Process with Retentate Recycling

In the conventional activated sludge process with biological sludge recycled from the final sedimentation clarifier, shown in Figure 27.11, the mean cell residence time or sludge retention time is... 

Equation 10 shows that for the conventional process (no recycle) the value of 6c is equal to the hydraulic retention time (0), Thus, effluent quality and treatment efficiency for a given waste are determined by the choice of a value of 0. The ability to vary 6c independently of 0 in the recycle process (Equation 11) is the principal advantage of the recycle process compared with the conventional process. This capability allows one to maintain a certain value of 6c and hence microbial growth rate chosen according to the desired effluent quality by varying the rate of return of microorganisms to the reactor. This can be accomplished with a short 6 and hence small reactor volume. 

Figure 7.2 Simplified flow sheet of a two-stage membrane process with and without retentate recycling.

Figures 7.7 to 7.10 present the technical and economic KPIs as function of the overall CO2 recovery for a two stage membrane process with retentate recycling.

Figure 7.7 Specific energy demand of a two stage membrane process with retentate recycling as a function of the overall CO2 recovery for different flue gas pressures (curly brackets) and different feed pressures of the second stage (dotted line 3 bar, dashed line 4 bar, solid line 5 bar) 0.2 bar permeate pressure in both stages.

Nevertheless, O2 concentrations of 1.1 to 1.5 vol.% were observed in the CO2 product stream. Thus, in analogy to membrane processes without retentate recycling, membrane systems with retentate recycling fail to meet strict O2 concentration limits as required for enhanced oil recovery. 

Most membrane processes operate by means of cross-flow filtration, in which only part of the fluid passes through the membrane as filtrate (or, more correctly, permeate, since some membrane processes operate by permeation rather than filtration) the retained part, the concentrate or retentate, conseqnently becomes more concentrated in particulate or solute species. Membrane systems are frequently operated in a closed loop, with the retentate recycled, and final concentrate is taken from the loop in proportion to the added feed suspension. Whereas microfiltration utilizes both through-flow and cross-flow filtration, cross-flow is the nsnal mode for the other membrane filtration processes, and has thereby grown to its present level of importance. 

First Carbonation. The process stream OH is raised to 3.0 with carbon dioxide. Juice is recycled either internally or in a separate vessel to provide seed for calcium carbonate growth. Retention time is 15—20 min at 80—85°C. OH of the juice purification process streams is more descriptive than pH for two reasons first, all of the important solution chemistry depends on reactions of the hydroxyl ion rather than of the hydrogen ion and second, the nature of the C0 2 U20-Ca " equiUbria results in a OH which is independent of the temperature of the solution. AH of the temperature effects on the dissociation constant of water are reflected by the pH. 

Effective and simple immobilization of enzymes can be obtained by the cross-linking of enzyme aggregates, so-called CLEAs [55]. In this way, essentially any enzyme, including crude preparations, can be transformed into a heterogeneous type of material, insoluble in both water and organic solvents, that is stable and recyclable with high retention of the enzyme s original activity [56], These enzyme preparations are, therefore, of special value for both bio-bio and bio-chemo cascade processes. 

Rautenbach and MeUis [75] describe a process in which a UF-membrane fermentor and a subsequent NF-treatment of the UF-permeate are integrated. The retentate of the NF-step is recycled to the feed of the UF-membrane reactor (Fig. 13.8). This process has been commercialised by Wehrle-Werk AG as the Biomembrat -plus system [76] and is well suited for the treatment of effluents with recalcitrant components. The process also allows for an additional treatment process, like adsorption or chemical oxidation of the NF-retentate, before returning the NF-retentate to the feed of the UF-membrane fermentor. Usually, the efficiency of these treatment processes is increased as the NF-retentate contains higher concentrations of these components. Pilot tests with landfiU leachates [75] and wastewater from cotton textile and tannery industry have been reported [77]. An overview of chemical oxygen demand (COD) reduction and COD concentrations in the permeate are shown in... 

Mixed fertilizer (subcategory G) treatment technology consists of a closed-loop contaminated water system, which includes a retention pond to settle suspended solids. The water is then recycled back to the system. There are no liquid waste streams associated with the blend fertilizer (subcategory G) process, except when liquid air scrubbers are used to prevent air pollution. Dry removals of air pollutants prevent a wastewater stream from being formed. 

Using this protocol, primary aliphatic amines, secondary aliphatic amines, and diamines could be converted into the corresponding urea derivatives in moderate yields. Additionally, catalytic efficiency of cations derived from various bases decreases in the order of > diamines > primary amines > secondary amines > aniline, probably being due to the steric effect and basicity. The catalyst could also be recovered after a simple separation procedure, and reused over five times with retention of high activity. This process presented here could show much potential application in industry due to its simplicity and ease of catalyst recycling. 

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