Cross-flow membrane modules

Filtrations for CWM were carried out with a cross flow membrane separation system. It consists of a cross flow membrane module, a feed pump, a recirculation pump and process pipes. The feed steam contained 1000 ppm PMMA with a mean particle size of 0.8... 

The specifications of the prototype cross-flow membrane module designed according to [113] are as follows membrane area, 5.0 m nominal pore size, 0.3 mm fiber material PP (Accurel Q3/2) and fiber diameter (inside/out-side), 0.6/1.0 mm. A feasibility study has demonstrated that CO2 can be produced economically from flue gas on a large scale [103]. The economic analysis carried out in the study indicates that, at a production capacity of 10 tonnes of CO2 per hour, production costs will be around US 50 per tonne of CO2. At this cost level, the process is cheaper than, for example, CO2 delivered by truck or current CO2 production processes. 

Microfiltration (MF) is a membrane filtration in which the filter medium is a porous membrane with pore sizes in the range of 0.02-10 pm. It can be utilized to separate materials such as clay, bacteria, and colloid particles. The membrane structures have been produced from the cellulose ester, cellulose nitrate materials, and a variety of polymers. A pressure of about 1-5 atm is applied to the inlet side of suspension flow during the operation. The separation is based on a sieve mechanism. The driving force for filtration is the difference between applied pressure and back pressure (including osmotic pressure, if any). Typical configurations of the cross-flow microfiltration process are illustrated in Fig. 2. The cross-flow membrane modules are tubular (multichannel), plate-and-frame, spiral-wound, and hollow-fiber as shown in Fig. 3. The design data for commercial membrane modules are listed in Table 1. 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

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. 

The prototype shell-and-tube type cross-flow filtration modules (Pall Corp.) used for filtration tests are welded into a stainless steel shell enclosure. The modules have an inlet (filtrate) and outlet (retentate) port (both at tube sides) with Vi-inch tubing ends, and a permeate port, located near the midpoint of the shell side of the unit. The stainless steel filter membranes have a nominal pore size of 0.1 pm. The surface of the filter media is coated with a proprietary submicron layer of zirconia. 

The objective in membrane design is to pack as much permeation surface area into as small a space as possible to minimize operation requirements. Depending on the application, various membrane designs are used, such as flat sheet, disc tube, hollow fiber, spiral wound, and ceramic (17). Module design has a measurable effect on the hydrodynamic performance of the cross-flow membrane device. The advantages and disadvantages of different membrane modules are summarized in Table 1. 

A cross-flow nanofillration module (SEPA CFII, GE Osmonics, Miime lis, MN) was used for this process with a maximum operating pressure of 7.0 MPa. The sur ce area of the membrane is 140 cm. The holdup volume of the membrane unit is 70 mL. The fermentation broth was placed in a 5-L fermentation vessel to control the temperature, agitation, and pH. A bench-top pump (M03-S, Hydra cell, MinneapoUs, MN) was used to pump the fermentation broth through the cross-flow membrane separation unit and recycle back to the fermentor (Fig. 2). The permeate was collected on a digital balance attached to a laptop computer with a RS-COM version 2.40 system (A D, Milpitas, CA) that recorded the amount of permeate collected every 0.5 min. The fermentation brofli was kept at constant temperature (37 °C), pH (5.5), and agitation (200 rpm). Transmembrane pressures of 1.4, 2.1, and 2.8 MPa were used in the nanofiltration tests. Each condition was tested twice, and each test lasted for 2 h. Samples of the original broth (before separation), permeate, and letentate were collected for analysis. 

Although it has been reported that an external dc electric field can induce an electrophoretic back transport that can significantly enhance flux in cross flow membrane filtration, its commercial implementation appears to be restricted by a number of factors. These include lack of suitably inexpensive corrosion-resistant electrode materials, concerns about energy consumption, and the complexity of module manufacture. 

Characteristics of membrane modules are summarised in Table 1.12. The spiral wound (SW) module shown in Figure 1.18 is used in all RO and NF applications. The RO hollow-fibre (FIF) module, similar to the one shown in Figure 1.19, is now manufactured by only by Toyobo, and is used for seawater desalination. UF HF membrane (see Figure 1.3) was used extensively in the dairy industry, but it has largely been replaced by SW modules. However, cross-flow HF modules are commorJy used in food processing and industrial wastewater treatment [18, 31]. 

Hollow fibers have proved to be useful to separate labeled hapten from the complex labeled hapten-Ab after a competitive on-line HPLC-immunochromato-graphic detection method [121]. First, hapten eluting from the HPLC column was made to react with Ab. Then, labeled hapten was added to react with the excess of Ab. Separation of free labeled hapten from labeled hapten-Ab was performed using a hollow-fiber module. The cross-flow membrane of this module allowed separation based on size, as opposed to a restricted-access media (RAM) column in which size and hydrophobicity account for separation [122]. No restriction on polarity and size of the label and no need for regeneration of the separation device arc characteristic of the use of hollow fibers. Although the method has been used for analysis of haptens, it could be used to analyze proteins as it has been shown that proteins of 40kDa can be separated from the corresponding protein-Ab complex if the appropriate hollow fiber is chosen. 

The factors to consider in the selection of cross-flow filtration include the cross-flow velocity, the driving pressure, the separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, and the hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

The concept of cross-flow microfiltration is shown in Figure 16.11, which represents a cross-section through a rectangular or tubular membrane module. The particle-containing fluid to be filtered is pumped at a velocity in the range 1-8 m/s parallel to the face of the membrane and with a pressure difference of 0.1-0.5 MN/m2 (MPa) across the membrane. The liquid penneates through the membrane and the feed emerges in a more concentrated form at the exit of the module.1617 All of the membrane processes are listed in Table 16.2. Membrane processes are operated with such a cross-flow of the process feed. 

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... 

Membrane modules can be configured in various ways to produce a plant of the required separation capability. A simple batch recirculation system has already been described in cross-flow filtration. Such an arrangement is most suitable for small-scale batch operation, but larger scale plants will operate as feed and bleed or continuous single pass operation (Figure 16.20). 

The cross-flow filtration method is applied mainly to hyper- and ultrafiltration as well as to some microfiltration.8 In cross-flow filtration the slurry solution or suspension fed to the filter flows parallel to the filter medium or membrane. The filtration product (permeate or filtrate) leaves the filtration module at right angles to the filter medium (the membrane). The traditional perpendicular flow filtration (where the flow of the suspension is directed at right angles to the filter medium and the permeate leaves the filter medium in the same direction) entails filter cake buildup, whereas cross-flow filtration is intended to prevent such filter... 

Figure 4.17 (a) Cross-, (b) co- and (c) counter-flow schemes in a membrane module and the changes in the concentration gradients that occur across a median section of the membrane... 

The benefit obtained from counter-flow depends on the particular separation, but it can often be substantial, particularly in gas separation and per-vaporation processes. A comparison of cross-flow, counter-flow, and counter-flow/sweep for the same membrane module used to dehydrate natural gas is shown in Figure 4.18. Water is a smaller molecule and much more condensable than methane, the main component of natural gas, so membranes with a water/methane selectivity of 400-500 are readily available. In the calculations shown in Figure 4.18, the membrane is assumed to have a pressure-normalized... 

Figure 4.18 Comparison of (a) cross-flow, (b) counter-flow and (c) counter-flow sweep module performance for the separation of water vapor from natural gas. Pressure-normalized methane flux 5 x 10 6cm3(STP)/cm2 s cmHg membrane selectivity, water/methane 200...

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. 

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