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Ind. Eng. Chem. Res. 2003, 42, 5526-5534
Fixed-Bed Alkylation Using SLP-Type Catalyst in a Chromatographic-Type Reactor Concept Sven I. Hommeltoft* Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark
Isobutane alkylation plays an important role in modern FCC-based refineries, where it serves the dual purpose of disposing of the LPG cut from the FCC and providing a source of highly desirable stock for gasoline blending. Concern about the processes used in the refineries today has resulted in the development of the fixed-bed alkylation (FBA) technology, which is a compromise between the chemistry of isobutane alkylation and the desire for safe catalyst handling. The FBA reactor uses an SLP-type catalyst, which consists of a strong Brønsted acid supported on a fixed support bed in a well-defined catalyst zone. The acid slowly migrates through the support bed as the reaction progresses. Such reactors have been tested in three differently sized pieces of equipment, including a 10 bpd scale-up pilot, which was used to demonstrate the feasibility of scaling up of the reactor. Together with the FBA reactor technology, a new on-site catalyst recovery technology centered around a stripping tower and new reactor effluent treatment technology have been developed. The FBA technology is sturdy and flexible with respect to both operating conditions and feedstock and can also be adapted to make products other than alkylate gasoline, thereby expanding the use of alkylation beyond gasoline production into the production of middle distillates for jet and diesel fuels. Introduction Alkylation of isobutane by light olefins such as butenes and propene was initially introduced into the first refineries around the beginning of World War II.1-4 At that time, it was an important source of high-octane aviation gasoline for allied warplanes, but since the end of World War II, isobutane alkylate has found increasing use as a blending component for motor gasoline. Alkylate is typically produced from the LPG cut from fluid catalytic cracker (FCC) units, and the alkylation unit plays an important role as a disposal unit for the C3-C4 cut from the FCC.5 Alkylate is a highly desirable blending component for motor gasoline. Isobutane alkylate has a high motor octane number and an ideal boiling point range for motor gasoline. It contains no aromatic compounds or olefins and is essentially free of sulfur and other impurities. Overall, isobutane alkylate is close to being the ideal blending component for reformulated gasoline at a time when the specifications are being tightened to make motor gasoline cleaner and less toxic to the environment and to man. Since the introduction of the first commercial unit in the late 1930s, commercial technologies have been based on the use of either sulfuric acid or anhydrous HF catalysts. The use of often very large quantities of these strong Brønsted acids in isobutane alkylation service has been the cause of safety and environmental concerns. In response to these concerns, considerable effort has been focused on the development of alternative safer and more environmentally benign technologies. Solid catalyst systems have received much attention over a long period of time,6,7 but solid catalysts have some inherent weaknesses that make them difficult to use for isobutane alkylation outside the laboratory. The new technologies in the area of isobutane alkylation that have been successful enough to reach pilot scale have, * E-mail:
[email protected]. Fax: (+45) 45 27 2999.
in most cases, involved some kind of compromise between the desire for increased safety and the alkylation chemistry, which favors liquid catalysts.5 The fixed-bed alkylation (FBA) technology, which was developed as an alternative to the established technologies, is an example of such a compromise. It employs a supported liquid phase (SLP) type catalyst consisting of a liquid Brønsted acid supported on a porous support material.8-11 The SLP-type catalyst system is based on the same chemical reaction and thus has the same selectivity as is known for the liquid catalyst systems. The use of a solid support provides better control of the liquid catalyst, thereby improving the safety. This type of catalyst system is flexible and versatile, with potential applications reaching well beyond isobutane alkylation. This article will take the reader through some of the considerations made in connection with the development of FBA technology, which applies a conceptually new reactor and catalyst system. A short review of some key aspects of the chemistry, which is important to the understanding of the reactor and catalyst system, is included. Subsequently, the reader will be led through a description of the reactor concept and how it was tested from screening in a bench-scale unit through a 0.5 bpd process pilot and to a 10 bpd reactor scale-up pilot. A few comments are made regarding the overall process, including the necessity of auxiliary technologies for effluent treatment and acid recovery, both of which have been developed in parallel. Finally, some thoughts are given concerning the flexibility of the reactor concept. Isobutane Alkylation Chemistry and Choice of Catalyst System From an environmental and safety point of view, a true solid catalyst would probably be the preferred choice for a new catalyst system for isobutane alkyla-
10.1021/ie0209531 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5527
tion. Substantial efforts of many research groups have been devoted to the search for a solid isobutane alkylation catalyst.5-7 Nevertheless, no true solid catalyst has so far been able to challenge the established use of HF and sulfuric acid catalysts for isobutane alkylation in the refining industry. To understand the origin of the difficulties connected with the solid catalysts, one must recognize some key features of isobutane alkylation that pose a challenge to anyone searching for alternative catalyst systems. These include the importance of kinetically controlled product selectivities, combined with the two-step reaction mechanism of the alkylation process and the formation of acid-soluble oils that passivate the acid catalyst.12,13 The product formed by the alkylation of isobutane with butenes is characterized by a high selectivity toward highly branched high-octane isoalkanes. The total product blend might consist of up to 80% isooctanes, and as much as 90% of these can be trimethylpentanes with octane numbers around 100. At thermodynamic equilibrium, less than 15% of the isooctanes are trimethylpentanes,14 and the content of C8’s in the total product mixture is also much lower at equilibrium than in the alkylate. Clearly, the selectivity of isobutane alkylation is determined not by thermodynamic equilibrium but by the kinetics of the reaction chemistry. Consequently, the reaction path and thus the reaction mechanism play a decisive role in determining the product selectivities. Alkylation is accompanied by several other acid-catalyzed reactions, such as olefin oligomerization, product cracking, and product isomerization, that result in the formation of inferior products. Consequently, isobutane alkylation is more sensitive to the choice of catalyst than many other catalytic processes. The mechanism usually described in the literature for isobutane alkylation involves a carbenium ion chain mechanism initiated by protonation of the olefin.6,7,15 This mechanism offers no satisfactory explanation for the selectivity toward alkylation as opposed to olefin oligomerization, nor does it explain why many solid acid catalysts tend to passivate rapidly even with the purest feedstocks. It has been found that the reaction actually proceeds in two steps involving the initial formation of an ester-type intermediate as illustrated in eqs 1 and 2.12 The existence of ester-type reaction intermediates has been known since the very early years of isobutane alkylation.16,17 The advantage of operating sulfuric acid alkylation as a two-step reaction, in which the esters are formed in one reactor and subsequently used for alkylation in a second reactor, was suggested in the 1970s.18-20 However, the profound effect of the ester intermediates on the product selectivity in Brønsted acid catalyzed isobutane alkylation was highlighted only recently.12
olefin + HX h [R+, X-] h RX
(1)
RX + isobutane 9 8 alkylate + HX acid
(2)
The formation of ester intermediates plays an important role in the selectivity of isobutane alkylation because it limits olefin oligomerization. When the olefin encounters the strong Brønsted acid alkylation catalyst, a rapid reaction takes place in which the acid (HX) reacts with the olefin to form an ester (RX) (step 1).
Because of the fast ester formation reaction, the acid activity is kept low as long as olefin is present. The ester subsequently reacts with isobutane in an acid-catalyzed reaction to form alkylate through a complex series of carbenium ion reactions similar to the reactions traditionally presented as the mechanism of isobutane alkylation (step 2). Because the reaction of the ester with isobutane requires a strong acid catalyst, this reaction takes place only when the acid activity is high, which is the case only when the olefin is absent. Hence, olefin oligomerization is prevented. However, the ester formation also represents an intermediate passivation of the acid. In the second step, when the ester intermediates react with isobutane in an acid-catalyzed reaction to form alkylate, the acid trapped in the first step as ester intermediate is liberated. In the liquid Brønsted acid catalyst systems, the mobility of the acid and ester intermediates ensures adequate access to additional “acid sites” to catalyze the conversion of the ester intermediates initially formed. The alkylation of the isobutane by the esters predominantly takes place in the liquid acid phase. For a solid catalyst with fixed acid sites, the ester intermediates will also be fixed and unable to move away from an olefin-rich environment. It is easy to envision how the conversion of all acid sites to ester intermediates in a certain region of the catalyst can cause a passivation of this region by leaving no extra acid activity to catalyze the subsequent reaction. The solid catalyst is thus much more prone to passivation by alkylation chemistry than the liquid catalyst systems. Furthermore, studies have indicated that the presence of small amounts of acidsoluble oil (ASO) increases the sensitivity toward passivation by the alkylation reaction.13 Catalyst passivation is always an issue in catalytic processes, but in isobutane alkylation, the passivation of the acid catalyst requires special attention. The acid passivation is, in part, caused by the formation of acidsoluble oil (ASO), which passivates the acid catalyst relatively rapidly, thus creating a need for easy catalyst activity maintenance.21,22 Impurities in the feed such as water, oxygenates, sulfur compounds, etc., also result in passivation of the acid catalyst, and much of the ASO formed in today’s alkylation processes actually originates from impurities such as dienes in the alkylation feedstock. Removal of these impurities by pretreatment of the feed reduces the acid passivation considerably. However, the alkylation chemistry itself includes pathways leading to the formation of small amounts of ASO even when the feed is completely pure. This chemistry includes side reactions to the alkylation reaction and cracking of the alkylate product,23-25 neither of which can be completely avoided. The ASO consists of cyclic diene molecules, which passivate the acid through formation of a 2:1 complex according to the stoichiometry (HX)2ASO, where HX is the Brønsted acid catalyst.24 In addition to the inevitable ASO formation, no feed pretreatment is 100% effective, and occasional upsets in the feed pretreatment are encountered just as in any other refinery operation. Easy maintenance of the catalyst activity is thus important. One of the strengths of the liquid acid catalysts used in the established technologies is the fact that the passivated catalyst can be withdrawn from the alkylation reactor for recovery or regeneration elsewhere without interrupting the alkylation operation.
5528 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Figure 1. FBA reactor concept with migrating catalyst zone.
In summary, it can be said that solid catalysts are difficult to apply in isobutane alkylation because of an inherent tendency to passivate. Liquid catalysts are better suited to match the chemistry, but their use has caused concern as it involves handling of large quantities of liquid acids blended with LPG. A supported liquid phase (SLP) type catalyst represents a compromise combining the best of both systems. It combines the chemistry of the liquid catalysts with improved safety by adsorption of the acid on a solid support. Reactor Concept The fixed-bed alkylation (FBA) process uses an SLPtype catalyst in a fixed-bed operation. The catalyst has some special features that distinguish it from most other SLP catalysts. In FBA, the catalyst is not evenly spread over the support bed but concentrated in a well-defined catalyst zone. This catalyst zone slowly migrates through the support bed in the direction of the hydrocarbon flow but at a much lower speed than the hydrocarbon flow. The basic concept is illustrated in Figure 1. The migration of the catalyst zone is closely coupled to the alkylation chemistry.8,9 In the upstream end of the catalyst zone, where the olefin-rich feed meets the acid zone, acid reacts with olefin to form ester intermediates. The esters have a weaker interaction with the support than the acid. Thus, the esters tend to migrate with the hydrocarbon flow further down the acid zone, where they react with isobutane in an acid-catalyzed reaction to form alkylate and liberate the acid. In this way, the acid is moved into the acid zone from the upstream end. Typically, the acid will be deposited in the part of the support bed that is already saturated and will therefore tend to be pushed downstream with the hydrocarbon flow toward the downstream end of the catalyst zone. The migration of the catalyst zone resembles the migration of a substrate band on a chromatographic column as known from liquid chromatography. The twostep alkylation reaction can, in this case, be perceived as a chromatographic reaction in the sense that the chromatographic elution is caused by the formation of the ester intermediates and thus directly connected with the reaction itself. Using this terminology, the FBA reactor can consequently be called a chromatographic reactor, even though in the past this term was used to refer to a fixed-bed reactor in which the products are continuously separated by chromatographic separation.26 The moving catalyst zone in the FBA reactor differs from a substrate band in regular chromatography
Figure 2. Details of the migrating catalyst zone.
in one significant way: no band broadening is observed in FBA. The reason for this is the fact that the feed stream containing the olefin is a better “eluant” than the olefin-free product stream. Consequently, the catalyst zone in FBA is constantly pushed together from the upstream end.8,9 The part of the support bed that is downstream of the migrating catalyst zone acts as an adsorbent for any catalyst that is dissolved in the product stream leaving the catalyst zone. Catalysts that are difficult to use in the back-mix reactor types applied in industry today because of their solubility in hydrocarbons can be used in the chromatographic reactor in FBA. In addition to improving the safety of the alkylation process, the fixed-bed reactor design also provides good control of the contact between the catalyst and the hydrocarbon stream. As mentioned above, there might be a selectivity advantage in performing the alkylation chemistry in two separate steps, forming esters in the first step and converting these esters to alkylate in the second step. The FBA reactor has this consecutive reaction step sequence as a built-in feature because the olefin reacts to form ester intermediates when the feed stream hits the catalyst zone and the ester intermediates subsequently migrate into the catalyst zone with the hydrocarbon flow as described above. It is also well-known that the product alkylate, if exposed to the acid catalyst for extended periods of time, tends to undergo cracking, isomerization, and disproportionation reactions to less desirable products.23,25 The FBA reactor concept makes it possible to set the desired contact time to achieve optimum performance and product selectivities. An added feature of the FBA reactor is that ASOpassivated spent acid, which has a weaker interaction with the support than the active acid, tends to move downstream of the acid zone and settle at the far downstream end of the catalyst zone in an ASO-rich zone (ASO zone in Figure 2). This feature keeps both the reaction zone and the support free of spent acid and generally makes the catalyst system very robust toward variations in feed composition and impurities in the feed. The separation of the ASO zone from the rest of the catalyst zone is much like a chromatographic separation and differs only from regular separation chromatography in the fact that the spent acid zone stays with the catalyst zone. The part of the catalyst zone between the reaction zone and the ASO zone is referred to as the fresh acid zone in Figure 2. The catalyst in this zone does not participate in the alkylation but serves as a buffer
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5529 Table 1. RON Determined by Motor Tests of Alkylates Produced in the Bench-Scale Unita by Alkylation of Isobutane with Various Pure Olefins at Various Conditions27 olefin propene propene 1-butene 1-butene 2-butene 2-butene isobutene/2-butene, 3/7 wt/wt isobutene/2-butene, 3/7 wt/wt 1-pentene 1-pentene 2-methyl-2-butene
isobutane/olefin bath temperature (wt/wt) (°C) RON 12.3 12.3 9 20 9 20 9
0 20 0 30 0 30 -10
91.6 92.2 97.1 94b 97.6 94b 95.1
Figure 3. Simplified process flow diagram for the 0.5 bpd process pilot.8 Table 2. Composition of Olefin Feeds (wt %)
20
30
7 7 7
0 20 -10
93b 88.1 89 93
a Generally, the bench-scale data show that the quality of the alkylate obtained in the FBA reactor is similar to that achieved with the commercially established process based on HF or H2SO4. b RON estimated from GC data.
against a situation where the entire catalyst zone is passivated and converted into ASO zone, at which point the alkylation reaction would no longer take place. The migration of the acid zone makes it possible to easily remove passivated acid for recovery outside the reactor system. Thus, the catalyst activity can be maintained without interruption of the alkylation operation, a key feature that has been important to the success of the established technologies for the past 60 years. Philosophically, it can be said that one of the key differences between FBA and a true solid catalyst system is that the FBA technology exploits the alkylation chemistry to facilitate the acid activity maintenance, whereas for a true solid catalyst, the chemistry is part of the reason for the catalyst passivation. Reactor Testing and Product Qualities The facilities used for testing of the FBA reactor for isobutane alkylation include a 100 g/h bench-scale unit, a 0.5 bpd process pilot, and a 10 bpd scale-up pilot. The bench-scale unit is a cooled reactor unit in which the reaction temperature is controlled by submerging the reactor in a bath. It is useful for fast screening and preliminary tests for quick evaluation of product qualities. This unit has been a primary tool in the testing of catalyst formulations and pure feeds. Product qualities for the alkylation of isobutane with a series of pure olefins at different temperatures in the bench-scale unit are shown in Table 1. For low-temperature alkylation, the octane numbers for the different olefin feedstocks are similar to what would be expected for alkylation of the same olefins at similar conditions using sulfuric acid as the catalyst. The highest octane numbers are achieved with the n-butenes. Alkylation with propene gives lower octane products as is expected given that dimethylpentanes, which constitute a significant part of the product, have lower octane numbers than isooctanes. Mixtures of isobutene and 2-butene yield products of lower octane quality than 2-butene, reflecting a higher content of heavier material as a consequence of the lower stability of the ester intermediate, as discussed above. The 0.5 bpd process pilot uses an adiabatic reactor in which the adiabatic temperature increase caused by the
propene 1-butene 2-butene isobutene pentenes propane n-butane isobutane n-pentane isopentane C6+ butadiene
feed 1
feed 2
feed 3
feed 4
0.1 2.3 63.4 0.3 0 0.1 33 0.7 0 0.1 0.1 0
0.2 15.6 29.2 19.8 0.4 0 7.5 26.7 0 0.3 0 0.4
6.7 8.2 18.9 6.0 1.1 3.7 10.2 37.4 0.1 7.3 0.3 0.04
17.7 9.3 18.5 7.3 5.2 7.8 7.5 19.3 0.4 6.5 0.1 0.14
exothermic alkylation reaction is observed over the reaction zone. The temperature increase of the reaction is controlled by a cooled reactor effluent recycle. A simplified flowsheet of the process pilot is shown in Figure 3. This pilot is fully automated and normally operates 24 h/day 7 days/week. The automation includes on-line monitoring and computer-controlled maintenance of the catalyst activity. The 0.5 bpd process pilot has been the primary tool for demonstration of the stability, operability, and reliability of the FBA process. This process pilot has operated for more than 22 000 h on a wide range of feedstocks. Solid acid catalyst systems, even though they are able to process pure olefins in some cases, often fail when they are exposed to a real refinery feedstock because of the content of impurities in such feeds. To demonstrate that the FBA process can operate on feeds containing impurities, the processing of real feedstocks from a number of refineries around the world has thus been given a high priority. Of the 22 000 h of operating experience in the process pilot, more than 9000 h has been on a variety of real feedstocks acquired from different refineries around the world. Results from tests on four industrial feeds acquired from four different refineries and processed at different temperatures in the 0.5 bpd pilot are chosen to illustrate the typical dependence of product quality on feed composition, operating temperature, and isobutane/ olefin ratio. The compositions of the four feeds are listed in Table 2.8 All of the feeds were passed through a dryer to remove water but were otherwise used as received from the refineries without further pretreatment. Feed 1 is an n-butene feed. This particular feed came from a Dimersol unit, but the results are representative of what would be achieved with an MTBE raffinate. Feed 2 is a full C4 cut from an FCC operation and thus differs from feed 1 in its content of isobutene. Feed 2 also has a fairly high butadiene content (0.4%). Feed 3 is a C4 cut containing some propene, and feed 4 is a full-range C3-C5 FCC olefin feed containing both propene and amylenes.
5530 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 Table 3. 8,28 Product Octane Values for Refinery Feedstocks Processed in the Process Pilot product quality feed
I/O (wt/wt)
inlet tempa (°C)
RON
MON
1 2 2 2 2 3 3 4 4
15 14 7 14 7 17 8 15 10
5 8 8 31 31 5 5 5 40
98 95 93 92 90 95 94 93 90
95 92 92 91 90 93 92 91 88
a
Temperature at the inlet of the adiabatic reactor.
Octane values determined by ASTM engine tests for products produced in the 0.5 bpd process pilot from the four feedstocks are reported in Table 3.8,28 The operability of the process pilot was essentially independent of the feed and operating conditions, demonstrating the sturdiness of the FBA technology. However, the product quality reflects the variation in feed quality and operating conditions. Thus, the lower octane number for feed 2 as compared to feed 1 reflects the content of isobutene in feed 2, which tends to yield less desirable products than the n-butenes because of the lower stability of the ester intermediates as explained above. Feed 3 has almost the same octane value as feed 2. The propene present in feed 3 should result in a lower octane, but feed 3 also has a lower isobutene content than feed 2, which should give a higher octane value, and the two effects almost cancel out. Feed 4 gives the lowest octane of the four feeds, reflecting the high content of propene and the presence of pentenes, which tend to pull the octane numbers down. Generally, the products produced at low temperature (5-8 °C), corresponding to refrigerated operation, have a quality similar to that of the product of a sulfuric acid catalyzed process. Increasing the temperature to nonrefrigerated conditions (30-40 °C) results in a decrease in product quality of approximately 3 numbers for feeds 2 and 4. However, a higher operating temperature also represents significant savings with respect to both investment and operation because refrigeration is eliminated. The pilot testing shows that FBA is fully capable of operation at nonrefrigerated conditions. The sensitivity of the product quality to the operating conditions is dependent on the feed. High-quality feeds tend to exhibit higher sensitivity toward temperature than feeds that, even under ideal conditions, give lowoctane products. For alkylation of amylenes (C5 cut from the FCC), there is essentially no penalty to pay with respect to product quality for operation at 40-50 °C. Even though operation temperatures higher than 50 °C are usually not feasible for isobutane alkylation, the catalyst system is capable of operation at significantly higher temperatures. Reactor Scale-up Scale-up of the FBA reactor has been examined in a 10 bpd scale-up pilot. It is of particular importance that the catalyst zone is distributed evenly across the reactor cross section to ensure a well-defined contact time also on a larger scale. If the catalyst zone moves faster in one area of the bed than in other areas, there is a risk of olefin breakthrough and thus of incomplete conver-
sion. To check for the effect of an increase of reactor scale on the evenness of the catalyst zone distribution, the catalyst zone was monitored as it passed through the 10 bpd reactor column using two three-channel densitometers. The densitometer is a noncontact on-line instrument that uses gamma-ray absorption to measure the average density of the medium through which the gamma-ray beam is passing on its way from source to detector. The densitometers used in the scale-up tests were from ICI Tracerco using 10 mCi cesium 137 gamma-ray sources and PRI-116/121 detectors. Figure 4 shows the densitometer readings for a set of three densitometers at one level in the reactor recorded during scale-up pilot run 2 as the catalyst zone passes by the three detector beams. The densitometer reading is not a direct measure of the absolute density. It is a relative measurement showing changes in the density across the cross section of the reactor as the catalyst zone passes by. The detectors are placed in such a way that the beams have an angle of 20° relative to one another, as shown in the inset in Figure 4. The densitometer reading shows an increase in density across the reactor as the downstream end of the catalyst zone moves into the detector beam. In the same way, the densitometers show a reduction in density when the tail end of the catalyst zone moves out of the detector beam. The important observation is that both the increase and the reduction of the density readings occur simultaneously for the three beams. This shows that the catalyst zone is level and evenly distributed across the cross section of the bed. The leading end or downstream end of the catalyst zone that passes the detector first contains most of the spent acid in the catalyst zone, and because the spent acid has a lower density than the active acid catalyst, the density reading should, in principle, reflect this. Under normal operating conditions, the content of spent acid in the catalyst is very low because spent acid is withdrawn as part of the catalyst activity maintenance. It is therefore difficult to see the spent acid zone in the densitometer data from scale-up pilot run 2 during which spent acid catalyst was withdrawn as it was formed. However, if the spent acid withdrawal is inactivated and spent acid is allowed to accumulate, the catalyst zone will collect a spent acid zone in the downstream leading end. This is illustrated in Figure 5, which shows densitometer readings accumulated during scale-up pilot run 3, in which the spent acid was allowed to build up in the catalyst zone. The results also show that, even in the presence of significant quantities of spent acid in the catalyst zone, the distribution of the catalyst zone across the reactor remains level and well-defined. Because the focus of the 10 bpd pilot was on scale-up issues relating to the reactor and the catalyst zone, this pilot was not equipped with a fractionation section. Product alkylate was consequently not isolated. The overall conclusion drawn from the scale-up pilot tests is that the catalyst zone remains level and welldefined when the reactor is scaled up. Process Layout Although the reactor section plays a vital role for the alkylation process, the process contains many other pieces of equipment, some of which are both larger and more expensive than the reactor section. For the FBA technology, the reactor section is relatively inexpensive
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5531
Figure 4. Densitometer profiles of the catalyst zone. The densitometer beam arrangement relative to the reactor cross section is shown in the inset in the upper right-hand corner.
Figure 5. Densitometer profiles of the catalyst zone during run 3 in which spent acid has been allowed to accumulate.
as compared with, for instance, those required for fractionation and refrigeration. The FBA process flow diagram (Figure 6) contains the same basic elements as other isobutane alkylation processes. The olefin feed to the alkylation process normally originates from the fluid catalytic cracking (FCC) unit
and contains various impurities that passivate the acid catalyst. Typically, a C4 feed can be saturated with water and contains 1-3000 ppm of dienes together with variable amounts of other impurities such as oxygenates and sulfur compounds. In principle, there are two ways of dealing with these feed impurities: either by pre-
5532 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
total quantity of gasoline formed in this ASO transformation is only 0.1% relative to the alkylate production and thus insignificant for the overall product composition. However, by removing the dienes responsible for much of the interaction between the acid and the ASO, this chemical transformation weakens the interaction between the acid and the oil and consequently facilitates the acid recovery. Flexibility and Applicability
Figure 6. Simplified process flow diagram for the FBA process.
treating the feed to prevent the impurities from reaching the catalyst or by treatment in the acid recovery unit, where the impurities will be present as part of the spent acid. Because the FBA reactor is capable of processing feeds with high impurity levels, the choice of whether to use feed pretreatment is primarily a question of economics. The optimum choice of feed pretreatment depends on the composition of the feed but will typically consist of drying and removal of dienes by selective hydrogenation. After pretreatment, the feed is mixed with the isobutane recycle stream from the fractionator section and fed to the reactor section. The isobutane/olefin ratio of the feed entering the reactor loop is typically around 10 wt/wt. In the reactor, the olefin is quantitatively converted into alkylate, and the reactor effluent is essentially olefin free. The FBA reactor is adiabatic, and the temperature is controlled by a cooled or refrigerated reactor effluent recycle. The process is capable of operating both refrigerated and at a temperature at which the refinery cooling water temperature is enough for cooling, which in most cases means an operating temperature of 30-40 °C. The net reactor effluent leaving the reactor effluent recycle loop is treated in an effluent treatment section to remove part-per-million levels of acid catalyst, which are recycled to the process. The dry and acid-free effluent is subsequently fed to the fractionation section, from which alkylate is withdrawn and excess isobutane is recycled to the reactor loop. n-Butane, which is present in the feed and inert in the alkylation reaction, is also withdrawn from the fractionation section. Propane, which is present in the feed when propylene is used for alkylation, can be withdrawn downstream of the refrigeration compressor in refrigerated units. Catalyst activity maintenance is needed in all alkylation processes, and FBA is no exception. In the FBA process, withdrawal of passivated acid catalyst from the reactor section and recycling of recovered catalyst to the reactor section maintain the catalyst activity. Both of these operations take place while the reaction is in progress. The spent acid is sent to the acid recovery section, where it is recovered in its active form. The heart of the acid recovery section is a stripping unit operating at near atmospheric pressure in which the acid is stripped from the spent acid by a hydrocarbon stripping agent. During this operation, the ASO is chemically transformed in such a way that the cyclic diene molecules are, to a large extent, converted into a mixture of isoalkanes, aromatics, and naphthenes, much of which boils in the gasoline distillation range. The
FBA technology is sturdy as well as flexible. The sturdiness means that feeds with relatively high acid passivation potentials can be processed. Thus, the process is capable of alkylating even very dirty feedstocks. The amylene cut from an FCC operation was thus alkylated in the 0.5 bpd pilot with no pretreatment other than drying, and propene from a coker operation with a mercaptan content of more than 400 ppm could also be alkylated without any operational difficulties. The flexibility of the technology goes beyond isobutane alkylation. Alkylation of isopentane proceeds smoothly to yield products of gasoline range but with a relatively low octane value.27,29 Typically, the octane number of isopentane alkylate is in the lower 70’s, whereas isopentane itself has a value of (R + M)/2 ) 91. The reason for considering the alkylation of isopentane would be as a means of controlling vapor pressure. Concerns about MBTE reaching the groundwater from leaking gasoline storage tanks might force refiners to replace MBTE with ethanol. Ethanol, when blended into gasoline, has a very high blending Reid vapor pressure (RVP).30 Consequently, replacement of MBTE with ethanol might force refiners to remove other volatile components such as isopentane from the gasoline pool to comply with vapor pressure specifications. Isopentane alkylation might be the best solution for disposal of a surplus of isopentane. Additionally, it is possible to operate an FBA reactor in such a way that much of the isopentane is converted into isobutane, which can subsequently be used in isobutane alkylation to yield high-octane alkylate. Heavier isoalkanes such as isohexane and isoheptane can also be alkylated in the FBA reactor. However, as the isoalkanes become heavier, the product becomes less suitable for blending into gasoline. More of the product ends up in the middle distillate boiling range and thereby opens a route to production of middle distillate by alkylation.27 The middle distillate produced in this way matches the ASTM specifications for jet A, and the heavier cuts with initial boiling points (IBPs) > 200 °C have cetane numbers above 40, making them useful for blending into road diesel. FBA can also be used to alkylate aromatic hydrocarbons. The aromatic hydrocarbons react much more rapidly than the isoalkanes, so when both are present, the olefin will preferentially alkylate the aromatic compounds. This makes it possible to selectively alkylate benzene in the C6 cut of reformate into a higher boiling point range, thus opening a new route to removing the benzene from this stream. Because of the high activity of aromatic alkylation, even ethylene can be used as an olefinic alkylating agent to make ethylsubstituted aromatics with 100% olefin conversion, whereas ethylene to a large extent behaves as a catalyst poison in isobutane alkylation. The chromatographic reactor concept of the FBA process can be used with many different catalyst
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5533
formulations involving a wide selection of acid catalysts. Perfluorinated alkane sulfonic acids such as triflic acid supported on silica27 are suitable for isobutane alkylation, but the selectivity can, in some cases, be improved by changing the acid catalyst. For instance, perfluorinated alkoxyalkane sulfonic acids such as perfluoro 2-ethoxyethane sulfonic acid provide a noticeable improvement, as compared with pure triflic acid, in the octane number of the alkylate at temperatures corresponding to nonrefrigerated conditions.31 The product selectivity can be shifted toward production of middle distillate by using CF3CFHSO3H as the acid catalyst.32 As a curiosity, it can be mentioned that the safety of HF-catalyzed alkylation can even be improved by supporting the HF on a suitable carrier in a mobile catalyst zone in a chromatographic FBA reactor.33 However, as for the new additive technologies for HF alkylation, in which additives are used to limit the HF vapor release in the case of an emergency, adsorption of the HF on a suitable support in an FBA reactor still involves the presence of significant quantities of HF in the refinery. One of the main targets of the development of FBA has been to give the refiners an alternative to HF-catalyzed alkylation. Conclusion and Perspective In summary, FBA is a very versatile and robust technology that is also highly flexible and tuneable. The reactor concept is based on the use of an SLP-type catalyst that slowly migrates through the reactor in a way similar to the movement of the substrate band in a chromatographic column. The concept combines alkylation chemistry and easy catalyst activity maintenance known from the liquid catalyst concept with the improved safety of fixed-bed operation. Because passivated acid catalyst tends to move away from the active upstream end of the catalyst band, the reactor concept has a built-in self-purification action, which makes it sturdy toward the upsets and feed impurities that normally represent a serious challenge to solid alkylation catalysts. Spent acid is readily withdrawn from the reactor loop without interruption of the alkylation operation, and the acid catalyst is recovered from the spent catalyst in a small unit on site. The reactor concept can be used with a range of different acid catalyst formulations, including acids that, because of their solubility in hydrocarbons, cannot be used in backmix type reactors such as the ones used in industry today. A wide range of different feedstocks can be processed under a much broader range of operating conditions than the ones currently used in isobutane alkylation. Overall, the reactor concept secures the technology a high degree of flexibility and sturdiness. The development of a new technology involves many aspects that go beyond the principles of and chemistry behind what is happening in the reactor. In the development of FBA technology, scale-up issues have been addressed, and necessary auxiliary technologies have been developed. In continuation of the development of the new reactor concept, new technologies for acid activity maintenance and reactor effluent treatment have been developed. FBA technology includes both of these. Having the acid recovery on site means that transportation of spent acid for external recovery or regeneration can be avoided.
Nomenclature and Abbreviations ASO ) acid-soluble oil ASTM ) American Society for Testing and Materials FBA ) fixed-bed alkylation FCC ) fluid catalytic cracking GC ) gas chromatography IBP ) initial boiling point LPG ) liquefied petroleum gas, typically C3-C4 hydrocarbons MTBE ) methyl tertiary-butyl ether MON ) motor octane number RON ) research octane number (R + M)/2 ) average of RON and MON RVP ) Reid vapor pressure, vapor pressure at 100 °F SLP ) supported liquid phase
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Received for review December 2, 2002 Revised manuscript received July 18, 2003 Accepted July 22, 2003 IE0209531
