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Fouling During Product Improvement Processes
Chapter 15
Fouling During Product Improvement Processes 15.1 INTRODUCTION Chemically, petroleum is an extremely complex mixture of hydrocarbon compounds, usually with minor amounts of nitrogen-, oxygen-, and sulfur-containing compounds as well as trace amounts of metal-containing compounds, while on the other hand heavy feedstocks (residua, heavy oil, extra heavy oil, and tar sand bitumen) are even more complex (Chapter 2) (Speight, 2001, 2005, 2013, 2014a, 2015). In addition, the properties of these feedstocks vary widely and are not conducive to modern-day use in the raw state. A variety of processing steps are required to convert these feedstocks to saleable products (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011a, 2014a). The production of liquid product streams by distillation (Chapter 9) or by cracking processes (Chapters 10-13) are only the first of a series of steps that leads to the production of marketable products. Several other unit processes are involved in the production of a final product and such processes may be generally termed product improvement processes since they are not used directly on the raw feedstock, but are used on primary product streams that have been produced from the feedstock (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014a,b). It is, therefore, the purpose of this chapter to present the concepts behind these secondary processes with examples of where fouling might occur.
15.2 REFORMING 15.2.1 General Aspects Reforming processes (molecular rearrangement processes) are processes in which the molecular structure of the feedstock is reorganized to enhance the properties of the product thereby changing the properties of the product relative to the feedstock (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2008, 2014a). In the process, there are four major types of chemical reactions which occur: (1) dehydrocyclization of paraffin Fouling in Refineries. http://dx.doi.org/10.1016/B978-0-12-800777-8.00015-2 © 2015 Elsevier Inc. All rights reserved.
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376 Fouling in Refineries
TABLE 15.1 Composition of Catalytic Reformer Product Gas Constituent
% by Volume
Hydrogen
75-85
Methane
5-10
Ethane
5-10
Propane
5-10
Butane
<5
Pentane and higher
<2
compounds to aromatic compounds, (2) hydrocracking of high-molecular-weight paraffins to low-molecular-weight paraffins, (3) isomerization of n -paraffins to iso-paraffins, and (4) dehydrogenation of naphthenes to aromatics. In fact, dehydrogenation is a main chemical reaction in catalytic reforming, and hydrogen gas is consequently produced in large quantities (Table 15.1). The hydrogen is recycled through the reactors where reforming takes place to provide the atmosphere necessary for the chemical reactions and also prevents the carbon from being deposited on the catalyst, thus extending its operating life. An excess of hydrogen above whatever is consumed in the process is produced, and as a result, catalytic reforming processes are unique in that they are the only petroleum refinery processes to produce hydrogen as a by-product. In the thermal reforming process, a feedstock such as naphtha (end point: 205 °C, 400 °F) or straight-run gasoline is heated to 510-595 °C (950-1100 °F) in a furnace with pressures on the order of 400-1000 psi. As the heated naphtha leaves the furnace, it is cooled or quenched by the addition of cold naphtha. The material then enters a fractional distillation tower where any heavy products are separated. The remainder of the reformed material leaves the top of the tower to be separated into gases and higher octane reformate. Because of the high temperature that is the potential for even the lowest molecular weight feedstock to produce coke (from any aromatic constituents or aromatic constituents formed during the process). Catalytic reforming processes (moving-bed processes, fluid-bed processes, and fixed-bed processes) which have largely replaced thermal reforming processes, use catalytic reactions to process primarily low-octane heavy straight-run (from the crude distillation unit) naphtha into high-octane aromatic naphtha. Like the thermal reforming process, the catalytic reforming process (Figure 15.1) converts low-octane naphtha into high-octane naphtha (reformate). When thermal reforming could produce reformate with research octane numbers of 65-80 depending on the yield, catalytic reforming produces reformate with octane numbers on the order of 90-95. Catalytic reforming is
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Reactor
Reactor
Reactor
Feedstock Furnace
Furnace
Furnace
Fractionator
Light hydrocarbons Hydrogen Recycle
Separator Reformate
FIGURE 15.1 The platforming process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
conducted in the presence of hydrogen over hydrogenation-dehydrogenation catalysts, which may be supported on alumina or silica-alumina. Depending on the catalyst, a definite sequence of reactions takes place, involving structural changes in the feedstock. The moving-bed and fluid-bed processes use mixed nonprecious metal oxide catalysts in units equipped with separate regeneration facilities. Fixed-bed processes use predominantly platinum-containing catalysts in units equipped for cycle, occasional, or no regeneration. The catalysts used in catalytic reforming processes are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions, that is, the production of aromatics, participation in hydrocracking, and rapid hydrogenation of carbon-forming precursors. In order for the catalyst to have an activity for isomerization of both paraffins and naphthenes and to participate in paraffin dehydrocyclization, it must have an acid activity. The balance between these two activities is most important in a reforming catalyst. In fact, in the production of aromatics from cyclic saturated materials (naphthenes), it is important that hydrocracking be minimized to avoid loss of the desired product and, thus, the catalytic activity must be moderated relative to the case of gasoline production from a paraffinic feed, where dehydrocyclization and hydrocracking play an important part.
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15.2.2 Fouling The most common impurities that give rise to process fouling are sulfur compounds that are derived from the sulfur compounds that occur in crude oil (Speight, 2014a), such as sulfide derivatives (RSR1) and the foul-smelling mercaptan derivatives (thiol derivatives, RSH). Oxygen compounds in the form of carboxylic acids (RCO2H) and phenols (ArOH, where Ar is an aromatic group) may also be present, especially in fractions derived from high-acid crude oils (Chapter 2). Nitrogen-containing compounds derived from those that occur in crude oil are also present. Furthermore, olefins (RCHCHR1) must also be eliminated from a feedstock or aromatics removed from a solvent, and these olefins and aromatics are considered impurities. Similarly, polymerized material, asphaltic material, or resin constituents may be impurities, depending on whether their presence in a finished product is harmful. Under the high-hydrogen partial pressure conditions used in catalytic reforming, sulfur compounds are readily converted into hydrogen sulfide, which, unless removed, builds up to a high concentration in the recycle gas and influences the performance of the catalyst (catalyst fouling). Hydrogen sulfide is a reversible poison for platinum and causes a decrease in the catalyst dehydrogenation and dehydrocyclization activities. In the first catalytic reformers, the hydrogen sulfide was removed from the gas cycle stream by absorption in, for example, diethanolamine. Sulfur is generally removed from the feedstock by the use of a conventional desulfurization over cobalt-molybdenum catalyst. An additional benefit of desulfurization of the feed to a level of <5 ppm sulfur is the elimination of hydrogen sulfide (H2S) corrosion problems (corrosion fouling) in the heaters and reactors. Thus, for the process, the feedstock must be clear of any sulfur-containing contaminants otherwise fouling of the catalyst will occur with a rapid decrease in catalyst activity. In addition, like the thermal reforming process, the high temperature employed for catalytic reforming raised the potential for even the lowest molecular weight feedstock to produce coke (from any aromatic constituents or aromatic constituents formed during the process) leading to catalyst fouling. Organic nitrogen compounds are converted into ammonia under reforming conditions, and this neutralizes acid sites on the catalyst (catalyst fouling) and thus represses the activity for isomerization, hydrocracking, and dehydrocyclization reactions. Straight-run materials do not usually present serious problems with regard to nitrogen, but feeds such as coker naphtha may contain around 50 ppm nitrogen and removal of this quantity may require high-pressure hydrogenation (800-1000 psi) over nickel-cobalt-molybdenum on an alumina catalyst. The yield of gasoline of a given octane number and at given operating conditions depends on the hydrocarbon types in the feed. For example, high- naphthene stocks, which readily give aromatic gasoline, are the easiest to reform and give the highest gasoline yields. Paraffinic stocks, however, which depend on the more difficult isomerization, dehydrocyclization, and hydrocracking
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reactions, require more severe conditions and give lower gasoline yields than the naphthenic stocks. The end point of the feed is usually limited to about 190 °C (375 °F), partially because of increased coke deposition on the catalyst as the end point during processing at about 15 °C (27 °F) (catalyst fouling). The efficiency of the catalytic process is currently limited by catalyst deactivation, mainly due to sulfur poisoning and fouling of the catalyst. Regeneration protocols will be necessary to enhance catalysts lifetime and therefore increase the overall process efficiency. Besides catalyst deactivation by sulfur poisoning, catalyst fouling through the deposition of coke and intermediate reaction products on the active metal and in the pores of the catalyst support (pore mouth poisoning/plugging) is another typical pathway of catalyst deactivation. This has been observed in various noble metal catalysts used in steam reforming, liquid phase hydrogenation, and hydrothermal reforming of bio-feedstocks (Bartholomew, 1982; Coll et al., 2001; Besson and Gallezot, 2003; Zöhrer and Vogel, 2013). This is especially true if biomass is used as a co-feedstock (Coll et al., 2001; Speight, 2011b). Deactivation by coking and fouling is often remedied by treatment with steam or hydrogen at high temperatures which, however, requires sufficiently stable catalyst supports and adequate reactors. Activated carbon, a catalyst support often used in hydrothermal reforming of organics due to its stability under these reaction conditions, presents a highly porous structure that is very susceptible to fouling and coking through entrapment of reactants in its micro- and mesopores, leading to coke formation and subsequent plugging of these pores. Furthermore, coke and tar formation can be a major issue during processing of organic feedstocks such as glycerol, glucose, or fermentation residues in suband supercritical water. A related process, steam reforming that is used to convert hydrocarbons such as methane and naphtha to hydrogen and carbon monoxide (Speight, 2014a) also deserves comment here. Large-scale reformers use naphtha as the feedstock in petrochemical plants, but higher molecular weight feedstocks (such as kerosene and light gas oil) are less suitable for steam reforming because of their higher sulfur content and propensity to cause carbon fouling of the catalysts. In addition to catalyst fouling, reboiler fouling has been identified during reformer operations—the result of coke laydown (Brons and Wiehe, 2000).
15.3 ISOMERIZATION 15.3.1 General Aspects Isomerization processes are used to provide additional feedstock (alkylate) for alkylation units or high-octane naphtha as gasoline blending stock. The reaction may take place in the vapor phase, with the activated catalyst supported on a solid phase, or in the liquid phase with a dissolved catalyst. Straight-chain paraffins (n-butane, n-pentane, n-hexane) are converted to respective iso-compounds
380 Fouling in Refineries Reactor Hydrogen make-up
Hydrogen recycle
High pressure separator
Stripper
Fuel gas
Off gas
Unstabilized light distillate
Feed Desulfurized product FIGURE 15.2 Butane isomerization. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
by continuous catalytic (aluminum chloride, noble metals) processes which yields naphtha components with a high-octane rating in this lower boiling range (Speight, 2014a). For example, in the process, pure butane or a mixture of isomeric butanes (Figure 15.2), is mixed with hydrogen (to inhibit olefin formation) and passed to the reactor, at 110-170 °C (230-340 °F) and 200-300 psi. The product is cooled, the hydrogen separated and the cracked gases are then removed in a stabilizer column. The stabilizer bottom product is passed to a super-fractionator where the normal butane is separated from the iso-butane. More modern processes use supported metal catalysts, have been developed, for use in high-temperature processes which operate in the range 370-480 °C (700-900 °F) and 300-750 psi (20-51 atmospheres), while aluminum chloride plus hydrogen chloride are universally used for the low-temperature processes. Nonregenerable aluminum chloride catalyst is employed with various carriers in a fixed-bed or liquid contactor. Platinum or other metal catalyst processes utilized fixed-bed operation and can be regenerable or nonregenerable. The reaction conditions vary widely depending on the particular process and feedstock, 40-480°C (100-900°F) and 150-1000 psi (10-68 atmospheres).
15.3.2 Fouling Isomerization reactions are usually reversible reactions and attain equilibrium at lower temperature with highest concentration of isomer products. In the process, as in any process, it is essential to inhibit side reactions (such as cracking and olefin formation) that will lead to catalyst fouling and, hence, loss of catalyst activity. The role of catalyst in isomerization is, therefore, extremely important. The intensity of unwanted side reactions diminishes at lower temperatures as higher temperature favors unwanted cracking, hydrogenation, and polymerization reactions. For that reason, isomerizing catalysts must ensure the optimal rate of reactions at as low temperature as possible. To prevent coke deposition,
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isomerization is carried out at an elevated pressure in a hydrogen atmosphere. Industrial processes are carried out at a temperature of 400-480 °C (750-895 °F). In addition, Friedel-Crafts reactions that are by aluminum chloride are often responsible for fouling observed on the shell side of a reformer reboiler unit (Brons and Wiehe, 2000). In addition, corrosion products such as iron oxide and/or iron sulfide (Speight, 2014b) can increase the lay-down and subsequent coking of high-molecular-weight constituents that are formed as by-products from the aluminum chloride reactions. Under the conditions of the reaction, the high-molecular-weight materials tend to deposit on the reboiler tubes and degrade to coke over time. Reduction of this type of fouling could be achieved by reducing system corrosion (Brons and Wiehe, 2000).
15.4 ALKYLATION 15.4.1 General Aspects Alkylation processes combine low-molecular-weight olefins (primarily a mixture of propylene and butylene) with isobutene in the presence of a catalyst, either sulfuric acid (Figure 15.3) or hydrofluoric acid (Figure 15.4) (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014a). The product (alkylate) and is composed of a mixture of high octane, branched-chain paraffinic hydrocarbons and is a premium blending stock for gasoline manufacturing because it has exceptional antiknock properties and is clean burning. In cascade-type sulfuric acid (H2SO4) alkylation units, the olefin feedstock (propylene, butylene, amylene, and fresh iso-butane) enters the reactor and contacts the concentrated sulfuric acid catalyst (in concentrations of 85-95% v/v for good operation and to minimize corrosion and any ensuing fouling). The reactor Recycle isobutane
Acid settler
Deisobutanizer
Reactor
Caustic scrubber
Feedstock
Alkylate
Recycle acid Fresh acid
Reject acid
FIGURE 15.3 The sulfuric acid alkylation process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
382 Fouling in Refineries Recycle isobutane Propane Fresh acid
Acid purifier
Depropanizer
Settler Feedstock (olefins, isobutane)
Deisobutanizer
Reactor
Alkylate Caustic washer
Acid oils
FIGURE 15.4 The hydrofluoric acid alkylation process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/ otm_iv_2.html.
is divided into zones, with olefins fed through distributors to each zone, and the sulfuric acid and iso-butane flowing over baffles from zone to zone. The reactor effluent is separated into hydrocarbon and acid phases in a settler (the acid is returned to the reactor) and the hydrocarbon phase is hot-water washed with caustic for pH control before being successively depropanized, deisobutanized, and debutanized. Alkylate obtained from the deisobutanizer can then be sent to the blending operation for gasoline production or be rerun to produce blending stock for aviation-grade gasoline. Hydrogen fluoride is also used for alkylation and the advantage of using hydrogen fluoride is that it is more readily separated and recovered from the product. However, hydrofluoric acid alkylation units require special engineering design, operator training, and safety equipment precautions to protect operators from accidental contact with hydrofluoric acid which is an extremely hazardous substance—even more so than sulfuric acid.
15.4.2 Fouling Fouling and corrosion in alkylation units are closely linked to feedstock quality and operating conditions. In fact, feedstock treating is the first line of defense in keeping contaminants including sulfur, water, diolefins, and other compounds, out of the unit. Since sulfuric acid and hydrofluoric acid are potentially hazardous chemicals, loss of coolant water, which is needed to maintain process temperatures, could result in a process upset leading to fouling through uncontrolled reactions. In the sulfuric acid-based alkylation process, the acid is continually cycled through the process; but as it cycles, it becomes diluted and contaminated from impurities in the hydrocarbon feeds. The alkylation reactors typically operate at temperatures 2-21 °C (35-70 °F, maximum) to minimize polymerization of
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the olefins to form undesirable hydrocarbons for the sulfuric acid process. The concentration of the sulfuric acid catalyst is important to the efficiency of the alkylation reaction, when the concentration of the acid decreases to approximately 88%, a portion of the contaminated acid is withdrawn and replaced with fresh acid. The contaminated, dilute sulfuric acid is then regenerated to its original purity and concentration. Corrosion and fouling in sulfuric acid units may occur from the breakdown of sulfuric acid esters or where caustic is added for neutralization. These esters can be removed by fresh acid treatment and hot-water washing. To prevent corrosion from hydrofluoric acid, the acid concentration inside the process unit should be maintained above 65% and moisture below 4%. In any case, if corrosion occurs, flakes and/or particle of metal can be expected to appear in the product leading to fouling of the reactor. Hydrofluoric acid exists in a vapor state at ambient conditions and this dictates that extreme precaution is necessary to ensure that this toxic substance is contained inside the process equipment. The hydrofluoric acid process, which is less sensitive to polymerization at warmer temperatures, typically operates at reactor temperatures of 21-38 °C (70-100 °F). Iso-butane concentrations are maintained very high (i.e., at ratios of 4:1 or more above the reaction requirements) in the reactor vessels to ensure that all of the olefins are reacted. The reactor effluent is distilled to separate the propane, iso-butane, and alkylate boiling fractions. The propane is routed to propane product treating, the iso-butane is recycled back to the alkylation reactors and the alkylate is routed to gasoline blending, or in some cases to additional solvent refinery processing. Water is a major contaminant that promotes corrosion in several ways— corrosion is a function of hydrogen fluoride concentration and temperature, and that high rates of corrosion can occur as the acid becomes more dilute. Also, the presence of water reduces acid strength and contributes to the formation of corrosive acid-soluble oils, which must be removed. In fact, dissolved polymerization products (which contribute to fouling) must be removed from the acid, usually as thick dark oil. Sulfur is one of several other contaminants that affect corrosion on the unit and hence contribute to fouling. Sulfur in the feedstock reduces the acid strength by reacting to form low-molecular-weight acid-soluble oil is difficult to remove in the acid regenerator and contributes to acid loss and corrosion. The effect of the sulfur compounds (such as hydrogen sulfide and carbonyl sulfide) is independent of the molecular form of the sulfur, so that the total sulfur content of the feedstock should be used in the estimation of the potential for corrosion and fouling. Diolefins such as butadiene (CH2CHCHCH2) enter with the olefin feed and react to form acid-soluble oils. The diene concentration in olefin feed streams have been increasing with the increase in reactor severity of fluid- catalytic cracking units and should be periodically monitored to ensure that they do not exceed recommended limits in the feedstock.
384 Fouling in Refineries
On the issue of corrosion (and corrosion fouling), oxygen dissolves in, and tends to stay with, the hydrofluoric acid and accelerates corrosion of reactor walls and pipes. Sources of oxygen include entry into the unit through imported feedstocks, wet gas compressors associated with the fluid-catalytic cracking unit, malfunction of a caustic treating unit, or using oxygen-contaminated nitrogen when charging fresh acid into the unit. Also, since iron fluoride is a product of the corrosion process, fluoride scale can return to the towers with the reflux and lay down on the trays (tower fouling) or in foul reboilers (reboiler fouling). Significant deposition may lead to accelerated tower fouling, impacting fractionation and resulting in the inability to maintain purity of the iso-butane. In addition, if acid carryover has occurred or if has exceeded 66 °C (150 °F), the tower top, trays, and overhead line may experience higher rates of corrosion and fouling. Sludge produced during the use of sulfuric acid as a treating agent is mainly of two types: (1) sludge from light oils (gasoline and kerosene) and (2) sludge from lubricating stocks, medicinal oils, and the like. In the treatment of the latter oils it appears that the action of the acid causes precipitation of asphaltene constituents and resin constituents, as well as the solution of color-bearing and sulfur compounds. Sulfonation and oxidation-reduction reactions also occur, but to a lesser extent since much of the acid can be recovered. In the desulfurization of cracked distillates, however, chemical interaction is more important, and polymerization, ester formation, aromatic-olefin condensation, and sulfonation also occur. Nitrogen bases are neutralized, and the acid dissolves naphthenic acids; thus the composition of the sludge is complex and depends largely on the oil treated, acid strength, and the temperature. Sulfuric acid sludge from iso-paraffin alkylation and lubricating oil treatment are frequently decomposed thermally to produce sulfur dioxide (which is returned to the sulfuric acid plant) and sludge acid coke. The coke, in the form of small pellets, is used as a substitute for charcoal in the manufacture of carbon disulfide. Sulfuric acid coke is different from other petroleum coke in that it is pyrophoric in air and also reacts directly with sulfur vapors to form carbon disulfide. Other foulants related to acid sludge include: (1) black acids, (2) brown acids, (3) liver oil, (4) mahogany acids, (5) mayonnaise, and (6) sulfonic acids. Unless removed, these by-products will aye fouling and serious disruption of the respective processes. Environmental disposal in an acceptable manner is an absolute necessity. By way of definition, black acid(s) [black soap(s)] is a mixture of the sulfonates found in acid sludge which are insoluble in naphtha, benzene, and carbon tetrachloride; highly soluble in water, but insoluble in 30% sulfuric acid; in the dry, oil-free state, the sodium soaps are black powders. Brown acids constitute the oil-soluble fraction of petroleum sulfonates found in acid sludge which can be recovered by extraction with naphtha solvent. Brown-acid sulfonates are somewhat similar to mahogany acids (mahogany sulfonates), but are more water soluble. In the dry, oil-free state, the sodium soaps are light-colored
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powders. Liver oil is the intermediate layer of dark-colored, oily material, insoluble in weak acid and in oil, which is formed when acid sludge is hydrolyzed. Mahogany acids are oil-soluble sulfonic acids formed by the action of sulfuric acid on petroleum distillates. They may be converted to their sodium soaps (mahogany soaps) and extracted from the oil with alcohol for use in the manufacture of soluble oils, rust preventives, and special greases. The calcium and barium soaps of these acids are used as detergent additives in motor oils. Mayonnaise is the colloquial name given to low-temperature (often acid) sludge which can be a black, brown, or gray deposit that has a soft, mayonnaise-like consistency. Sulfonic acids are acids obtained by of petroleum or a petroleum product with strong sulfuric acid.
15.5 POLYMERIZATION 15.5.1 General Aspects Polymerization (in the petroleum refining sense of the word) is used to convert propene and butene to high-octane naphtha to be sued as a gasoline blending component. The reactions typically take place under high pressure in the presence of a phosphoric acid catalyst. The feedstock must be (1) free of sulfur, which poisons the catalyst, (2) free of basic materials, which neutralize the catalyst; and oxygen, which affects the reactions—failure to take both issues into account will lead to reactor and catalyst fooling. The propene and butene feed is washed first with caustic to remove mercaptans (molecules containing sulfur), then with an amine solution to remove hydrogen sulfide, then with water to remove caustics and amines, and finally dried by passing through a silica gel or molecular sieve dryer. Thermal polymerization is not as effective as catalytic polymerization, but has the advantage that it can be used for saturated materials that cannot be induced to react by catalysts. The process consists of vapor-phase cracking of, for example, propane and butane followed by prolonged periods at high temperature (510-595 °C, 950-1100 °F) for the reactions to proceed to near completion (Figure 15.5). However, olefins can also be conveniently polymerized by means of an acid catalyst. Thus, the treated, olefin-rich feed stream is contacted with a catalyst (sulfuric acid, copper pyrophosphate, and phosphoric acid) at 150-220 °C (300-425 °F) and 150-1200 psi (10-81 atmospheres), depending on feedstock and product requirement. The feedstock usually consists of propylene (propene (CH2CH=CH2) and butylenes (butenes, various isomers of C4H8)) from cracking processes or might even be selective olefins for dimer, trimer, or tetramer production: nCH 2 = CH 2 ® H - ( CH 2 CH 2 )n - H In this process, n is usually 2 (dimer), 3 (trimer), or 4 (tetramer) the molecular size of the product is limited to give products boiling in the gasoline
386 Fouling in Refineries
Flash drum
C3/C4 olefin feed
RECYCLE DRUM
Stabilizer
Quench
C3/C4
Feed drum
Recycle Poly gasoline
FIGURE 15.5 Polymerization process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
range constituents. This is in contrast to polymerization that is carried out in the polymer industry where n may be on the order of several hundred. Thus, polymerization in the true sense of the word is usually prevented, and all attempts are made to terminate the reaction at the dimer or trimer (three monomers joined together) stage. The four-carbon to twelve-carbon compounds that are required as the constituents of liquid fuels are the prime products. However, in the petrochemical section of a refinery, polymerization, which results in the production of (for example) polyethylene, is allowed to proceed until the products having the required high molecular weight have been produced.
15.5.2 Fouling Phosphates are the principal catalysts used in the catalytic polymerization process—the commercially used catalysts are liquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The latter is the least active, but the most used and easiest one to regenerate by washing and recoating. The serious disadvantage is that tar must occasionally be burned off the support to remove any foulants. The process using liquid phosphoric acid catalyst is far more responsible to attempts to raise production by increasing temperature than the other processes.
15.6 PRODUCT BLENDING The modern petroleum refinery consists of a very complex mix of high- technology processes which efficiently convert the wide array of crude oils into hundreds of specification products we use daily. Each refinery has its own unique processing configuration as a result of the logistics and associated economics related to its specific crude oils and products markets. The refiner
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must continuously optimizing the mix of product volumes and this is accomplished through executing decisions regarding parameters as varied as crude oil feedstock selection, adjustments in product cut-points, and reactor severities in individual processes. Additional options include changing the dispositions of intermediate product streams to alternative processing units, or alternative finished product blends. In fact, many refinery products are typically the result of blending several component streams or blending stocks. In most cases, product blending is accomplished by controlling the volumes of blend stocks from individual component storage tanks that are mixed in the finished product storage tank. Samples of the finished blend are then analyzed by laboratory testing for all product specifications prior to shipping. Alternatively, in-line blending refers to pipeline shipments in which the finished product is actually blended directly into the product pipeline (as opposed to a standing product storage tank). The most commonly recognized blending operations occur in the gasoline production section of the refinery. The various gasoline streams are so that specifications (dependent upon geographic location, environmental regulations, and weather patterns) can be met. Gasoline blending involves combining of the components that make up motor gasoline. The components include the various hydrocarbon streams produced by distillation, cracking, reforming, and polymerization, tetraethyl lead, and identifying color dye, as well as other special-purpose components, such as solvent oil and anti-icing compounds. The physical process of blending the components is simple, but determination of how much of each component to include in a blend is much more difficult. The physical operation is carried out by simultaneously pumping all the components of a gasoline blend into a pipeline that leads to the gasoline storage, but the pumps must be set to deliver automatically the proper proportion of each component. Baffles in the pipeline are often used to mix the components as they travel to the storage tank. Selection of the components and their proportions in a blend is the most complex problem in a refinery. Many different hydrocarbon streams may need to be blended to produce quality gasoline. Each property of each stream is a variable, and the effect on the product gasoline is considerable. For example, the low-octane number of straight-run naphtha limits its use as a gasoline component, although its other properties may make it desirable. The problem is further complicated by changes in the properties of the component streams due to processing changes. For example, an increase in cracking temperature produces a smaller volume of higher octane cracked naphtha, but before this cracked naphtha can be included in a blend, adjustments must be made in the proportions of the other hydrocarbon components. Similarly, the introduction of new processes and changes in the specifications of the finished gasoline dictate reevaluation of the components that make up the gasoline (Gibbs, 1989). Gasoline blending is not the only blending operations and other product blending operations are also in operation in a refinery. The applicable
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s pecifications vary by product, but typically include properties pertinent to the behavior of the product in use. Many product specifications do not blend linearly by component volumes. In these circumstances, the finished blend properties are predicted using experience-based algorithms for the applicable blending components. The usual practice is to blend crude product fractions having similar characteristics, although fluctuations in the properties of the individual fractions may cause significant variations in the properties of the blend over a period of time. However, incompatibility of the blend stocks can occur if, for example, a paraffin-base blend stock is blended with an asphalt-base blend stock (especially in fuel oil production) which can cause sediment formation thereby complicating the product (Mushrush and Speight, 1995, 1998). Blending of two or more blend stocks can result unstable mixes which precipitate species such as resin constituents or, more likely asphaltene constituents and result in rapid fouling. For this reason, it is necessary to develop and use compatibility tests that will assist in predicting the proportions and the order of blending of products and avoid incompatibility under the relevant operating conditions (Saleh et al., 2005).
REFERENCES Bartholomew, C., 1982. Carbon deposition in steam reforming and methanation. Catal. Rev. Sci. Eng. 24, 67–112. Besson, M., Gallezot, P., 2003. Deactivation of metal catalysts in liquid phase organic reactions. Catal. Today 81, 547–559. Brons, G., Wiehe, I.A., 2000. Mechanism of reboiler fouling during reforming. Energy Fuel 14 (1), 2–5. Coll, R., Salvado, J., Farriol, X., Montané, D., 2001. Steam reforming model compounds of biomass gasification tars: conversion at different operating conditions and tendency towards coke formation. Fuel Process. Technol. 74, 19–31. Gary, J.H., Handwerk, G.E., Kaiser, M.J., 2007. Petroleum Refining: Technology and Economics, fourth ed. Marcel Dekker Inc., New York (Chapter 9). Gibbs, L.M., 1989. Gasoline. Oil Gas J. 87 (17), 60. Hsu, C.S., Robinson, P.R. (Eds.), 2006. Practical Advances in Petroleum Processing, vols. 1 and 2. Springer Science, New York. Mushrush, G.W., Speight, J.G., 1995. Petroleum Products: Instability and Incompatibility. Taylor & Francis, Washington, DC. Mushrush, G.W., Speight, J.G., 1998. Instability and incompatibility of petroleum products. In: Speight, J.G. (Ed.), Petroleum Chemistry and Refining. Taylor & Francis, Washington, DC (Chapter 8). Parkash, S., 2003. Refining Processes Handbook. Gulf Professional Publishing, Elsevier, Amsterdam, Netherlands. Saleh, Z.S., Sheikholeslami, R., Watkinson, A.P., 2005. Blending effect on fouling of four crude oils. In: Proceedings of the 6th International Conference on Heat Exchanger Fouling and Cleaning—Challenges and Opportunities (ICHEFCC0'05), Kloster Irsee, Germany. pp. 37–46. Speight, J.G., 2001. Handbook of Petroleum Analysis. John Wiley & Sons Inc., Hoboken, NJ.
Fouling During Product Improvement Processes Chapter | 15 389 Speight, J.G., 2005. Natural bitumen (tar sands) and heavy oil. Coal, Oil Shale, Natural Bitumen, Heavy Oil and Peat, from Encyclopedia of Life Support Systems (EOLSS). Developed under the Auspices of the UNESCO. EOLSS Publishers, Oxford, United Kingdom. http://www.eolss. net. Speight, J.G., 2008. Synthetic Fuels Handbook: Properties, Processes and Performance. McGrawHill, New York. Speight, J.G., 2011a. The Refinery of the Future. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G. (Ed.), 2011b. The Biofuels Handbook. Royal Society of Chemistry, London, United Kingdom. Speight, J.G., 2013. Heavy and Extra Heavy Oil Upgrading Technologies. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G., 2014a. The Chemistry and Technology of Petroleum, fifth ed. CRC Press, Taylor & Francis Group, Boca Raton, FL. Speight, J.G., 2014b. Oil and Gas Corrosion Prevention. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G., 2015. Handbook of Petroleum Product Analysis, second ed. John Wiley & Sons Inc., Hoboken, NJ. Speight, J.G., Ozum, B., 2002. Petroleum Refining Processes. Marcel Dekker Inc., New York. Zöhrer, H., Vogel, F., 2013. Hydrothermal catalytic gasification of fermentation residues from a biogas plant. Biomass Bioenergy 53, 138–148.
Fouling During Product Improvement Processes 15.1 INTRODUCTION Chemically, petroleum is an extremely complex mixture of hydrocarbon compounds, usually with minor amounts of nitrogen-, oxygen-, and sulfur-containing compounds as well as trace amounts of metal-containing compounds, while on the other hand heavy feedstocks (residua, heavy oil, extra heavy oil, and tar sand bitumen) are even more complex (Chapter 2) (Speight, 2001, 2005, 2013, 2014a, 2015). In addition, the properties of these feedstocks vary widely and are not conducive to modern-day use in the raw state. A variety of processing steps are required to convert these feedstocks to saleable products (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2011a, 2014a). The production of liquid product streams by distillation (Chapter 9) or by cracking processes (Chapters 10-13) are only the first of a series of steps that leads to the production of marketable products. Several other unit processes are involved in the production of a final product and such processes may be generally termed product improvement processes since they are not used directly on the raw feedstock, but are used on primary product streams that have been produced from the feedstock (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014a,b). It is, therefore, the purpose of this chapter to present the concepts behind these secondary processes with examples of where fouling might occur.
15.2 REFORMING 15.2.1 General Aspects Reforming processes (molecular rearrangement processes) are processes in which the molecular structure of the feedstock is reorganized to enhance the properties of the product thereby changing the properties of the product relative to the feedstock (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2008, 2014a). In the process, there are four major types of chemical reactions which occur: (1) dehydrocyclization of paraffin Fouling in Refineries. http://dx.doi.org/10.1016/B978-0-12-800777-8.00015-2 © 2015 Elsevier Inc. All rights reserved.
375
376 Fouling in Refineries
TABLE 15.1 Composition of Catalytic Reformer Product Gas Constituent
% by Volume
Hydrogen
75-85
Methane
5-10
Ethane
5-10
Propane
5-10
Butane
<5
Pentane and higher
<2
compounds to aromatic compounds, (2) hydrocracking of high-molecular-weight paraffins to low-molecular-weight paraffins, (3) isomerization of n -paraffins to iso-paraffins, and (4) dehydrogenation of naphthenes to aromatics. In fact, dehydrogenation is a main chemical reaction in catalytic reforming, and hydrogen gas is consequently produced in large quantities (Table 15.1). The hydrogen is recycled through the reactors where reforming takes place to provide the atmosphere necessary for the chemical reactions and also prevents the carbon from being deposited on the catalyst, thus extending its operating life. An excess of hydrogen above whatever is consumed in the process is produced, and as a result, catalytic reforming processes are unique in that they are the only petroleum refinery processes to produce hydrogen as a by-product. In the thermal reforming process, a feedstock such as naphtha (end point: 205 °C, 400 °F) or straight-run gasoline is heated to 510-595 °C (950-1100 °F) in a furnace with pressures on the order of 400-1000 psi. As the heated naphtha leaves the furnace, it is cooled or quenched by the addition of cold naphtha. The material then enters a fractional distillation tower where any heavy products are separated. The remainder of the reformed material leaves the top of the tower to be separated into gases and higher octane reformate. Because of the high temperature that is the potential for even the lowest molecular weight feedstock to produce coke (from any aromatic constituents or aromatic constituents formed during the process). Catalytic reforming processes (moving-bed processes, fluid-bed processes, and fixed-bed processes) which have largely replaced thermal reforming processes, use catalytic reactions to process primarily low-octane heavy straight-run (from the crude distillation unit) naphtha into high-octane aromatic naphtha. Like the thermal reforming process, the catalytic reforming process (Figure 15.1) converts low-octane naphtha into high-octane naphtha (reformate). When thermal reforming could produce reformate with research octane numbers of 65-80 depending on the yield, catalytic reforming produces reformate with octane numbers on the order of 90-95. Catalytic reforming is
Fouling During Product Improvement Processes Chapter | 15 377
Reactor
Reactor
Reactor
Feedstock Furnace
Furnace
Furnace
Fractionator
Light hydrocarbons Hydrogen Recycle
Separator Reformate
FIGURE 15.1 The platforming process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
conducted in the presence of hydrogen over hydrogenation-dehydrogenation catalysts, which may be supported on alumina or silica-alumina. Depending on the catalyst, a definite sequence of reactions takes place, involving structural changes in the feedstock. The moving-bed and fluid-bed processes use mixed nonprecious metal oxide catalysts in units equipped with separate regeneration facilities. Fixed-bed processes use predominantly platinum-containing catalysts in units equipped for cycle, occasional, or no regeneration. The catalysts used in catalytic reforming processes are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions, that is, the production of aromatics, participation in hydrocracking, and rapid hydrogenation of carbon-forming precursors. In order for the catalyst to have an activity for isomerization of both paraffins and naphthenes and to participate in paraffin dehydrocyclization, it must have an acid activity. The balance between these two activities is most important in a reforming catalyst. In fact, in the production of aromatics from cyclic saturated materials (naphthenes), it is important that hydrocracking be minimized to avoid loss of the desired product and, thus, the catalytic activity must be moderated relative to the case of gasoline production from a paraffinic feed, where dehydrocyclization and hydrocracking play an important part.
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15.2.2 Fouling The most common impurities that give rise to process fouling are sulfur compounds that are derived from the sulfur compounds that occur in crude oil (Speight, 2014a), such as sulfide derivatives (RSR1) and the foul-smelling mercaptan derivatives (thiol derivatives, RSH). Oxygen compounds in the form of carboxylic acids (RCO2H) and phenols (ArOH, where Ar is an aromatic group) may also be present, especially in fractions derived from high-acid crude oils (Chapter 2). Nitrogen-containing compounds derived from those that occur in crude oil are also present. Furthermore, olefins (RCHCHR1) must also be eliminated from a feedstock or aromatics removed from a solvent, and these olefins and aromatics are considered impurities. Similarly, polymerized material, asphaltic material, or resin constituents may be impurities, depending on whether their presence in a finished product is harmful. Under the high-hydrogen partial pressure conditions used in catalytic reforming, sulfur compounds are readily converted into hydrogen sulfide, which, unless removed, builds up to a high concentration in the recycle gas and influences the performance of the catalyst (catalyst fouling). Hydrogen sulfide is a reversible poison for platinum and causes a decrease in the catalyst dehydrogenation and dehydrocyclization activities. In the first catalytic reformers, the hydrogen sulfide was removed from the gas cycle stream by absorption in, for example, diethanolamine. Sulfur is generally removed from the feedstock by the use of a conventional desulfurization over cobalt-molybdenum catalyst. An additional benefit of desulfurization of the feed to a level of <5 ppm sulfur is the elimination of hydrogen sulfide (H2S) corrosion problems (corrosion fouling) in the heaters and reactors. Thus, for the process, the feedstock must be clear of any sulfur-containing contaminants otherwise fouling of the catalyst will occur with a rapid decrease in catalyst activity. In addition, like the thermal reforming process, the high temperature employed for catalytic reforming raised the potential for even the lowest molecular weight feedstock to produce coke (from any aromatic constituents or aromatic constituents formed during the process) leading to catalyst fouling. Organic nitrogen compounds are converted into ammonia under reforming conditions, and this neutralizes acid sites on the catalyst (catalyst fouling) and thus represses the activity for isomerization, hydrocracking, and dehydrocyclization reactions. Straight-run materials do not usually present serious problems with regard to nitrogen, but feeds such as coker naphtha may contain around 50 ppm nitrogen and removal of this quantity may require high-pressure hydrogenation (800-1000 psi) over nickel-cobalt-molybdenum on an alumina catalyst. The yield of gasoline of a given octane number and at given operating conditions depends on the hydrocarbon types in the feed. For example, high- naphthene stocks, which readily give aromatic gasoline, are the easiest to reform and give the highest gasoline yields. Paraffinic stocks, however, which depend on the more difficult isomerization, dehydrocyclization, and hydrocracking
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reactions, require more severe conditions and give lower gasoline yields than the naphthenic stocks. The end point of the feed is usually limited to about 190 °C (375 °F), partially because of increased coke deposition on the catalyst as the end point during processing at about 15 °C (27 °F) (catalyst fouling). The efficiency of the catalytic process is currently limited by catalyst deactivation, mainly due to sulfur poisoning and fouling of the catalyst. Regeneration protocols will be necessary to enhance catalysts lifetime and therefore increase the overall process efficiency. Besides catalyst deactivation by sulfur poisoning, catalyst fouling through the deposition of coke and intermediate reaction products on the active metal and in the pores of the catalyst support (pore mouth poisoning/plugging) is another typical pathway of catalyst deactivation. This has been observed in various noble metal catalysts used in steam reforming, liquid phase hydrogenation, and hydrothermal reforming of bio-feedstocks (Bartholomew, 1982; Coll et al., 2001; Besson and Gallezot, 2003; Zöhrer and Vogel, 2013). This is especially true if biomass is used as a co-feedstock (Coll et al., 2001; Speight, 2011b). Deactivation by coking and fouling is often remedied by treatment with steam or hydrogen at high temperatures which, however, requires sufficiently stable catalyst supports and adequate reactors. Activated carbon, a catalyst support often used in hydrothermal reforming of organics due to its stability under these reaction conditions, presents a highly porous structure that is very susceptible to fouling and coking through entrapment of reactants in its micro- and mesopores, leading to coke formation and subsequent plugging of these pores. Furthermore, coke and tar formation can be a major issue during processing of organic feedstocks such as glycerol, glucose, or fermentation residues in suband supercritical water. A related process, steam reforming that is used to convert hydrocarbons such as methane and naphtha to hydrogen and carbon monoxide (Speight, 2014a) also deserves comment here. Large-scale reformers use naphtha as the feedstock in petrochemical plants, but higher molecular weight feedstocks (such as kerosene and light gas oil) are less suitable for steam reforming because of their higher sulfur content and propensity to cause carbon fouling of the catalysts. In addition to catalyst fouling, reboiler fouling has been identified during reformer operations—the result of coke laydown (Brons and Wiehe, 2000).
15.3 ISOMERIZATION 15.3.1 General Aspects Isomerization processes are used to provide additional feedstock (alkylate) for alkylation units or high-octane naphtha as gasoline blending stock. The reaction may take place in the vapor phase, with the activated catalyst supported on a solid phase, or in the liquid phase with a dissolved catalyst. Straight-chain paraffins (n-butane, n-pentane, n-hexane) are converted to respective iso-compounds
380 Fouling in Refineries Reactor Hydrogen make-up
Hydrogen recycle
High pressure separator
Stripper
Fuel gas
Off gas
Unstabilized light distillate
Feed Desulfurized product FIGURE 15.2 Butane isomerization. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
by continuous catalytic (aluminum chloride, noble metals) processes which yields naphtha components with a high-octane rating in this lower boiling range (Speight, 2014a). For example, in the process, pure butane or a mixture of isomeric butanes (Figure 15.2), is mixed with hydrogen (to inhibit olefin formation) and passed to the reactor, at 110-170 °C (230-340 °F) and 200-300 psi. The product is cooled, the hydrogen separated and the cracked gases are then removed in a stabilizer column. The stabilizer bottom product is passed to a super-fractionator where the normal butane is separated from the iso-butane. More modern processes use supported metal catalysts, have been developed, for use in high-temperature processes which operate in the range 370-480 °C (700-900 °F) and 300-750 psi (20-51 atmospheres), while aluminum chloride plus hydrogen chloride are universally used for the low-temperature processes. Nonregenerable aluminum chloride catalyst is employed with various carriers in a fixed-bed or liquid contactor. Platinum or other metal catalyst processes utilized fixed-bed operation and can be regenerable or nonregenerable. The reaction conditions vary widely depending on the particular process and feedstock, 40-480°C (100-900°F) and 150-1000 psi (10-68 atmospheres).
15.3.2 Fouling Isomerization reactions are usually reversible reactions and attain equilibrium at lower temperature with highest concentration of isomer products. In the process, as in any process, it is essential to inhibit side reactions (such as cracking and olefin formation) that will lead to catalyst fouling and, hence, loss of catalyst activity. The role of catalyst in isomerization is, therefore, extremely important. The intensity of unwanted side reactions diminishes at lower temperatures as higher temperature favors unwanted cracking, hydrogenation, and polymerization reactions. For that reason, isomerizing catalysts must ensure the optimal rate of reactions at as low temperature as possible. To prevent coke deposition,
Fouling During Product Improvement Processes Chapter | 15 381
isomerization is carried out at an elevated pressure in a hydrogen atmosphere. Industrial processes are carried out at a temperature of 400-480 °C (750-895 °F). In addition, Friedel-Crafts reactions that are by aluminum chloride are often responsible for fouling observed on the shell side of a reformer reboiler unit (Brons and Wiehe, 2000). In addition, corrosion products such as iron oxide and/or iron sulfide (Speight, 2014b) can increase the lay-down and subsequent coking of high-molecular-weight constituents that are formed as by-products from the aluminum chloride reactions. Under the conditions of the reaction, the high-molecular-weight materials tend to deposit on the reboiler tubes and degrade to coke over time. Reduction of this type of fouling could be achieved by reducing system corrosion (Brons and Wiehe, 2000).
15.4 ALKYLATION 15.4.1 General Aspects Alkylation processes combine low-molecular-weight olefins (primarily a mixture of propylene and butylene) with isobutene in the presence of a catalyst, either sulfuric acid (Figure 15.3) or hydrofluoric acid (Figure 15.4) (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al., 2007; Speight, 2014a). The product (alkylate) and is composed of a mixture of high octane, branched-chain paraffinic hydrocarbons and is a premium blending stock for gasoline manufacturing because it has exceptional antiknock properties and is clean burning. In cascade-type sulfuric acid (H2SO4) alkylation units, the olefin feedstock (propylene, butylene, amylene, and fresh iso-butane) enters the reactor and contacts the concentrated sulfuric acid catalyst (in concentrations of 85-95% v/v for good operation and to minimize corrosion and any ensuing fouling). The reactor Recycle isobutane
Acid settler
Deisobutanizer
Reactor
Caustic scrubber
Feedstock
Alkylate
Recycle acid Fresh acid
Reject acid
FIGURE 15.3 The sulfuric acid alkylation process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
382 Fouling in Refineries Recycle isobutane Propane Fresh acid
Acid purifier
Depropanizer
Settler Feedstock (olefins, isobutane)
Deisobutanizer
Reactor
Alkylate Caustic washer
Acid oils
FIGURE 15.4 The hydrofluoric acid alkylation process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/ otm_iv_2.html.
is divided into zones, with olefins fed through distributors to each zone, and the sulfuric acid and iso-butane flowing over baffles from zone to zone. The reactor effluent is separated into hydrocarbon and acid phases in a settler (the acid is returned to the reactor) and the hydrocarbon phase is hot-water washed with caustic for pH control before being successively depropanized, deisobutanized, and debutanized. Alkylate obtained from the deisobutanizer can then be sent to the blending operation for gasoline production or be rerun to produce blending stock for aviation-grade gasoline. Hydrogen fluoride is also used for alkylation and the advantage of using hydrogen fluoride is that it is more readily separated and recovered from the product. However, hydrofluoric acid alkylation units require special engineering design, operator training, and safety equipment precautions to protect operators from accidental contact with hydrofluoric acid which is an extremely hazardous substance—even more so than sulfuric acid.
15.4.2 Fouling Fouling and corrosion in alkylation units are closely linked to feedstock quality and operating conditions. In fact, feedstock treating is the first line of defense in keeping contaminants including sulfur, water, diolefins, and other compounds, out of the unit. Since sulfuric acid and hydrofluoric acid are potentially hazardous chemicals, loss of coolant water, which is needed to maintain process temperatures, could result in a process upset leading to fouling through uncontrolled reactions. In the sulfuric acid-based alkylation process, the acid is continually cycled through the process; but as it cycles, it becomes diluted and contaminated from impurities in the hydrocarbon feeds. The alkylation reactors typically operate at temperatures 2-21 °C (35-70 °F, maximum) to minimize polymerization of
Fouling During Product Improvement Processes Chapter | 15 383
the olefins to form undesirable hydrocarbons for the sulfuric acid process. The concentration of the sulfuric acid catalyst is important to the efficiency of the alkylation reaction, when the concentration of the acid decreases to approximately 88%, a portion of the contaminated acid is withdrawn and replaced with fresh acid. The contaminated, dilute sulfuric acid is then regenerated to its original purity and concentration. Corrosion and fouling in sulfuric acid units may occur from the breakdown of sulfuric acid esters or where caustic is added for neutralization. These esters can be removed by fresh acid treatment and hot-water washing. To prevent corrosion from hydrofluoric acid, the acid concentration inside the process unit should be maintained above 65% and moisture below 4%. In any case, if corrosion occurs, flakes and/or particle of metal can be expected to appear in the product leading to fouling of the reactor. Hydrofluoric acid exists in a vapor state at ambient conditions and this dictates that extreme precaution is necessary to ensure that this toxic substance is contained inside the process equipment. The hydrofluoric acid process, which is less sensitive to polymerization at warmer temperatures, typically operates at reactor temperatures of 21-38 °C (70-100 °F). Iso-butane concentrations are maintained very high (i.e., at ratios of 4:1 or more above the reaction requirements) in the reactor vessels to ensure that all of the olefins are reacted. The reactor effluent is distilled to separate the propane, iso-butane, and alkylate boiling fractions. The propane is routed to propane product treating, the iso-butane is recycled back to the alkylation reactors and the alkylate is routed to gasoline blending, or in some cases to additional solvent refinery processing. Water is a major contaminant that promotes corrosion in several ways— corrosion is a function of hydrogen fluoride concentration and temperature, and that high rates of corrosion can occur as the acid becomes more dilute. Also, the presence of water reduces acid strength and contributes to the formation of corrosive acid-soluble oils, which must be removed. In fact, dissolved polymerization products (which contribute to fouling) must be removed from the acid, usually as thick dark oil. Sulfur is one of several other contaminants that affect corrosion on the unit and hence contribute to fouling. Sulfur in the feedstock reduces the acid strength by reacting to form low-molecular-weight acid-soluble oil is difficult to remove in the acid regenerator and contributes to acid loss and corrosion. The effect of the sulfur compounds (such as hydrogen sulfide and carbonyl sulfide) is independent of the molecular form of the sulfur, so that the total sulfur content of the feedstock should be used in the estimation of the potential for corrosion and fouling. Diolefins such as butadiene (CH2CHCHCH2) enter with the olefin feed and react to form acid-soluble oils. The diene concentration in olefin feed streams have been increasing with the increase in reactor severity of fluid- catalytic cracking units and should be periodically monitored to ensure that they do not exceed recommended limits in the feedstock.
384 Fouling in Refineries
On the issue of corrosion (and corrosion fouling), oxygen dissolves in, and tends to stay with, the hydrofluoric acid and accelerates corrosion of reactor walls and pipes. Sources of oxygen include entry into the unit through imported feedstocks, wet gas compressors associated with the fluid-catalytic cracking unit, malfunction of a caustic treating unit, or using oxygen-contaminated nitrogen when charging fresh acid into the unit. Also, since iron fluoride is a product of the corrosion process, fluoride scale can return to the towers with the reflux and lay down on the trays (tower fouling) or in foul reboilers (reboiler fouling). Significant deposition may lead to accelerated tower fouling, impacting fractionation and resulting in the inability to maintain purity of the iso-butane. In addition, if acid carryover has occurred or if has exceeded 66 °C (150 °F), the tower top, trays, and overhead line may experience higher rates of corrosion and fouling. Sludge produced during the use of sulfuric acid as a treating agent is mainly of two types: (1) sludge from light oils (gasoline and kerosene) and (2) sludge from lubricating stocks, medicinal oils, and the like. In the treatment of the latter oils it appears that the action of the acid causes precipitation of asphaltene constituents and resin constituents, as well as the solution of color-bearing and sulfur compounds. Sulfonation and oxidation-reduction reactions also occur, but to a lesser extent since much of the acid can be recovered. In the desulfurization of cracked distillates, however, chemical interaction is more important, and polymerization, ester formation, aromatic-olefin condensation, and sulfonation also occur. Nitrogen bases are neutralized, and the acid dissolves naphthenic acids; thus the composition of the sludge is complex and depends largely on the oil treated, acid strength, and the temperature. Sulfuric acid sludge from iso-paraffin alkylation and lubricating oil treatment are frequently decomposed thermally to produce sulfur dioxide (which is returned to the sulfuric acid plant) and sludge acid coke. The coke, in the form of small pellets, is used as a substitute for charcoal in the manufacture of carbon disulfide. Sulfuric acid coke is different from other petroleum coke in that it is pyrophoric in air and also reacts directly with sulfur vapors to form carbon disulfide. Other foulants related to acid sludge include: (1) black acids, (2) brown acids, (3) liver oil, (4) mahogany acids, (5) mayonnaise, and (6) sulfonic acids. Unless removed, these by-products will aye fouling and serious disruption of the respective processes. Environmental disposal in an acceptable manner is an absolute necessity. By way of definition, black acid(s) [black soap(s)] is a mixture of the sulfonates found in acid sludge which are insoluble in naphtha, benzene, and carbon tetrachloride; highly soluble in water, but insoluble in 30% sulfuric acid; in the dry, oil-free state, the sodium soaps are black powders. Brown acids constitute the oil-soluble fraction of petroleum sulfonates found in acid sludge which can be recovered by extraction with naphtha solvent. Brown-acid sulfonates are somewhat similar to mahogany acids (mahogany sulfonates), but are more water soluble. In the dry, oil-free state, the sodium soaps are light-colored
Fouling During Product Improvement Processes Chapter | 15 385
powders. Liver oil is the intermediate layer of dark-colored, oily material, insoluble in weak acid and in oil, which is formed when acid sludge is hydrolyzed. Mahogany acids are oil-soluble sulfonic acids formed by the action of sulfuric acid on petroleum distillates. They may be converted to their sodium soaps (mahogany soaps) and extracted from the oil with alcohol for use in the manufacture of soluble oils, rust preventives, and special greases. The calcium and barium soaps of these acids are used as detergent additives in motor oils. Mayonnaise is the colloquial name given to low-temperature (often acid) sludge which can be a black, brown, or gray deposit that has a soft, mayonnaise-like consistency. Sulfonic acids are acids obtained by of petroleum or a petroleum product with strong sulfuric acid.
15.5 POLYMERIZATION 15.5.1 General Aspects Polymerization (in the petroleum refining sense of the word) is used to convert propene and butene to high-octane naphtha to be sued as a gasoline blending component. The reactions typically take place under high pressure in the presence of a phosphoric acid catalyst. The feedstock must be (1) free of sulfur, which poisons the catalyst, (2) free of basic materials, which neutralize the catalyst; and oxygen, which affects the reactions—failure to take both issues into account will lead to reactor and catalyst fooling. The propene and butene feed is washed first with caustic to remove mercaptans (molecules containing sulfur), then with an amine solution to remove hydrogen sulfide, then with water to remove caustics and amines, and finally dried by passing through a silica gel or molecular sieve dryer. Thermal polymerization is not as effective as catalytic polymerization, but has the advantage that it can be used for saturated materials that cannot be induced to react by catalysts. The process consists of vapor-phase cracking of, for example, propane and butane followed by prolonged periods at high temperature (510-595 °C, 950-1100 °F) for the reactions to proceed to near completion (Figure 15.5). However, olefins can also be conveniently polymerized by means of an acid catalyst. Thus, the treated, olefin-rich feed stream is contacted with a catalyst (sulfuric acid, copper pyrophosphate, and phosphoric acid) at 150-220 °C (300-425 °F) and 150-1200 psi (10-81 atmospheres), depending on feedstock and product requirement. The feedstock usually consists of propylene (propene (CH2CH=CH2) and butylenes (butenes, various isomers of C4H8)) from cracking processes or might even be selective olefins for dimer, trimer, or tetramer production: nCH 2 = CH 2 ® H - ( CH 2 CH 2 )n - H In this process, n is usually 2 (dimer), 3 (trimer), or 4 (tetramer) the molecular size of the product is limited to give products boiling in the gasoline
386 Fouling in Refineries
Flash drum
C3/C4 olefin feed
RECYCLE DRUM
Stabilizer
Quench
C3/C4
Feed drum
Recycle Poly gasoline
FIGURE 15.5 Polymerization process. Source: OSHA Technical Manual, Section IV, Chapter 2: Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html.
range constituents. This is in contrast to polymerization that is carried out in the polymer industry where n may be on the order of several hundred. Thus, polymerization in the true sense of the word is usually prevented, and all attempts are made to terminate the reaction at the dimer or trimer (three monomers joined together) stage. The four-carbon to twelve-carbon compounds that are required as the constituents of liquid fuels are the prime products. However, in the petrochemical section of a refinery, polymerization, which results in the production of (for example) polyethylene, is allowed to proceed until the products having the required high molecular weight have been produced.
15.5.2 Fouling Phosphates are the principal catalysts used in the catalytic polymerization process—the commercially used catalysts are liquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The latter is the least active, but the most used and easiest one to regenerate by washing and recoating. The serious disadvantage is that tar must occasionally be burned off the support to remove any foulants. The process using liquid phosphoric acid catalyst is far more responsible to attempts to raise production by increasing temperature than the other processes.
15.6 PRODUCT BLENDING The modern petroleum refinery consists of a very complex mix of high- technology processes which efficiently convert the wide array of crude oils into hundreds of specification products we use daily. Each refinery has its own unique processing configuration as a result of the logistics and associated economics related to its specific crude oils and products markets. The refiner
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must continuously optimizing the mix of product volumes and this is accomplished through executing decisions regarding parameters as varied as crude oil feedstock selection, adjustments in product cut-points, and reactor severities in individual processes. Additional options include changing the dispositions of intermediate product streams to alternative processing units, or alternative finished product blends. In fact, many refinery products are typically the result of blending several component streams or blending stocks. In most cases, product blending is accomplished by controlling the volumes of blend stocks from individual component storage tanks that are mixed in the finished product storage tank. Samples of the finished blend are then analyzed by laboratory testing for all product specifications prior to shipping. Alternatively, in-line blending refers to pipeline shipments in which the finished product is actually blended directly into the product pipeline (as opposed to a standing product storage tank). The most commonly recognized blending operations occur in the gasoline production section of the refinery. The various gasoline streams are so that specifications (dependent upon geographic location, environmental regulations, and weather patterns) can be met. Gasoline blending involves combining of the components that make up motor gasoline. The components include the various hydrocarbon streams produced by distillation, cracking, reforming, and polymerization, tetraethyl lead, and identifying color dye, as well as other special-purpose components, such as solvent oil and anti-icing compounds. The physical process of blending the components is simple, but determination of how much of each component to include in a blend is much more difficult. The physical operation is carried out by simultaneously pumping all the components of a gasoline blend into a pipeline that leads to the gasoline storage, but the pumps must be set to deliver automatically the proper proportion of each component. Baffles in the pipeline are often used to mix the components as they travel to the storage tank. Selection of the components and their proportions in a blend is the most complex problem in a refinery. Many different hydrocarbon streams may need to be blended to produce quality gasoline. Each property of each stream is a variable, and the effect on the product gasoline is considerable. For example, the low-octane number of straight-run naphtha limits its use as a gasoline component, although its other properties may make it desirable. The problem is further complicated by changes in the properties of the component streams due to processing changes. For example, an increase in cracking temperature produces a smaller volume of higher octane cracked naphtha, but before this cracked naphtha can be included in a blend, adjustments must be made in the proportions of the other hydrocarbon components. Similarly, the introduction of new processes and changes in the specifications of the finished gasoline dictate reevaluation of the components that make up the gasoline (Gibbs, 1989). Gasoline blending is not the only blending operations and other product blending operations are also in operation in a refinery. The applicable
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s pecifications vary by product, but typically include properties pertinent to the behavior of the product in use. Many product specifications do not blend linearly by component volumes. In these circumstances, the finished blend properties are predicted using experience-based algorithms for the applicable blending components. The usual practice is to blend crude product fractions having similar characteristics, although fluctuations in the properties of the individual fractions may cause significant variations in the properties of the blend over a period of time. However, incompatibility of the blend stocks can occur if, for example, a paraffin-base blend stock is blended with an asphalt-base blend stock (especially in fuel oil production) which can cause sediment formation thereby complicating the product (Mushrush and Speight, 1995, 1998). Blending of two or more blend stocks can result unstable mixes which precipitate species such as resin constituents or, more likely asphaltene constituents and result in rapid fouling. For this reason, it is necessary to develop and use compatibility tests that will assist in predicting the proportions and the order of blending of products and avoid incompatibility under the relevant operating conditions (Saleh et al., 2005).
REFERENCES Bartholomew, C., 1982. Carbon deposition in steam reforming and methanation. Catal. Rev. Sci. Eng. 24, 67–112. Besson, M., Gallezot, P., 2003. Deactivation of metal catalysts in liquid phase organic reactions. Catal. Today 81, 547–559. Brons, G., Wiehe, I.A., 2000. Mechanism of reboiler fouling during reforming. Energy Fuel 14 (1), 2–5. Coll, R., Salvado, J., Farriol, X., Montané, D., 2001. Steam reforming model compounds of biomass gasification tars: conversion at different operating conditions and tendency towards coke formation. Fuel Process. Technol. 74, 19–31. Gary, J.H., Handwerk, G.E., Kaiser, M.J., 2007. Petroleum Refining: Technology and Economics, fourth ed. Marcel Dekker Inc., New York (Chapter 9). Gibbs, L.M., 1989. Gasoline. Oil Gas J. 87 (17), 60. Hsu, C.S., Robinson, P.R. (Eds.), 2006. Practical Advances in Petroleum Processing, vols. 1 and 2. Springer Science, New York. Mushrush, G.W., Speight, J.G., 1995. Petroleum Products: Instability and Incompatibility. Taylor & Francis, Washington, DC. Mushrush, G.W., Speight, J.G., 1998. Instability and incompatibility of petroleum products. In: Speight, J.G. (Ed.), Petroleum Chemistry and Refining. Taylor & Francis, Washington, DC (Chapter 8). Parkash, S., 2003. Refining Processes Handbook. Gulf Professional Publishing, Elsevier, Amsterdam, Netherlands. Saleh, Z.S., Sheikholeslami, R., Watkinson, A.P., 2005. Blending effect on fouling of four crude oils. In: Proceedings of the 6th International Conference on Heat Exchanger Fouling and Cleaning—Challenges and Opportunities (ICHEFCC0'05), Kloster Irsee, Germany. pp. 37–46. Speight, J.G., 2001. Handbook of Petroleum Analysis. John Wiley & Sons Inc., Hoboken, NJ.
Fouling During Product Improvement Processes Chapter | 15 389 Speight, J.G., 2005. Natural bitumen (tar sands) and heavy oil. Coal, Oil Shale, Natural Bitumen, Heavy Oil and Peat, from Encyclopedia of Life Support Systems (EOLSS). Developed under the Auspices of the UNESCO. EOLSS Publishers, Oxford, United Kingdom. http://www.eolss. net. Speight, J.G., 2008. Synthetic Fuels Handbook: Properties, Processes and Performance. McGrawHill, New York. Speight, J.G., 2011a. The Refinery of the Future. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G. (Ed.), 2011b. The Biofuels Handbook. Royal Society of Chemistry, London, United Kingdom. Speight, J.G., 2013. Heavy and Extra Heavy Oil Upgrading Technologies. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G., 2014a. The Chemistry and Technology of Petroleum, fifth ed. CRC Press, Taylor & Francis Group, Boca Raton, FL. Speight, J.G., 2014b. Oil and Gas Corrosion Prevention. Gulf Professional Publishing, Elsevier, Oxford, United Kingdom. Speight, J.G., 2015. Handbook of Petroleum Product Analysis, second ed. John Wiley & Sons Inc., Hoboken, NJ. Speight, J.G., Ozum, B., 2002. Petroleum Refining Processes. Marcel Dekker Inc., New York. Zöhrer, H., Vogel, F., 2013. Hydrothermal catalytic gasification of fermentation residues from a biogas plant. Biomass Bioenergy 53, 138–148.