Thermal cracking Introduction

Transcription

1 5.3 Thermal cracking Introduction Thermal cracking is the thermal decomposition of straight-run and recycled heavy s at temperatures between about 450 and 540 C under moderate pressure conditions. This operation usually represents more severe thermal processing conditions than visbreaking. Large oil molecules are thermally decomposed into smaller, lower boiling molecules. Thermal cracking enables a refiner to produce cracked naphtha and cracked light products from heavy s (sometimes termed waxy distillate), thus reducing the amount of heavy oils produced. Thermal cracking, as practised commercially, normally involves the recycle of unconverted heavy s, usually to near extinction (Fig. 1). Products from a thermal cracker are: a) thermal tar; b) unconverted heavy ; c) light ; d) naphtha; e) gas. Thermal cracking was invented before 1914 as primarily a means of increasing gasoline production. Today, this use of thermal cracking is essentially obsolete, since from the 1940s thermal cracking has been progressively replaced by Fluidized Catalytic Cracking (FCC) as the main conversion process for producing gasoline in a complex refinery. Today in Western Europe, use of thermal cracking is mainly restricted to some of the refineries without FCC process units. A common fractionation system with a visbreaker unit is generally employed (Fig. 2) which produces heavy s for recycle to the thermal cracker. The recycle ratio (of recycled heavy s to fresh feed) is often adjusted to maximize cracked production. The thermal cracking process can be regarded as mixed-phase; although the reacting heavy is in the liquid phase, some vapour generation occurs as gasoline range and lighter components are formed in the cracking reactions. Usual kinetic considerations apply in terms of temperature, pressure and residence time, but the absence of a catalyst simplifies both chemistry and engineering considerations. Thermal cracking reactions are carried out with a relatively short contact time of a few minutes in the tubes of a furnace coil. The furnace provides the slightly endothermic heat of reaction, as well as heating the reactor contents from furnace inlet to outlet temperature. Main operating variables are the outlet temperature, unit throughput and recycle ratio. These, together with the feedstock type, affect the severity of cracking, which is usually measured in terms of conversion per pass (see Chapter 5.2). Sufficient pressure is required at the furnace inlet to keep and heavier reaction constituents in the liquid phase at the furnace outlet (minimum coil outlet pressure, typically 7 bar). Some thermal cracker licensors prefer significantly higher pressures to minimize the amount of coking and, hence, maximize run lengths between decoking. The maximum reaction temperature is limited by the selected metallurgy (alloy steel) for the furnace tubes. fresh feed recycle thermal cracker furnace Fig. 1. Simplified flow diagram of reactor section of a thermal cracker. heavy quench to fractionation section VOLUME II / REFINING AND PETROCHEMICALS 239

2 THERMAL CONVERSION PROCESSES Fig. 2. Block flow diagram of combined thermal cracker/visbreaker unit. residue gas naphtha HGO quench heavy (HGO) recycle tar heavy recycle Process evolution Thermal cracking was first commercialized by William Burton in the USA (Burton, 1913), using a pressurized heated tank (boiler), condenser and receiver in a batch process to crack paraffinic s in the boiling range F ( C) to gasoline. Development of thermal cracking processes followed, spurred on by the rapid growth of the automobile industry. A continuous process was then invented by Jesse Dubbs and his son, Carbon Petroleum Dubbs, by circa 1919 (Dubbs,1921); the so-called Dubbs Process was then further improved by Gustav Egloff (Egloff, 1925). Oil charge was fed to a heating coil and then delivered to a vapour separator vessel (later termed an evaporator). Egloff s improvement involved the relatively mild heating of heavier oils in a separate coil. This then permitted the use of topped crude as a charge to the process. The topped crude was fractionated in a separate distillation column, and the bottoms could then be subjected to a separate mild cracking operation in a coil operating at lower temperatures. This formed the basis of the Dubbs Process (Murphree and Ciprios, 1962). Various other rival thermal cracking processes, such as the Winkler-Koch and Donnelly, were also developed at about the same time (Nelson, 1941), and soon the rival processes were caught up in patent litigation. Further developments followed, for commercial thermal crackers over the next few decades included: Limitation in conversion per pass to 10-20% to slow down coke deposition and gas production with recycle of unconverted to the heating coil. Charge feed became a mixture of fresh and recycle charge. Use of a suitable quench stream (typically heavy atmospheric ) to limit reaction time and, hence, overcracking. Development of combination processes involving a combination of thermal cracking and visbreaking. This then allowed a feedstock of atmospheric residue as practised in the Dubbs process (with Egloff s improvement) which also used a downflow soaking chamber to complete the reaction. The use of a two radiant cell furnace design containing separate radiant heating and soaking sections. Post-Second World War vehicle engines demanded a higher octane, and the invention of the fluidized catalytic cracker led to the demise of conventional thermal cracking. Gasoline produced by thermal cracking of heavy has an octane rating expressed as (RON+MON)/2 (where RON = Research Octane Number and MON = Motor Octane Number) of typically between about 60-70, whereas a modern FCC produces gasoline with a (RON+MON)/2 of approximately 86. By the early 1960s, conventional thermal cracking on its own to produce gasoline as the primary product was largely obsolete. M.W. Kellogg Ltd. (MWKL) and its parent, Kellogg Brown & Root (KBR), have designed a significant number of thermal crackers over the last 50 or more years, often in combination with visbreakers (see Chapter 5.2). The use of downflow soaker drums had, by this point, largely been discontinued. Thermal cracking has survived commercially into the new millennium in Europe and elsewhere, typically, in a combined thermal cracking/visbreaking process. The emphasis of the process and its yields has, however, shifted from the production of gasoline range products to operations to maximize production of diesel range material (light and heavy cracked gas oils), as finished diesels are very much in demand in Europe. The fresh feeds to the unit are, typically, separate feeds of heavy s (to a thermal cracker 240 ENCYCLOPAEDIA OF HYDROCARBONS

3 THERMAL CRACKING coil) and residue (to a single pass visbreaker). The recycle ratio for the thermal cracker is defined as: recycle ratio = fresh heavy feed recycle cracked heavy s fresh heavy feed A common fractionation system with the visbreaker is used with heavy atmospheric (and also ) cracked s, forming the recycle back to the thermal cracker coil. Maximum cracked production is achieved by suitable optimization of the conversion per pass and recycle ratio in the thermal cracker coil, with use of a higher recycle than that used for maximum gasoline operation Chemistry, thermodynamics and kinetics The main reactions taking place in thermal processing are many, and their interactions complex (see Chapter 5.2). Thermal cracking of heavy s is carried out under medium severity conditions with a maximum temperature of about 540 C (more severe temperatures than the mild conditions of visbreaking, but less severe than the thermal cracking of lighter hydrocarbons). Two types of thermal cracking reaction predominate (Murphree and Ciprios, 1962; Asinger,1968): Carbon-carbon bond scission, which usually results in the formation of smaller paraffinic and olefinic molecules; dehydrogenation (C H bond scission), giving rise to the formation of olefinic molecules, aromatics and hydrogen. More details of the reactions related to carboncarbon bond scission, dehydrogenation, isomerization and polymerization are given in Chapter 5.2, as the reaction mechanisms are essentially the same for both thermal cracking and visbreaking. At low conversion per pass, the overall reaction kinetics are first order (see Chapter 5.2). The typical feedstock of or heavy atmospheric to a thermal cracker is lighter than residue fed to a visbreaker. This means that at a given reaction temperature, there is a lower tendency to form coke in thermal cracking than in visbreaking. Feedstock conversion Conversion per pass is defined by KBR & MWKL as the volume per cent conversion per pass of feed converted to C range material from 350 C TBP material. Light gases and range components are excluded from this measure of conversion. Conversion for a given feedstock is selected to achieve a stable fuel oil product. Reaction severity Reaction severity can be measured in terms of the conversion per pass. The higher the conversion, the higher the severity. Conversion is influenced by coil exit temperature (main operating variable), feedstock type and operating pressure (an important design/operating variable). Fuel oil stability This can be measured by a P (Peptizing)-Value Test. Stability of a fuel oil can, more generally, be confirmed using internationally accepted hot filtration tests such as ISO and ISO (see Chapter 5.2. for more information on the P Value Test and the impact of chemistry on fuel oil stability). Yield data A typical set of yield data for the design of an earlier commercial visbreaker/thermal cracker complex in the 1960s is given in Table 1. This plant fed a mix of Libyan/local atmospheric residues to a single pass coil visbreaker and recycle of cracked waxy distillate ( ) from a flasher to a separate recycle waxy distillate furnace coil (Fig. 3). The set of yields were obtained for a recycle ratio of Conversion per pass for the (already partly cracked) heavy s in the recycle coil was moderate with a coil exit temperature of 485 C. Conversion per pass in the visbreaker was set by fuel oil stability with a coil exit temperature of 477 C. Higher conversions and, hence, higher yields are possible in the thermal cracker of a modern MWKL combination thermal cracker/visbreaking unit. This is achieved by using partly fresh feed (straight run heavy s) and partly recycle feed in the thermal cracking coil, rather than just a recycle stock. In this case, the feed can be described as being less refractory, permitting a higher level of conversion. Table 1. Product yields for design of 1960s combined thermal cracker/visbreaker Product Yield (wt per cent) Dry gas 2.2 C C C 11.9 Gas oil 29.2 Blend stock 8.9 Vacuum tar 46.8 Total 100 VOLUME II / REFINING AND PETROCHEMICALS 241

4 THERMAL CONVERSION PROCESSES atmospheric residue recycled waxy distillate quench The effect of increased operating pressure is to reduce the propensity to form coke by side polymerization/condensation reactions. However, the improvements in run-length between decoking and yields need to be balanced against the increased capital costs of designing the thermal cracker furnace and feed system for higher pressures. Different companies in the industry have different views on the choice of operating pressure for a thermal cracker Processes visbreaker furnace termal cracker furnace Fig. 3. Reaction section of early 1960s Kellogg visbreaker/thermal cracker complex. to common fractionator system Dubbs process The Dubbs process of the Universal Oil Products (UOP) is the original two-coil cracking plus downflow soaker drum process (Murphree and Ciprios,1962) which produces cracked products from atmospheric residue, and which was effectively made obsolete by the 1940s/1950s due to the advent of the FCC. Shell thermal gasoil process This is a combined residue and waxy distillate ( ) conversion process (Douwes et al., 1999) with a configuration similar to the MWKL process referred to in Section 5.3.2, except that the process uses an upflow soaker drum and a cyclone downstream of the visbreaker coil. Typical feedstock is atmospheric (or ) residue. Combined KBR and MWKL coil thermal cracking/visbreaking process A modern KBR & MWKL thermal cracking/visbreaking process is described in Fig. 4. This uses partly fresh feed (straight run heavy s) and partly recycle feed in the thermal cracking coil, rather than only heavy cracked recycle stocks (used in the 1960s MWKL unit). This permits a higher conversion per pass than if just a recycle waxy distillate stock is used, with a resultant increase in cracked distillate production. Typical yields for the thermal cracking furnace are given in Table 2. For typical yields over the visbreaker furnace, see Chapter 5.2. Thermal Cracker (TC) feed consists of fresh Heavy Gas Oil (HGO) and/or Heavy Vacuum Gas Oil (HVGO) feeds, which are supplemented with HGO and HVGO, recycled from the visbreaker(vb)/thermal cracker atmospheric and columns. The TC feed is heated through a VB tar /TC feed exchanger before entering the TC Furnace. The TC furnace is a two-pass, twin cell box furnace arrangement. The TC charge first picks up heat in the convection section, and is then further heated in the radiant heater cell before entering the soaker cell. Soaker cell firing is regulated to give an outlet temperature of approximately C, depending on the required conversion and run length requirements. The outlet pressure is in the range bar. Thermally cracked products leave the soaker cell and enter the transfer line, and quench is immediately provided through a quench valve of special design by recirculated HGO from the main fractionator. The furnace effluent then enters a low pressure thermal tar separator (usually termed as an evaporator ). The effluent vapour from the separator is subjected to further cooling in the transfer line, then enters the flash zone of the main fractionator. Bottoms liquid from the thermal tar separator is further quenched (with cooled visbroken bottoms) and routed, either directly to the Table 2. Typical product yields for thermal cracking of derived from a high sulphur crude C 4 Product Yield (wt per cent) 8.8 C C 28.8 Light 46.8 HGO/HVGO 0.0 Thermal tar ( 530 C) 15.6 Total 100 These yields are typical for a particular feedstock and conversion level. 242 ENCYCLOPAEDIA OF HYDROCARBONS

5 THERMAL CRACKING Fig. 4. Simplified flow diagram of a commercial thermal cracker/visbreaker unit. residue HGO/HVGO recycles fresh HGO quench visbreaker furnace TC furnace gas unstabilized naphtha kerosene to side stripper LGO to side stripper HGO quench HGO recycle HGO quench overheads to ejector system LVGO product HVGO recycle slop wax VB, TC feed BFW tar steam to fuel oil blending flasher, or to a preflash tower ahead of the flasher. Feeds into the flash zone of the main fractionator comprise vapours from both TC and VB evaporators. Liquid flowing down from the flash zone combines with a cooled tower bottoms flux stream to become fractionator bottoms, once light-ends are stripped out by injected low pressure steam. The vapour leaving the top of the flash zone is cooled in the evaporator section by a spray of HGO pumpback reflux. The vapour enters the top section of the main fractionator, where a separation into column overheads, LGO and HGO is effected through the baffles and fractionation trays or packed beds. HGO liquid is drawn off the chimney tray by pump. A hot portion of the circulating HGO stream constitutes the pumpback reflux to the main fractionator. Cold HGO is used as pumparound to the fractionator, or as recycle feed via another surge drum and pump to the feed surge drum. The LGO draw-off is steam stripped in an LGO stripper and then further cooled. The fractionator overhead vapour is partially condensed before entering a fractionator reflux drum. In this drum, liquid hydrocarbon is separated from visbreaker gas and sour water. Part of the hydrocarbon liquid from the drum is pumped to the top tray of the main fractionator as reflux, and the balance of the hydrocarbon liquid (unstabilized VB naphtha product) is pumped to battery limits for processing elsewhere. VOLUME II / REFINING AND PETROCHEMICALS 243

6 THERMAL CONVERSION PROCESSES Flashed vapour from the drum is typically compressed and routed elsewhere. Sour water is pumped to the battery limit. Main fractionator bottoms, together with tar separator bottoms, form the feed to a flasher tower. Typically, the flasher contains four beds plus stripping trays that separate the atmospheric tar into overhead vapour (to ejector system), LVGO, HVGO, slop wax and tower bottoms (visbroken tar). The slop wax stream is typically recycled as a quench flux medium to the main fractionator, or can be added to the bottoms. Part of the LVGO is used as reflux (bed two), and part of the HVGO is also used as reflux to the wash section (bed four). Stripping steam is injected below the bottom tray. The tower bottoms, at about 370 C, are pumped through filters and cooled against VB and TC feed preheat exchangers. Cooled tower bottoms are also recycled to the base of the main fractionator as a quench to minimize coking, and to the drums located downstream of the furnace effluent quenches. References Asinger F. (1968) Mono-olefins. Chemistry and technology, Oxford, Pergamon Press. Burton W.M. (1913) US Patent to Standard Oil of Indiana. Douwes B.A. et al. (1999) Shell thermal conversion technology in modern power integrated refinery schemes, in: Proceedings of the National Petroleum Refiners Association annual meeting, San Antonio (TX), March. Dubbs C.P. (1921) US Patent Egloff G. (1925) US Patent to UOP. Murphree E.V., Ciprios G. (1962) Cracking and reforming, in: Institute of Petroleum, Modern petroleum technology, London, IP, Chapter 9, Nelson W.L. (1941) Petroleum refinery engineering, New York-London, McGraw-Hill. David Bosworth Kellogg, Brown & Root Houston, Texas, USA 244 ENCYCLOPAEDIA OF HYDROCARBONS