Fluid Coking Heated by the produced coke Cracking reactions occur inside the heater and the fluidized- bed reactor. The fluid coke is partially formed in the heater. Hot coke slurry from the heater is recycled to the fluid reactor to provide the heat required for the cracking reactions. Fluid coke is formed by spraying the hot feed on the already- formed coke particles. Reactor temperature is about 520°C, and the conversion into coke is immediate, with complete disorientation of the crystallites of product coke. The burning process in fluid coking tends to concentrate the metals, but it does not reduce the sulfur content of the coke.
Characteristics of fluid coke: high sulfur content, low volatility, poor crystalline structure, and low grindability index. Flexicoking, integrates fluid coking with coke gasification. Most of the coke is gasified. Flexicoking gasification produces a substantial concentration of the metals in the coke product.
Flow diagram of an Exxon flexicoking unit:5 (1) reactor, (2) scrubber, (3) heater, (4) gasifier, (5) coke fines removal, (6) H2S removal.
CATALYTIC CONVERSION PROCESSES Catalytic Reforming To improve the octane number of a naphtha. Aromatics and branched paraffins have high octane ratings than paraffins and cycloparaffins. Many reactions: e.g. dehydrogenation of naphthenes and the dehydrocyclization of paraffins to aromatics. Catalytic reforming is the key process for obtaining benzene, toluene, and xylenes (BTX). These aromatics are important intermediates for the production of many chemicals.
Reformer Feeds heavy naphtha fraction produced from atmospheric distillation units. Naphtha from other sources such as those produced from cracking and delayed coking may also be used. Before using naphtha as feed for a catalytic reforming unit, it must be hydrotreated to saturate the olefins and to hydrodesulfurize and hydrodenitrogenate sulfur and nitrogen compounds. Olefinic compounds are undesirable because they are precursors for coke, which deactivates the catalyst. Sulfur and nitrogen compounds poison the reforming catalyst. The reducing atmosphere in catalytic reforming promotes forming of hydrogen sulfide and ammonia. Ammonia reduces the acid sites of the catalyst, while platinum becomes sulfided with H2S.
Important is : – Types of hydrocarbons in the feed. – Naphthene content – The boiling range of the feeds Feeds with higher end points (≈200°C) are favorable because some of the long-chain molecules are hydrocracked to molecules in the gasoline range. These molecules can isomerize and dehydrocyclize to branched paraffins and to aromatics, respectively.
Reforming Catalysts Bi-functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. platinum, is the best known hydrogenation- dehydrogenation catalyst Alumina, (acid sites) promote carbonium ion formation The two types of sites are necessary for aromatization and isomerization reactions.
Pt/Re catalysts are very stable, active, and selective. Trimetallic catalysts of noble metal alloys are also used for the same purpose. The increased stability of these catalysts allowed operation at lower pressures. Reforming Reactions Reforming Catalysts Aromatization
The reaction is endothermic i.e. higher temp and lower pressures. Effect of temp on the conversion and selectivity:
Catalytic Cracking Catalytic cracking (Cat-cracking): To crack lower-value stocks and produce higher-value light and middle distillates. To produce light hydrocarbon gases, which are important feedstocks for petrochemicals. To produce more gasoline of higher octane than thermal cracking. This is due to the effect of the catalyst, which promotes isomerization and dehydrocyclization reactions. Feeds vary from gas oils to crude residues Polycyclic aromatics and asphaltenes peoduce coke.
Catalytic Catalysts Acid-treated clays were the first catalysts used. Replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Lewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a (II) oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation.
Catalytic Catalysts Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites are cracking catalysts ) is heated to temperatures in the range of 400–500°C, Lewis acid sites are formed.
Zeolite Catalysts Highly selective due to its smaller pores, which allow diffusion of only smaller molecules through their pores, and to the higher rate of hydrogen transfer reactions. However, the silica-alumina matrix has the ability to crack larger molecules. Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. Deactivation may be reversible or irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms: zeolite dealumination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium.
Cracking Reactions A major difference between thermal and catalytic cracking is that reactions through catalytic cracking occur via carbocation intermediate, compared to the free radical intermediate in thermal cracking. Carbocations are longer lived and accordingly more selective than free radicals. Acid catalysts such as amorphous silica-alumina and crystalline zeolites promote the formation of carbocations. The following illustrates the different ways by which carbocations may be generated in the reactor:
Aromatization Reactions Dehydrocyclization reaction. Olefinic compounds formed by the beta scission can form a carbocation intermediate with the configuration conducive to cyclization. Once cyclization has occurred, the formed carbocation can lose a proton, and a cyclohexene derivative is obtained. This reaction is aided by the presence of an olefin in the vicinity (R–CH=CH2).
Cracking Process Most catalytic cracking reactors are either fluid bed or moving bed. In FCC, the catalyst is an extremely porous powder with an average particle size of 60 microns. Catalyst size is important, because it acts as a liquid with the reacting hydrocarbon mixture. In the process, the preheated feed enters the reactor section with hot regenerated catalyst through one or more risers where cracking occurs. A riser is a fluidized bed where a concurrent upward flow of the reactant gases and the catalyst particles occurs.
The reactor temperature is usually held at about 450–520°C, and the pressure is approximately 10–20 psig. Gases leave the reactor through cyclones to remove the powdered catalyst, and pass to a fractionator for separation of the product streams. Catalyst regeneration occurs by combusting carbon deposits to carbon dioxide and the regenerated catalyst is then returned
Typical FCC reactor/regenerator
Isomerization Reactions leading to skeltal rearrangements over Pt catalysts
Hydrocracking A hydrogen-consuming reaction that leads to higher gas production Hydrdealkylation A cracking reaction of an aromatic side chain in presence of hydrogen
Deep Catalytic Cracking Deep catalytic cracking (DCC) is a catalytic cracking process which selectively cracks a wide variety of feedstocks into light olefins. It produces more olefines than FCC.
Hydrocracking Process It is a cracking process in presence of hydrogen. The feedstocks are not suitable for catalytic cracking because of their high metal, sulfur, nitrogen, and asphaltene contents. The process can also use feeds with high aromatic content. Products from hydrocracking processes lack olefinic hydrocarbons. The product slate ranges from light hydrocarbon gases to gasolines to residues. The process could be adapted for maximizing gasoline, jet fuel, or diesel production.
Hydrocracking Catalysts and Reactions Bifunctional noble metal containing zeolites are used. This promote carbonium ion formation. Catalysts with strong acidic activity promote isomerization. The hydrogenation-dehydrogenation is promoted by catalysts such as cobalt, molybdenum, tungsten, vanadium, palladium, or rare earth elements. As with catalytic cracking, the main reactions occur by carbonium ion and beta scission, yielding two fragments that could be hydrogenated on the catalyst surface. The first-step is formation of a carbocation over the catalyst surface:
The carbocation rearrange, eliminate a proton to produce an olefin, or crack at a beta position to yield an olefin and a new carbocation. -Products from hydrocracking are saturated. i.e. gasolines from hydrocracking units have lower octane ratings. They have a lower aromatic content due to high hydrogenation activity. -Products from hydrocracking units are suitable for jet fuel use. Hydrocracking also produces light hydrocarbon gases (LPG) suitable as petrochemical feedstocks.
Hydrocracking Process Mostly single stage, with the possibility of two operation modes. Once-through and a total conversion of the fractionator bottoms by recyling. In once-though operation, low sulfur fuels are produced and the fractionator bottom is not recycled. In the total conversion mode the fractionator bottom is recylced to the inlet of the reactor. In the two-stage operation, the feed is hydrodesulfurized in the first reactor with partial hydrocracking. Reactor effluent goes to a high-pressure separator to separate the hydrogen- rich gas, which is recycled and mixed with the fresh feed. The liquid portion from the separator is fractionated, and the bottoms of the fractionator are sent to the second stage reactor.
Hydrocracking reaction conditions vary widely, depending on the feed and the required products. Temperature and pressure range from 400 to 480°C and 35 to 170 atmospheres. Space velocities in the range of 0.5 to 2.0 hr-1 are applied. Flow diagram of a Cheveron hydocracking unit:29 (1,4) reactors, (2,5) HP separators, (3) recycle scrubber (optional), (6) LP separator, (7) fractionator.
Hydrodealkylation Process Designed to hydrodealkylate methylbenzenes, ethylbenzene and C9 + aromatics to benzene. The petrochemical demand for benzene is greater than for toluene and xylenes. After separating benzene from the reformate, the higher aromatics are charged to a hydrodealkylation unit. The reaction is a hydrocracking one, where the alkyl side chain breaks and is simultaneously hydrogenated.
Consuming hydrogen is mainly a function of the number of benzene substituents. Dealkylation of polysubstituted benzene increases hydrogen consumption and gas production (methane).
Hydrotreatment Processes Hydrotreating is a hydrogen-consuming process to reduce or remove impurities such as sulfur, nitrogen, and some trace metals from the feeds. It also stabilizes the feed by saturating olefinic compounds. Feeds could be any petroleum fraction, from naphtha to crude residues. The feed is mixed with hydrogen, heated to the proper temperature, and introduced to the reactor containing the catalyst.
Hydrotreatment Catalysts and Reactions The same as those developed in Germany for coal hydrogenation. The cobalt-molybdenum/alumina is an effective catalyst. hydrodenitrogenation
Alkylation Process To produce large hydrocarbon molecules in the gasoline fraction from small moleucles. (branched hydrocarbons). Normally acid catalyzed using H 2 SO 4 or abhydrous HF. The product is known as the alkylate.
Some recent research has been devoted to replace the homogeneous acid catalysts by heterogeneous solid catalysts employing zeolites and alumina, or zirconia.
Isomerization process Small volume but important refinery process. Acid catalyzed. To produce branched alkanes. Bifunctional catalysts activated by inorganic chelorides are used. Pt/zeolite is a typical isomerization catalyst. Oligomerization of Olefines (Dimerization) To produce polymer gasoline with high octane number. Acid catalyzed. By phosphoric or sulfuric acid. The feed is Propylne-propane or propykene-butane mixture. The alkane is used as diluent.