Permeate flux, units

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

To help understand the performance of membranes, brief explanations of a few terminologies are in order. Permeability of a membrane is determined by dividing permeate flux by the transmembrane pressure. It indicates the membrane s throughput per unit area (flux) per unit pressure difierence. An important factor afiecting flux and retention ability of the membrane is the direction of the feed flow relative to the membrane surface. In through-flow configuration, the feed flow is perpendicular to the membrane surface. In cross-flow configuration, the feed stream flows parallel to the membrane... 

A commercially available cellulose acetate film which we would now describe as homogeneous or isotropic, gave the results shown in Row 2 of Table I. The volumetric permeation rate of water per unit membrane area, called the water permeation flux Jl mVm day, and the water permeation constant. A, m m day atm were both very low, but a salt rejection of 94 percent was obtained. We define ... 

In ultrafiltration and reverse osmosis, in which solutions are concentrated by allowing the solvent to permeate a semi-permeable membrane, the permeate flux (i.e. the flow of permeate or solvent per unit time, per unit membrane area) declines continuously during operation, although not at a constant rate. Probably the most important contribution to flux decline is the formation of a concentration polarisation layer. As solvent passes through the membrane, the solute molecules which are unable to pass through become concentrated next to the membrane surface. Consequently, the efficiency of separafion decreases as fhis layer of concentrated solution accumulates. The layer is established within the first few seconds of operation and is an inevitable consequence of the separation of solvent and solute. 

As most of the membrane separation processes arc operated in the crossflow mode for the reasons discussed earlier (Section 5.5.1 Crossflow Configuration), the crossflow velocity has marked effects on the permeate flux. A higher crossflow velocity typically results in a higher flux. The rate of flux improvement as a function of the crossflow velocity usually can be described by the following equation with specific units ... 

Besides some measures of separation efficiency such as the separation factor and extent of separation defined above, some quantity indicative of the throughput rate of a membrane system is needed to compliment the permselectivity of the membrane. It is quite common and practical in the membrane technology to use a phenomenological expression to relate the permeate flux (Ja in the unit of cm (STP)/s-cm7) of a given gas (A) through the membrane to the driving force, the transmembrane pressure difference (Ap) as follows ... 

Permeate flux is defined as the permeation rate per unit of membrane area. Thus, maximizing the flux will reduce system size and cost. Permeate flux can be calculated as follows (15) ... 

Important for practical implemetation of zeolitic membranes is the acquisition of permeation data of single components and mixtures and the interpretation of these data in the form of macroscopic models. These models describe the permeation flux of components as a function of partial pressure, composition and temperature. Once good models exist separation units can be designed for the separation of multicomponent mixtures. 

The next step for application is the development of handsome modules that can be combined in larger units for separation or catalytic membrane reactors. In separation one strives to high surface-area-to-volume ratios in catalytic membrane reactors this ratio will depend on the volumetric catalytic rates compared to the required permeation fluxes. 

The foregoing convoluted conversion of units will give a new value for the permeate flux V" in the following units ... 

Pulsate flows were applied to mineral microfiltrations membranes during apple juice filtration [36] illustrating the advantage of this method to enhance permeability compared to steady flow regime. With carefully chosen pulsations permeate flux increased up to 45% at 1 Hz pulsation frequency. Moreover well defined pulsations decreased the hydraulic power dissipated in the retentate per unit volume by up to 30%. In an other work on cross-flow filtration of plasma from blood [37] permeate flux increase was also observed when pressure and flow pulsations at 1 Hz are superimposed on the retentate. 

Hydraulic permeability, A 70 to 10,000 g/s-m. MPa. Pressure 0.3 to 0.5 MPa for ceramic. Capacity/unit 0.001 to 1 L/s. Liquid permeate flux 0.001 to 0.2 L/s-m with the permeate flux through ceramic membranes 2 to 3 times higher than through symmetric polymeric or sintered metal membranes and 5 to 10 times higher than through asymmetric polymeric membranes because ceramic operates at higher pressure. 

Finally, the separation of the Rh-distearylamine/TPPTS catalyst system by membranes was successfully tested on pilot-plant scale with crude aldehyde from the hydroformylation of DCP. The unit was continuously operated over a period of 12 weeks. No decrease in permeate flux and catalyst activity was observed. In contrast to laboratory-scale results, the Rh concentration must be increased to 100 ppm... 

Since flux and selectivity increase with increasing transmembrane pressure difference and the modules can tolerate pressure differences of about 100 bars, the reader might question why the first unit is operated with a transmembrane pressure difference of only 60 bars. The reason is that the H2 recovery system has been added to an existing plant and that the first stage of the synthesis compressor would not accept the permeate flux of both units. 

According to Table 6.5, a further reduction of the specific separation costs is feasible if the effective membrane thickness can be reduced, for example, by employing optimized" asymmetric membranes.31 It should be noted, that the optimal feed concentration of the pervaporation unit is not only determined by the design of the extractive distillation but by the effective membrane thickness (or specific membrane costs) as well. With increasing permeate fluxes, or lower specific membrane costs, the optimal feed concentration is shifted to higher benzene concentrations. 

When concentration polarization occurs, permeate fluxes become invariant with the transmembrane pressure, and increases in the permeation rate can be achieved only by enlarging the membrane surface area or by improved fluid management. The design of the UF membrane unit in the polarized regime must relate the flux to the optimal level of reactor VSS or total suspended solids (TSS) according to Michaels gel theory, to give... 

Performances of PV membranes are represented by parameters such as separation factor, flux of permeates, and service life. The separation factor of a membrane is a measure of its permeation selectivity (permselectivity) and is defined as the ratio of the concentration of components in the permeate mixture to that in the feed mixture. The component flux is the amount of a component permeating per unit time and unit area, and is given by the product of the permeability coefficient of the membrane and the driving force. The driving force is the gradient in the chemical potential of the components between the feed and the permeate side of the membrane. These values are influenced by operating variables such as temperature, composition of each component in the feed mixture, and permeate side pressures (see Fig. 107). 

In a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [37] as shown in Figure 9.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side Qp which results in two outgoing flows related to retentate Q, and permeate gp sides, respectively. The permeation flux J per membrane surface unit is directly calculated from Q. Two important parameters account for hydrodynamic working conditions of a module, one is the flow velocity, v, in the module calculated as the ratio of the incoming flow <2/ (mVs) by the hydraulic section of the module Q (m ), the other is the transmembrane pressure, P. ... 

An important factor that influences the performance of membrane filtration is its mode of operation. According to Gekas et al. [48], the filtration mode for most unit operations with manbranes, among which processes of NF and UF are in cross-flow mode, in which the feed stream is parallel to the manbrane surface, while the permeate flux is transverse to it. Thus, the flows of feed and permeate are intertwined, justifying the terminology cross-flow. Moreover, in the conventional mode of filtration, the feed stream is perpendicular to the m brane or filter media and is known as dead end or flow through. 

A number of researchers have produced capillary tubing and hollow-fibers from materials of sufficient strength to avoid the need for a porous support. These materials are typically melt spun into hair-size fibers having dense walls of sufficient strength to obviate the need for a supporting layer.. While the dense layer offers substantial resistance and limits the permeation flux, the hair-size of the hoUow-fibers enables designs that accommodate high membrane surface area densities (area per unit volume) [32]. 

Fig. 4.6 Permeate flux dependence on the feed flow rate at different TOABr concentrations. The crossflow unit was operated at 30 °C and 30 bar pressure. (Reprinted from [32] with permission from Elsevier.)...

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