Development of HS-FCC (High Severity FCC) Process

Transcription

1 2 International 5 Development of HS-FCC (High Severity FCC) Process Masaki Yatsuzuka (Petroleum Energy Center) Yuichiro Fujiyama (Central Technical Research Laboratory, Nippon Mitsubishi Oil Corporation) 1. Introduction Development of high-severity fluid catalytic cracking (hereinafter, HS-FCC ) began in 1994 as a new FCC process for increasing production of light olefins. At the initial stage of development, catalyst development and optimization of reaction conditions took place at the Central Technical Research Laboratory of the former Nippon Oil Co., Ltd., as a supported study by the Petroleum Energy Center (hereinafter PEC ). Later in 1996, development of the HS-FCC process became a theme of international cooperative research by PEC with Saudi Arabia. In this joint research project, development proceeded to the process itself, a 3 B/D demonstration plant will be constructed and the conduct of demonstrative operations is targeted using the plant. In order to propel the joint research project, a joint research center was established at the King Fahad University of Petroleum and Minerals in Saudi Arabia and a HS-FCC specialized working group was established at the PEC in Japan. The research covered a broad spectrum, including reaction analysis, catalyst development, circulation fluidized bed design and more, but in this paper, the overall status of development of the HS-FCC process will be discussed. Table 1 Time Schedule Up To FY 2 Item Basic Research Evaluation of HS-FCC economy Schedule year MAT testing Development of HS-FCC simulator Concept design of HS-FCC process HS-FCC Pilot Research Design/Assembly Installation/Test operation Operational research Cold Flow Model Research Design/Construction Operational research 1

2 2. Optimization of Reaction Conditions and Catalyst Development Concurrently, by shortening the contact time period, over-cracking and other secondary reactions were curtailed. Because the reaction takes place at high temperature, HS-FCC uses two times the amount of C/O ratio used by conventional FCC, or even more. High C/O ratio magnifies the contribution by catalytic cracking and it has the effect of curtailing thermal cracking, which becomes predominant at high temperatures. In the present study, a MAT (Micro-Activity Test: in accordance with ASTMD-397) and a circulation fluidized bed type pilot plant on the scale of.1 B/D were used to investigate reaction conditions. The pilot plant was equipped with a riser (up-flow type reactor) the same as in conventional FCC. From the results of experiments in which these devices were used, the reaction conditions of HS-FCC were determined within the range of reaction temperature at 55 to 65 C and contact time period at.5 to several seconds. In the development of catalyst, emphasis was put more on olefin selectivity than on cracking activity and the amount of acid in the catalyst was adjusted. In addition, an additive for increased production of olefin was developed. By combining these reaction conditions, catalyst and additive, a high olefin yield and gasoline yield could be obtained (Fig. 1). Yield mass (%) Ethylene Propylene Butenes Gasoline 2 LCO+HCO Others Conventional HS-CC Conditions FCC Mode-1 Mode-2 Catalyst Commercial Developed Figure 1 Results of pilot plant study (riser) 2

3 3. Selection of Reactor In the riser type reactor used with conventional FCC, the phenomenon called back mixing is known to occur. When back mixing occurs, not only does the distribution of catalyst/gas retention time become widespread. Additionally, the distribution of catalyst concentration in the direction of the reaction tube radius becomes uninformed 1, 2). The deviation in the grain speed/concentration distribution within the riser in the course of back mixing becomes especially severe at times of high grain flow rate 3). Because HS-FCC is operated at high C/O ratio, as described previously, this condition becomes markedly disadvantageous. A broad catalyst/gas retention time distribution, as found in riser, is undesirable because HS-FCC needs a very short contact time to avoid hydrogen transfer, over-cracking and other secondary. Accordingly, it was decided to use a down-flow type reactor for HS-FCC. In order to verify the superiority of the down-flow type reactor, a pilot plant on the.3 B/D scale, equipped with a down-flow reactor, was constructed at the joint research center in Saudi Arabia. The performance of this plant was compared with that of a riser type pilot device. In order to abstract and investigate the differences in performance between down-flow reactor and up-flow reactor, reaction results were compared with both pilot plants under the same reaction conditions as the HS-FCC conditions (high temperature, short contact time, high C/O ratio). Gasoline yield in proportion to light olefin yield is presented in Fig. 2. As severity is further elevated, the light olefin yield increases, but if the severity is elevated excessively, the region of over-cracking is reached and gasoline yield starts to decline. Using the down-flow reactor, as compared to the riser reactor, the gasoline yield trended at a higher value because gasoline cracking is curtailed. And with the same gasoline yield, the yield of light olefin was about 4% higher. Using the down-flow reactor, the yields of light olefin and gasoline were high, but dry gas yield was reduced (Fig. 3). These facts suggest that over-cracking and thermal cracking reactions can be curtailed by using a down flow reactor. 6 Down Flow Reactor Gasoline mass (%) 4 2 Riser Figure Light Olefin mass (%) Comparison of two types of reactors 3

4 1 Dry Gas (H2-C2) mass (%) Down Flow Reactor Riser Conversion (< 221 C) mass (%) Figure 3 Dry gas yields 4. Design of Catalyst Circulation System In order to use a down flow reactor, a catalyst circulation system including a stripper/regenerator was designed. An outline of the HS-FCC reaction system is presented in Fig. 4. A cold flow model was constructed in the Central Technical Research Laboratory of Nippon Mitsubishi Oil Corporation on consignment of PEC, to verify the performance of the catalyst circulation system. This device was equivalent in size to the 3 B/D demonstration plant, measuring about 2 m in height. It was designed for pressure balance by having the catalytic layer static pressure in the stripper compensate for nearly all the pressure loss in each component. The regeneration tower was a combustor type in which the top is connected to the lift line. The entire volume of air for regeneration is directed as such to the lift line and used as lift medium. For this reason, catalyst can be lifted without using surplus gas or motive power. From cold flow experiments, it was learned that unless the regenerator is operated under specific conditions, classification of catalyst grains takes place. Using this device, experiments were conducted under various conditions, and it was found that with a large volume of catalyst circulation, equivalent to C/O ratio = 42, a balance in pressure is achieved and operation is possible. 4

5 Figure 4 Configuration of reactor and regenerator section 5. Synopsis Demonstration studies are scheduled from the year 2 to 24. Plans for construction of demonstration plant within the oil refining compound of Saudi refinery and negotiations covering construction conditions, etc., are currently in progress. Through technological cooperation with Saudi side which holds such extensive influence throughout Saudi Arabia, contributions can be made to strengthening of ties between both nations, and if demonstration studies are completed successfully, Japan s unique FCC process, which increases olefin production, can be applied. The next stage beyond the research stage -- the stage of demonstration plant construction/operation -- will soon be reached. In progressing to equipment operation, even more careful attention will have to be given to safety. We want to proceed with developments cautiously. 1) Z. Wang, et al, Powder Technology, 7, 271 (1992). 2) K. Kato, et al., 3rd International Conference on CFB, Niigata, October, (199), p145. 3) Y. Yang, et al., 3rd International Conference on CFB, Niigata, October, (199), p21. Copyright 2 Petroleum Energy Center all rights reserved. 5