Flux reduction

Effects of ribs or spacers. A spacer rib test was conducted by the Savannah River Laboratory (Mirshak and Towell, 1961). The results show that critical heat flux reduction due to contact of a longitudinal spacer rib can be as much as 32%. 

Measurements and modelling of the UV flux reduction at the earth s surface due to aerosols. 

W.R. Bowen and A.O. Sharif, Transport through microfiltration membranes—particle hydrodynamics and flux reduction. J. Colloid interface Sci. 168 (1994) 414-421. 

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

In addition to the Navier-Stokes equations, the convective diffusion or mass balance equations need to be considered. Filtration is included in the simulation by preventing convection or diffusion of the retained species. The porosity of the membrane is assumed to decrease exponentially with time as a result of fouling. Wai and Fumeaux [1990] modeled the filtration of a 0.2 pm membrane with a central transverse filtrate outlet across the membrane support. They performed transient calculations to predict the flux reduction as a function of time due to fouling. Different membrane or membrane reactor designs can be evaluated by CFD with an ever decreasing amount of computational time. 

Fane et al. (1982) discussed the possibility of UF flux enhancement by particulates. It was found that rigid particles larger than 1 pm could enhance flux. Cohesive and compressible particles, even if large, would cause flux reduction. Milonjic et al. (1996) filtered hematite suspensions and found that increased pressure and stirring lead to a increased flux. Chudacek and Fane (1984) measured deposit layers of several pm on UF membrane by macrosolutes and silica colloids. 

MF and UF have been compared as a pretreatment step for NF in colour removal. MF obtained poor colour removal (80%) and flux reduction is higher for the higher flux membranes (MF) (Chang et al (1995)). Chellam et al (1997) compared MF and UF (10 kDa) as pretreatment for NF and found no difference, but both performed significantly better than conventional pretreatment, which was attributed to coUoid removal. A low flux and high recover operation was recommended for NF. UF has been used as a pretreatment to RO in water treatment (Kamp (1995)). Amy et al. (1993) stated that to guarantee low fouling of NF, pretreatment with MF or UF was required. MF was only moderately... 

Chabot and coworkers studied the effects of gas phase impurities such as CO, CO2, and CH4 on the hydrogen flux through a PdAg alloy foil membrane [16] over a range of gas compositions and temperatures. The degree of inhibition (hydrogen flux reduction) they reported for 9.5mole% CO in H2 feed gas mixture was similar to what we observed. Inhibition was a minimum between 350 and 400°C. Above 400°C, CO methanation was also observed. 

For a PdAu membrane (GTC-31), no flux reduction was observed for the WGS mixture compared to a pure H2 feed gas at the same 25 psig partial pressure difference and 400°C. A typical pure H2 flux was 245 SCFH/ft = 0.93 mol/m s for a 100 psig H2 feed gas pressure at 400°C. This flux exceeds the 2010 IX)E Fossil Energy pure hydrogen flux target. The H2/N2 pure gas selectivity of membrane GTC-31 was about 1,000 at a partial pressure difference of 100 mi. 

Alloys help alleviate the problems associated with flux reduction from impurity adsorption on the surface of palladium membranes [74, 122]. Steam, CO, and H2S inhibit the hydrogen flux through palladium membranes by chemisorption on the surface. There is some indication that palladium alloys are affected less by CO poisoning [111, 117, 123-125]. Recently, work on Pd-Cu alloys has increased... 

In concentrating WPs, DF is employed, in which water or buffer is continually added to the retentate while lactose and minerals are simultaneously removed in the filtrate, to increase WP purity [4,52,61]. This is commonly done in constant-volume mode where water or buffer is added to the retentate at the same rate as permeation. There is an optimum protein concentration in the retentate at which DF can be commenced where the trade-off between permeate flux and the number of diavolumes is balanced and only the minimum membrane area or process time is necessary [62,63]. Using 20 kDa MWCO polysulfone membrane sheets, Nilsson [18] found that, in the UF of reconstituted whey protein concentrate (WPC)-80, the relative flux reduction (RFR) increased with protein concentration and then plateaued up to about 3.2% protein concentration in the retentate. Beyond this concentration, the RFR increased sharply. Cheryan and Kuo [22] showed that at 335 kPa TMP and 50°C, the flux approached a minimum when the retentate reached about 3% protein concentration using polysulfone spiral wound membrane while the flux in the polysulfone hollow fiber is four times higher (Figure 19.1). This suggests that DF may be effectively carried out until 3% protein concentration in the retentate is reached. 

Concentrates Battery Paste Secondary Materials Fluxes Reductant Coal... 

In the above calculations and considerations diffusion through the nonporous layer of the membrane was assumed to be the rate-determining process and thus the only transport resistance. In every membrane process, however, additional transport steps at the feed occur, usually summarized as polarization . By the preferential transport of one component out of a mixture through the membrane the fluid layer directly adjacent to the membrane surface will be depleted of that component, and its concentration will be lower than that in the bulk of the feed mixture. This (unknown) lower concentration determines the sorption and thus the effective activity and partial vapor pressure of the component directly at the feed side of the membrane. The flux reduction caused by the additional resistance for the transport of matter by diffusion through the liquid layer adjacent to the feed side of the membrane is known as concentration polarizatioif, and effective in all membrane processes. Due to the phase change... 

A similar effect has been observed for many other hydrocarbons, i.e. adsorption and subsequent hydrogen flux reduction. Post-inspection of the surface often reveals carbonaceous surface contamination, like carbon, CH4 " or propylene. Secondary effects of CO adsorption is the catalytic decomposition and formation of carbon, carbide (Pdi xC )" " or carbonate phases. While reversible CO adsorption decreases with increasing temperature, catalytic decomposition appears to increase with temperature. The tendency of deposit forming due to catalytic decomposition is reported to be counteracted by the presence of steam " mitigating the flux reduction. 

Coating as a protection has also been suggested and demonstrated." " Recently, it was reported that flux reduction in the presence of H2S could not be observed in a Pd-based membrane having a surface protective film." The coating route, probably in combination with alloy optimization, holds promise for the use of Pd-based membranes in gas mixtures containing reactive components. 

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