Active Contactor

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

CMRs have also been used to improve catalytic processes by enhancing contact between reactants and catalyst. The membrane acts here as an active contactor (Section A9.3.3.3). 

Equilibrium-restricted reactions (Section A9.3.3.1) have until now been the main field of research on CMRs. Other types of application, such as the controlled addition of reactants (Section A9.3.3.2) or the use of CMRs as active contactors (Section A9.3.3.3), seem however very promising, as they do not require permselective membranes and often operate at moderate temperatures. Especially attractive is the concept of active contactors where the membrane being the catalyst support becomes an active interface between two non-miscible reactants. Indeed this concept, initially developed for gas-liquid reaction [79] has been recently extended to aqueous-organic reactants [82], In both cases the contact between catalyst and limiting reactant which restricts the performance of conventional reactors is favored by the membrane. 

The concept of combining membranes and reactors is being explored in various configurations, which can be classified into three groups, related to the role of the membrane in the process. As shown in Figure 25.12, the membrane can act as (a) an extractor, where the removal of the product(s) increases the reaction conversion by shifting the reaction equilibrium (b) a distributor, where the controlled addition of reactant(s) limits side reactions and (c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone. In the first two cases, the membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

FIGURE 9.29 Roles of the membrane in membrane reactors (a) Extractor the removal of produces) increases the reaction conversion by shifting the reaction equilibrium, (b) Distributor the controlled addition of reactant(s) limits side reactions, (c) and (d) Active contactors the controlled diffusion of reactant(s) to the catalytic membrane can lead to an engineered catalytic zone. 

Figure 1.11 Principles of membrane reactors to enhance the reaction process (a,b) membrane as a product extractor (c,d) membrane as a reactant distributor (e,f) membrane as an active contactor.

The membrane serves as an active contactor (Figure l.ll(e,f)). Reactants are supplied to the catalyst by the controlled diffusion in the membrane, hence a well-defined reaction interface (or region) between two reactant streams is created. The reactants can be provided from one side or from opposite sides of the membrane. Furthermore, more reactive sites can be provided due to the easy access of reactants to the catalyst, and thus the catalyst s efficiency can be increased greatly. 

In porous MRs, the membrane may function as an extractor, a distributor, or an active contactor, as listed in Table 2.6. The extractor mode corresponds to the earlier applications of MRs and has been applied to increase the conversion of a number of equilibrium-limited reactions, such as alkane dehydrogenation, by selectively extracting the hydrogen produced. Other H2-producing reactions - such as water gas shift (WGS), steam reforming of methane, and the decomposition of H2S and HI -have also been investigated successfully with the MR extractor mode. The H2 perm-selectivity of the membrane and its permeability are two important factors controlling the efficiency of the processes [17]. 

Removal of Refractory Organics. Ozone reacts slowly or insignificantly with certain micropoUutants in some source waters such as carbon tetrachloride, trichlorethylene (TCE), and perchlorethylene (PCE), as well as in chlorinated waters, ie, ttihalomethanes, THMs (eg, chloroform and bromoform), and haloacetic acids (HAAs) (eg, trichloroacetic acid). Some removal of these compounds occurs in the ozone contactor as a result of volatilization (115). Air-stripping in a packed column is effective for removing some THMs, but not CHBr. THMs can be adsorbed on granular activated carbon (GAG) but the adsorption efficiency is low. 

Fig. 1. Alternative wastewater treatment technologies, where GAC = granular activated carbon, PAC = powdered activated carbon, POTW = publicly owned treatment works, and RBC = rotating biological contactor (— ), wastewater return flows (—— ), sludge.

In a countercurrent-type column contactor, stable operation is possible as long as the rate of arrival of droplets in any section does not exceed the coalescence rate at the main interface once this value is exceeded, droplet backup will occur at the interface and slowly build back into the column active area, a condition known as flooding. This is an inoperable condition. 

Activated carbon is available in powdered form (200-400 mesh) and granular form (10-40 mesh). The latter is more expensive but is easier to regenerate and easier to utilize in a countercurrent contactor. Powdered carbon is applied in well-mixed shiny-type contactors for... 

Two-level diffuser contactors, which involve application of ozone-rich gas to the lower chamber. Lower chamber off-gases are applied to the upper chamber. Offgas treatment from contactors is an important consideration. Methods employed for off-gas treatment include dilution, destruction via granular activated carbon, thermal or catalytic destruction, and recycling. 

Conventional treatment The preliminary treatment, sedimentation, flotation, trickling filter, rotating biological contactor, activated sludge and chlorination of wastewater. 

Karigi F, Dincer AR (1998) Saline wastewater treatment by halophile supplemented activated sludge culture in an aerated rotating biodisc contactor. Enz Microbial Technol 122 427 133... 

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