Transmembrane pressure determination

The transmembrane pressure determines the solvent flux through the membrane. Increased flux will cause an increase in solute concentration at the membrane surface until a limiting layer (gel, osmotic pressure, etc) is formed and flux no longer increases with pressure as under pure water conditions. 

The pressure difference between the two membrane sides, namely the transmembrane pressure, determines the position of the gas-liquid interface along the membrane cross-section, since the phase-phase displacement will take place only in pores where the transmembrane pressure is greater than the breakthrough pressure. Therefore, in catalytic membrane reactors operating in contactor mode a strict control of the transmembrane pressure is very important. 

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... 

The performance of a hollow-fiber or sheet bioreactor is primarily determined by the momentum and mass -transport rate [15,16] ofthe key nutrients through the biocatalytic membrane layer. Thus, the operating conditions (transmembrane pressure, feed velocity), the physical properties of membrane (porosity, wall thickness, lumen radius, matrix structure, etc.) can considerably influence the performance of a bioreactor, the... 

Fluid-dynamic operating conditions, such as axial or angular velocity (i.e., shear stress that determines drag force value) and transmembrane pressure (that determines disperse-phase flux, for a given disperse-phase viscosity and membrane... 

The flux solvent evolution of pure water with the transmembrane pressure across NF/LPRO membranes are reported in Fig. 6. The linear dependence of fluxes with the transmembrane pressure shows that Darcy s law is verified. As expected, the hydraulic permeabilities determined from the slopes (Table 3) show higher values for NF than LPRO membranes, due to their larger pore size. The NF90 membrane shows the higher hydraulic permeability with Z p = 14.8 Lh 1 m 2bar 1. 

Table 5. R0bs values determined for NF90, NF270 and BW30 membranes with different transmembrane pressure applied and two different salts (NaCI and Na2S04)...

Microfiltration is a unit operation for the separation of small particles. The separation limits are between 0.02 and 10 (jum particle dimensions. Microfiltration can be carried out in a dead-end mode and a cross-flow mode. In downstream processing, the cross-flow filtration is carried out continuously or discontinuously. The most important parameters that determine the productivity of cross-flow microfiltration are transmembrane pressure, velocity, particle size and surface, viscosity of the liquid and additives such as surfactants, and changing the surface and surface tension. 

Among the operating parameters of a membrane system, the fluid temperature, transmembrane pressure (TMP) and feed concenu ation play important roles in determining the permeate flux. 

A membrane can be defined as a thin and selective barrier that enables the transport or the retention of compounds between two media. In the case of ceramic membranes, the usual driving force for transport is a pressure gradient between the feed and strip compartments (transmembrane pressure). The treated phases can be liquid or gas. For porous membranes, the pore size mainly manages the cutoff of the membrane. However, for retention of the smallest entities by the smallest pores, the transport mechanisms are more complex than simple sieving. Specific physical and chemical interactions (electrostatic repulsion, physisorp-tion, capillary condensation, etc.) become preponderant and determine the membrane selectivity. Table 25.1 summarizes the characteristics of the main processes in which ceramic membranes are involved. 

The objective of the experiments carried out in this pilot plant was to determine the optimal values of some operational parameters, such as biomass concentration, transmembrane pressure, flow rate, and hydrauhc retention time, the last defined as the relationship between the volume of the bioreactor and the daily flow discharge of permeate. The working plan was as follows ... 

Membrane structures for MF include screen filters, which collect retained matter on the surface, and depth filters, which trap particles at constrictions within the membrane. Depth filters have a much sharper cutoff, resulting in enhanced separation factors. For example, a Nuclepore membrane of type 2 can separate a male-determining sperm from a female-determining sperm (Seader and Henley, 2006). Nuclepore MF membranes come in pore sizes from 0.03 to 8.0 microns with water permeate flux rates, at 294 K and a transmembrane pressure difference of 70 kPa, ranging from 15 to 350,000 L/m2-h. 

While gel formation and precipitation are reported frequently as the source of fouling in all membrane processes, only a small amount of work has been done on a quantitative determination of gel layer concentration and the solubility of natural organics. Naturally, flux or transmembrane pressure are important for concentration polarisation, which seems to be the major factor in gel formation. The MTC can also describe this, as was shown above. Organic characteristics such as solubility and hydrophobicity require further investigation as do their interaction with ions. The solubilities of HS and their complexes with salts are relatively unknown, as was discussed in Chapter 2. 

Energy requirements can be an important cost in membrane applications due to the requirement of a transmembrane pressure. The power input is determined by water or feed flow Q f and applied pressure P (see equation (8.1)). 

The equilibritun flux rate correlates very well with wall shear stress, Figure 10.21, under laminar and turbulent flow conditions. However, the e q>erimenfa]ly determined values of deposit depth are appreciably greater than those predicted by solving Equation (10.34) at low transmembrane pressures, but the deposition inodel becomes acceptable at transmembrane pressures greater than 1 bar. 

Selection of a membrane the membrane should have both the capability to retain the catalyst and to partially reject organic species, enabling control of the residence time in the reaction environment. In order to select a suitable membrane, rejection should be determined during operation of the photoreactor. Parameters such as transmembrane pressure, solution pH, molecular size of the pollutants and products/by-products of their degradation should be especially considered. In the case of NF an improvement of permeate quality could be obtained by taking advantage of the Donnan exclusion effect, provided that the membrane is selected properly. 

In practice, the cross membrane filtration potential is modified by a very brief jump of transmembrane pressure and, in consequence, of the permeate rate, which creates a jump of filtration potential, too (Fig. 2). For a given pH, that procedure is repeated with different pressure jump values. The streaming potential is obtained by dividing filtration potential jump value by transmembrane pressure jump value. The same test done at different pH, enables to determine the isoelectric point of a membrane (Fig. 3) and to have an idea on electrostatic charges density and sign. 

Figure 3.5 shows the curve for an experiment carried out with a membrane obtained with 20% polysulfone in dimethyl formamide (DMF) and precipitated in a coagulation bath composed of 50% water and 50% DMF. The test fluid was obtained by mixing six dextrans of different molecular weights (17, 40, 70, 100, 200, 500 kDa) from Leuconostoc (Fluka). The concentration of each one in solution was 1.0 g/L. From the curve obtained, a MWCO of 28kDa could be determined. Transmembrane pressure (driving force) was set to 9 bar. 

We prapared four kinds of water solution which had a coefficient of viscosity equal to Ht value 20%, 30%, 40%, 50% using PEG6000. We measured the arterial blood pressure, the filtration pressure, the plasma entrance pressure, TMP (transmembrane pressure), the liquid pressure, and the dialysis venous pressure. Fig.4(a)-(d) show the changes in pressures in those points by the different equivalent Ht values. We determined the pressures after setting the system operation as follow. 

For constant flux feed, the fouling index I can be determined from the slope of the linear region in a plot of transmembrane pressure (AF) versus time using Eq. (6.7). The MFI can then be calculated from Eq. (6.8) ... 

The structural analysis of membrane-associated peptides comprises two steps (a) the elucidation of the three-dimensional fold of the peptide and (b) the determination of the membrane-peptide interface. We will use our results gained for the 36 amino acid residue neuropeptide Y (NPY) [83] to demonstrate the approaches that can be used. NPY regulates important pharmacological functions such as blood pressure, food intake or memory retention and hence has been subject of many investigations (for a review see Ref. [84]). It targets the so-called Y receptors that belong to the class of seven transmembrane receptors coupled to G-proteins (GPCRs). 

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