Process without Retentate Recycling

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

Figure 7.3 Specific energy demand of a two-stage membrane process without 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. 

Multiple crops—In the crystallization of some BPCs, multiple crops are sometimes utilized to maximize the amount of material isolated. Even where the cost of the materials being isolated is not high, the ability to increase the overall yield through the preparation of second, third, or even fourth crops is frequently a routine part of the process. A related technique is to recycle the mother liquors without additional treatment from the crystallization to the beginning of the process. Whether through multiple crops or recycling of the mother liquor, both of these processes result in the concentration and/or retention of impurities. The validation of these practices must be a part of the development effort for the process, and reconfirmed on the commercial scale. 

The product stream, containing H2S, H2, and S2, leaves through the top of the catalytic reactor and enter the first membrane module. Hydrogen is removed in the separation module and the retentate is cooled to the dew point temperature to separate sulfur. The decomposition gas leaving the third separator is recycled to the Claus reactor to treat the unconverted H2S and produce the required reaction heat through the Claus process. Hydrogen streams are cooled, compressed and further cooled before a final purification with a dedicated PSA. Since no natural gas is used the proposed scheme allows hydrogen production without CO2 emissions. 

The hydraulic retention time (HRT) is defined as the theoretical amount of time that the liquid is resident within the reactor. For completely mixed systems without biomass recycling, the HRT is the same as the SRT. For systems designed to encourage greater biomass retention, the SRT is generally longer that the HRT. Separating both parameters in this manner improves process stability and efficiency. 

The distillation process is particularly suitable in the cases where the products are miscible with the ionic hquid, as in the case of the Rh-catalyzed hydroformylation of methyl-3-pentenoate in which the reaction mixture is monophasic. The use of an ionic hquid as a solvent results in the almost complete retention of the regioselec-tivity, whicdi is influenced by the ligand, and in significant enhancement of the lifetime and overall productivity of the catalyst. The catalyst recycling and product isolation were achieved by a distillation process under reaction conditions. In these cases, the immobilized catalyst is stabilized by the ionic hquid under the thermal stress of the distihation. The catalyst can be reused several times without additional regeneration process and without loss in activity and selectivity [12]. 

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