computational fluid dynamics modeling of high density … · computational fluid dynamics modeling...

sAmerican Journal of Oil and Chemical Technologie

ISSN Print: 2326- 6570 ; Online: 2326- 6589 Page: 1-6 Volume 1, Issue 1, March 2013

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Computational Fluid Dynamics Modeling of High Density Poly Ethylene

Catalytic Cracking Reactor

Bagher Anvaripour‚ Mohammad Shah Bin‚ Zahra Maghareh Chemical Engineering Department‚ University of Putra‚ Malaysia

Abstract: The reactor of High Density Poly Ethylene –HDPE – conversion into liquid fuel was studied and modeled. The calculation software: CFD FLUENT was applied for this study. The software is a powerful tool in simulating chemical reactions. The simulation of HDPE catalytic cracking reactor was implemented by using this software and inserting both kinetic and thermodynamic conditions of the reaction in Isothermal state. To validate the simulation, the software output graphs including product compositions were put under comparisons with GC gas chromatography graphs. On the other hand, the non-catalytic reaction was modeled and the results were compared to catalytic cracking outputs. On the basis of the received data by simulation, the product content in catalytic cracking had higher values than thermal cracking reaction. Keywords: Computation software FLUENT, simulation, modeling, catalytic cracking, Isothermal kinetic conditions, HDPE 1. Introduction: Based on the latest studies, the best method of waste plastics disposal can be served as their conversion into lighter hydrocarbons. To accomplish this method, the cracking processes (chemical decompositions, here: pyrolysis) can be utilized. These processes are implemented in two cases of catalytic cracking and non-catalytic cracking [1, 2]. The reaction can be handled by heating the polymer in an inert medium –paralyzing– where the process is called Thermal Cracking. In the case of using a suitable catalyst, the process conditions are simplified which cause the production content to be raised. This case is meaningfully called Catalytic Cracking. Most of researches on this field deal with Thermal cracking [3]. Due to the difficulties involved in this method, this study –paper– can have a control degree of products efficiencies including products distributions by using a well designed reactor along with a suitable catalyst engagement; as well, the reaction temperature can be reduced to amounts of not- hardly procured. In this study, the experimental application of HDPE cracking reactor was assessed laboratorial the first; and then secondly the simulation of the process of HDPE cracking into light Hydrocarbons was modeled by computational software CFD Fluent. And finally the third; for certainty of simulation validity, comparisons and jurisdictions between experiment results and simulation outputs were brought into account. 2. Experiments: At first effective parameters on HDPE decomposition into lighter Hydrocarbons were identified and their corresponding impacts were evaluated. Experiment initial design was based on temperature and catalyst fraction in order to catalytically crack HDPE to middle distillates [4]. According to the design, the reaction was implemented in a non-stirring reactor with electric thermal jacket as a semi-continuous atmospheric media. In all experiments the polymer weight was kept constant equal to 100 gram HDPE . A definite amount of catalyst in each experiment entered the reactor simultaneously with feed entrance. Heating to the reactor started, after evacuations of the reactor inner air and substituting Nitrogen as an inert gas. Owing to absence of stirring, different thermal stages were opted to optimize the mixing process. After procuring the desired temperature in the reactor, the polymer cracking reaction initiated and products exhausted the reactor as gaseous phase. With attention to studies of references [5, 6, 7, 8], temperature of 390 0 c was opted as the first temperature stage for the reaction. The other temperature stages were elevated with intervals of 200 c up to the terminal stage of 550 0 c. The option of temperature stages according to the design is shown in table 1.

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Petrotex



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After the experiments accomplished in determined conditions, assessing the obtained results showed that temperature of 430 0c and catalyst fraction of 30% were identified as the best determinations for the most preferred production. In table 2 are shown the GC Mass gas chromatography results of the liquid production in states of 430 and 450 0 c for two cases of catalytic cracking and non-catalytic cracking. After achieving the reaction results experimentally, now we shift to simulation of the HDPE cracking reactor computationally. 3. Modeling of HDPE reactor of cracking into light Hydrocarbons: In this part, the reactor of the desired reaction is to be modeled by CFD software: FLUENT 6.1.18. This powerful software is able to simulate chemical reactions with respect to the reactions’ activation energies and the reactions’ constants in studied temperatures. Then the software presents the concentration profiles of the reactions’ constituents [9]. 3.1. Applied conditions in simulation of reactor for HDPE catalytic cracking into Petrol: The reactor for Poly Ethylene conversion to light Hydrocarbons is modeled with considering the following conditions. These conditions were provided with accordance to the experimental conditions in previous part to have proper comparisons of the output results with experimental GC results.

1. The product compositions fell between C1 to C16 and the remaining solid was assumed to be totally coke. 2. The inserted Poly Ethylene in simulation had properties of the Poly Ethylene with 8000 carbon chains. 3. The reaction was assumed to be homogenous with constant volume. 4. The reaction mechanism was assumed as aA-bB-cC-dD-…. [10]. 5. The reaction occurred in single phase of gaseous phase. 6. The reaction activation energy equaled E=70.125 KJmole-1 and the reaction constant equaled A=6555.4 [11]. 7. The reaction occurred in an Isothermal temperature of 430 0c. 8. Operational pressure was atmospheric. 9. The catalyst fraction over the reactants was 30 over 100. 10. The flow stream in the reactor was Laminar. 11. The reactor was made of Steel Carbon 50%. 12. The reactor dimensions were 25*60.

3.2. Modeling the reactor for HDPE non-catalytic cracking: In this part of the study, HDPE thermal cracking reactor is modeled. The thermodynamic conditions of the reaction in this part was similar to previous part with the difference that the reactant in this part included 100 gram HDPE with presence of no catalyst. Due to the absence of the catalyst, the activation energy equaled 33660 Jmole-1 and reaction constant equaled 3445 [12]. Ultimately after modeling, products in these two cases were compared for determination and valuations of the effects of catalyst functions; furthermore, the outputs were compared with the experimental results for verifications of simulation validity. 4. Results and discussion: For modeling the reaction, in both cases was utilized two dimensional rectangular meshes with a number of 6171which is shown in Figure 1. This meshing grid was implemented by pre-processing software Gambit in form of 2D. With respect to kinetic and thermodynamic conditions of the reaction in 430 0c and catalyst fraction of 3catalyst/10reactants, products’ contours can be observed as Figure 2. As well, the products’ contents are available in Figure3 in mole fractions. As it is found in Figure2, most of products in this reaction comprised Light Saturated Hydrocarbons which were in accordance with the analysis results. This was a verification to validate the simulations to an acceptable criterion. Of course as the contours represent; because of absence of a carrier for products, it was assumed that the products did not exhaust the reactor. Thus the purpose of the study was the amounts and compositions of the products in the reactor and so their comparisons with experimental data.



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Figure 1:Meshing grids of the reactor for HDPE cracking by Gambit Software. Now in Figure 2 are shown the yielded amounts of products on the basis of the proposed mechanism and the present kinetic conditions:

Figure 2: Mass fraction contours of the products for HDPE Catalytic cracking As Figure 3 represents, the production includes mostly Saturated Hydrocarbons, i.e. HDPE has a great tendency to be catalytically cracked into lighter hydrocarbons. This result is very harmonious to the results for products’ analysis in Table 2.



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Figure3:Comparison of production contents in HDPE catalytic cracking In following, the non-catalytic cracking product graphs are to be put under assessment. In Figure 4 are shown the contours of the reaction products. As it is viewed in these graphs, the percentage of middle distillates i.e. the desired and optimum production of the cracking process in thermal non-catalytic case had less amounts than the case of catalytic cracking which preserved the validity of the modeling.



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Figure 4: Contours of HDPE thermal cracking Comparing the received contours of Figures 2 and 4, it is perfectly obvious that the amount of HDPE conversion into lighter Hydrocarbons was greater in catalytic cracking with comparison to thermal non-catalytic case. This matter is harmoniously visible through Figures 3 and 5 too.



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Figure 5: Comparison of product contents of HDPE different cases cracking

5. Conclusion: Paying attention to implemented experiments and also the results of simulation for HDPE conversion into light middle distillates, it was found out that the desired conversion can be well received and so perceived by the determined cracking processes [1, 2, 3, and 4]. Throughout lab studies and experimental assessments it was operationally resulted that the most desired and the most substantial products of the decomposition process were the family of Light Saturated Hydrocarbons i.e. Alkanes. The synonymous result was then achieved by simulations accomplished by the CFD software: FLUENT 6.1.18. It would be of great notice that the study had the multiple and various aspects of theory, kinetics, thermodynamics, experiments and computational modeling simultaneously. 7. References: [1]B.Roozbehani Catalyst construction report for heavy polymer disposals conversions to Naphtha. Research and development center of Bandar Imam petrochemical plant. [2]Chemical recycling of post-consumer polymer waste over fluidizing cracking catalysts for producing chemicals and hydrocarbon fuels. Ta-Tung Wei, Ken-Jer Wu, Sheau-Long Lee, Yeuh-Hui Lin. Resources, Conservation and Recycling 54 (2010) 952–961 [3]Characteristics of liquid product from the pyrolysis of waste plastic mixture at low and high temperatures: Influence of lapse time of reaction Kyong-Hwan Lee, Dae-Hyun Shin. Waste Management 27 (2007) 168–176. [4] Fei Ding, Lian Xiong, Cairong Luo, Hairong Zhang, Xinde Chen. Kinetic study of low-temperature conversion of plastic mixtures to value added products. Journal of Analytical and Applied Pyrolysis 94 (2012) 83–90. [5]S.M. Al-Salem, P. Lettieri. Kinetic Study of High Density Polyethylene (HDPE) Pyrolysis. Chemical Engineering Research and Design 88(2010)599–1606. [6] John Doe "Catalytic Digradation of High Density Polyethylene overe a copper oxide Catalyst" Department of Chemical Engineering Ohio University December 2000. [7] K .R. Venkatesh , G.D.Holder , J.W.Tierney , I . Wender . Dept. of Chem & Petroleam Engineering ,1994. [8] A.Buekens , H . Huang "Catalytic plastics Cracking for Recovery of Gasoling Range Hydrocarbons from Plastic Wastes" , Resources , Conservation and Recycling 23(1998) 163-181 [9] Zhang Zhibo , Suehiro Nishio et al. , "Thermal and Chemical Recycle of Waste Polymers" , Catalysis Today 29 (1996)303-308 [10] Garforth , A.Etal , "Production Hydrocarbons By Catalytic Degradetion of HDPE in a laboratory Fluidised . Bed Reactor" , Applied Catalysis A:General 169(1988)331-342

computational fluid dynamics modeling of high density … · computational fluid dynamics modeling...

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