Permeate stream

The more permeable component is called the. st ga.s, so it is the one enriched in the permeate stream. Permeability through polymers is the product of solubihty and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 22-23 after Breck, Zeolite Molecular Sieves, Wiley, NY, 1974, p. 636). 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

The task of synthesizing an optimal RON can be stated as follows For a given feed flowrate, Qf. and a feed concentration, Cp. it is desired to synthesize a minimum cost system of reverse osmosis modules, booster pumps and energy-recovery turbines Chat can separate the feed into two streams an environmentally acceptable permeate and a retentate (reject) stream in which the undesired species is concentrated. The permeate stream must meet two requirements ... 

Concentration of solute in feed stream (kmol/m ) Concentration of solute in permeate stream (fcraol/m ) Concentration of solute in reject stream (kmol/m )... 

Water flux through the membrane (kmol/m ) Pressure of feed stream entering a module (atm) Pressure of permeate stream leaving a module (atm) Pressure of reject stream leaving a module (atm) Pressure difference across the membrane (atm)... 

Multiple-Component Separation Separation Factor Consistent with the characterization of different separation methods, one can define a separation factor a,j (also called selectivity) for components i andj that compares their relative concentrations in the permeate stream to those in the feed stream ... 

TFF membrane systems generally use a common feed distributed among parallel modules with a collection of common retentate and common permeate streams. In some applications, it is also useful to plumb TFF modules with the retentate in series where the retentate flow from one module provides the feed flow to the next module. This type of configuration is equivalent to increasing the length of the retentate channel. Permeate flows may or may not be plumbed together. 

We have seen several examples of a technique for separation of gas mixtures which, in contrast with most commercial processes, requires no physical transfer of solvent, handling of solids, or cycling of temperature or pressure. The energy requirements can also be far lower The thermodynamic minimum work of separation is, under isothermal conditions, the free energy difference between the process stream and byproduct, or permeate, stream. When this difference is due only to the partial pressure difference of component 1, it becomes ... 

After an activation period of 4 h, the conversion showed a maximum of 40% followed by a steady decrease in conversion (Figure 4.38). Overnight, the pressure was decreased to 6 MPa and the needle valve on the permeate side was closed. This shutdown procedure caused the catalyst to precipitate and no reaction occurred anymore. The precipitated catalyst can be used for a new cycle by pressurization of the membrane reactor, redissolving the catalyst. At the end of the third run the conversion had dropped to 33%. A TON of 1.2xl05 in 32 h (t 62 min) was obtained. ICP-AAS analysis of the permeate stream indicated complete retention of the catalyst. The authors propose possible traces of oxygen as the cause of the decrease in activity of the catalyst. 

The first SRS unit was built as a demonstration plant and has been in operation since September 1997. The basic principle of operation is that a solution of sodium chloride and sodium sulphate in contact with a nanofiltration membrane at high pressure, will separate into a sulphate-lean permeate stream and a sulphate-rich concentrate stream. 

In the example shown in Fig. 11.3, a feed-stream containing a sodium sulphate concentration of 20 g l-1, with a flow rate of 3.43 m3 h 1, is split into two streams by the filter, namely a concentrate stream of 0.8 m3 h 1, with 83 g 1 1 of sodium sulphate and a permeate stream of 2.63 m3 h-1, with a sodium sulphate concentration of 0.82 g l-1. The permeate stream is returned to the brine saturators and the concentrate stream is sent to effluent. 

The membrane performance for separations is characterized by the flux of a feed component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a feed mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity or separation factor. Selectivity (0 can be defined as the ratio of the permeabilities of the feed components across the membrane (i.e., a/b = Ta/Tb, where A and B are the two components). The permeability and selectivity of a membrane are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent... 

After extraction, the solute-laden CLAs need to be separated from the mother liquor so that they can be back stripped. Hence attempts were made to filter the solute-rich CLAs from the aqueous phase using cross-flow microfiltration [70]. The filtration characteristics of the CLAs as indicated by the flux, CLA size, and concentration showed that they are completely retained by the membrane and do not foul the membrane surface. Using this system, the CLAs could easily be concentrated up to 30% w/v at low pressures, and the permeate stream remained totally clear. The CLAs appear to maintain their structural integrity because only 3 mg dm of SDS was... 

One option involves the condensation of (or part of) the permeate under atmospheric instead of vacuum conditions. This requires the use of dry-vacuum pumps , able to compress the permeate vapour from vacuum to atmospheric pressure, after which condensation is performed at a higher temperature [23]. In this case, the operating conditions have to be carefully monitored since these pumps may lead to unsuitable heating of the vapour and eventually aroma deterioration, despite the low residence time. Alternatively, the use of liquid ring vacuum pumps where the service liquid can take some of the aromas from the permeate stream has been proposed [24]. 

Pervaporation may certainly play an important role for replacement of evaporative techniques as well as aroma-recovery processes based on solvent extraction, in particular when the labelling natural is considered crucial. Some of the most relevant technical challenges discussed herein have to be addressed in order to render organophilic pervaporation a competitive process (Fig. 19.4). In particular, the way of capturing the target aromas from the permeate stream has to be reanalysed in terms of minimising energy consumption and labour-intensive operations. 

Membrane Rejection. Both cellulose acetate and FT-30 composite membranes were evaluated for rejection of solutes. Sodium chloride rejections were confirmed and listed in Table III. Typical organic rejections of model compounds are listed in Tables IV and V for cellulose acetate and FT-30 composite membranes, respectively. Rejections were measured during screening and concentration tests solute levels were in the parts-per-billion range. Measurement of feed and permeate stream solute concentrations provided the necessary information to calculate solute rejection. Eq 1 was used to calculate rejection values. 

Ultrafiltration can be utilized to separate the emulsion and dissolved oil from water. The specific ultrafiltration membrane polymer and pore-size requirement are determined by the oil chemistry however/ the oil can typically be concentrated up to 60 - 80%, and in some cases, incinerated to recover energy in the form of heat. The permeate stream may be pure enough to be re-used, or may require treatment with reverse osmosis prior to re-use. 

In the cross-flow module illustrated in Figure 4.18(a) the average concentration of water on the feed side of the membrane as it decreases from 1000 to 100 ppm is 310 ppm (the log mean). The pooled permeate stream has a concentration of 6140 ppm. The counter-flow module illustrated in Figure 4.18(b) performs substantially better, providing a pooled permeate stream with a concentration of 13 300 ppm. Not only does the counter-flow module perform a two-fold better separation, it also requires only about half the membrane area. 

Another factor that affects membrane system design is the degree of separation required. The usual target of a gas separation system is to produce a residue stream essentially stripped of the permeable component and a small, highly concentrated permeate stream. These two requirements cannot be met simultaneously a tradeoff must be made between removal from the feed gas and enrichment in the permeate. The system attribute that characterizes this trade-off is called the stage-cut. The effect of stage-cut on system performance is illustrated in Figure 8.15. 

Figure 8.16 shows that when 90% of the VOC in the feed stream is removed, the permeate stream will contain approximately 4 % of the permeable component. In many cases, 90 % removal of VOC from the feed stream is insufficient to allow the residue gas to be discharged, and enrichment of the component in the permeate is insufficient also. 

If the main problem is insufficient VOC removal from the feed stream, a two-step system as shown in Figure 8.17 can be used. In a two-step system, the residue stream from the first membrane unit is passed to a second unit, where the VOC concentration is reduced by a further factor of 10, from 0.1 to 0.01 %. Because the concentration of VOC in the feed to the second membrane unit is low, the permeate stream is relatively dilute and is recirculated to the feed stream. 

Figure 8.18 A two-stage system to produce a highly concentrated permeate stream. Selectivity, 20 pressure ratio, 20...

Figure 8.20 shows another type of recycle design in which a recycle loop increases the concentration of the permeable component to the point at which it can be removed by a second process, most commonly condensation [38], The feed stream entering the recycle loop contains 1 % of the permeable component as in Figures 8.16-8.19. After compression to 20 atm, the feed gas passes through a condenser at 30 °C, but the VOC content is still below the condensation concentration at this temperature. The membrane unit separates the gas into a VOC-depleted residue stream and a vapor-enriched permeate stream, which is recirculated to the front of the compressor. Because the bulk of the vapor is recirculated, the concentration of vapor in the loop increases rapidly until the pressurized gas entering the condenser exceeds the vapor dew point of 6.1 %. At... 

In the process shown in Figure 8.21, a two-step membrane design is used to reduce the cost of recompressing the hydrogen permeate stream to the very high... 

Natural gas is usually produced from the well and transported to the gas processing plant at high pressure, in the range 500-1500 psi. To minimize recompression costs, the membrane process must remove impurities from the gas into the permeate stream, leaving the methane, ethane, and other hydrocarbons in the high-pressure residue gas. This requirement determines the type of membranes that can be used for this separation. Figure 8.30 is a graphical representation of the factors of molecular size and condensability that affect selection of membranes for natural gas separations. 

A typical flow diagram of a membrane system for C3+ liquids recovery is also shown in Figure 8.34. The natural gas is fed to modules containing a higher-hydrocarbon-selective membrane, which removes the higher hydrocarbons as the permeate stream. This stream is recompressed and cooled by a cold-water exchanger to condense higher hydrocarbons. The non-condensed bleed... 

The competitiveness of membrane systems in this application is very sensitive to the selectivity of the membranes for propane, butane and higher hydrocarbons over methane. If the membranes are very selective (propane/methane selectivity of 5-7, butane/methane selectivity of 10-15), the permeate stream from the main set of modules will be small and concentrated, minimizing the cost of the recompressor. Currently, silicone rubber membranes are being considered for this application, but other, more selective materials have been reported [55],... 

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