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Membrane-Based Separation Processes
Published in Pau Loke Show, Chien Wei Ooi, Tau Chuan Ling, Bioprocess Engineering, 2019
Kit Wayne Chew, Bervyn Qin Chyuan Tan, Jiang Chier Bong, Kevin Qi Chong Hwang, Pau Loke Show
Due to the different rates of permeation of constituents, however, there is a gradual build-up in concentration of nonpermeating components as more permeable components pass through the membrane, which results in layer formation on the membrane surface. This phenomenon is known as concentration polarization. Concentration polarization reduces the concentration difference of permeating components across the membrane, resulting in a subsequent decrease in the flux as well as in the selectivity of the membrane. Membrane fouling is also a major limitation that affects all the membrane technology performance, including microfiltration, as a result of the deposition of solutes or particles on the surface (external fouling) or in the membrane pores (internal fouling). Intense chemical cleaning or replacement of the membrane may be required if the fouling is severe. The pore size of a microfiltration membrane is usually larger than the molecule; thus, biomolecules like proteins are less likely to deposit at the membrane surface compared to other membrane application such as ultrafiltration membranes (Charcosset, 2012b).
Design of Highly Compact and Cost-Effective Water Purification Systems for Promoting Rural and Urban Welfare
Published in Sundergopal Sridhar, Membrane Technology, 2018
B. Govardhan, Y.V.L. Ravikumar, Sankaracharya M. Sutar, Sundergopal Sridhar
Concentration polarization is defined as an accumulation of solute deposit on a membrane surface. Consequences of concentration polarization are increase in osmotic pressure, resistance to solvent flow, increase in solution viscosity and subsequently fouling or blocking of membrane pores, thereby reducing the flux, rejection and membrane life. Concentration polarization cannot be avoided in the membrane process, but it can be minimized by various methods. Chemical treatment methods can minimize fouling, and in situations involving gel or cake formation on the membrane surface, hydrodynamics has to be changed in the feed channel to improve mass transfer. This can be done either by steady state technique using high cross-flow velocity or unsteady state by introducing turbulence promoters such as spacers, inserts, etc., in the feed flow path. Fouling is a term that indirectly describes the performance loss of a membrane, which becomes physically or chemically changed by the process fluids leading to reduced and poor quality output. Fouling can also be called loss in flux, which cannot be reversed while the process is in operation.
Desalination
Published in Frank R. Spellman, Hydraulic Fracturing Wastewater, 2017
Similar to the flow of water through a pipe (see Figure 7.1A, B), concentration polarization is the phenomenon of increased solute (e.g., salt) concentration relative to the bulk solution that occurs in a thin boundary layer at the membrane surface on the feed side (Figure 2.1C). Let’s look first at Figure 7.1A, which shows that flow may be laminar (streamline), and then look at Figure 7.1B, where the flow may be turbulent. Laminar flow occurs at extremely low velocities. The water moves in straight parallel lines, called streamlines or laminae, which slide upon each other as they travel rather than mixing up. Normal pipe flow is turbulent flow, which occurs because of friction encountered on the inside of the pipe. The outside layers of flow are thrown into the inner layers, and the result is that all of the layers mix and are moving in different directions and at different velocities, although the direction of flow is forward. Figure 7.1C shows the hydraulic boundary layer formed by fluid flow through a pipe. Concentration polarization has a negative effect on the performance of an RO membrane; specifically, it reduces the throughput of the membrane (Kucera, 2010). Flow may be steady or unsteady. For our purposes, we consider steady-state flow only; that is, most of the hydraulic calculations in this text assume steady-state flow.
A study on near zero liquid discharge approach for the treatment of reverse osmosis membrane concentrate by electrodialysis
Published in Environmental Technology, 2020
Cigdem Balcik-Canbolat, Cisel Sengezer, Hacer Sakar, Ahmet Karagunduz, Bulent Keskinler
A number of experiments were performed to investigate the effect of concentrate and diluate flow rate at constant voltages. The ED experiments were carried out using different feed flow rates (20, 30, 40 L/h) and with an applied voltage of 10 V. Flow rates were chosen considering the previous similar studies [19,28]. The process was performed at room temperature. In theory, the flow rate can cause an increase in desalination efficiency, but this increase is generally limited. Thus, operation flow rate must be investigated for a suitable range. From Figures 6 and 7, flow rate did not have a significant effect on deionization in the studied range of flow rates. It was concluded that the desalination efficiencies were almost the same at 20, 30 and 40 L/h. Low flow rates can cause concentration polarization at boundary layer of the membrane surface. Accordingly, the low flow of the feed solution gives enough time to organic or inorganic ions to accumulate on the membrane surface [19]. So, operating at higher flow rates can reduce the effect of the concentration polarization due to the turbulence through the ED stack. Hence, the 40 L/h of flow rate was determined as the optimum flow rate considering polarization problems. Similar results were observed by Chandramowleeswaran and Palanivelu [29] and Jing et al. [30]. Liu et al. [31] reported that the influences of flow rate on the water permeate performance or ions rejection were not so obvious. It was also stated that higher flow rate induces higher turbulence and results in lower scaling potential.
Valorisation of sodium lignosulfonate by ultrafiltration of spent sulphite liquor using commercial polyethersulfone membrane
Published in Indian Chemical Engineer, 2022
Kaushik Nath, Vinay B. Patel, Haresh K. Dave, Suresh C. Panchani
Membrane permeate flux and solute rejection are the two important performance indices of any pressure-driven membrane process. The steady-state permeate flux was determined by measuring the volume of permeate collected in a given span of time following Equation (1): where is the volumetric permeate flux in m3 m−2 s−1, is the effective membrane area in m2 and is the permeate flow rate in m3 s−1. Solute rejection coefficient under convective flow through a membrane can be estimated by where is the solute (here lignosulfonate) concentration in the permeate stream and is that in the bulk feed or concentrate. This rejection coefficient is considered to be the observed rejection coefficient as both the solute concentration of permeates and bulk could be measured directly. But in UF, like all other pressure-driven membrane processes, a gradual buildup of solute takes place on the membrane surface. A concentration profile is developed in the concentrate side of the membrane, ultimately leading to a phenomenon of concentration polarisation. Since, the surface solute concentration at the membrane is different from that in the bulk; the observed rejection coefficient () as stated by Equation (2) fails to portray the actual or true rejection experienced by the membrane. Therefore, we introduced another term true rejection coefficient (R) as described by where is the concentration of solute at the membrane wall. Incorporating the concept of concentration polarisation and considering film theory to hold good, a material balance at membrane surface can be expressed as [20]
Separation of sago starch from model suspensions by tangential flow filtration
Published in Chemical Engineering Communications, 2019
Samantha Siong Ling-Chee, Octavio Carvajal-Zarrabal, Cirilo Nolasco-Hipólito, Mohammad Omar Abdullah, Esaki Shoji, María Guadalupe Aguilar-Uscanga, Zayn Al-Abideen Gregory, Shafri Samawi
As TMP increased, flux declined due to the fibers in the suspension clogging the entrance of the membrane cassettes. As can be seen in Figure 3, starch is contained in the stems, which are disrupted during the rasping process to release the starch. Fiber particles are bigger than starch granules, clogging the entrances of the membrane cassettes separately from the fouling caused by granules. However, multiple layers of membranes in the cassette provide enough membrane area to allow the filtration process to proceed. It was clear that a conditioning pretreatment is necessary to eliminate the fibers prior to starting the process of filtration, as is often done prior to microfiltration and ultrafiltration (Middlewood and Carson, 2012). Once fibers were removed, the membrane functioned as normal. The fouling phenomena could occur due to concentration polarization from the starch granules as expected. Concentration polarization refers to the emergence of concentration gradients at a membrane/solution interface resulting from selective transfer of some components through the membrane under the effect of transmembrane driving forces. This effect is the typical effect explained in many articles on membrane technology. Our process was improved substantially since we used a stirred tank. The study with and without mixers helped a lot to understand that the phenomena of concentration polarization is of less impact on the flux of the membrane. It can as well be said that a static mixer exerts its most favorable effect at low pressures and lower flow rates and by the fact that in the said circumstances the concentration polarization of the membrane is reduced, since the turbulence created by the mixer does not allow the sedimentation of the particles of starch (Šaranović, et al., 2011). Moreover, the nature of the feed particles and the flux behavior suggest that classic mass-transfer models based on Brownian diffusion and film theory are not applicable to starch suspensions. The resistance model is the most accepted model to describe the data obtained when processing this kind of suspension (Wiesner et al., 1992; Ousman and Bennasar, 1995; Shukla et al., 2000). Wiesner et al. (1992) proposed simple models to describe the variation of permeate flux with time when the limiting resistance was due to the membrane, pore blocking or cake layer; these models were subsequently applied by Lim and Bai (2003) to describe microfiltration of activated-sludge wastewater. In the present research this model is the most acceptable, since no temperature was used, no other components in considerable concentration were present and the process was controlled exclusively by the variation of the concentration as a result of the TMP applied and membrane area used respecting the process time.