Science and technology of novel processes

Review Article Science and technology of novel processes for deep desulfurization of oil refinery streams: a review q I.V. Babich * , J.A. Moulijn Faculty of Applied Sciences, Delft University of Technology, Delft ChemTech, Julianalaan 136, 2628 BL Delft, The Netherlands Received 15 March 2002; revised 15 July 2002; accepted 9 October 2002; available online 14 November 2002 Abstract Oil refinery related catalysis, particularly hydrodesulfurization (HDS) processes, is viewed as a mature technology and it is often stated that break-throughs are not to be expected. Although this could be a justified compliment to those who developed this area, at the same time it could also stifle potential new ideas. The applicability and perspectives of various desulfurization technologies are evaluated taking into account the requirements of the produced fuels. The progress achieved during recent years in catalysis-based HDS technologies (synthesis of improved catalysts, advanced reactor design, combination of distillation and HDS) and in ‘non-HDS’ processes of sulfur removal (alkylation, extraction, precipitation, oxidation, and adsorption) is illustrated through a number of examples. The discussed technologies of sulfur removal from the refinery streams lead to a wealth of research topics. Only an integrated approach (catalyst selection, reactor design, process configuration) will lead to novel, efficient desulfurization processes producing fuels with zero sulfur emissions. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Oil refinery; Sulfur removal; Hydrodesulfurization 1. Introduction A modern refinery is a highly integrated industrial plant, the main task of which is to efficiently produce high yields of valuable products from a crude oil feed of variable composition. Employing different physical and chemical processes such as distillation, extraction, reforming, hydrogenation, cracking and blending the refinery converts crude oil to higher value products. The main products are liquid petroleum gas, gasoline, jet and diesel fuels, wax, lubricants, bitumen and petrochemicals. Energy and hydrogen for internal and external use are also produced in a refinery. Currently, refineries meet changing societal needs concerning product specifications and quality by upgrading existing technologies and continuously developing advanced technologies [1]. Changes in refining processes are made in response to external driving forces taking into account the inherent limitations of the refinery (Fig. 1). Environmental restrictions regarding the quality of transportation fuels produced and the emissions from the refinery itself are currently the most important issues, as well as the most costly to meet. The primary goal of the recently proposed regulations (by the Directive of the European Parliament [2] and the Environmental Protection Agency (EPA) Clean Air Act (Tier 2) [3]) is to reduce the sulfur content of transportation fuels. The CO 2 emitted by the refinery into the atmosphere is limited by the Kyoto protocol [4]. According to various estimation models, $10– 15 billions in the European refinery industry and up to $16 billion in US and Canadian refineries will be invested in direct response to the new environmental clean-fuel legislation [5,6]. Gasoline, diesel and non-transportation fuels account for 75–80% of the total refinery products. Most of the desulfurization processes are therefore dealing with the streams forming these end products. Sulfur present in the fuels leads to SO x air pollution generated by vehicle engines. In order to minimize the negative health and environmental effects of automotive exhaust emissions, 0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 001 6 -2 3 61( 0 2)0 0 324 - 1 Fuel 82 (2003) 607–631 www.fuelfirst.com q Published first on the web via fuelfirst.com—http://www.fuelfirst.com * Corresponding author. Present address: Faculty of Chemical Technology, University of Twente, Postbus 217, 7500 AE, Enschede, The Netherlands. Tel.: þ31-53-489-35-36; fax: þ 31-53-489-46-83. E-mail address: i.v.babych@ct.utwente.nl (I.V. Babich). the sulfur level in motor fuels is minimized. New sulfur limits of 30–50 ppm for gasoline and diesel marketed in the European community and the USA will be introduced starting from January 1, 2005 [2,3,5,7,8]. Germany has even passed legislation limiting the sulfur in diesel and gasoline to 10 ppm as of November, 2001 [9]. In fact, zero-emission and, as a consequence, zero levels of S are called for worldwide in coming 5–10 years. Such ultra low-sulfur fuels requirements have consequences for the refinery. Efficiency of the desulfurization technologies becomes a key point. Conventional hydrodesulfurization (HDS) processes cannot currently produce such zero sulfur level fuels, while maintaining other fuel requirements such as oxygen content, vapor pressure, benzene content, overall aromatics content, boiling range and olefin content for gasoline, and cetane number, density, polynuclear aromatics content, and distillation 95% point for diesel fuel [2,3,5,7,8]. 1.1. Gasoline Gasoline is formed by blending straight run naphtha (isomerate, reformate and alkylate products), naphtha from fluid catalytic cracking (FCC) units and coker naphtha. Most sulfur in gasoline comes from the FCC naphtha. Treatment of FCC gasoline is, therefore, essential. The sulfur content of the other gasoline forming refinery streams is not a problem for the current environmental regulations, but to produce gasoline of # 30 ppm S the refinery will be obliged to treat them as well. A relatively high level of sulfur removal can be reached by using conventional or advanced CoMo and NiMo catalysts. However, simultaneous hydrogenation of olefins should be minimized because it reduces the octane number. Also aromatics are not desired in the final gasoline product. Process applicability is determined by its efficiency in terms of end product yield and specifications. Instead of further improving traditionally applied catalysis-based HDS technologies in small steps, now might be the right time for advanced desulfurization technologies which provide effective sulfur removal and simultaneously increase the octane number. 1.2. Diesel Diesel fuel is formed from straight run diesel, light cycle oil from the FCC unit, hydrocracker diesel, and coker diesel. Nowadays, diesel is desulfurized by hydrotreating all blended refinery streams. To get diesel with less sulfur content the hydrotreating operation has to be more severe. For straight run diesel, sulfur removal is the only point of concern in hydrotreating since the other diesel specifications (e.g. cetane number, density, and polyaromatics content) are satisfactorily met. Hydrocracker diesel is usually relatively high in quality and does not require additional treatment to reduce the sulfur content. As with gasoline, the diesel produced by the FCC and coker units contains up to 2.5 wt% sulfur. Both the FCC and coker diesel products have very low cetane numbers (slightly above 20), high densities, and high aromatics and polyaromatics content (about 80 –90%). In addition to being desulfurized, these streams must be upgraded by high pressure and temperature processes requiring expensive catalysts. Another problem is that at high temperature the hydrogenation–dehydrogenation equilibrium shifts toward aromatics. As with gasoline desulfurization, there are many options for developing and applying advanced desulfurization technologies with simultaneous upgrading to higher diesel specifications. 1.3. Non-transportation fuels Non-transportation fuels are formed from vacuum gas oils and residual fractions from coking and FCC units. The sulfur content requirement for non-transportation fuels is less strict than for gasoline and diesel because industrial fuels are used in stationary applications where sulfur emissions can be avoided by combustion gas cleaning processes. In particular, high temperature solid adsorbents based on zinc titanate [10–12] or manganese/alumina [13– 15] are currently receiving much attention. In practice, the major process is the capture of SO x with CaO producing CaSO 4 [16– 19]. Of course, for non-transportation fuels Fig. 1. External and internal factors influencing modern refineries. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631608 HDS technologies can also be applied without considering other fuel specifications that must be met for gasoline and diesel fuels. It is also important to note that in some cases the HDS process requirements are between those for transportation and non-transportation fuels. For instance, in large ships and power plants ample space is available for dedicated equipment aiming at reduction of emission of SO x and soot that makes the requirement to sulfur content less strict. It has to be expected that the sulfur level requirements will become more and more strict in the near future, approaching zero sulfur emissions from burned fuels. The next generation of engines, especially fuel cell based engines, will also require fuels with extremely low (preferably zero) sulfur content. Therefore, scientists and engineers involved in improving current refinery technologies and developing advanced technologies should shoot for complete sulfur removal from refinery products. The applicability of various desulfurization technologies should be evaluated taking into account all requirements for the produced fuels. The most effective options for ultra deep desulfurization should be chosen since removing all sulfur from the fuels might be too expensive or result in refinery CO 2 emissions which are too high [9,20]. The aim of this paper is to analyze different desulfurization technologies for crude oil and refinery streams and to formulate challenges for innovative research. We purport that break-through innovations in oil refinery related desulfurization are still possible. The desulfurization processes currently employed in some refineries and in semi-commercialized and laboratory proven approaches are discussed. Special attention is paid to development and application of new desulfurization reactors and some examples of advanced options for reactor design are mentioned. In a separate chapter, structured monolithic catalytic reactors are discussed since they can be readily applied to desulfurization processes. 2. Classification of desulfurization technologies There is a no universal approach to classify desulfurization processes. The processes can be categorized by the fate of the organosulfur compounds during desulfurization, the role of hydrogen, or the nature of the process used (chemical and/or physical). Basedonthewayinwhichtheorganosulfur compounds are transformed, the processes can be divided into three groups depending on whether the sulfur compounds are decomposed, separated from refinery stream without decomposition, or both separated and than decomposed (Fig. 2). When organosulfur compounds are decomposed, gaseous or solid sulfur products are formed and the hydrocarbon part is recovered and remains in the refinery streams. Conventional HDS is the most typical example of this type of process. In other processes, the organosulfur compounds are simply separated from the refinery streams. Some processes of this type first transform the organosulfur compounds into other compounds which are easier to separate from the refinery streams. When streams are desulfurized by separation, some desired product can be lost and disposal of the retained organosulfur molecules is still a problem. In the third type of process, organosulfur compounds are separated from the streams and simultaneously decomposed in a single reactor unit rather than in a series of reaction and separation vessels. These combined processes, which provide the basis for many technologies currently proposed for industrial application, may prove very promising for producing ultra-low sulfur fuels. Desulfurization by catalytic distillation is the fascinating example of this type of process. Desulfurization processes can be also classified in two groups, ‘HDS based’ and ‘non-HDS based’, depending on the role of hydrogen in removing sulfur. In HDS based processes, hydrogen is used to decompose organosulfur compounds and eliminate sulfur from refinery streams while non-HDS based processes do not Fig. 2. Classification of desulfurization processes based on organosulfur compound transformation. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631 609 require hydrogen. Different combinations of refinery streams pre- or post-distilling treatments with hydrotreating to maintain desired fuel specifications can also be assigned as HDS based processes since HDS treatment is one of the key steps. The two above-mentioned classifications overlap to some extent. Most sulfur elimination processes, with the exception of selective oxidation, are HDS based. The organosulfur compound separation processes are usually non-HDS based since they do not require hydrogen if concentrated sulfur-rich streams are not subsequently hydrotreated. Finally, desulfurization processes can be classified based on the nature of the key physico-chemical process used for sulfur removal (Fig. 3). The most developed and commercialized technologies are those which catalyti- cally convert organosulfur compounds with sulfur elimination. Such catalytic conversion technologies include conventional hydrotreating, hydrotreating with advanced catalysts and/or reactor design, and a combination of hydrotreating with some additional chemical processes to maintain fuel specifications. Technologies of this type are discussed further in the Section 3. The main feature of the technologies of the second type is the application of physico-chemical processes different in nature from catalytic HDS to separate and/or to transform organosulfur compounds from refinery streams. Such technologies include as a key step distillation, alkylation, oxidation, extraction, adsorption or combination of these processes. These processes will be discussed in Section 4. 3. Catalysis based HDS technologies 3.1. Conventional HDS: catalysts and reactivity Catalytic HDS of crude oil and refinery streams carried out at elevated temperature and hydrogen partial pressure converts organosulfur compounds to hydrogen sulfide (H 2 S) and hydrocarbons. Detailed analysis of the HDS process is presented in the literature [21,22] so we discuss only the general aspects here. The conventional HDS process is usually conducted over sulfided CoMo/Al 2 O 3 and NiMo/Al 2 O 3 catalysts [21]. Their performance in terms of desulfurization level, activity and selectivity depends on the properties of the specific catalyst used (active species concentration, support properties, synthesis route), the reaction conditions (sulfiding protocol, temperature, partial pressure of hydrogen and H 2 S), nature and concentration of the sulfur compounds present in the feed stream, and reactor and process design. Organosulfur compounds are usually present in almost all fractions of crude oil distillation. Higher boiling point fractions contain relatively more sulfur and the sulfur compounds are of higher molecular weight. Therefore, a wide spectrum of sulfur-containing compounds should be considered from the viewpoint of their reactivity in the hydrotreating processes. In Table 1 some of the organosulfur compounds of interest, namely, mercaptans, sulfides, disulfides, thiophenes and benzothiophenes (BT), and their alkylated derivatives are mentioned with the proposed mechanism of sulfur removal. Of course, for deep desulfurization of refinery streams, polynuclear organic sulfur compounds are also of interest. However, as they are rather stable under conventional HDS conditions we decided not to list them in Table 1. Moreover, their desulfurization reaction pathway is more complex com- pared with alkylated dibenzothiophene, and is not well understood. The reactivity of organosulfur compounds varies widely depending on their structure and local sulfur atom environment. The low-boiling crude oil fraction contains mainly the aliphatic organosulfur compounds: mercaptans, sulfides, and disulfides. They are very reactive in conventional hydrotreating processes and they can easily be completely removed from the fuel. Other processes like Fig. 3. Desulfurization technologies classified by nature of a key process to remove sulfur. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631610 Merox can be applied to extract mercaptans and disulfides from gasoline and light refinery streams [23]. For higher boiling crude oil fractions such as heavy straight run naphtha, straight run diesel and light FCC naphtha, the organosulfur compounds pre-dominantly contain thiophenic rings. These compounds include thiophenes and benzothiophenes and their alkylated derivatives. These thiophene containing compounds are more difficult than mercaptans and sulfides to convert via hydrotreating. The heaviest fractions blended to the gasoline and diesel pools— bottom FCC naphtha, coker naphtha, FCC and coker diesel—contain mainly alkylated benzothiophenes, diben- zothiophenes (DBT) and alkyldibenzothiophenes, as well as polynuclear organic sulfur compounds, i.e. the least reactive sulfur compounds in the HDS reaction. HDS of model organosulfur compounds as well as industrial fuels have been the subject of many investigations (see, for example, [21,22,24–29]). As reaction conditions, reactor type, catalyst, and feed composition vary from study to study, the observed data do not always agree. However, some general conclusions about reaction mechanism and catalyst efficiency can be made based on the published data. HDS of thiophenic compounds proceeds via two reaction pathways (Table 1). Via the first pathway the sulfur atom is directly removed from the molecule (hydrogenolysis pathway). In the second pathway the aromatic ring is Table 1 Typical organosulfur compounds and their hydrotreating pathway Type of organic sulfur compound Chemical structure Mechanism of hydrotreating reaction a Mercaptanes R–S–H R– S – H þ H 2 ! R – H þ H 2 S Sulfides R 1 –S–R 2 R 1 –S– R 2 þ H 2 ! R 1 – H þ R 2 –H þ H 2 S Disulfides R 1 –S–S–R 2 R 1 –S– S – R 2 þ H 2 ! R 1 – H þ R 2 – H þ H 2 S Thiophene Benzothiophene Dibenzothiophene a Reaction pathway for alkylated thiophene, benzothiophene and dibenzothiophene is similar to the reaction of nonalkylated counterparts. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631 611 hydrogenated and sulfur is subsequently removed (hydrogenation pathway). Both pathways occur in parallel employing different active sites of the catalyst surface. Which reaction pathway pre-dominates depends on the nature of the sulfur compounds, the reaction conditions, and the catalyst used. At the same reaction conditions, DBT reacts preferably via the hydrogenolysis pathway whereas for DBT alkylated at the 4 and 6 positions both the hydrogenation and hydrogenolysis routes are significant [26,28]. The reactivity of sulfur compounds in HDS follows this order (from most to least reactive): thiophene . alkylated thiophene . BT . alkylated BT . DBT and alkylated DBT without substituents at the 4 and 6 positions . alkylated DBT with one substituent at either the 4 or 6 position . alkylated DBT with alkyl substituents at the 4 and 6 positions [25,29]. Deep desulfurization of the fuels implies that more and more of the least reactive sulfur compounds must be converted. Since from study to study the parameters of the HDS process differ, the reported values of catalyst activity and selectivity vary a lot. For example, in a continuous-flow reactor, a NiMo catalyst was found to be more active than a CoMo catalyst for desulfurizing 4,6-dimethyldibenzothio- phene (DMDBT) [30]. In contrast, desulfurization of the same sulfur compounds in a batch reactor has been reported to be more efficient with a CoMo catalyst [31]. However, despite the differences in the experimental data, some general conclusions about the performance of NiMo and CoMo based catalysts can be made [21,22]. Conventional CoMo catalysts are better for desulfurization via the hydrogenolysis pathway since the CoMo hydrogenation activity is relatively low and, as a result, relatively little hydrogen is consumed. This makes CoMo catalysts attractive in HDS of unsaturated hydrocarbon streams like FCC naphtha. In contrast, NiMo catalysts possess high hydrogenation activity. Therefore, they are preferable for HDS of refinery streams that require extensive hydrogenation. 3.2. Advanced HDS: catalyst, reactor and processing Deep desulfurization of refinery streams becomes possible when the severity of the HDS process conditions is increased. Unfortunately, more severe conditions result not only in a higher level of desulfurization but also in undesired side reactions. When FCC gasoline is desulfurized at higher pressure, many olefins are saturated and the octane number decreases. Higher temperature processing leads to increased coke formation and subsequent catalyst deactivation. It is also important to note that in practice the severity of the operating conditions is limited by the HDS unit design. Instead of applying more severe conditions, perhaps HDS catalysts with improved activity and selectivity can be synthesized. Ideal hydrotreating catalysts should be able to remove sulfur, nitrogen and, in specific cases, metal atoms from the refinery streams. At the same time they must also improve other fuel specifications, such as octane/cetane number or aromatics content, which are essential for high fuel quality and meeting environmental legislation stan- dards. Hydrotreating efficiency can also be increased by employing advanced reactor design such as multiple bed systems within one reactor, new internals in the catalytic reactor or new types of catalysts and catalyst support (e.g. structured catalysts). The best results are usually achieved by a combination of the latter two approaches, namely, using an appropriate catalyst with improved activity in a reactor of advanced design. 3.2.1. Advanced HDS catalysts To improve catalyst performance, all steps in the catalyst preparation—choice of a precursor of the active species, support selection, synthesis procedure and post-treatment of the synthesized catalysts—should be taken into account. Different approaches have resulted in new catalyst formu- lations (Fig. 4) and some examples are considered here. Applying a new catalyst manufacturing technology, Akzo Nobel introduced in 1998 new, highly active CoMo Fig. 4. Different approaches to improve HDS catalyst performance [32–36, 40 –50]. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631612 and NiMo catalysts [51–53] referred to as STARS (Super Type II Active Reaction Sites). Under usual HDS operating conditions, these catalysts are claimed to desulfurize refinery streams down to 2–5 ppm of sulfur and to significantly reduce polyaromatic content and improve the cetane number and density of diesel fuels. Both CoMo and NiMo catalysts can be used for deep desulfurization but their efficiency is determined by the feedstock properties [51]. The CoMo STARS catalysts are preferable for streams with relatively high sulfur levels of 100–500 ppm and perform better than NiMo catalysts at low pressure. In contrast, the NiMo STARS catalysts are especially suitable for fuels with low sulfur levels (below 100 ppm) at high pressure. Commercial results of STARS catalysts are reported to be promising. They show a stable high level of desulfurization during a long-term run of 400 days on stream. The CoMo STARS catalyst makes it possible to run a conventional HDS unit with output sulfur levels of 10 – 20 ppm for feed rates up to 30% higher than those for which the unit was designed without revamping the equipment [53,54]. Another Akzo Nobel catalyst preparation technology has been claimed to result in extremely active hydrotreating catalysts—the so-called NEBULA catalysts (NEBULA, NEw BULk Activity) [55]. In these catalysts, which are also active in sulfided form, the active phase and the carrier are different in nature from conventional HDS catalysts. The hydrogen consumption is relatively high and these catalysts are suitable for diesel hydrotreating both at medium severity conditions and at high pressure. NEBULA catalysts have already been applied in two commercial units. A similar approach—to enhance catalyst activity by modifying the preparation route—was employed by Criterion Catalysts and Technologies and resulted in Criterion’s CENTINEL catalyst family [56]. The CENTI- NEL catalysts are claimed to combine superior hydrogenation activity and selectivity. At lower H 2 pressures and for high sulfur content streams, CoMo CENTINEL catalysts are preferable. For high H 2 pressures and low sulfur content (below 50 ppm) NiMo CENTINEL catalysts are more useful. Combining new types of active catalytic species with advanced catalyst supports such as ASA (amorphous silica– alumina) [37,38] can result in extremely high desulfurization performance. The application of ASA-supported noble metal based catalysts for second-stage deep desulfurization of gas oil is an example [37,38]. The Pt and PtPd catalysts are very active in the deep HDS of pre-hydrotreated straight-run gas oil under industrial conditions. These catalysts are able to reduce sulfur content down to 6 ppm while simultaneously reducing aromatics to 75% of their initial amount [57]. The PtPd/ASA catalysts are excellent for feeds with low or medium sulfur content and low aromatics levels (Fig. 5). At higher aromatics levels, the Pt/ ASA catalysts perform better than PtPd/ASA. At high sulfur levels, the ASA supported noble metal catalysts are poisoned by sulfur and NiW/ASA catalysts become preferable for deep sulfur removal and dearomatization. Application of noble metal catalysts for deep HDS is limited by their sulfur resistance. Therefore, noble metal catalysts are usually used when most of the organosulfur compounds and H 2 S have been removed from the process stream. A new concept of HDS catalyst design has been proposed to increase the sulfur resistance of noble metal hydrotreating catalysts [39]. The proposed catalyst is bifunctional. It combines catalyst supports with bimodal pore size distribution (e.g. zeolites) and two types of sulfur resistant active sites. The first type of active sites, placed in large pores, is accessible for large organosulfur compounds and is sensitive to sulfur inhibition (sulfur resistant sites of the type I). The second type of active sites, placed in small pores, is not accessible for organosulfur compounds and is stable against poisoning by H 2 S (sulfur resistant sites of type II). Since hydrogen can easily access the sites located in the small pores, it can be adsorbed dissociatively and transported between pore systems to regenerate the poisoned metal sites of type I. Auto regeneration is ensured, so the HDS activity remains high even for feeds with high sulfur content. The concept looks very interesting, although successful application has not yet been demonstrated. Moreover, a number of questions of scientific interest should be solved. Appropriate design of active sites of different sulfur resistance is one of the key feature of this concept. Supports with appropriate texture and surface chemistry must be developed. For example, monolith supports with washcoats of regular structure (discussed in Section 5) might be attractive. 3.2.2. New reactor systems 3.2.2.1. Counter-current operation. Aside from improving the catalysts, upgrading hydrotreating equipment is an option. Conventionally used hydrotreating reactors are fixed-beds with co-current supply of oil streams and hydrogen, resulting in unfavorable H 2 and H 2 S concentration profiles through the reactor. Due to high H 2 S concentration at the reactor outlet, the removal of the last ppm S is inhibited. Counter-current operation can provide a more preferable concentration profile. In counter-current reactor operation Fig. 5. Classification of ASA based catalysts for deep HDS of the feed of different composition [57]. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631 613 mode, the oil feed is introduced into the reactor at the top and hydrogen is introduced at the bottom of the reactor, in the place where its presence is most desired. H 2 S is removed from the reactor at the top, avoiding possible recombination of H 2 S and olefins at the reactor outlet. A commercial example of this approach is the hydrotreating process based on SynSat Technology, which combines Criterion’s SynSat catalysts and ABB Lummus’ reactor technologies [58,59]. The general process scheme is shown in Fig. 6. In the first stage, the feed and hydrogen co-currently contact the catalyst bed and the bulk of the organosulfur compounds is converted. This is followed by the removal of H 2 S from the reactant flow. The second stage of the reactor system operates in the counter-current mode providing more favorable concentration profiles of H 2 S and H 2 over the length of the reactor. Such a configuration allows for application of catalysts that are intrinsically very active but sensitive to sulfur poisoning, such as the noble metal based catalysts. The Scanraff’s SynSat gas oil hydrotreating unit in Sweden uses a noble-metal catalyst in the second stage of the process. Industrial application of SynSat Technologies illustrates the ability of the counter-current approach not only to remove sulfur, but also to remove nitrogen and aromatics as well. It was reported that a sulfur level of 1 ppm and an aromatics level of 4 vol% could be attained [58]. 3.2.2.2. Ebullated bed reactors. The ebullated bed reactor [60] is an example of other types of reactors aimed at HDS of heavy refinery streams, processing of which results in very fast catalyst deactivation due to coke formation. This type of reactor also has good heat transfer so overheating of the catalyst bed is minimized and less coke forms. An ebullated bed is used by a.o. IFP (Institut Français du Pe ´ trole, France) in the so-called T-Star process to desulfurize heavy feedstocks such as deep cut heavy vacuum gas oils, coker gas oils, and some residues [61]. In this unit, the catalyst particles are fluidized by the feed and hydrogen and are therefore well mixed with the feed stream. Bed plugging and channeling are avoided and the unit operates nearly isothermally with a constant low-pressure drop. It is also very convenient that the catalyst activity can be controlled by adding and with- drawing catalyst particles. In comparison with fixed-bed HDS catalysts, the additional requirement for T-Star catalysts is that they be mechanically stable and resist attrition. Integration of the T-Star process with inline hydrotreating produces diesel with less than 50 ppm sulfur and FCC feed with 1000–1500 ppm sulfur, which will result in FCC gasoline sulfur of 30–50 ppm [61]. As an example of the processes employing a ‘special reactor design’ and modified catalyst system for HDS of a large variety of feedstocks, the so-called Prime processes (Prime-G, Prime-G þ , and Prime-D) developed by IFP must be mentioned [62,63]. They combine mild operating conditions with relatively high space velocities utilizing a dual catalyst system. The Prime HDS technology results in minimal olefin saturation in the case of FCC gasoline desulfurization, and polyaromatics reduction and cetane number improvement in the case of gas oils treatment. The Prime technology enables over 98% desulfurization of the entire FCC naphtha cut. Prime reactors fit easily into any refinery configuration and currently five units are in operation. 3.2.3. Combinations of hydrotreating with other reactions Sulfur removal by HDS processes is usually accompanied by other hydrogenation reactions, which are particularly undesired for FCC gasoline streams where olefins are present. Olefin saturation during hydrotreating results in octane loss of the final gasoline pool. Different options of FCC gasoline treatment before or after desulfurization in the HDS unit can be considered to compensate for the loss of octane. 3.2.3.1. Aromatizing and hydrotreating. Aromatizing of the cracked gasoline before HDS treatment was proposed by Phillips Petroleum Co. [64]. By combining pre-aromatiza- tion of FCC gasoline streams with conventional HDS, sulfur content decreases from 300 to 10 ppm and the octane number increases from 89 to 100. Despite almost complete olefin saturation, octane is boosted by increasing the aromatics amount in the end product up to 68 wt%. However, it is fair to state that a high level of aromatics in the final product makes application of the proposed technology less attractive since new environmental rules require a limited amount of aromatics in the gasoline. Fig. 6. Co-current/counter-current Syn Technology process scheme. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631614 3.2.3.2. Hydrotreating and octane boosting (ISAL process 1 ). The ISAL process, which was specially developed for hydrotreating FCC gasoline, combines conventional HDS with post-treatment of the products to minimize the decrease in octane number [65–67]. As in conventional hydrotreating, it saturates olefins present in the feed, but the resulting octane loss is compensated by octane-enhancing reactions. The key point of the process is the catalyst formulation. Due to improved catalyst desulfurization activity and nitrogen and sulfur tolerance, the ISAL process employs one fixed-bed reactor unit with the catalyst system divided in a multiple bed configuration. For example, typically combination of CoMoP/Al 2 O 3 and GaCr/H-ZSM-5 catalysts is applied [68]. The flow scheme of the ISAL process is similar to that of a conventional hydrotreating process. As a result, the ISAL process can be easily implemented as a new process unit or as a revamp of existing hydroprocessing units. It was very efficient at reducing sulfur from 1450 ppm in a naphtha feed to 10 ppm in the final product with almost no decrease in octane number [67,68]. 3.2.4. Catalytic distillation To avoid octane loss with deeper desulfurization, the FCC gasoline stream can be fractionated by distillation before desulfurization and each fraction can be desulfurized at appropriately severe conditions. This option is efficient since the olefins are mainly concentrated in the low-boiling fraction of the FCC naphtha whereas the sulfur compounds are mainly present in the high-boiling fraction. Moreover, the nature of sulfur compounds in light and heavy naphtha fractions is different and, therefore, they can be hydrotreated advantageously at different selective conditions, preserving olefins in the final product. But realizing this approach requires multiple hydrotreating reactors—one reactor per fraction. Combining distillation and reaction in a single vessel is a breakthrough. The elegant technology of sulfur removal employing distillation and HDS (catalytic distillation (CD)) has been introduced by CDTech Company [69– 71]. The process is based on simultaneously desulfurizing and splitting the FCC naphtha stream into fractions with different boiling points. The simplified CDHDS process flow is shown in Fig. 7. The main feature of the process is that, depending on the FCC naphtha properties and desired product specification, a distillation column is loaded with a hydrotreating catalyst at different levels of the column or throughout the whole column. Desulfurization conditions are different for light and heavy fractions, their severity being nicely controlled by the boiling temperature of the naphtha fraction. The lighter fractions, which contain most of the olefins and easily removable sulfur compounds, are subjected to desulfurization at lower temperatures at the top of the column. That leads to higher desulfurization selectivity and less hydrocracking and/or saturation of olefinic compounds. The higher boiling portions, containing heavily desulfurized sulfur compounds, are subjected to desulfurization at higher temperatures at the bottom of the distillation column reactor. The reaction zone cannot overheat since the heat released during the HDS reaction is used to boil the hydrocarbon stream. This leads to nearly perfect heat integration. The CDHDS process efficiency has been demonstrated at Motiva’s Port Arthur, Texas Refinery with the application of a commercially available catalyst loaded in a proprietary distillation structure provided by CDTech [72]. Over the first four months of operation, the technology showed a stable desulfurization level of about 90% with an average octane number loss of less than 1. To improve process feasibility and increase product yield a two stage CDTech w process including CDHydro (production of sweet light cut naphtha with very low mercaptan content and increased octane) and CDHDS processes has been proposed [70,71]. It is claimed that the technology of CDTech w is about 25% less expensive than the conventional HDS process, making it very attractive for refineries. 4. ‘Non-HDS’ based desulfurization technologies Technologies that do not use hydrogen for catalytic decomposition of organosulfur compounds are discussed here as non-HDS based desulfurization technologies. The following approaches are considered to be attractive for attaining high levels of sulfur removal by shifting the boiling point of sulfur-containing compounds, separating by extraction or adsorption, and decomposition via selective oxidation. Fig. 7. Simplified flow scheme for CDHDS based technology. 1 The name of the technology comes from ‘isomerization’ and ‘Salazar’ - name of the technology inventor. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631 615 4.1. Shifting the boiling point by alkylation When the boiling temperature of organosulfur compounds is shifted to a higher value, they can be removed from light fractions by distillation and concentrated in the heavy boiling part of the refinery streams. British Petroleum used this approach in a new advanced technology process for desulfurizing FCC gasoline streams—olefinic alkylation of thiophenic sulfur (OATS) [73–75]. The process employs alkylation of thiophenic compounds via reaction with olefins present in the stream: 2 As a result the boiling temperature of the sulfur containing hydrocarbon compounds increases. In comparison with thiophene (boiling point around 85 8C), alkylated thiophenes such as 3-hexylthiophene or/and 2-octylthio- phene have a much higher boiling point (221 and 259 8C, respectively). This enables them to be easily separated from the main gasoline stream by distillation. The high-boiling compounds produced can be blended into the diesel pool and desulfurized by conventional hydrotreating as the octane number is not important for diesel. The OATS technology consists of a pre-treatment section, an OATS reactor, and a product separation unit (Fig. 8) [73]. Thiophenic sulfur is alkylated in an OATS reactor employing acidic OATS catalysts such as BF 3 , AlCl 3 , ZnCl 2 , or SbCl 5 deposited on silica, alumina or silica– alumina supports [78]. After the OATS reactor, the feed is sent to a conventional distillation column where it is separated into a light sulfur-free naphtha and a heavy sulfur-rich stream. The light naphtha is directly sent to the gasoline pool and the heavy stream is preferably hydrotreated. The hydrotreater is not an essential part of the OATS technology, but its application after the fractionator increases the product yield. Employing the OATS technology, over 99.5% of the sulfur can be removed from the gasoline stream [74,79]. Demonstration exper- iments showed sulfur reduction in gasoline from 2330 ppm to less than 20 ppm with only two octane number loss [74]. Another advantage of the OATS process is that less hydrogen is consumed since only a relatively low volume of the FCC gasoline stream is hydrotreated. The efficiency of the OATS process can be limited by competing processes—alkylation of aromatic hydrocarbons and olefin polymerization. Fortunately, under the conditions employed alkylation of the sulfur-containing compounds occurs more rapidly than that of aromatics. One of the disadvantages of the OATS process is that the alkylated sulfur compounds produced require more severe hydrotreating conditions to eliminate sulfur. To our knowledge, there is no information in the open literature about catalyst durability and other key process characteristics. It seems that many issues must be studied and proven before OATS technology can be commercialized. 4.2. Desulfurization via extraction Extractive desulfurization is based on the fact that organosulfur compounds are more soluble than hydrocarbons in an appropriate solvent. The general process flow is Fig. 8. The OATS process flow scheme. 2 If CH 3 I or AgBF 4 is used as an additional alkylation agent, S- alkylsulfonium salts are formed and sulfur is removed from fuel oil as precipitates [76,77]. As a result, fuel oil can be desulfurized to less than 30 ppm S. The desulfurization level can be further increased by increasing the alkylating agent/sulfur ratio. Taking into account the high cost of alkylating agents, this approach does not seem to be economically feasible on an industrial scale. Another disadvantage is the decrease in olefin concentration due to their reaction with the alkylating agents. I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607–631616 [...]... capabilities of these technologies regarding sulfur removal and protection of other stream specifications attract attention of scientists and engineers and promise the further development and improvement of these processes FCC diesel desulfurization (desulfurization unit 6 in Fig 15) can be done by HDS, using proper process conditions, catalysts with advanced activity and improved formulation, and new reactor and. .. and reactive adsorption; and gas –liquid – solid processes like moving bed application for hydrodemetalization and sulfur removal by reactive adsorption Catalyst preparation and extrusion should be developed further for specific applications, optimizing the structure and active phase distribution Hydrodynamics and transport processes have to be described better to design reliable processes In many of. .. hazards A number of scientific questions should be addressed before downhole catalytic upgrading of crude oil becomes reality, namely the placement of the catalysts into an appropriate zone of the well, mobilization of the reactants (crude oil, H2, and CO) at the catalyst surface, and creation optimal process conditions ( p, T ) Feasibility of each of these options has been proven separately and their application... certainly emerge, while some of those presented here will be skipped from further research Acknowledgements We thank Dr S Eijsbouts (Akzo Nobel, The Netherlands) and Prof J.A.R van Veen (Shell Research and Technology Centre, The Netherlands) for fruitful discussions during the review preparation The authors wish to thank to Dr Tracy Gardner (Delft University of Technology, The Netherlands) for help with the... [138] Hatziantoniou V, Andersson B Ind Engng Chem Fundam 1984; 23:82 631 [139] Irandoust S, Andersson B Chem Engng Sci 1988;43:1983 ¨ [140] Irandoust S, Andersson B, Bengtsson E, Siverstrom M Ind Engng Chem Res 1989;28:1489 [141] Andersson B, Irandoust S, Cybulski A Modeling of monolith reactors in three-phase processes In: Cybulski A, Moulijn JA, editors Structured catalysts and reactors p 267 Chemical... streams is attractive due to the availability of the reacting gas and its low price The main issues of the direct selective oxidation process are operation safety and the formation of by-products (CO2, CO, etc.) We checked the thermodynamic feasibility of selective oxidation of thiophene and benzothiophene assuming the formation of SO2 and hydrocarbons using air as an oxidant It appears to be thermodynamically... predicting the future is not the strongest point of scientists and engineers; so, this field should be closely watched 7 Challenging research topics The above discussed technologies of sulfur removal from refinery streams leads to a wealth of research topics Not all of them are challenging, of course Several topics have the character of demonstration of known science The most challenging ones are given in... applications which are not far out of reach for monoliths Several options exist for application of monoliths in oil refineries They include, but are not limited to, gas phase processes for removal of the last ppm S from gasoline and effluent gases; gas – liquid phase processes aimed at deep desulfurization, denitrogenation and dearomatization and hydrocracking (co- and counter-current) employing catalytic... desulfurization of the feed and upgrading of other diesel specifications like increase of cetane number and decreases in polynuclear aromatics and density A wide range of non-HDS approaches can also be used for deep diesel desulfurization This concerns adsorptive desulfurization and extractive/oxidative separation technologies To be efficiently applied they should be directed to the removal of heavy sulfur... easier) the process will be inefficient and environmentally unfavorable Transformation of S-compounds with sulfur elimination is traditionally the technology of choice A lot of experience exists in HDS, both in catalyst development and process design Usually sulfur is removed from the organic compounds and transformed into H2S The normal process step is conversion of the formed H2S into elemental sulfur . Article Science and technology of novel processes for deep desulfurization of oil refinery streams: a review q I.V. Babich * , J.A. Moulijn Faculty of Applied Sciences, Delft University of Technology, . (synthesis of improved catalysts, advanced reactor design, combination of distillation and HDS) and in ‘non-HDS’ processes of sulfur removal (alkylation, extraction, precipitation, oxidation, and adsorption). example of the processes employing a ‘special reactor design’ and modified catalyst system for HDS of a large variety of feedstocks, the so-called Prime processes (Prime-G, Prime-G þ , and Prime-D)

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