Water Gas Shift Reactor an Industrial Case

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  4 Modeling and Simulation of Water Gas Shift Reactor: An Industrial Case Douglas Falleiros Barbosa Lima 1 , Fernando Ademar Zanella 1 , Marcelo  Kaminski Lenzi 2  and Papa Matar Ndiaye 2  1 Refinaria Presidente Getúlio Vargas –  REPAR / PETROBRAS 2 Universidade Federal do Paraná –  UFPR Brazil 1. Introduction Recently, refineries finished products hydro treatment became critical due to changes in fuel regulations. These changes are related to the specification of more clean fuels with special focus in sulfur content reduction. In order to achieve these goals more hydro treatment is needed. Hydrogen is the main raw material to hydro treatment units. In refining plants, a hydrogen generation unit usually is necessary to supply the hydrogen demand to all demanding processes. The process known as steam reform unit is the most widely adopted technology. In large scale, it has the highest energetic efficiency and the best cost-benefit ratio (Borges, 2009). In this process the hydrogen conversion is carried out in two reactors in series. The first one, the steam reform reactor converts steam and a hydrocarbon (naphtha or natural gas) into syngas . In the sequence, a reactor known as water gas shift reactor (WGSR) converts the carbon monoxide present in syngas  into carbon dioxide and more hydrogen is generated. Consequently, the WGSR, an intermediate step of hydrogen generation process, plays a key role in a petrochemical plant due to hydrogen increasing demand. 1.1 Hydrogen generation unit The hydrogen generation unit, based on the steam reform technology, is responsible for approximately 95% of generated hydrogen (Borges, 2009). A simplified unit block diagram is showed in figure 1.1.1. Fig. 1.1.1. Hydrogen generation unit –  process block diagram. First, sulfur is removed from the hydrocarbon stream (usually natural gas), in order to prevent catalyst poisoning and deactivation with the use of a guard bed. Steam is mixed in the main stream in a fixed steam to carbon molar basis. The steam reform reactor (SRR) is a multitubular catalyst filled furnace reactor where the hydrocarbon plus steam are converted into syngas at high temperatures (700ºC –  850ºC) according to the following reaction:    CO +  H   2 O ⇔  CO 2 +  H  2   Δ H  298 o = − 41,1 kJ / mol  (1) The reaction (1) is endothermic and reaction (2) is moderately exothermic. Both are reversible reactions. In the SRR, (1) is the main reaction and generates most of the hydrogen. The reaction (2) due to its endothermic nature occurs in a lower extension in the SRR. The syngas stream composition is CO and H 2 , in this process CO 2  and H 2 O are also present in gas state. The main purpose of the water gas shift reactor (WGSR) is to carry out the reaction (2) reducing the CO fraction and increasing the hydrogen yield. Finally, the WGSR stream is conducted to a purification section, where hydrogen purity is increased according to the process needs. 2. The water gas shift reaction 2.1 kinetic rate expression The water gas shift reaction (reaction 1) is a heterogeneous reaction (gas). According to (Smith et al., 2010) in this kind of application, there are two options in the WGSR step. Using a high temperature shift (HTS) catalyst based reactor or a series of HTS followed by a low temperature shift catalyst based reactor (LTS) with intercooling stage to increase the overall conversion and high purity hydrogen is needed (Newsome, 1980). The chapter focuses on a HTS ferrochrome catalyst industrial reactor modeling. The HTS usually is an iron oxide –  chromium oxide based catalyst. Also reaction promoters such as Cu may be present in catalyst composition. Operational temperatures vary from 310ºC to 450ºC. Inlet temperatures are usually kept at 350ºC to prevent the catalyst bed temperature from damage. Exit CO concentrations are in the order of 2% to 4%. Industrial reactors can operate from atmospheric pressure to 8375 kPa. Sulfur is a poison for Fe-Cr catalysts. LTS reactors are copper based catalyst. Typical compositions include Cu, Zn, Cr, Ni and Al oxides. Recent catalysts can be operated at medium temperatures around 300ºC and superficial velocity was varied from 0.05 m/s to 0.305 m/s. Copper is more sensitive to catalyst thermal sintering and should not be operated at higher temperatures. Sulfur is also a poison to LTS reactors. Typical exit concentration is of 0,1% of CO. The reaction is operated adiabatically in industrial scale, where the temperature increases along the length of the reactor. According to Arrhenius law of kinetics, increasing temperature increases the reaction rate. By the other side, the thermodynamic of equilibrium or Le Châtelier principle states that increasing the temperature of an exothermic reaction shifts the reaction to reactants side decreasing its equilibrium conversion. Therefore the water gas shift reaction is a balance between these effects and the reactor optimal operational point takes into account the tradeoff between kinetics and equilibrium driving forces. In (Chen et al., 2008) experimental data indicates that increasing temperature in HTS will promote the performance of WGSR. For the LTS, the reaction is not excited if the reaction is bellow 200ºC. Once the temperature reaches 200 ºC the reaction occurs, but the CO conversion decreases with increasing temperature. This fact reveals that that the water gas shift reactions with the HTS and the LTS are governed by chemical kinetics and thermodynamic equilibrium, respectively in industrial conditions. (Smith et al., 2010) classifies the reaction kinetic models in microkinetic approach and the empirical method
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