Retentate stream

Fluid Stream Designations For the generalized membrane module shown in Fig. 20-45, a feed stream enters a membrane module while both a permeate and a retentate stream exit the module. The permeate (or filtrate) stream flows through the membrane and has been depleted of retained components. The term filtrate is commonly used for NFF operation while permeate is used for TFF operation. The retentate (or concentrate) stream flows through the module, not the membrane, and has been enriched in retained components. 

Unlike sterilizing and virus removal filters, tangential flow filtration (TFF) filters are often reused. Flow and integrity tests are necessary to ensure the filter remains the same after usage and cleaning. Consistency of filtrate and retentate streams is validated using relevant validated assays that are specific for each process and product. 

Membrane reactors (MRs) are an interesting alternative to traditional reactors (TRs) owing to their characteristic of product separation during the reaction progress. The simultaneous separation shows some advantages related to the process of both permeate and retentate downstreams and on the reaction (rate) itself. In fact, the load of the downstream separation is significantly lower because both (permeate and retentate) streams leaving the MR are concentrated in more and fewer permeable species, respectively. In addition, separation/purification is not required in the special case of pure permeate. 

Most membrane operations indicated in Table I are run as continuous steady state processes with a feed, permeate, and retentate stream (see Fig. 1). For example, in dialysis, a feed stream comprising blood with urea and other metabolic by-products passes across the upstream face of a membrane while an electrolyte solution without these by-products passes across the lower face of the membrane. A flux of by-products (A) occurs into the downstream where it is taken away as a permeate and the purified blood leaves as nonpermeate. 

To improve process economics, an integrated process shown conceptually in Fig. 27 has been proposed. A per-vaporation subsystem is equipped with a membrane selectively permeable to water and ammonia, but rejects ethanol and ethyl lactate. The retentate stream carrying these reactants may be returned to the reactor to help drive the reaction toward completion. 

The high-pressure gas (retentate) stream leaves the membrane steam reformer at 625°C and 30 bar, while the H2-rich permeate stream leaves the membrane steam reformer at 555°C and 1.5 bar. Pure nitrogen from an air separation plant is supplied as a sweep gas on the permeate side of the membrane. 

The hydrogen content of the retentate stream is too small to make hydrogen recovery economically feasible. On the other hand, the heat content of the retentate stream is reused. The stream is cooled by heat exchange with part of the reformer feedstock and subsequently used for preheating the water feed of the saturation column. 

Although the use of membrane reactors for the retention of the enzyme is mostly applied in continuous processes, some authors used a membrane batch reactor in order to reuse the enzyme in consecutive cycles [11, 74]. Flock et al. used a membrane unit coupled to the reactor with recycling of both permeate and retentate streams to the reactor vessel. A valve at the outlet of the membrane maintained pressure within the range fixed by the manufacturer [74]. Pasta et al. operated a reactor with the membrane inside, emptied the reactor content at scheduled times, and thereafter, replenished it with fresh solution of the substrate and the oxidizing system [11]. 

From Eqs. (7-1) and (7-2), it follows that the separation factor is purely based on the compositions of the entering and exit streams regardless of their flows. Another measure of the separation efficiency of a membrane process is the extent of separation proposed by Rony [1968]. In the context of applying this index of separation efficiency between two comfionents, it is assumed that there is no difficulty in separating the third component Thus the segregation fractions, fiy, are obtained from the molar flow rates of the permeate and retentate streams on the basis of only two components. The extent of separation is defined as the absolute value of a determinant of a binary separation matrix consisting of the segregation fractions as follows ... 

The majority of gas separation applications use pressure difference as the driving force for the membrane separation. As such, the issues of sealing the ends of membrane elements and connecting the elements and the module or process piping are critical in providing gas-tight or essentially leakproof conditions. The seals and connections are necessary to prevent remixing of the permeate and the retentate streams. 

In a simple membrane reactor, basically the membrane divides the reactor into two compartments the feed and the permeate sides. The geometries of the membrane and the reaction vessel can vary. The feed may be introduced at the entrance to the reactor or at intermediate locations and the exiting retentate stream, for process economics, may be recycled back to the reactor. Furthermore, the flow directions of the feed and the sweep (including permeate) streams can be co-current or counter-current or some combinations. It is obvious that there are numerous possible process and equipment configurations even for a geometrically simple membrane reactor. 

Flow Configurations of Feed, Permeate and Retentate Streams... 

The flow patterns of the feed, permeate and retentate streams can greatly influence the membrane reactor performance. First of all, the crossflow configuration distinctly differs from the flow>through membrane reactor. In addition, among the commonly employed crossflow arrangements, the relative flow direction and mixing technique of the feed and the permeate have significant impacts on the reactor behavior as well. Some of these effects are the results of the contact time of the reactant(s) or produces) with the membrane pore surface. 

For those cases where the permeability of reactant A is in between those of the two products, B and C, both the conversion and extent of separation increase with increasing permeation rate or permeation to reaction rate ratio (Table 11.9). The corresponding optimal compressor load (recycle flow rate to feed flow rate) also increases with the rate ratio. The top (permeate) stream is enriched with the most permeable product (i.e., B) while the bottom (retentate) stream is enriched with the least permeable product (i.e., C). It is noted from Table 11.9 that the optimal compressor loads for achieving the highest conversion and extents of separation can be quite different and a decision needs to be made for the overall objective. 

The flow directions (e.g., co-currcni, counter-current and flow-through) and flow patterns (e.g., plug flow, perfect mixing and fluidized bed) of feed, permeate and retentate streams in a membrane reactor can significantly affect the reaction conversion, yield and selectivity of the reaction involved in different ways. These variables have been widely investigated for both dense and porous membranes used to carry out various isothermal and non-isothermal catalytic reactions, particularly dehydrogenation and hydrogenation reactions. 

Recycling some portion of the permeate or retentate stream and introducing feed at intermediate locations are effective methods for improving the reaction conversion. The relative permeabilities of the reactant(s) and product(s) and whether the permeate or retentate is recycled all affect the effectiveness of these measures for conversion enhancement. To compensate for the variations in the transmembrane pressure difference and consequently in the permeation rate, the concept of a location-dependent membrane ]x rmeability has been proposed. The effects of this approach and the average permeation rate arc discussed. 

Membrane modules can be operated in the dead-end or cross-flow modes (see Figure 18.8). Dead-end ultrafiltration is used mostly for laboratory-scale applications and industrial ultrafiltration processes are usually carried out in the cross-flow mode. The main advantage of cross-flow ultrafiltration is the lower extent of concentration polarization. The cross-flow mode also allows recirculation of the retentate stream to the feed tank followed by its mixing with fresh feed that leads to several operational advantages. 

The characteristics of permeate and retentate streams in terms of upper limits at the exit of RO plant were presented in Table 30.2. The concentration of salt in permeate is lower than 0.1 g/dm. The concentration of some specific elements as heavy metals has to be in conformity with the limits of impurities for wastes discharged to the inland waters. Total specific activity for (3 and y emitters is lower than 10 kBq/m, while for a emitters it is lower than 1 kBq/m (the limits for liquid waste). The total salt concentration in retentate is limited by ability of binding the solution with the concrete, the specific radioactivity by nuclear... 

The process conducted in batch-type counter-flow apparatus (Figure 30.17) equipped with capillary PP Acccurel membranes showed good effectiveness of membrane distillation for purification of radioactive waste. Permeate obtained was pure water. All solutes together with radioactive compounds were rejected by the hydrophobic membrane. At tenfold volume reduction of the initial portion of waste, approximately tenfold concentration of radioactivity in the retentate stream was reached, while radioactivity of permeate retained on the level of namral background (Figure 30.18). As was observed in experiments small sorption in the system took place. However, permeate was free of radioactive substances and other dissolved compounds, the concentration and radioactivity factors sometimes slightly differed from volume reduction factors. 

For the model validation and the analysis of the heat integration in the hybrid pervaporation distillation process, a laboratory plant has been built at the TU -Berlin and prepared for the connection with the distillation column (see fig. 3). With this plant experiments with a flat PVA-based (Polyvinylalcohol from GKSS) hydrophilic membrane have been done. A heat exchanger has been built within the pervaporation module. The temperature in the heat exchanger has been necessary to avoid the temperature drop between feed and retentate streams in the pervaporation process. In the process a 2-Propanol/ Water mixture has been separated. The concentration of 2-Propanol in the feed is between 80 and 90 % in weight and the temperature range in the experiments was between 70 and 90°C. The feed flow is turbulent and the system fully insulated to avoid heat looses. The pressure in the permeate side has been kept at 30 mbar and the feed pressure at 1.5 bar. 

During the process, a stream of feed enters the module with a specific content at a specific flow rate. By passing through the membrane module, the feed stream is separated into two streams, a retentate stream and a permeate stream. The retentate stream is the fraction of the feed that retains in the feed stream and the permeate stream is the fraction that passes through the membrane. 

In the cross-flow operation, the inlet feed stream entering the module at a certain composition and it flows parallel to the membrane surface. The composition of the stream changes along the module, and the stream is separated into two parts a permeate stream and a retentate stream. Flux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module configuration and cross-flow velocities. 

In the single-pass system, the feed stream passes through the system only once, and there is no recycling. In a recycling system, a recirculation pump is used to recycle the retentate stream. For small-scale applications, a batch system can be used, as shown in Fig. 19. 

In cases 1 to 4 part of the reactor effluent is split off by the membrane as permeate. The retentate stream, depleted in hydrogen, is then fed to the next reactor. After the fourth reactor membrane permeate and reactor effluent are mixed again to be treated further in the downstream section of the process. 

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