Modeling of fluidized bed reactor for ethylene polymerization: effect of parameters on the single-pass ethylene conversion

International Journal of Industrial Chemistry, Aug 2013

Background In this study, we present the developments in modeling gas-phase catalyzed olefin polymerization fluidized bed reactors (FBR) using chromium catalyst technique. The model is based on the two-phase theory of gas-solid fluidization: bubble phase and emulsion phase. The model has proved to be the suitable model in many of past studies. In the proposed model, the bed is divided into several sequential sections. The effect of important reactor parameters such as superficial gas velocity, catalyst injection rate, catalyst particle growth, and minimum fluidization velocity on the dynamic behavior of the FBR has been discussed. The conversion of product in a fluidized bed reactor is investigated and compared with the actual data from the plant site. Results A good agreement has been observed between the model predictions and the actual plant data. It has been shown that about 0.28% difference between the calculated and actual conversions has been achieved. Conclusions The study showed that the computational model was capable of predicting the hydrodynamic behavior of gas-solid fluidized bed flows with reasonable accuracy.

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Modeling of fluidized bed reactor for ethylene polymerization: effect of parameters on the single-pass ethylene conversion

Hassan Farag 0 Mona Ossman 1 2 Moustapha Mansour 0 Yousra Farid 0 0 Chemical Engineering Department, Faculty of Engineering, Alexandria University , Alexandria, Egypt 1 City for Scientific Research CSAT , Borg Elarab, Alexandria, Egypt 2 Petrochemical Engineering Department, Faculty of Engineering, Pharos University , Alexandria, Egypt Background: In this study, we present the developments in modeling gas-phase catalyzed olefin polymerization fluidized bed reactors (FBR) using chromium catalyst technique. The model is based on the two-phase theory of gas-solid fluidization: bubble phase and emulsion phase. The model has proved to be the suitable model in many of past studies. In the proposed model, the bed is divided into several sequential sections. The effect of important reactor parameters such as superficial gas velocity, catalyst injection rate, catalyst particle growth, and minimum fluidization velocity on the dynamic behavior of the FBR has been discussed. The conversion of product in a fluidized bed reactor is investigated and compared with the actual data from the plant site. Results: A good agreement has been observed between the model predictions and the actual plant data. It has been shown that about 0.28% difference between the calculated and actual conversions has been achieved. Conclusions: The study showed that the computational model was capable of predicting the hydrodynamic behavior of gas-solid fluidized bed flows with reasonable accuracy. - Background The fluidized bed reactor has a unique physical design, with gas and polymer particles flowing in opposite directions. It consists of metallic catalyst particles that are fluidized by the flow of ethylene gas, and catalyst particles (pre-polymer) are suspended in the ethylene fluid as ethylene gas is pumped from the bottom of the reactor bed to the top. The gas is fed from the base of the reactor and splits into two phases: bubble phase and emulsion phase. Pre-polymer particles are fed in near the top of the reactor, and while the polymerization reaction occurs, the particles grow, increasing in weight and size. Particle segregation occurs in the reactor according to particle weight, so the full-grown polymer particles are removed at the base of the reactor. Nonreacted gases leave the reactor after passing through the disengagement zone [1]. Pre-polymerization is generally carried out in continuous stirred-tank reactors (CSTR) located before the fluidized bed reactor. Catalyst particles are fed into the CSTR along with ethylene and co-monomers to yield pre-polymer. Afterwards, these pre-polymerized catalyst particles are fed into the FBR to complete the ethylene polymerization. A diagram of the industrial fluidized bed reactor system using a CSTR as the pre-polymerization reactor is shown in Figure 1 [2]. In effect, the catalyst is not actually consumed; it is simply incorporated with the polyethylene product as polyethylene molecules remain stuck to the catalyst particle from which they were produced. The conversion of ethylene is low for a single pass through the reactor, and it is necessary to recycle the unreacted ethylene. Unreacted ethylene gas is removed off the top of the reactor, where it is expanded and decompressed to separate the catalyst and low molecular weight polymer from the gas. After purification, ethylene gas is then recompressed and recycled back into the reactor. Granular polyethylene is gradually removed from the bottom of the reactor as soon as reasonable conversions have been achieved. Typically, a residence time of 3 to 5 h results in a 97% conversion of ethylene. Figure 1 Industrial polyethylene production diagram (BP Chemical Technology, London). The flow in the fluidized bed reactor can be mathematically modeled using mole balances [1]. An accurate model describing the movement of gas/ solid and pressure distribution around a rising bubble was then proposed by Davidson and Harrison [3].This model describes the gas flow through a three dimensional fluidized bed mainly in spherical or semi-spherical shape bubbles through the core; however, depending on the emulsion gas velocity, the region around the bubble may be surrounded by a cloud as a result of emulsion gas circulation between the dense solid phase and the core of the bubble. Choi and Ray [4] proposed a two-phase model including bubble and emulsion phases with constant bubble size. McAuley et al. [5] proposed a single phase model by modifying Ray's model with additional assumptions. In a comparison between the two models, they have shown that the single phase assumption does not make considerable difference in the results obtained from the models. Hatzantonis et al. [6] in a research work developed the two-phase model by considering the bubble growth effect on hydrodynamic behavior of the reactor and have shown that the developed model has a better agreement with industrial data than single-phase and two-phase models with constant bubble size. Gros (...truncated)


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Hassan Farag, Mona Ossman, Moustapha Mansour, Yousra Farid. Modeling of fluidized bed reactor for ethylene polymerization: effect of parameters on the single-pass ethylene conversion, International Journal of Industrial Chemistry, 2013, pp. 20, Volume 4, Issue 1, DOI: 10.1186/2228-5547-4-20