used in coke cutting (hydroblasting) operations is illustrated in Figure 2. 

Figure 1. Delayed Coker Unit -Cutaway to Depict Drum In Filling and Migration Mode (Left) and Drum In Cutting Mode (Right).

Figure 2. Delayed Coker Unit - Coke Drums and Hydroblast Systems.

Accident Investigation

Once removed from the coke drums, the coke is transported away from the receiving area. From here, the coke is either exported from the refinery or crushed, washed, and stored prior to export.

The following specific operations and more general situations and conditions contribute most significantly to the hazards associated with DCU operations:

Specific operation hazards ← Coke drum switching ← Coke drum head removal

← Coke cutting (hydroblasting operation)

Emergency and general operational hazards ← Coke transfer, processing, and storage ← Emergency evacuation

← Toxic exposures, dust irritants, and burn trauma

The hazards associated with these specific operations and DCU operations, in general, are explained below to share lessons learned and increase awareness of the situations and conditions that are most prone to serious accidents. Following this section, the joint CSA/SHIB describes actions that can be taken to help minimize the risks associated with these situations and conditions.

Specific Operation Hazards

Coke Drum Switching

Most DCU operations consist of several DCU modules, each typically alternating between two coke drums in the coking/decoking sequence. Some DCU modules include a third drum in this sequence. Each drum includes a set of valving, and each module includes a separate set of valving. Differences in valving among drums and among modules may be difficult to distinguish and can lead to unintended drum inlet or outlet stream routing. Similarly, valve control stations, for remotely activated valves, may not always clearly identify the operating status of different drums and modules. Activating the wrong valve because of mistakes in identifying the operational status of different drums and modules has led to serious incidents.

Coke Drum Head Removal

Conditions within the drum, during and after charging, can be unpredictable. Under abnormal conditions, workers can be exposed to the release of hot water, steam and coke, toxic fumes, and physical hazards during removal of the top and bottom drum heads. The most frequent and/or severe hazards associated with this operation are described below:

← Geysers/eruptions - Under abnormal situations, such as feed interruption or anomalous shortcircuiting during steaming or quenching, hot spots can persist in the drum. Steam, followed by water, introduced to the coke drum in preparation for head removal can follow established channels rather than permeate throughout the coke mass. Because coke is an excellent insulator, this can leave isolated hot areas within the coke. Although infrequent, if the coke within the drum is improperly drained and the coke bed shifts or partially collapses, residual water can contact the isolated pockets of hot coke, resulting in a geyser of steam, hot water, coke particles, and hydrocarbon from either or both drum openings after the heads have been removed.

← Hot tar ball ejection - Feed interruption and steam or quenching water short-circuiting can also cause “hot tar balls,” a mass of hot (over 800°F) tar-like material, to form in the drum. Under certain circumstances, these tar balls can be rapidly ejected from the bottom head opening.

← Undrained water release - Undrained hot water can be released during bottom head removal, creating a scalding hazard.

← Shot coke avalanche - Sometimes, the coke forms into a multitude of individual, various sized, spherical shaped chunks known as “shot coke,” rather than a single large mass. In this situation, the drum contents are flowable and may dump from the drum when the bottom head is removed, creating an avalanche of shot coke.

← Platform removal/falling hazard - Some DCUs require the removal of platform sections to accommodate unheading the bottom of the drum. This can introduce a falling hazard.

Coke Cutting (Hydroblasting Operation)

Coke-cutting or -hydroblasting involves lowering from an overhead gantry a rotating cutter that uses high pressure (2000 to 5000 psig) water jets. The cutter is first set to drill a bore hole through the coke bed. It is then reset to cut the coke away from the drum interior walls. Workers around the gantry and top head can be exposed to serious physical hazards, and serious incidents have occurred in connection with hydroblasting operations. Some of the most frequent and/or severe hazards are described below:

← If the system is not shut off before the cutting nozzle is raised out of the top drum opening, a high pressure water jet can be exposed and seriously injure, even dismember a nearby worker.

← Fugitive mists and vapors from the cutting and the quench water can contain contaminants that pose a health hazard (see section on Toxic Exposures, Dust Irritants and Burn Trauma, below).

← The water hose can burst while under high pressure, resulting in whipping action that can seriously injure nearby workers.

← The wire rope supporting the drill stem and water hose can fail (part), allowing the drill stem, water hose, and wire rope to fall onto work areas.

← Gantry damage can occur, exposing workers to falling structural members and equipment.

Emergency and General Operational Hazards

Coke Transfer, Processing, and Storage

The following coke conveyance, processing, and storage operations have presented safety and health hazards for DCU workers:

← The repositioning of rail cars by small locomotives or cable tuggers to receive coke being cut from a drum can create physical hazards for workers in the rail car movement area.

← Mechanical conveyors and coke crushers may contain exposed moving parts that can cause fracture or crush type injuries at pinch points.

← Fires are common in coke piles and rail cars. Large chunks of coke can contain pockets of unquenched material at temperatures well above the ignition point. When fractured and exposed to air, this material can ignite. Fires have also been attributed, although less frequently, to reactions that lead to spontaneous combustion.

← Combustion products and/or oxygen depletion resulting from spontaneous fires can create hazardous conditions for workers in confined spaces.

← Wet coke in an enclosed area has been reported to have absorbed oxygen from the surrounding air under certain circumstances. This can make the area oxygen deficient and cause asphyxiation.

Emergency Evacuation

The delayed coking process is very labor intensive. Each batch process cycle requires 25 or more

manual operations (valve, winch operation, drum heading, etc.), and many DCUs operate with three or more sets of drums. Tasks are performed at several levels on the coke drum structure. The upper working platform (frequently called the “cutting deck”) is generally well over 120 feet above ground. During an emergency, evacuation from the structure can be difficult.

In addition, moisture escaping from drum openings during cold weather can produce fog. This can obscure vision and make walkways, and hand rails wet and slippery, creating additional difficulties during emergency evacuation.

Toxic Exposures, Dust Irritants, and Burn Trauma

DCU workers can be exposed to coke dust and toxic substances in gases and process water around DCU operations. Workers can also be exposed to physical stress and other hazardous conditions. The following exposures to toxic substances, irritants, and hazardous conditions have been associated with DCU operations, in general:

← Hot water, steam, and liquid hydrocarbon (black oil) can escape from a coke drum and cause serious burn trauma. Contact with black oil can cause second or third degree burns. In addition, liquid hydrocarbon escaped from a coke drum can be well above its ignition temperature, presenting a fire hazard.

← Heat stress can be a health hazard during warm weather, particularly for those required to wear protective clothing while performing tasks on the coke drum structure.

← Hazardous gases associated with coking operations, such as hydrogen sulfide, carbon monoxide, and trace amounts of polynuclear aromatics (PNAs), can be emitted from the coke through an opened drum or during processing operations.

← If allowed to accumulate and become airborne, dust around a DCU may exceed acceptable exposure limits and become a hazard.

Controlling the Hazards

Evaluating hazardous conditions, modifying operations to control hazards, actively maintaining an effective emergency response program, and familiarizing workers about risks and emergency procedures will help reduce the frequency and severity of serious incidents associated with DCU operations.

Specific Operation Hazards

Coke Drum Switching

No one system has proven effective in eliminating all incidents associated with incorrect valve activation due to mistaken coke drum or module identification; however, the following actions have been reported as beneficial:

← Conduct human factors analyses to identify, evaluate, and address potential operator actions that could compromise the safe operation of the coke drum system.

← Provide interlocks for automated or remotely activated valve switching systems.

← Provide interlocks for valves that are manually operated as part of the switching/decoking cycle to avoid unanticipated valve movement.

← Color code and clearly label valves and control points to guard against incorrect identification.

← Provide indicator lights at valve and valve control stations to help the operator determine which is the correct valve station for the intended operator action.

← Use the “buddy system” (employees working in pairs) to help verify accurate valve or switch identification.

← Conduct periodic and documented training focusing on the importance of activating the correct valve or switch and the consequence of incorrect activation.

Coke Drum Head Removal It can be difficult to anticipate the presence of either a hot spot or a hot tar ball in the coke drum prior to drum head removal. In light of this possibility and the potential for serious incidents, it is prudent to:

← Be alert to any operating abnormalities or variations during charging, steaming, or quenching that may forewarn a hot spot or tar ball. Have a contingency plan to deal with such issues before proceeding with coke drum head removal and coke cutting.

← Always assume the possibility of a hot-spot induced geyser or the release of hot tar balls or undrained hot water, and incorporate protective operational measures in drum unheading operations. Further control the hazard by establishing restricted areas; minimizing the number of workers in restricted areas; minimizing the time spent by essential workers in restricted areas; and maintaining readiness for a rapid evacuation.

← Consider equipment upgrades to further control the hazards associated with geysers and release of hot tar balls and undrained hot water during drum head removal, such as installing protective shrouds and automating both top and bottom head removal operations to keep workers away from these unprotected areas.

← Consider emergency steam/cooling water sources in the event of loss of primary steam/cooling water supply or because of drum inlet flow path obstruction.

← Provide temporary guardrails to prevent employees from falling while platform plating is removed for bottom head removal.

← Consider installation of vapor ejectors to draw vapors away from the open top head area.

Coke Cutting (Hydroblasting Operation) The following actions could help control hazards associated with coke cutting operations:

← Install an enclosed cutter’s shack for worker protection--preferably supplied with air from a remote source to maintain slight positive pressure.

← Ensure that personnel who must be on the coke drum structure when a drum is open wear prescribed personal protective equipment.

← Conduct training in recognition and prevention of worker heat stress.

← Make sure the interlocks will work to shut off and prevent restart of the cutting water pump any time that the cutting head is raised above a predetermined point within the coke drum. Consider installing redundant switches to provide an additional level of protection against extracting a cutting head that is under pressure.

← Verify the adequacy of the inspection and maintenance program for cutting water hoses, wire ropes, and hoists.

← Establish a gantry structure inspection and maintenance program. Periodically verify that gantry structures have not been weakened due to corrosive conditions, such as mist exiting from the top nozzle, that could lead to gantry collapse.

← Install drill stem free fall arresters.

Emergency and General Operational Hazards

Coke Transfer, Processing, and Storage

The following actions could help control hazards associated with coke conveyance, processing, and storage operations:

← Establish and enforce restricted areas (e.g., areas where heavy equipment movement and possible lash path of a wire rope from failed equipment may occur) to prevent personnel entry and, ultimately, injury.

← Establish and periodically verify the operability of an alarm system that activates immediately before and during heavy equipment (rail car, bridge crane, or conveyor) movement.

← Verify conformance with a safe entry permit system to ensure that appropriate measures are taken prior to and during entry into any enclosed area or vessel where coke may be present.

← Establish personnel protective measures to protect against inhalation or personal contact with coke dust or potentially contaminated mists from water used for cutting, quench, or coke conveyance (see section on Toxic Exposures, Dust Irritants, and Burn Trauma, below).

Emergency Evacuation - Preparations and Procedures

Despite best efforts to prevent incidents, DCU operators should anticipate the need for emergency evacuation and other response measures, operate in a manner that will minimize the severity of an incident, and prepare for and implement emergency procedures to protect worker safety.

The following specific actions are recommended: ← Review and address weaknesses associated with the location and suitability of emergency

escape routes. Protected stairways, preferably detached from the coke drum structure, are the most effective conventional means of emergency escape route (egress) from tall structures, such as those serving the coke drums. Consider installing horizontal walkways to adjacent structures. Some refineries are exploring the use of commercially available escape chutes. Also, slip resistant walking surfaces will help prevent falling during an emergency evacuation.

← Establish or verify the operability of an evacuation signal (Scram Alarm) to expedite personnel clearing the structure in the event of an emergency. Alarm signal actuation (triggering) stations should be deployed at work areas and along the escape routes.

← Install water sprays to protect work stations and emergency escape routes. Include activation stations at work stations and along the escape route.

← Provide heat shields to protect work stations and escape routes. Ensure that the shield will not interfere with evacuation and will not entrap fugitive vapors.

← Conduct regular emergency exercises to test the plan as well as to ensure familiarity with emergency signals, evacuation routes, and procedures.

Toxic Exposures, Dust Irritants, and Burn Trauma

The following actions could help control exposures to toxic substances, irritants, physical stress, and hazardous conditions associated with DCU operations, in general:

← Configure coke drum inlets and outlets with doubleblock valve and steam seal isolation to reduce the likelihood of unanticipated leakage.

← Establish burn trauma response procedures, including procedures for interacting with emergency medical service providers and the burn trauma center that would be used in the event of a burn incident.

← Conduct burn trauma simulation exercises to ensure appropriate use of the emergency response procedures and the training level of relevant personnel.

← Evaluate health exposure potential and establish appropriate protective measures based on an industrial hygiene survey plan that anticipates variations in the range of DCU feed stocks and operating conditions.

← Shovel, sweep, vacuum, and provide proper ventilation to keep exposures to dust around a DCU to within acceptable limits.

Information Resources

Internet resources - The search entry, “Delayed Coker Unit,” yields many sources of information that are believed to be useful. However, neither EPA nor OSHA control this information and cannot guarantee the accuracy, relevance, timeliness or completeness of all facets of the information.

Further, the citation to these resources is not intended to endorse any views expressed, or services offered by the author of the reference or the organization operating the service identified by the reference. The following are examples of informative additional reading.

← http://www.coking.com - focuses on coking best practices, safety, reliability, and communications within the DCU industry.

← http://www.fireworld.com/magazine/coker. html – describes a May 1999 coking unit fire and offers recommendations on fire protection.

← DELAYED COKING PROCESS & APPARATUS –

← (Patent Pending)← The Technology called “Improved Delayed Coker← Unit (IDCU)”← “IDCU” Technology has revolutionized the Delayed Coker← processing by eliminating in-situ De-Coking process.← “IDCU” Technology is equally applicable to the Operating← Delayed Coker Units and the New Ones.← NAPHTHA

← OFF GAS

← Heater Coke Drum A/B Fractionator

← STEAM

← COKER FEED

← KEROSENE

← LCGO

← HCGO

← START

← Crusher

← Figure 1

← STEAM

← BFW

← COKE

← STEAM STEAM

← Improved Delayed Coker Unit (IDCU)

← Introduction: The IDCU, IMPROVED DELAYED COKER UNIT, is a new and

← improved designed of the already proven Delayed Coking Technology, presently

← in use at many facilities across the world← Application: Improved Delayed Coking Technology is a

Delayed and Fluid← coker combination process to convert heavy hydrocarbons

(vacuum residuum,← extra heavy oil or bitumen) to full range lighter liquid

products and coke.← Description: Improved Delayed Coking Technology greatly

shortens the required← duration of the alternating drum fill, decoking cycles and

eliminates the need to← perform drum quenching, draining, unheading, hydraulic

decoking, reheading,← and pressure testing procedures in the decoking cycle. This

patent pending← technology significantly improves coker efficiency,

profitability, reliability and← safety.← Operation: De-coking cycle includes simply a steam-out stage

and a coke← product removing step. The coke product produced in the

coking drums is a hot,← solid, flowable material from which heat can be recovered

for producing steam.← The flowable coke is directed to the pit or trucks.← Reaction proceeds at lowered cracked oil partial pressure by

injecting steam into← the drum, keeping petroleum pitch in a homogeneous liquid

state and stripping← the liquid out. Unlike a conventional Delayed Coker, a higher

cracked oil yield← can be obtained.← Driving force to remove the coke from drum, includes a

pressure system to keep

← the drum at constant pressure during the De-coking process, a heavy duty

← Crusher located at the drum bottom to provide smaller particle size coke, and a

← lifting steam system to move the coke through the exchangers.

← Benefits of “IDCU”← Increases Drum Capacity in excess of 200%.← Greatly reduces coking cycle from 18 hours to 6 hours.← Eliminates the potential damage to the coking drum,

longer drum life.← Significant Heat Recovery for Steam Production← Eliminates Hydraulic De-coking, Heading and De-heading

system,← blowdown and related operations.← Significant reduction in utility usage, O&M costs← Significant Lower Capital Cost for New Installations← Significant Increase in Volume of Liquid Products← Significant Improvement in Quality of Liquid Products← Reduces coke yield.← Handles virtually any pumpable hydrocarbon feed← Operation Conditions: Typical range are:← Hater Outlet temperature, _F 950 -1000← Coke drum pressure, psig 10-50← Recycle ratio, vol/vol feed, % 0← FeedStock: Typical Middle East vac. Residue (Gravity, 7.3

_API, 23← wt% Concarbon, 4.8wt% sulfur):← Yields:← Light Ends, Wt% 7.8← Naphtha (C5-350 _F), wt% 14.3← Gasoil (350-650 _F), wt% 52.7← Coke, wt% 25.3← Economics:← Investment ( based on 12,500 bpsd )← 2Q 2006 US Gulf, $ per bpsd 2,500- 3,500← Installation : All existing delayed Coker can be converted to

IDCU with

← minimum expense within a short time. Unit shut down is not required.

← For more information, please contact us:← US Cokertech, LLC← Office: 405-701-8300← Cell : 713-825-5666← E-mail: kazem.ganji@cokertech.us

Delayed cokingMain Articl

e

Talk

Related   A rticles  [?]

Bibliography  [?]

External   Links  [?]

  

[edit intro]

Delayed coking is one of the chemical engineering unit processes used in many petroleum

refineries. In brief, the process heats the residual oil from the vacuum distillation unit in a

petroleum refinery to its thermal cracking temperature in the heat transfer tubes of a furnace.

This cracks the long chain hydrocarbon molecules of the residual oil into hydrocarbon gases,

coker naphtha, coker gas oil and petroleum coke.[1][2][3].

Some delayed coking units produce as much as 5,000 tons of coke per day.[4]

Contents

[hide]

1 Flow diagram and process description

o 1.1 Typical schematic flow diagram

o 1.2 Process description

1.2.1 Typical 24-hour coke drum cycle

2 Petroleum coke specifications

   

3 Uses of petroleum coke

4 History

5 Other processes for producing petroleum coke

6 References

Flow diagram and process description

The flow diagram and description in this section are based on a typical delayed coking unit with

two coke drums. However, larger units have tandem pairs of drums, some with as many as 6

drums, each of which may have diameters of up to 10 meters and overall heights of up to 43

meters.[5]

Typical schematic flow diagram

(GNU) Image: Milton Beychok A typical schematic flow diagram of a delayed coking unit

Process description

Residual oil from the vacuum distillation unit (sometimes including high-boiling oils from other

sources within the refinery) is pumped into the bottom of the distillation column called the main

fractionator. From there it is pumped, along with some injected steam, into the fuel-fired furnace

and heated to its thermal cracking temperature of about 480 °C. Thermal cracking begins in the

pipe between the furnace and the coke drums, and finishes in the coke drum that is on-stream.

The injected steam helps to minimize the deposition of coke within the furnace tubes.

Pumping the incoming residual oil into the bottom of the main fractionator, rather than directly

into the furnace, preheats the residual oil by having it contact the hot vapors in the bottom of the

fractionator. At the same time, some of the hot vapors condense into a high-boiling liquid which

recycles back into the furnace along with the hot residual oil.

As cracking takes place in the drum, gas oil and lighter components are generated in vapor

phase and separate from the liquid and solids. The drum effluent is vapor except for any liquid

or solids entrainment, and is directed to main fractionator where it is separated into the desired

boiling point fractions.

The solid coke is deposited and remains in the coke drum in a porous structure that allows flow

through the pores. Depending upon the overall coke drum cycle being used, a coke drum may

fill in 16 to 24 hours.

After the drum is full of the solidified coke, the hot mixture from the furnace is switched to the

second drum. While the second drum is filling, the full drum is steamed out to reduce the

hydrocarbon content of the petroleum coke, and then quenched with water to cool it. The top

and bottom heads of the full coke drum are removed, and the solid petroleum coke is then cut

from the coke drum with a high pressure water nozzle, where it falls into a pit, pad, or sluiceway

for reclamation to storage.

The yield of coke from the delayed coking process ranges from about 18 to 30 percent by

weight of the feedstock residual oil, depending the composition of the feedstock and the

operating variables.

Typical 24-hour coke drum cycle

There are a number of coke drum cycles, varying from 12 to 24 hours. However, the one typically used is

the 24-hour cycle:

24-Hour Delayed Coking Cycle[1][6]

Step Time, hours

On-stream drum fills with coke 24.0

Switch feed from full drum to empty drum 0.5

Steam-out of the full drum 2.0

Cool the full drum by filling with water 4.0

Drain water out of full drum 2.0

Remove top and bottom heads of full drum 0.5

Drill pilot hole from top to bottom of coke using high-pressure water 0.5

Cut coke out completely also using high-pressure water 4.0

Replace top and bottom heads of the empty drum 1.0

Steam test and purge to remove air from empty drum 1.0

Warm-up by routing hot vapors from on-line drum into top of empty drum 6.0

Open vapor valve completely and switch feed into empty drum 1.0

Contingency time to allow for slippage in above steps 1.5

Total overall time 48.0

Petroleum coke specifications

The table below lists the specifications for raw petroleum coke produced in a delayed coker and

the corresponding specification after the raw coke has been calcined in a rotary kiln at 1200 to

1400 °C. The raw coke is classified as sponge coke and often referred to as green coke.[7]

Depending on the feedstock composition and certain operating variables, other forms of coke

may be produced such as shot coke and needle coke. However, sponge coke is the form most

typically formed and this article does not discuss the other forms.

Typical Specifications for Sponge Coke[8]

ComponentGreen cokeas produced

Coke calcinedat 1300 °C

Fixed carbon, wt % 86 − 92 99.5

Sulfur, wt % <2.5 <2.5

Volatile matter, wt % 8− 14 0.5

Moisture, wt % 6-14 0.1

Ash, wt % 0.25 0.4

Iron 0.01 0.02

Nickel 0.02 0.03

Silicon 0.02 0.02

Vanadium 0.02 0.03

Uses of petroleum coke

The product coke from a delayed coker has many commercial uses and applications.[9] [10][11] The

largest use is as a fuel.

The uses for green coke are:

As fuel for space-heaters, large industrial steam generators, fluidized bed steam

generators, Integrated Gasification Combined Cycle (IGCC) units and cement kilns

In silicon carbide foundries

For producing blast furnace coke

The uses for calcined coke are:

As anodes in the production of aluminum

In the production of titanium dioxide

As a carbon raiser in cast iron and steel making

Producing graphite electrodes and other graphite products such as graphite brushes

used in electrical equipment

In carbon structural materials

History

Petroleum coke was first made in the 1860's in the early oil refineries in Pennsylvania which

boiled oil in small, iron distillation stills to recover kerosene, a much needed lamp oil. The stills

were heated by wood or coal fires built underneath them, which over-heated and coked the oil

near the bottom. After the distillation was completed, the still was allowed to cool and workmen

could then dig out the coke and tar.[11]

In 1913, William Merriam Burton, working as a chemist for the Standard Oil of Indiana refinery at

Whiting, Indiana, was granted a patent[12] for the Burton thermal cracking process that he had

developed. He was later to become the president of Standard Oil of Indiana before he retired.

In 1929, based on the Burton thermal cracking process, Standard Oil of Indiana built the first

delayed coker. It required very arduous manual decoking.[11]

In the late 1930's, Shell oil developed hydraulic decoking using high-pressure water at their

refinery in Wood River, Illinois. That made it possible, by having two coke drums, for delayed

decoking to become a semi-continuous process.[11]

From 1955 onwards, the growth in the use of delayed coking increased. As of 2002, there were

130 petroleum refineries worldwide producing 172,000 tons per day of petroleum coke.[13]

Included in those worldwide data, about 59 coking units were operating in the United States and

producing 114,000 tons per day of coke.[13]

Other processes for producing petroleum coke

There are other petroleum refining processes for producing petroleum coke, namely the Fluid

Coking and Flexicoking processes[14][15] both of which were developed and are licensed by Exxon

Mobil Research and Engineering. The first commercial unit went into operation in 1955. Forty-

three years later, as of 1998, there were 18 of these units operating worldwide[16] of which 6 were

in the United States.

There are other similar coking processes, but they do not produce petroleum coke. For

example, the Lurgi-VZK Flash Coker which produces coke by the pyrolysis of biomass.[1