Heavy Oils Processing

Overview

• After desalting and dehydration, crude is separated into fractions by distillation.

• The distilled fractions can not be used directly.• The reason for such a complex set of processes is

the difference between the crude oil properties and the needs of the market.

• Another reason for complexity is environmental. Legislation demands cleaner products and is the major drive for process improvement and development of novel processes.

Flow scheme of a typical refinery

Physical and chemical processes

PhysicalChemical

Thermal CatalyticDistillationSolvent extractionPropane deasphaltingSolvent dewaxingBlending

VisbreakingDelayed cokingFlexicoking

HydrotreatingCatalytic reformingCatalytic crackingHydrocrackingCatalytic dewaxingAlkylationPolymerizationIsomerization

CokingCoking is a severe method of thermal cracking used to upgrade heavy

residuals ("bottom-of- the-barrel”) into lighter products or distillates.

Coke can be formed from the condensation of polynuclear aromatics (such as n-butylnapthalene)

Some thermodynamics

For reaction 3

Delayed Coking• Coking is a severe method of thermal cracking used to upgrade heavy

residuals ("bottom-of- the-barrel”) into lighter products or distillates.• It is the process mostly used today – other processes compared with

delayed coking• First developed in 1928, in early refineries extensive thermal cracking

would result in deposit of unwanted coke in the heaters.

Coking

• Solution: raise rapidly the temperature of the residue above the coking point without depositing the coke in the heater itself. Provision of an insulated surge drum downstream of the heater so that the coking took place after the heater, but before subsequent processing.

• The next step was to add a second coke drum, which doubled the run length and led to the development of the art of switching coke drums while still maintaining operation .In the early 1930s the drums were limited in size to 10 ft in diameter. Coke drums as large as 30 ft in diameter have recently been installed

Delayed Coking

Delayed Coking

Coking section

• Reduced-crude or vacuum-residue fresh feed is preheated by exchange against gas oil products before entering the coker-fractionator bottom surge zone.

• The fresh feed is mixed with recycle condensed in the bottom section of the fractionator and is pumped by the heater charge pump through the coker heater, where the charge is rapidly heated to the desired temperature level for coke formation in the coke drums.

Coking Section

• Steam is often injected into each of the heater coils to maintain the required minimum velocity and residence time and to suppress the formation of coke in the heater tubes.

• The vapor-liquid mixture leaving the furnace enters the coke drum, where the trapped liquid is converted to coke and light-hydrocarbon vapors.

• The total vapors rise upward through the drum and leave overhead.

• A minimum of two drums is required for operation. One drum receives the furnace effluent, which it converts to coke and gas while the other drum is being decoked.

Fractionation Section

• The coke-drum overhead vapors flow to the coker fractionator and enter below the shed section.

• The coke-drum effluent vapors are often "quenched" and "washed" with hot gas oil pumped back to the trayed wash section above the sheds. These operations clean and cool the effluent-product vapors and condense a recycle stream at the same time.

• This recycle stream, together with the flesh feed, is pumped from the coker fractionator to the coking furnace

Delayed coker maximum drum size

US Coking plant statistics

Effect of process variables• Increasing pressure will increase coke formation and slightly increase

gas yield. • However, refinery economics require operating at minimum coke

formation. New units are built to work at 1 bar gauge (15 psig), while existing units work at 2.4 bar gauge (35 psig). In a case of production of needle coke, a pressure of 150 psig is required. Recycle ratio is used to control the endpoint of the coker gas oil. It has the same effect as pressure. Units are operating at a recycle ratio as low as 3%.

• Feedstock variables are the characterization factor and the Conradson carbon which affect yield production. Sulphur and metal content are usually retained in the coke produced. Engineering variables also affect the process performance. These include mode of operation, capacity, coke removal and handling equipment.

Decoking Scedule

• Steaming: The full coke drum is steamed out to remove any residual-oil liquid. This mixture of steam and hydrocarbon is sent first to the fractionator and later to the coker blowdown system, where the hydrocarbons (wax tailings) are recovered.

• Cooling: The coke drum is water-filled, allowing it to cool below 93°C. The steam generated during cooling is condensed in the blowdown system.

• Draining: The cooling water is drained from the drum and recovered for reuse.

• Unheading: The top and bottom heads are removed in preparation for coke removal.

Decoking Schedule

• Decoking: Hydraulic decoking is the most common cutting method. High-pressure water jets are used to cut the coke from the coke drum. The water is separated from the coke fines and reused.

• Heading and testing: After the heads have been replaced, the drum is tightened, purged, and pressure-tested.

• Heating up: Steam and vapors from the hot coke drum are used to heat up the cold coke drum. Condensed water is sent to the blowdown drum. Condensed hydrocarbons are sent to either the coker fractionator or the blowdown drum.

• Coking: The heated coke drum is placed on stream, and the cycle is repeated for the other drum.

Decoking schedule

• Cycles are typically 36-hour coking cycles, composed of 18 hours of coking and 18 hours of decoking, they are often referred to as 18-hour cycles.

• Refiners sometimes operate on "short cycles," which have cycle times less than the design cycle. This has an operating advantage. It allows the refiner to increase the unit through- put by filling the coke drums faster (14-16 typically)

Feedstocks

• Heavy residues such as vacuum residue or occasionally atmospheric residue are the feedstocks which are most commonly used in delayed coking. For special applications in which high-quality needle coke is desired, certain highly aromatic heavy oils or blends of such heavy oils may be used instead

Predicting yields

• Because the correlations used to predict coking yields are, in general, considered to be proprietary information to the companies which have developed these correlations, relatively little information is given in the published literature on how to predict coking yields

Prediction yields

Gas(C4-) wt% = 7.8 + 0.144*(wt%CCR)Naphtha wt% = 11.29 + 0343 * (wt% CCR)Coke wt% = 1.6 * (wt% CCR)...

Gary, J.H., and Handwerk, G.E. (2001). ‘‘Petroleum Refining.’’ Marcel Dekker, New York.

Empirical correlations

The two impurities in the products from delayed coking which are of greatest concern are sulfur and metals

Types of coke produced

Coke amount can be up to 30 wt% in delayed coking. It is produced as green coke which requires calcination to remove the volatiles as fuel product. Green coke can also be used as fuel. The most common types of coke are: Sponge coke: Sponge coke is named for its sponge-like appearance. It is produced from feeds having low to moderate asphaltene content. Needle coke: This coke has a needle-like structure and is made from feed having no asphaltene contents such as decant oils from FCC. It is used to make expensive graphite electrodes for the steel industry. Shot coke: This coke is an undesirable product and is produced when feedstock asphaltene content is high and/or when the drum temperature is too high. Discrete mini-balls of 0.1–0.2 in. (2–5 cm) in diameter are produced.

Regular-Grade Coke Production• Virgin petroleum feedstocks have a large number of cross-linkages with less

than 6 carbon atoms. These feedstocks tend to produce isotropic or amorphous cokes and when they are visibly very porous they are called sponge coke

• Sponge coke derived from a petroleum feedstock that shows abundant pore structure

• electrodes for the aluminuium industry• fuel

Needle Coke Production Used in the manufacture of high-quality graphite electrodes for the steel industry. It owes this application to its excellent electrical conductivity, good mechanical strength at high temperatures, low coefficient of thermal expansion, low sulfur content, and low metal content.

A heavy feedstock which is highly aromatic and, in addition, is low in sulfur and low in metal is neededPolymerization and condensation of a large number of aromatic compounds with a low concentration of impurities leads to the formation of coke containing fewer cross-linkages and has a more crystalline appearance

OPERATING VARIABLES - Temperature

• Temperature is used to control the volatile combustible material (VCM) content of the coke product.

• In general produce coke is produced with a VCM ranging between 6.0 and 8.0 wt %.

• This results in a harder coke and, if structure and impurity levels are acceptable, in a more desirable aluminum-grade coke.

• At constant pressure and recycle ratio the coke yield decreases as the drum temperature increases.

Temperature

• The furnace supplies all the necessary heat to promote the coking reaction.

• If the temperature is too low, the coking reaction does not proceed far enough and pitch or soft-coke formation occurs

• If temperature is too high the coke formed is very hard and difficult to remove from the coke drum with hydraulic decoking equipment.

• Higher temperatures also increase the potential of coking the furnace tubes and/or transfer line.

• Typical Temperatures 460-525oC.

Pressure• Increasing pressure is to retain more of the heavy hydrocarbons are

retained in the coke drum. • This increases the coke yield and slightly increases the gas yield while

decreasing the pentane and heavier liquid-product yield. • The trend in the design of delayed cokers which maximize the yield of

clean liquid products is to design for marginally lower operating pressures.• The use of a heavier coker feed- stock which produces fuel-grade coke

having a market value 15 to 30 percent of that for aluminum-grade coke drives design economics to the absolute minimum coke yield, even though it results in an increased expense for vapor-handling capacity.

• Typical values of pressure used is 15-35 lb/in2.• As a result, units are currently being designed with coke-drum pressures

as low as 150 lb/in2.

Recycle Ratio

• As the recycle ratio is increased, the coke and gas yields increase while the pentane and heavier liquid yield decreases.

• The recycle ratio is used primarily to control the endpoint of the coker gas oil. The same economics which are forcing the operation of cokers to lower operating pressures are also at work on recycle ratios. Units operating at recycle ratios as low as 3 percent have been reported

Estimated Cost Investment

• the investment cost of delayed cokers as a function of tons per day of product coke as well as barrels per day of feed.

• Although a highly accurate investment cost for a delayed coker can be determined only by a detailed definitive estimate, it is often necessary when carrying out economic evaluations to develop a rough, preliminary budget-type estimate. This type of estimate typically has an accuracy of 30%.

• For a delayed coker a cost in the range $45,000 to $95,000/(short ton-day) of coke produced may be used for preliminary evaluations. This cost excludes the vapor-recovery unit and is based on the following assumptions.

Coke Drum

• Coke is hydraulically removed from the drum using a jet water pump, which produces a high-pressure (2500 to 4500 lb/in2gage) and high-volumetric-flow (900 to 1300 gal/min) water stream.

• Most cokers today use a combination tool, or two-mode drill bit, that first drills the pilot hole and then switches modes to cut the remainder of the coke from that drum.

• The cutting water and coke flow from the bottom of the drum, through the coke shroud, and into the coke handling area.

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The Flexicoking Process

38

The Flexicoking process

• Feed is preheated to about 310-370oC and sprayed into the reactor where it contacts a hot fluidised bed of coke

• The coke is recycled at a rate that maintains reactor fluid bed between 510-540oC

• The coke produced is deposited as thin films on the surface of the existing coke particles in the reactor fluidised bed

• Shorter times than delayed coking • Decrease yields of coke – higher amounts of aromatics

produced• This process is very similar to fluid coking

Fluidcoking

Visbreaking• Visbreaking is a mild form of thermal cracking that lowers the viscosity

of heavy crude-oil residues without affecting the boiling point range.• Residue 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.

Visbreaking

Visbreaking

• Coke is not produced• Reactions continue in the soaker – 2 phase

system

Visbreaking

• Alternatively, 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.