Chapter Seven: Solvent Deasphalting and Thermal Cracking (Bottom of the Barrel) Processes

Solvent Deasphalting

Solvent Deasphalting Solvent deasphalting is a physical process (extraction) NOT a thermal cracking (process)

Propane Deasphalting Flowsheet Propane/Vacuum residue ratio is 6:1 to 10:1 by vol.

Solvent Deasphalting Feed is Vacuum residue/bitumen Coke-forming tendencies of heavier distillation products are reduced by removal of asphaltenic materials by solvent extraction. Liquid propane is a good solvent. Butane, Pentane, Heptane or mixture of solvents are also commonly used. Vacuum residue is fed to a counter current deasphalting tower. Deasphalting is based on solubility of hydrocarbons in propane, i.e. the type of molecule; Alkanes dissolve in propane whereas asphaltenic materials (aromatic compounds), ‘coke-precursors’ do not. Asphalt is sent for thermal processing. Deasphalted oil can be used as Lube oil base feedstock (LBFS) or as feed to FCC unit.

Solvent Deasphalting DAO from propane deasphalting has the highest quality but lowest yield. Mixtures of propane & n-butane is more suitable for higher yield extraction. Using pentane may double or triple the yield from a heavy feed, but at the expense of contamination by metals and carbon residues that shorten the lifetime of downstream cracking catalysts. Choice of solvent & extraction conditions are critical.

Thermal Cracking Processes: Visbreaking Coking Flexi-coking

Why Thermal Processes Residual fractions (Bottom of Barrel) are the least valuable streams of a refinery. Nearly 45%-50% of the typical crude oils contain 370 °C+ fractions. Disposal problems due to stringent environmental regulations. Decreasing demand of fuel oil. Gradually increasing demand of middle distillates.

Thermal Cracking Reactions Thermal cracking reactions take place due to application of heat During the cracking process, large molecules decompose to smaller (lighter) molecules. In thermal cracking generally two types of reactions take place: 1) Primary reactions by decomposition of large molecules to smaller molecules, 2) Secondary reactions by which active products from primary cracking reactions further crack or react to form other molecules or polymerize to generate heavy products (coke).

Thermal Cracking Reactions . .

Thermal Cracking Reactions

Visbreaking

Typical Coil Visbreaker Layout Operating Pressure: 50-300 psig. Operating Temperature: 455-520°C. Liquid phase cracking process. Residue from Atmospheric /Vacuum distillation units can be used. Coil/Furnace type operating at high temperature and short residence time is used as a reaction medium. Soaker type can also be used, which operates at lower temperature and longer residence time. Feed

Coking

Coking DELAYED COKING: converts heavy feedstock to coke, oil & gas. The process has long residence time of 24 h. FLEXICOKING: Flexicoking is a combination of thermal cracking and coke gasification/ combustion.

Objectives of Delayed Coking Process heavy residuum to produce distillates, naphtha & gas oils that may be catalytically upgraded to more valuable products Attractive for heavy residuum that is not suitable for catalytic processes Metal, sulfur and other catalyst poisons generally end up in coke

Typical Delayed Coking Unit

Delayed Coking Technology Delayed Coking is one of the most commonly used process that upgrades residues to a wide range of lighter hydrocarbon gases and distillates through thermal cracking. The byproduct of delayed coking process is petroleum coke. The goal for delayed coking operation is to maximize the yield of clean distillates and minimize the yield of coke. Delayed coking technology is preferred for upgrading heavier residues due to its inherent flexibility to handle even the heaviest residues while producing clean liquid products. The main products of delayed coking operation is hydrocarbon gases (such as LPG), naphtha, Light Gas Oil (LGO) and Heavy Gas Oil (HGO).

Operating Variables Four operating parameters govern the yield pattern and product quality of Delayed Coking are: 1) Temperature 2) Pressure 3) Recycle Rate (RR) 4) Velocity Medium

Effects of Operating Variables High temperature: Favors cracking Increases the Formation of more distillates liquids Lowers the yield of coke & hydrocarbon gases High pressure: Decreases the yield of distillates liquids Increases the yield of coke & hydrocarbon gases High recycle rate: High residence time: Favors coke and gas formation Lead to over conversion and the reduction in the production of distillates.

Chemical Reactions during Delayed Coking There are three types of chemical reaction processes which occur continuously without any distinct steps in the coking process: Dehydrogenation : The initial reaction in carbonization involves the loss of hydrogen atom from an aromatic hydrocarbon and formation of aromatic free radical intermediate. Rearrangement Reactions: Thermal rearrangement usually leads to formation of more stabilized aromatic ring system which forms building block of graphite (coke) growth. Polymerization of aromatic radicals: Aromatic free radicals get polymerized in the process of coking reaction.

Delayed Coking Process Delayed Coking is a batch/ continuous process: Flow through the furnace coil is continuous. Feed is switched between two or more drums in batchwise fashion. Delayed coker drum cycle length varies from unit to unit. However, it is typically kept within 16 to 24 hours.

Feedstock of Delayed Coking Delayed Coker can process a wide variety of feedstocks: 1) Feedstock can have considerable amount of metal (Ni + V), sulfur, resins and asphaltenes 2) Contaminants present in feedstock mostly get eliminated with coke. Most typical feedstock is Vacuum Residue (530 °C+). Atmospheric residue is also occasionally processed. Typical feed contains: Sulfur: 5 to 6 w% Metals: ~0.1 w% Properties of coke mostly depends on feed quality.

Products of Delayed Coking Delayed coker produces desirable liquid products (naphtha and gas oil) and byproducts hydrocarbon gases (also called coker gases) and solid coke Coker gases stream is sent to the gas plant where C3 and C4 is recovered as LPG and the lighter end products can be used as fuel gas in the refinery. Produced naphtha contains high olefin content and this stream is usually sent to hydrotreater for stabilization. Light Gas Oil (LGO) is sent to diesel hydrotreater for production of diesel. Heavy Gas Oil (HGO) is sent to FCC for production of gasoline and other valuable distillate products.

Type of Coke Coke formed in Delayed Coker Unit can be classified into three different types Sponge Coke Needle Coke Shot Coke

Sponge Coke Sponge Coke is porous, irregular shaped lumps. It was named by its sponge like appearance. Vacuum Residue with low to moderate asphaltene produces sponge coke. Most of the sponge coke is used as fuel. Some sponge coke with low metal and sulfur content (< 2 w%) can be used to make anodes used in aluminum industries.

Needle Coke It is a premium coke from Delayed Coker named by its needle like appearance. It has microscopic, elongated, needle like structure. It has very low coefficient of thermal expansion and electrical resistance suitable for using as electrodes in steel making.

Shot Coke It is formed from high asphaltene content feedstock present at high coke drum temperature. Shot coke is undesirable product in delayed coking. Shot coke is usually blended with sponge coke to use as fuel.

Modern Trends in Delayed Coker Technology Maximize the production of liquid distillate. Minimize the production of coke. Produce heavy gas oil suitable for downstream catalytic processing. Optimize number and size of coke drums. Maximize air cooling and minimize water cooling. Low operating drum pressure. Reduction in Recycle Ratio (10% or lower). Shorter coke drum cycle

Delayed Coking: Equipment & Operating Modes Typical Equipments: Heater Coke Drum Fractionator Reflux Drum and Strippers Coke Drums run in two batch modes Filling Decoking Other Equipments operate in continuous mode

Delayed Coking: Fresh Feed & Furnace Fresh feed is sent to the bottom of fractionator before charging to the furnace Feed is heated in Furnace Outlet temperature about 496 0C Cracking starts at 426 0C Endothermic Reactions Superheating allows cracking reactions to continue in the coke drums Velocities in Furnace kept high to: Prevent coking in the furnace tubes Delay Coking Reactions until the Coke Drums (hence the term ‘Delayed Coking’ is used)

Coke Drums & Coking Flow Direction: Number of Coke Drums Furnace outlet enters from bottom of the coke drum Vapor flows up the coke drum Coke solidifies & precipitates in the drum. Coking reactions take about three hours for the coke to fully solidify Lower molecular weight products exit from the top of drum as vapors – quenched with oil and sent to bottom of fractionator Number of Coke Drums Typically 2 or 4. One set online in a coking step while other set is being decoked

Typical Delayed Coking Unit 35

Coke Drums & Coking To increase the throughput capacity Cycle Time Large & More Coke Drums are used Good control of coke fill height is necessary Cycle Time Trend is to decrease cycle time Typically 24 Hours cycle time 16 hours cycle is common & preferable 14 hour & 12 hour cycle is being designed Primary purpose of reducing cycle time is to increase throughput

Fractionator Feed Coke Drum Overhead is sent to bottom of fractionator above liquid level Fresh feed typically sent to bottom of fractionator below liquid level

Fractionator Products Top: Liquid Naphtha & Gas light ends. Middle: Light and heavy gas oil. Bottom: Recycled to coking

Flexi-coking Expensive process that have only a small portion of the coking market Continuous fluidized bed technology where coke particles are used as the continuous particulate phase with a Reactor & Burner. The coke product is gasified with steam & air to a low Btu gas containing H2S.

Simplified Flexi-coking Flowsheet Heavy residuum feed is introduced into the reactor vessel where it is thermally cracked. The heat for cracking is supplied by a fluidized bed of hot coke transferred to the reactor from the heater vessel. The vapor products of the reaction leave the reactor zone to enter the scrubber section. Fine coke and some of the heavy oil particles are removed from the cracked products in the scrubber zone and returned to mix with the fresh feed entering the reactor. Feed Steam The reactor products subsequently leave the scrubber and are sent to a fractionating facility. Steam is introduced to the bottom of the reactor to maintain a fluidized bed of coke and to strip the excess coke leaving the reactor free from entrained oil.

Flexi-coking Flexi-coking is an extinctive process. By continuous recycling of heavy oil stream, almost all the feed is converted into distillate fractions, refinery gas, and low Btu gas. There is a very small coke purge stream which amounts to about 0.4–0.8 wt% of fresh feed. When suitably hydrotreated the fractionated streams from the flexi-coker provide good quality products. Hydrotreated coker naphtha provides an excellent high-naphthene feed to the catalytic reformer.

End of Chapter Seven