Conversion Processes 1. THERMAL PROCESSES 2. CATALYTIC PROCESSES 1
Physical and chemical processes Physical Thermal Chemical Catalytic Distillation Solvent extraction Propane deasphalting Solvent dewaxing Blending Visbreaking Delayed coking Flexicoking Hydrotreating Catalytic reforming Catalytic cracking Hydrocracking Catalytic dewaxing Alkylation Polymerization Isomerization 2
Cracking Cracking is a petroleum refining process in which heavymolecular weight hydrocarbons are broken up into light hydrocarbon molecules by the application of heat and pressure, with or without the use of catalysts, to derive a variety of fuel products. Cracking is one of the principal ways in which crude oil is converted into useful fuels such as motor gasoline, jet fuel, and home heating oil. 3
1. Thermal Cracking Thermal cracking is a refining process in which heat (~800 C) and pressure (~700kPa) are used to break down, rearrange, or combine hydrocarbon molecules. The first thermal cracking process was developed around 1913. Distillate fuels and heavy oils were heated under pressure in large drums until they cracked into smaller molecules with better antiknock characteristics. This method produced large amounts of solid, unwanted coke. This early process has evolved into the following applications of thermal cracking: A. visbreaking B. steam cracking C. coking 4
A. Visbreaking Visbreaking is a mild form of thermal cracking that lowers the viscosity of heavy crude-oil residues without affecting the boiling point range. Residuum from the atmospheric distillation tower is heated (425-510ºC) at atmospheric pressure and mildly cracked in a heater. It is then quenched with cool gas oil to control over-cracking, and flashed in a distillation tower. Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils. Middle distillates may also be produced, depending on product demand. The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum-flashed in a stripper and the distillate recycled. 5
Vacuum residue can be cracked. The severity of the visbreaking depends upon temperature and reaction time (1-8 min). Usually < 10 wt% of gasoline and lighter products are produced. 6
B) Steam Cracking Process Steam cracking is a petrochemical process sometimes used in refineries to produce olefinic raw materials (e.g., ethylene) from various feedstock for petrochemicals manufacture. The feedstock range from ethane to vacuum gas oil, with heavier feeds giving higher yields of by-products such as naphtha. The most common feeds are ethane, butane, and naphtha. Steam cracking is carried out at temperatures of 1,500-1,600 F, and at pressures slightly above atmospheric. Naphtha produced from steam cracking contains benzene, which is extracted prior to hydrotreating. Residual from steam cracking is sometimes blended into 7 heavy fuels.
C) Coking Processes The bottom of the barrel has become more of a problem for refiners because the market for heavy residual fuel oils has been decreasing. Historically, the heavy residual fuel oils have been burned to produce electric power and to supply the energy needs of heavy industry, but more severe environmental restrictions have caused many of these users to switch to natural gas. Coking units convert heavy feedstocks into a solid coke and lower boiling hydrocarbon products which are suitable as feedstocks to other refinery units for conversion into higher value transportation fuels. Actually the coke formed contains some volatile matter or high-boiling hydrocarbons. To eliminate essentially all volatile matter from petroleum coke it must be calcined at approximately 1095 to 1260 C. 8
Coking is a severe method of thermal cracking used to upgrade heavy residuals into lighter products or distillates. Coking produces straight-run gasoline (coker naphtha) and various middle-distillate fractions used as catalytic cracking feedstock. The process completely reduces hydrogen so that the residue is a form of carbon called "coke." The two most common processes: 1. delayed coking 2. continuous (contact or fluid) coking. 9
C-1) Delayed coking The heated charge (typically residuum from atmospheric distillation towers) is transferred to large coke drums which provide the long residence time needed to allow the cracking reactions to proceed to completion. 1. Initially the heavy feedstock is fed to a furnace which heats the residuum to high temperatures (900-950 F) at low pressures (25-30 psi) and is designed and controlled to prevent premature coking in the heater tubes. 2. The mixture is passed from the heater to one or more coker drums where the hot material is held approximately 24 hours (delayed) at pressures of 25-75 psi, until it cracks into lighter products. 10
3. Vapors from the drums are returned to a fractionator where gas, naphtha, and gas oils are separated out. 4. The heavier hydrocarbons produced in the fractionator are recycled through the furnace. 5. After the coke reaches a predetermined level in one drum, the flow is diverted to another drum to maintain continuous operation. 6. The full drum is steamed to strip out uncracked hydrocarbons, cooled by water injection, and de-coked by mechanical or hydraulic methods. 7. The coke is mechanically removed by an auger rising from the bottom of the drum. Hydraulic decoking consists of fracturing the coke bed with high-pressure water ejected from a rotating cutter. 11
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C-2) Flexicoker Continuous (contact or fluid) coking is a moving-bed process that operates at temperatures higher than delayed coking. Thermal cracking occurs by using heat transferred from hot, recycled coke particles to feedstock in a radial mixer, called a reactor, at a pressure of 50 psi. Gases and vapors are taken from the reactor, quenched to stop any further reaction, and fractionated. Feed can be any heavy oil such as vacuum resid, coal tar, shale oil, or tar sand bitumen. The feed is preheated to about 600 to 700 F (315 to 370 C) and sprayed into the reactor where it contacts a hot fluidized bed of coke. This hot coke is recycled to the reactor from the coke heater at a rate which is sufficient to maintain the reactor fluid bed temperature between 950 and 1000 F (510 to 540 C). The coke recycle from the coke heater thus provides sensible heat and heat of vaporization for the feed and the endothermic heat for the cracking reactions. 13
The cracked vapor products pass through cyclone separators in the top of the reactor to separate most of the entrained coke particles (cyclone separators are efficient down to particle sizes about 7 microns, but the efficiency falls off rapidly as the particles become smaller) and are then quenched in the scrubber vessel located at the top of the reactor. Some of the high-boiling [925 F (495 C)] cracked vapors are condensed in the scrubber and recycled to the reactor. The balance of the cracked vapors flow to the coker fractionator where the various cuts are separated. Wash oil circulated over baffles in the scrubber provides quench cooling and also serves to reduce further the amount of entrained fine coke particles. 14
The coke produced by cracking is deposited as thin films on the surface of the existing coke particles in the reactor fluidized bed. The coke is stripped with steam in a baffled section at the bottom of the reactor to prevent reaction products, other than coke, from being entrained with the coke leaving the reactor. Coke flows from the reactor to the heater where it is reheated to about 1100 F (593 C). The coke heater is also a fluidized bed and its primary function is to transfer heat from the gasifier to the reactor. 15
Coke flows from the coke heater to a third fluidized bed in the gasifier where it is reacted with air and steam to produce a fuel gas product consisting of CO, H2, CO2, and N2. Sulfur in the coke is converted primarily to H2S, plus a small amount of COS, and nitrogen in the coke is converted to NH3 and N2. This gas flows from the top of the gasifier to the bottom of the heater where it serves to fluidize the heater bed and provide the heat needed in the reactor. The reactor heat requirement is supplied by recirculating hot coke from the gasifier to the heater. The system can be designed and operated to gasify about 60 to 97% of the coke product in the reactor. The overall coke inventory in the system is maintained by withdrawing a stream of purge coke from the heater. The coke fines collected in the venturi scrubber plus the purge coke from the heater represent the net coke yield and contain essentially all of the metal and ash components of the reactor feed stock. 16
Simplified flow diagram for a Flexicoker. 17
C-3) FLUID COKING Fluid coking is a simplified version of flexicoking. In the fluid coking process only enough of the coke (is about 20 to 25%) is burned to satisfy the heat requirements of the reactor and the feed preheat. Only two fluid beds are used in a fluid coker a reactor and a burner which replaces the heater. The primary advantage of the Flexicoker over the more simple fluid coker is that 1. Most of the heating value of the coke product is made available as low sulfur gas which can be burned without an SO2 removal system 2. The coke gas can be used to displace liquid and gaseous hydrocarbon fuels in the refinery process heaters and does not have to be used exclusively in boilers as is the case with fluid coke. 18
Coke End Uses Three typical types of coke are obtained : 1. sponge coke, 2. honeycomb coke 3. needle coke Depending upon the 1. reaction mechanism 2. time 3. Temperature 4. the crude feedstock 19
1. Fuel 2. Manufacture of anodes for electrolytic cell reduction of alumina 3. Direct use as chemical carbon source for manufacture of elemental phosphorus, calcium carbide, and silicon carbide 4. Manufacture of electrodes for use in electric furnace production of elemental phosphorus, titanium dioxide, calcium carbide, and silicon carbide 5. Manufacture of graphite 20
TYPES, PROPERTIES, AND USES OF PETROLEUM COKE 21