Retentate recycling

The different operation modes used in microbial fermentations are employed also in animal cell cultivation. Although many different classifications can be adopted, the most general is the one that considers the following operation modes batch, fed-batch, continuous, and perfusion, which is a continuous mode with cell recycle/retention (Castilho and Medronho, 2002). 

Figure 16. General ultrafiltration (UF) scheme used in HTLV process. Key 1, supply/recycle (retentate) vessel 2, sanitary pump 3, membrane UF unit 4, back-flush loop and pump 5, permeate vessel 6, containment room 7, enclosed glove box for sampling retentate 8, enclosed glove box for harvesting retentate and/or filtrate.

The protonated form of poly(vinyl amine) (PVAm—HCl) has two advantages over many cationic polymers high cationic charge densities are possible and the pendent primary amines have high reactivity. It has been appHed in water treatment, paper making, and textiles (qv). The protonated forms modified with low molecular weight aldehydes are usehil as fines and filler retention agents and are in use with recycled fibers. As with all new products, unexpected appHcations, such as in clear antiperspirants, have been found. It is usehil in many metal complexation appHcations (49). 

Retention and drainage additives are vital to the use of recycled fibers. Papermakers consider recycled fibers to behave like virgin fines, while recycled fines behave like filler. Drainage on the paper machine can be impeded and first-pass retention reduced by the use of recycled fiber (9). Additionally, the negative impact of contaminants found in recycled fibers can be minimized by the appropriate use of dispersants and other pitch-control additives. 

Chlorination can be carried out at 25°C or below. However, the reaction is exothermic and, as mills have used filtrate recycle, operating temperatures have unavoidably risen. Retention times are 30—60 minutes but decrease as temperature increases. In most mills the retention time cannot be changed because the tower is upflow in design. The normal pulp consistency is 3—4%, but the trend is toward higher (ca 10%) consistency or gas-phase chlorination. Target pH in the chlorination stage (also (7 ) is about 1.8. 

Cascade with recycle required to obtain one (relatively) pure permeate and enriched retentate. 

Pre-liming. Lime slurry, 0.25% lime on juice (0.250 g of CaO/100 g juice), is added to bring the pH of the mixture into the alkaline range. Insoluble calcium salts are precipitated as finely dispersed coUoids. Calcium carbonate in the form of recycled first carbonation sludge is added to provide coUoid absorption and stabilization. Temperature may be cool (50°C) or hot (80°C) depending on the temperature of the next step, or occasionally on the type of diffusion equipment. Retention time is 15 to 30 min. 

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. 

Effluent quahty from facultative lagoons is related primarily to the suspended solids created by living and dead microbes. The long retention period in the lagoons allows the microbes to die off, leaving a small particle that settles slowly. The release of nutrients from the dead microbes permits the algae to survive by recycling the nutrients. 

An anaerobic digester is a no-recycle complete mix reactor. Thus, its performance is independent of organic loading but is controlled by hydraulic retention time (HRT). Based on kinetic theoiy and values of the pseudo constants for methane bac teria, a minimum HRT of 3 to 4 days is required. To provide a safety factor and compensate for load variation as indicated earlier, HRT is kept in the range 10 to 30 days. Thickening of feed sludge is used to reduce the tank volume required... 

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

For those waste streams that can impact public health or the environment (if mismanaged), provide a summary of the treatment and disposal methods (for example, solvents are incinerated or recycled, lab wastes are incinerated) used to manage them and identify the on-site or off-site facilities used. Is the disposal of the waste adequately documented (for example, retention of manifests, bills of lading or transfer notes) ... 

A more serious limit to this implementation is due to the volume of the recycling pump and associated equipment such as flowmeters and pressure sensors. As the pump moves with respect to the zones, its volume leads to a dead volume dissymmetry, which can lead to a decrease extract and raffinate purities. This decrease can be significant for SMB with short columns and/or compounds with low retention. However, it can be easily overcome by using a shorter column or asynchronous shift of the inlets/outlets [54, 55]. This last solution is extremely efficient and does not induce extra costs because it is a purely software solution. 

Good rhodium retention results were obtained after several recycles. However, optimized ligand/metal ratios and leaching and decomposition rates, which can result in the formation of inactive catalyst, are not known for these ligands and require testing in continuous mode. As a reference, in the Ruhrchemie-Rhone-Poulenc process, the losses of rhodium are <10 g Rh per kg n-butyraldehyde. 

In plug flow reactor the value for Cs is reduced from 12 to 3 g-m 3, then the retention time and rate model with recycle ratio in a plug flow reactor can be written as ... 

A flow diagram of a simple cross-flow system is shown in Figure 16.12. This is the system likely to be used for batch processing or development rigs it is in essence a basic pump recirculation loop. The process feed is concentrated by pumping it from the tank and across the membrane in the module at an appropriate velocity. The partly concentrated retentate is recycled into the tank for further processing while the permeate is stored or discarded as required. In cross-flow filtration applications, product washing is frequently necessary and... 

A bench top polysulfone hollow fiber membrane (0.0325m ) with molecular weight cutoff (MWCO) of 30K (A/G Technology Corp., Needham MA) was used (24). UF was run in a total recycle mode at a rate of 1.2 L/min (flow speed of 0.73 m/sec), cross membrane pressure of 25 PSIG and 10 + 1°C. PE permeability is expressed as the fraction of PEU/mL in the permeate to PEU/mL in the retentate. Data presented are representative of at least duplicate replications. 

Continuous Operation Continuous operation (also called feed-and-bleed) involves the partial recycle of the retentate. The residence time and number of pump passes are in between those of single-pass and batch operation, depending on the fraction of retentate recycled. Several continuous units can be plumbed with the retentate flow feeding the next consecutive system. This configuration is commonly used for large-scale membrane systems. 

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