Chemical three-phase reactors

The present monoliths that are developed for emissions control in cars are not optimal for chemical three-phase reactors. Development of new monoliths, both metallic and ceramic, with higher cell densities and other geometries at much lower prices, can be expected when the market starts to grow. 

Batchwise operating three-phase reactors are frequently used in the production of fine and specialty chemicals, such as ingredients in drags, perfumes and alimentary products. Large-scale chemical industry, on the other hand, is often used with continuous reactors. As we developed a parallel screening system for catalytic three-phase processes, the first decision concerned the operation mode batchwise or continuous. We decided for a continuous reactor system. Batchwise operated parallel sluny reactors are conunercially available, but it is in many cases difficult to reveal catalyst deactivation from batch experiments. In addition, investigation of the effect of catalyst particle size on the overall activity and product distribution is easier in a continuous device. 

Of primary interest for the industrial application of monolith reactors is to compare them with other conventional three-phase reactors. Two main categories of three-phase reactors are slurry reactors, in which the solid catalyst is suspended, and packed-bed reactors, where the solid catalyst is fixed. Generally, the overall rate of reactions is often limited by mass transfer steps. Hence, these steps are usually considered in the choice of reactor type. Furthermore, the heat transfer characteristics of chemical reactors are of essential importance, not only due to energy costs but also due to the control mode of the reactor. In addition, the ease of handling and maintenance of the reactor have a major role in the choice of the reactor type. More extensive treatment of conventional reactors can be found in the works by Gianetto and Silveston [11], Ramachandran and Chaudhari [12], Shah [13,14], Shah and Sharma [15], and Trambouze et al. [16], among others. 

For catalytic hydrogenations we usually are concerned with three phases gaseous hydrogen, an, often dissolved, component to be hydrogenated in the liquid phase and a heterogeneous solid catalyst. Homogeneous catalysis is not very widespread. A number of good three phase reactors is available in process industries. Therefore we will restrict ourselves to the classic three phase reactors which already have proven their value in bulk chemicals processes. However, for fine chemicals applications a number of special requirements have to be met, which will be discussed in detail below. 

Gas-liquid-solid reactors with a trickle-flow regime are the most widely used type of three-phase reactors and are usually operated under steady-state conditions. The behavior of this kind of reactor under the other three-phase fixed-bed reactors is rather complex due to gas and liquid flow concurrently downward through a catalyst packing. For process intensification it is required to improve some of the specific process steps in such chemical reactors. Figure 4.1 shows an overview of different factors that influenced the trickle-bed reactor performance. 

Warna, J., Salmi, T Dynamic modeling of catalytic three-phase reactors. Computers Chemical Engineering, 1995, 20, 39... 

Nacef, S., Poncin, S., Bouguettoucha, A., and Wild, G. (2007), Drift flux concept in two-and three-phase reactors, Chemical Engineering Science, 62(24) 7530-7538. 

One of the most widely-used three-phase reactors is the trickle-hed reactor which is particularly favored hy the hydroprocessing industry. On the contrary, slurry systems are prefered in the chemical industry they are used in direct coal liquefaction processes and in Fischer-Tropsch synthesis. 

The intrinsic complexity of three phase systems creates some difficulties in the scale-up and in the prediction of performances of three phase reactors. But this complexity is also often a serious advantage, as the simultaneous occurrence of three phases offers such a large number of design possibilities that almost all technical and chemical problems (heat removal, temperature control, selectivity of the catalyst, deactivation, reactants ratio etc..,) can be solved by a proper choice of the equipment and of the operating conditions. For example, countercurrent flow of gas and liquid can be used to overcome thermodynamic limitations and solvent effects can be used to improve selectivity and resistance to poisoning of the catalyst. 

In a multi-phase catalytic reactor, the reacting species are dissolved in two different fluid phases (e.g., in a gas-liquid system or in liquid-liquid system) which are separated by a phase interface, and the catalyst is located in one of the fluid phases or in a dissolved form (as for example an homogeneous catalyst) or as a third heterogeneous phase (e.g., a solid phase). When a gas-liquid system is in the presence of a solid phase catalyst, the reactor is referred to as a three-phase reactor. Multi-phase reactors represent one of the most important classes of chemical reactors and they are widely used in many industrial sectors, as for example chemical, petrochemical, biotechnological, pharmaceutical and food processing industries (Barnett, 2006 Biardi and Baldi, 1999 Henkel, 2000 Nauman, 2008). Multi-phase reactors have typical industrial application in ... 

Akyurtlu, J.F.,Akyurtlu,A.,Hamrin, C.E.,1988. A study on the performance of the catalytic porous-waU three-phase reactor. Chemical Engineering Communications 66,169-187. 

Tarmy B, Chang M, Coulaloglou C, Ponzi P. Hydrodynamic characteristics of three-phase reactors. Chemical Engineer 407 18-23, 1984. 

In catalytic three-phase reactors, a gas phase, a liquid phase, and a solid catalyst phase coexist. Some of the reactants and/or products are in the gas phase under the prevailing conditions (temperature and pressure). The gas components diffuse through the gas-liquid interface, dissolve in the liquid, diffuse through the liquid film to the liquid bulk phase, and diffuse through the liquid film around the catalyst particle to the catalyst surface, where the chemical reaction takes place (Figure 6.1). If catalyst particles are porous, a chemical reaction and diffusion take place simultaneously in the catalyst pores. The product molecules are transported in the opposite direction. 

Let us consider the mass balance of two kinds of three-phase reactors bubble columns and tube reactors with a plug flow for the gas and the liquid phases, and stirred tank reactors with complete backmixing. Modeling concepts can be implemented in most existing reactors backmixing is typical for slurry reactors, bubble columns, and stirred tank reactors, whereas plug flow models describe the conditions in a trickle bed reactor. The interface between the gas and the liquid is supposed to be surroimded by gas and liquid films. Around the catalyst particles, there also exists a liquid film. In gas and liquid films, physical diffusion, but no chemical reactions, is assumed to take place. A volume element is illustrated in Figure 6.15. 

S.R. Son, S.D. Kim, Semi-continuous operation of chemical looping combustion with metal oxides supported on bentonite in an annular fluidized bed reactor, in Proceedings of the Asian Conference on Fluidized Bed and Three Phase Reactors (2006)... 

A hst of 74 GLS reacdions with hterature references has been compiled by Shah Gas-Liquid-Solid Reactions, McGraw-HiU, 1979), classified into groups where the solid is a reactant, or a catalyst, or inert. A hst of 75 reactions made by Ramachandran and Chaudhari (Three-Phase Chemical Reactors, Gordon and Breach, 1983) identifies reactor types, catalysts, temperature, and pressure. They classify the processes according to hydrogenation of fatty oils, hydrodesulfurization, Fischer-Tropsch reactions, and miscellaneous hydrogenations and oxidations. 

Epoxides such as ethylene oxide and higher olefin oxides may be produced by the catalytic oxidation of olefins in gas-liquid-particle operations of the slurry type (S7). The finely divided catalyst (for example, silver oxide on silica gel carrier) is suspended in a chemically inactive liquid, such as dibutyl-phthalate. The liquid functions as a heat sink and a heat-transfer medium, as in the three-phase Fischer-Tropsch processes. It is claimed that the process, because of the superior heat-transfer properties of the slurry reactor, may be operated at high olefin concentrations in the gaseous process stream without loss with respect to yield and selectivity, and that propylene oxide and higher... 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

Three-phase slurry reactors are commonly used in fine-chemical industries for the catalytic hydrogenation of organic substrates to a variety of products and intermediates (1-2). The most common types of catalysts are precious metals such as Pt and Pd supported on powdered carbon supports (3). The behavior of the gas-liquid-sluny reactors is affected by a complex interplay of multiple variables including the temperature, pressure, stirring rates, feed composition, etc. (1-2,4). Often these types of reactors are operated away from the optimal conditions due to the difficulty in identifying and optimizing the critical variables involved in the process. This not only leads to lost productivity but also increases the cost of down stream processing (purification), and pollution control (undesired by-products). 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

---