Effect of catalyst to oil weight ratio on gaseous product distribution during heavy oil catalytic pyrolysis

Transcription

1 Chemical Engineering and Processing 3 () Effect of catalyst to oil weight ratio on gaseous product distribution during heavy oil catalytic pyrolysis Xianghai Meng, Chunming Xu, Jinsen Gao, Qian Zhang State Key Laboratory of Heavy Oil Processing, University of Petroleum, Beijing 19, China Received 1 March 3; received in revised form 5 September 3; accepted September 3 Available online 19 November 3 Abstract As for catalytic pyrolysis of Daqing atmospheric residue over catalysts LCM-5 and CEP-1, this paper investigated the effect of catalyst to oil weight ratio on gaseous product distribution in a confined fluidized bed reactor. Cracked gas was determined by gas chromatograph. The results show that the effect of catalyst to oil weight ratio on yields of gaseous products depends on the catalyst used. For LCM-5, at a reaction temperature of 973 K, steam to oil weight ratio.63 and residence time 1.8 s, the total yield of light olefins exceeds 5 wt.% and increases linearly with catalyst to oil weight ratio. For CEP-1, at a reaction temperature 93 K, steam to oil weight ratio of.75 and residence time 1.7 s, the total yield of light olefins pass through a maximum of wt.% with the increase of catalyst to oil weight ratio. Experimental data are analyzed by least square method to produce experimental formulae for yields of ethylene, propylene, butylene and total olefins as a function of catalyst to oil weight ratio. Catalyst type is the key element in the pyrolysis reaction, affecting both the yields of products and the reaction mechanisms. 3 Elsevier B.V. All rights reserved. Keywords: Catalytic pyrolysis; ; Gaseous product; Olefin; Yield 1. Introduction Steam cracking in a tubular furnace is the primary process for light olefins production. However, it is only applicable to feedstocks such as: natural gas, liquefied petroleum gas, naphtha and part of the light distillation cut. In terms of world production capacity, the steam cracking process provides more than 9% of ethylene and about 7% of propylene [1,]. In the year of 1, total ethylene production increased 7% in the world, and 7.% in the Asian Pacific region. In the period of 1995, ethylene consumption in China increased 17.% [3]. In next years it is anticipated that the demand for light olefins will increase greatly. In order to meet the rising demand for light olefins, and to accommodate the trend to heavier feedstocks, the production of light olefins directly from heavy oil has become an important field of research. The key is to develop appropriate catalysts. Russia, Japan, France, USA and Denmark have done a lot of work in this field and have developed Corresponding author. Fax: address: mengxianghai@sina.com (X. Meng) /$ see front matter 3 Elsevier B.V. All rights reserved. doi:1.116/j.cep some catalysts capable of producing high ethylene yields in K []. For many years, research on heavy oil catalytic pyrolysis has attracted great interest in China. The Research Institute of Petroleum Processing (RIPP) [5,6] has developed several new processes and relevant catalysts. These processes are Deep Catalytic Cracking (DCC) and Catalytic Pyrolysis Process (CPP). Besides, Luoyang Petrochemical Engineering Corporation [7,8] has developed Heavy Oil Contact Cracking (HCC) process together with suitable catalysts. Through fundamental research, University of Petroleum Beijing [9,1] has developed much in this area and can provide theoretical guidance to the development of catalytic pyrolysis technologies and catalysts. Theoretically, high reaction temperature, steam to oil weight ratio and catalyst to oil weight ratio, together with short residence time are suitable for light olefin production in heavy oil catalytic pyrolysis processes. These factors vary in their effect on product distribution in the presence of different kinds of catalysts. This paper studied the effect of catalyst to oil weight ratio on gaseous products for catalytic pyrolysis of Daqing atmospheric residue in a confined fluidized bed reactor using catalysts LCM-5 and CEP-1. 转载

2 966 X. Meng et al. / Chemical Engineering and Processing 3 () Table 1 Properties of Daqing atmospheric residue Parameter Value Table Properties of catalyst LCM-5 and CEP-1 Catalyst Density (93 K) (g/cm 3 ).913 Viscosity (33 K) (Pa s).13 BMCI 36 H/C atomic ratio 1.79 Carbon residue (wt.%).3 Group analysis (wt.%) Saturates 59. Aromatics 9.1 Resin and asphaltenes Experimental.1. Feedstock and catalysts In this paper, Daqing atmospheric residue was used as the feedstock, the main properties of which are shown in Table 1. Catalysts LCM-5 (one catalyst for HCC) and CEP-1 (one catalyst for CPP) were utilized as pyrolysis catalysts, and their main properties are listed in Table... Apparatus A confined fluidized bed reactor was utilized as experimental unit. The useful volume of the reactor was 58 cm 3, and the dimension was as follows, the upper side was a cylinder, the inner diameter and the highness of which were 55 and 1 mm, respectively; whereas the lower part was a cone and the highness was 8 mm. The schematic diagram of experimental equipment is shown in Fig. 1. It was comprised of five sections: oil and steam input mechanisms, a reaction zone, temperature control, a product separation and collection system. Distilled water and feedstock were kept in separate vessels and LCM-5 CEP-1 Micro-reaction activity index 3 69 Packing density (g cm 3 ) Pore volume (m 3 g 1 ).11.8 Surface area (m g 1 ) Abrasion index (wt.%).8.1 Chemical content (wt.%) Al O Fe O Particle size distribution (wt.%) m. 1.9 m m >8 m pumped by different pumps. Variable amount of distilled water was pumped into the furnace to form steam, and then mixed with the feedstock pumped by another pump simultaneously at the outlet of a constant temperature box. The mixture was heated to approximately 773 K in a preheater before it entered the reactor, where catalytic pyrolysis reactions took place..3. Operating conditions A summary of run conditions for the main catalytic pyrolysis tests is shown in Table 3... Analytical methods An HP689 gas chromatograph with Chem Station software was used to measure the volume percentage of cracked gas components. (Here, two capillary columns, two packed columns and one porous polymeric column were used. A Fig. 1. Schematic diagram of the experimental unit: (1) Constant temperature box; () steam furnace; (3) feedstock; () electronic balance; (5) oil pump; (6) water tank; (7) water pump; (8) pre-heater; (9) reactor furnace; (1) thermocouple; (11) reactor; (1) inlet for catalyst; (13) filter; (1) condenser; (15) collect bottle for liquid products; (16) gas-collect tank; (17) beaker; (18) gas sample bag.

3 X. Meng et al. / Chemical Engineering and Processing 3 () Table 3 Range of operating conditions Parameters Value Reaction temperature a (K) Water inflow (ml/min) b 1 3 Steam to oil weight ratio.6.75 Residence time c (s) a Reaction temperature refers to the catalyst zone temperature. b is the ratio of catalyst mass in the bed to the overall mass of feedstock processed, here, operating time is kept constant. c Residence time is calculated by the useful volume of the reactor divided by the average volumetric velocity of oil gas in the reactor. Yields of olefins, wt% feed C 3 C 3 C (a)catalyst LCM-5 TCD was used to measure hydrogen, methane, carbon oxide and carbon dioxide at 53 K, and a FID was utilized to measure other components in cracked gas at 53 K.) The equation of state for ideal gases converts these data to mass percentage. The cracked liquid was analyzed with simulate distillation gas chromatogram to get the weight percentage of gasoline, diesel and heavy oil. The weight percentage of coke on catalyst was measured with a coke analyzer. 3. Results and discussion The material balance was investigated firstly, and the data are listed in Table. It is clear that the repeatability in experiments is very good, and the total weight yield of cracked gas, liquid and coke is usually above 9%. The optimized experimental conditions for catalysts LCM-5 and CEP-1 were determined in a series of initial experiments. Under these optimized conditions, reaction temperature, weight ratio of steam to oil and resistance time being kept constant at 973 K,.63 and about 1.8 s, respectively, for catalyst LCM-5, and at 93 K,.75 and about 1.6 s, respectively, for catalyst CEP-1, the effect of catalyst to oil weight ratio was studied via changing the mass of catalyst in the bed. Table Material balance of catalytic pyrolysis Catalyst LCM-5 CEP-1 Temperature (K) Steam to oil weight ratio Residence time (s) Yields of products (wt.%) Dry gas Liquefied petroleum gas Liquid Coke Material balance (wt.%) Loss (wt.%) Yields of olefins, wt% feed C 3 C 3 C (b)catalyst CEP-1 Fig.. Effect of catalyst to oil weight ratio on light olefin yields Effect of catalyst to oil weight ratio on light olefin yields The variation of light olefin yields with catalyst to oil weight ratio for LCM-5 and CEP-1 is shown on Fig.. As can be seen from Fig. (a), the yields of ethylene and overall light olefins increase slightly with increasing catalyst to oil weight ratio, whereas the yields of propylene and butylene are almost unchanged. Moreover, there is a good linear relationship between light olefin yields Y and catalyst to oil weight ratio λ. After analyzing the experimental data by least square method, the relationships between ethylene, propylene, butylene and overall light olefins with catalyst to oil ratio are obtained and as follows: = λ (1) 3 = λ () = λ (3) +C = 3 +C= = λ () It is clearly illustrated in Fig. (b) that the yield of ethylene increases slightly with catalyst to oil weight ratio, while the yields of propylene, butylene and total light olefins all pass through maxima. These results show that the yield of aimed

4 968 X. Meng et al. / Chemical Engineering and Processing 3 () products pass through a maximum and does not increase monotonically with catalyst to oil weight ratio. From Fig. (b), it is apparent that there is a quadratic relationship between yields of light olefins and catalyst to oil weight ratio. Treatment of the experimental data by the least quadratic multiplication technique provides the following expressions correlating yields of ethylene, propylene, butylene and overall light olefins with catalyst to oil weight ratio: = λ.5λ (5) 3 = λ.5λ (6) = λ.3λ (7) +C = 3 +C= = λ.15λ (8) Catalytic pyrolysis of hydrocarbons involves catalytic cracking on acid sites of catalyst and thermal cracking out acid sites. The micro-activity index of catalyst LCM-5 is only 3, indicating the presence of few acid sites. Here, the reaction is primarily thermal cracking, following a free radical mechanism and producing mainly ethylene. Then catalyst s primary function is to offer energy and location for catalytic pyrolysis reactions. However, the micro-activity index of catalyst CEP-1 is 69, indicating that acid sites predominate. In this case, reactions are mainly following a carbonium ion mechanism to yield much propylene and butylene rather than ethylene. Table 5 shows the effect of catalyst to oil weight ratio on the distribution of ethylene, propylene and butylene in total light olefins. For catalyst LCM-5, the weight percents of ethylene, propylene and butylene in total light olefins are about 5, 3 and %, respectively. As catalyst to oil weight ratio increases, the ethylene weight percent increases, while propylene weight percent decreases and butylene weight percent almost remains the same. For catalyst CEP-1, the weight percents of ethylene, propylene and butylene in total light olefins are about 36, 7 and 17%, respectively. With increasing catalyst to oil Table 5 Effect of catalyst to oil weight ratio on light olefin distribution Catalyst Catalyst to oil weight ratio Distribution in total light olefins (wt.%) Ethylene Propylene Butylene LCM CEP weight ratio, propylene weight percent increases, ethylene weight percent is virtually unchanged, but butylene weight percent pass through a maximum in 8 1. As can be seen from Table 5, catalyst LCM-5 favors ethylene production, while catalyst CEP-1 produces more propylene. 3.. Effect of catalyst to oil weight ratio on yields of hydrogen and methane Fig. 3 shows the influence of catalyst to oil weight ratio on the yields of hydrogen and methane. For LCM-5, methane yield increases slightly whereas hydrogen yield increases only marginally. For CEP-1, methane yield shows a minimum and hydrogen yield is almost unchanged. Increasing catalyst to oil weight ratio results in higher reaction degree of pyrolysis. In the case of catalyst LCM-5, the yields of hydrogen and methane increase to different degree, similar to the results for thermal pyrolysis of hydrocarbons, which further approves that catalytic pyrolysis on LCM-5 mainly follows a free radical mechanism. With catalyst CEP-1, methane yield shows a minimum with increasing catalyst to oil weight ratio. At low catalyst to oil weight ratio, catalyst acid sites per unit weight of hydrocarbons are relatively few. Also, catalyst activity becomes Yields of and C, wt% feed Yields of and C, wt% feed C C (a)catalyst LCM (b)catalyst CEP-1 Fig. 3. Effect of catalyst to oil weight ratio on and C yields.

5 X. Meng et al. / Chemical Engineering and Processing 3 () and C to overall light olefins and C to overall light olefins /( = ~C = ) C /( = ~C = ) (a)catalyst LCM-5 /( = ~C = ) C /( = ~C = ) (b)catalyst CEP-1 Fig.. Effect of catalyst to oil weight ratio on the relative content of and C to overall light olefins. lost when the surface is covered by coke during the process of catalytic pyrolysis. As a result, thermal pyrolysis plays a more and more important role, which leads to more methane production. As catalyst to oil weight ratio increases, the acid sites per unit weight of hydrocarbons increases, causing a reduction in the proportion of thermal pyrolysis, together with a concomitant decrease in methane yield. However, at extremely high value of catalyst to oil weight ratio, the degree of pyrolysis reaction becomes very high and yields of methane and hydrocarbon increase correspondingly. The influence of catalyst to oil weight ratio on the relative content of hydrogen and methane to overall light olefins is shown in Fig.. The ratios of hydrogen and methane to total light olefins of catalyst LCM-5 show minima at catalyst to oil weight ratios of Similar is to catalyst CEP-1 at catalyst to oil weight ratio of The desired products of catalytic pyrolysis are light olefins such as ethylene, propylene, etc. Hydrogen, methane, and other gaseous alkanes are side products and must be minimized, since they consume valuable hydrogen and cause difficulty during condensation and separation of cracked gas. The ratios of hydrogen and methane to overall light olefins are indicative of the relative amounts of side products in cracked gas and provide a useful reference for selection of catalyst to oil weight ratio. Light olefins content in cracking gas Catalyst LCM-5, 973K Catalyst CEP-1, 93K catalyst to oil weight ratio Fig. 5. Effect of catalyst to oil weight ratio on the content of total light olefins in cracked gas Effect catalyst to oil weight ratio on the content of total light olefin in cracked gas The effect of catalyst to oil weight ratio on the content of total light olefins in cracked gas is shown in Fig. 5. For catalyst LCM-5, the weight percent of total light olefins in cracked gas is approximately 77%, independent of the catalyst to oil weight ratio. But for catalyst CEP-1, it passes through a maximum of 77% at a catalyst to oil weight ratio of about 1. The results show that the yield of total light olefins is proportional to that of cracked gas and has less side production for catalyst LCM-5. From the analysis above, the conclusions can be drawn that the optimum weight ratios of catalyst to oil are in at 93 K for catalyst CEP-1, and in 1 at 973 K for catalyst LCM-5. However, other factors, such as, heat balance of the process, material quality, equipment and so forth, also play a significant part in the selection of catalyst to oil weight ratio.. Conclusions 1. The effective laws of catalyst to oil weight ratio on the yields of gaseous products vary with the kinds of catalysts.. Under optimized experimental conditions of reaction temperature, weight ratio of steam to oil and residence time of 973 K,.63 and about 1.8 s respectively, the total yield of light olefins (C = C= ) produced with catalyst LCM-5 exceeds 5 wt.%, and increases linearly with catalyst to oil weight ratio. The optimum weight ratio of catalyst to oil is in Catalyst CEP-1 has the best catalyst to oil weight ratio of At optimal reaction temperature, steam to oil weight ratio and residence time of 93 K,.75 and about 1.6 s, respectively, the total yield of light olefins (C = C= ) reaches a maximum of wt.%.. Experimental data are analyzed by least square method to give experimental equations for the yields of ethylene,

6 97 X. Meng et al. / Chemical Engineering and Processing 3 () propylene, butylene and total light olefins as a function of catalyst to oil weight ratio. 5. Catalyst is the key of heavy oil catalytic pyrolysis. Both distribution of products and reaction mechanisms vary greatly with different kinds of catalysts. Acknowledgements The authors thank the Innovation Foundation of China National Petroleum Corporation for financial support. References [1] X. Wang, F. Jiang, The characteristic and foreground of olefin production by heavy oil, Petrol. Process. Petrochem. 5 (7) (199) 1 8. [] H. Wang, B. Qian, Technic development of increasing production for propylene, Petrochem. Technol. 9 (9) () [3] X. Cao, in: Proceedings of the Petrochemical Technology Learning Conference on the Challenge and Countermeasure of Ethylene Industry in China, Chinese Chemical Association, 1. [] Q. Zeng, Ethylene produce of catalytic pyrolysis of hydrocarbons, Petrochem. Technol. 3 () (199) [5] C. Xie, R. Pan, Stugies on producing ethylene and properylene from heavy hydrocarbons by catalytic pyrolysis process, Petrol. Process. Petrochem. 5 (6) (199) 3 3. [6] P. Zhou, Deep catalytic cracking (DCC) technology, Petrochem. Technol. 6 (8) (1997) 5 5. [7] Y. Sha, Z. Cui, et al., Olefine production by heavy oil contact cracking, Petrochem. Technol. 8 (9) (1999) [8] X. Chen, Commercial prospect of heavy oil contact cracking (HCC) for ethylene production, Petrol. Ref. Eng. 3 (6) () 1. [9] Q. Zhang, Preparation and Evaluation of Residue Pyrolysis Catalysts for Olefin Production, Ms.D. thesis, University of Petroleum Beijing, 1. [1] X. Chen, Study on Catalytic Pyrolysis Reaction Kinetics and Reaction Regeneration Simulation of Daqing Residue. Ph.D. thesis, University of Petroleum Beijing,.