Outside-in Hollow Fiber Research Articles

Comparison of scale formation between inside-out and outside-in hollow fiber membrane distillation (MD) modules when concentrating hyper saline feed solutions

Comparison of scale formation between inside-out and outside-in hollow fiber membrane distillation (MD) modules when concentrating hyper saline feed solutions

Numerical and experimental investigation for cleaning process of submerged outside-in hollow fiber membrane.

Membrane fouling has limited extensive applications for hollow fiber membranes in water treatment. Backwashing and air scouring can effectively solve this problem in the submerged outside-in hollow fiber membrane system. In this study, variation of the fouling layer on the membrane surface during backwashing and the impact of shear stress caused by air scouring on fouling removal were investigated through computational fluid dynamics (CFD) simulation. The backwashing and air scouring process were simulated using CFD and the results were verified by experimental studies. The results of experimental studies are in accordance with the simulation results. During the backwashing process, the velocity profile inside the reactor was presented, and visualization of the particle movement to illustrate the dynamic peeling process of the fouling layer on the membrane surface was also shown. The formation of uneven cleaning reveals that the upper region of the fibers has an excellent cleaning effect during backwashing. After that, the supporting role of air scouring was investigated in the study. It is concluded that the lower part and the middle region of the fibers suffer greater shear stress by analyzing the velocity contours and vectors, and the analysis results indicated that air scouring can further remove membrane fouling.

Modelling of fouling of outside-in hollow-fiber membranes by TiO2 particles

Abstract The advantage of using suspended TiO2 particles in a photocatalytic membrane reactor is a higher surface area with respect to immobilized particles and, therefore, a higher catalytic activity. Detoxified water can be separated from the suspended catalyst by application of membrane processes such as microfiltration. Unfortunately, during the microfiltration, fouling of membranes by catalyst particles occurs, due to which the flux decreases or the transmembrane pressure increases, so cleaning by back-flushing of the permeate should be performed frequently. We have developed a mathematical model describing the pressure rise on a hollow-fiber module used for the constant flux, outside-in microfiltration of TiO2 dispersion. Unlike other models treating fouling of hollow-fiber membranes, our model incorporates a variation of the bulk concentration of TiO2 particles as a very important parameter with respect to the reaction. By the application of the model, we can predict some critical points of the photocatalytic membrane reactor operation such as the critical value of the suction pressure and the optimum time when cleaning of the membrane should be introduced.

Study on the flux distribution in a dead-end outside-in hollow fiber membrane module

In this study, model-based prediction and CFD simulation were performed to systematically study the flux distribution in a dead-end outside-in hollow fiber membrane module. A dimensionless number was proposed to determine where the maximum local flux was located. This number can serve as a guide to avoid deterioration of the flux distribution and enable the key point for cleaning to be determined. The number also indicated that fibers with a specific dimension must be packed with a packing density within the proper corresponding range to avoid poor hydrodynamic conditions in the module. A non-uniformity coefficient N was developed to characterize the flux distribution. The conceptual energy utilization η was suggested as the complement to N. A decrease in N caused by a change in one of the geometric parameters will always accompany an increase in η. Reducing the permeability of the membrane and the pressure drop in both the shell and lumen sides will improve the uniformity of the flux distribution. For the specific hollow fiber membrane module, to ensure better flux distribution and high energy utilization, the following parameters are recommended for the fiber: a length shorter than 2000mm, a shell void fraction larger than 0.4 and an internal diameter wider than 0.4mm. Fibers that have the parabolic-shaped membrane resistance distribution will theoretically achieve a completely uniform flux distribution.

Numerical Estimation of Hollow Fiber Membrane for Mobile Water Treatment

In the application of water treatment system, membrane has gained favour in the industry as well as in the research field. In pressure-driven category, ultrafiltration membrane with pore size of 3 to 10 nm is one of the choices for water treatment application. With the advantages of being compacted and self-supporting, hollow fiber membrane configuration has been widely used as ultrafiltration membrane. This is an important feature for a mobile water treatment system developed in this work. The mobile water treatment system is investigated in terms of its operational performance focusing for simple setup configuration. Mobility of the membrane treatment system in this work is aim to develop a stand-alone membrane water treatment system that can operates without electricity. Therefore, the system targeted to be a self-sufficient in rural areas where electricity and delivery of spare parts are difficult. A membrane filtration system with outside-in hollow fiber membrane is developed. The numerical approach of Response Surface Method (RSM) is used to estimate and optimize the flux performance in this work. The operating conditions i.e transmembrane pressure (TMP) as well as the local condition (water temperature) were considered in the numerical estimation. The initial numerical estimation found that the developed mobile system has permeate flux range from 0.422 L/m2h up to 3.035 L/m2h for local temperature of 20˚C to 35˚C and further optimization were discussed in this study.

Pilot study of inside-out and outside-in hollow fiber UF modules as direct pretreatment of seawater at low temperature for reverse osmosis

The essential point for a successful seawater reverse osmosis (SWRO) plant is pretreatment that provides consistent and high-quality feed to SWRO membranes. Advances in membrane technology and increasing requirements on water quality have stimulated the use of ultrafiltration (UF) for SWRO pretreatment. In this study, inside-out (module A) and outside-in (module B) hollow fiber UF modules were adopted as the pretreatment prior to SWRO desalination at Qingdao Jiaozhou Bay, the Yellow Sea of China. The performances of the two modules were compared to operation parameters and permeate qualities. Based on the flux data, TMP and permeability, the outside-in UF module consistently performed better with an air enhanced backwash than the inside-out one using a chemical enhanced backwash, which was not particularly efficient for the outside-in module. As to permeate quality, both modules met the demand of the SWRO feed.

Use of cake deposition to improve the efficiency of ultra- and microfiltration plants

Fundamental principles and topics of a new approach to designing ultra- and microfiltration plants in which cake deposition and its control are used to improve the membrane filtration efficiency are discussed. A general phenomenological mathematical model for the process of depth membrane filtration, which was suggested by the new approach, is formulated and methods of its numerical and approximate solutions are described. The available particular solutions to the DMF model are analyzed and classified, and recommendations for their practical use are given. The use of the new approach for increasing the efficiency of existing deadend outside-in hollow fiber membrane filters is also discussed. A general mathematical model is formulated for this type of deadend filters and methods of its numerical and approximate solutions are discussed. It is shown that the highest efficiency of outside-in hollow fiber membrane filters can be achieved when the collection efficiency of the membrane surface with respect to suspended particles is highest.

Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants

This paper describes pilot test results obtained on cooling tower blowdown of a power plant in HeBei province, using both 0.1 μm MF and 0.03 μm UF, which are outside-in hollow fiber membrane modules. In this pilot, the MF and UF modules were installed at the same system, although at different testing periods. Operation parameters such as flux, backwash (BW) time, BW frequency, frequency of air backwash (ABW) were changed; the MF and UF modules were operated at constant operating conditions during the same testing period. The study showed that both the MF and the UF membrane systems can produce permeate with high and consistent quality. The filtrate SDI 15 of the permeate produced by both systems was below 3 for most of the time (except the period of flocculation addition). This water quality meets the requirements of the reverse osmosis (RO) system. In-line rapid pre-coagulation using polyaluminium chloride (PAC) at a concentration of 2.5–5 ppm did not decrease the filtrate silt density index (SDI 15) and may have even led to a sharp increase in filtrate SDI15, especially when combined with polyacrylamide (PAM). In addition, the injection of polymerized ferric sulfate (PFS) and sodium hypochlorite (NaClO) to the pretreatment system feed water had little influence on the filtrate SDI 15. Other water quality parameters such as total Fe, Cu, COD and colloidal Si were also measured. The UF membrane module reached a higher level of FPI than the MF membrane module did, while the latter had more stable performance than the former. The MF/UF test has achieved stable membrane permeability, and both MF and UF modules were found suitable to provide adequate pretreatment prior to RO desalination.

Selection of membranes for deadend micro- and ultrafiltration outside-in hollow fiber filters

A mathematical model taking into account the adsorption of particles around a membrane pore mouth at the stage of standard blocking is developed. The model is used to study the effect of the average membrane pore size, the cutoff coefficient, and the ratio of the length of the pore inlet region to be blocked to the whole pore length on the product flow rate of a deadend outside-in hollow fiber filter. It is shown that the use of membranes with an average pore size higher than the cutoff diameter for the particles to be removed from the suspension will be profitable only for the first 15–20 min of operation. In this case, the membrane should have an average pore size as large as possible, but not exceeding a limit value at which the membrane selectivity can drop; the cutoff coefficient as low as possible; and a pore inlet region to be blocked as long as possible.

Phenomenological theory of depth membrane filtration

A phenomenological theory of depth membrane filtration (DMF), in which outside-in hollow fiber membranes are used as collectors of colloidal particles, is developed to study the performance of a hollow fiber membrane filter with two product streams, permeate (clarified liquid that passed through semipermeable membrane) and filtrate (clarified liquid due to the collection of suspended particles on the external surface of hollow fibers). The theory is based on the general phenomenological expression for the rate of particle deposition on the membrane surface, in which the deposition rate is proportional to the product of the particle deposition coefficient and the concentration of suspended particles, with the deposition coefficient being an arbitrary function of the mass of deposited particles and permeate velocity. The system of governing equations is solved by the generalized Crank–Nicholson finite-difference method and the approximate method using the averaging of the permeate velocity in the mass conservation equation. The effect of membrane permeability on the filter productivity is studied. It is shown that the approximate solution can be used as a relatively simple and accurate tool to study and design hollow fiber filters for DMF.

Deadend outside-in hollow fiber membrane filter: Mathematical model

A mathematical model describing the performance of deadend outside-in hollow fiber filters developed on the basis of Darcy's law and the theory of depth filtration accounting for the decrease in the liquid flow rate with filter depth coordinate due to permeate withdrawal is studied. The system of governing equations is solved with the generalized Crank–Nicholson method of central finite differences and approximate methods based on averaging certain process parameters, which use simple Laplace transform solutions for particular cases. It is shown that some of the approximate methods provide an accuracy acceptable for engineering calculations, implying that they can be used in designing more efficient filters.

Particle deposition in outside-in hollow fiber filters and its effect on their performance

Effect of transmembrane pressure (TMP), particle deposition and reentrainment coefficients, and specific cake resistance on the performance of a dead-end outside-in hollow fiber (DOIHF) filter and hollow fiber membrane (HFM) adsorber is studied using the adsorption–peptization model. It is shown that the increase in TMP causes a higher decline in the permeate velocity for both a DOIHF filter and HFM adsorber. The rise in the adsorption (particle deposition) coefficient can considerably increase the productivity (produced volume of clarified liquid) of these membrane devices. The increase in the peptization (particle reentrainment) coefficient can noticeably decrease their productivity. The volume of clarified liquid produced by the HFM adsorber is 1.6–2 times higher than that produced by the DOIHF filter for the same operation time and process parameters. The values of the particle deposition coefficient, particle reentrainment coefficient, and specific cake resistance are suggested to be adjusted by changing process parameters such as the ionic strength, TMP, and pH, as well as by choosing the material of hollow fibers that provides favorable (attractive) surface interactions between the membrane and particles.

Membrane Separation in Deadend Hollow Fiber Filters at Constant Transmembrane Pressure

A mathematical model of cake formation in deadend outside-in hollow fiber filters accounting for nonuniform cake deposition throughout the filter is developed. The model is used to study two possible mechanisms of cake growth: conventional (convective) and adsorption-peptization. The system of equations describing the cake formation is numerically solved with the generalized Crank-Nicholson finite-difference scheme. Comparison of the numerical solutions with experimental data shows that the cake formation in deadend out-side-in hollow fiber filters is governed by the adsorption-peptization mechanism.

Membrane filtration with reversible adsorption: Hollow fiber membranes as collectors of colloidal particles

A novel membrane separation process in which outside-in hollow fiber (HF) membranes with axes normal to the flow of the suspension under treatment are used as collectors of the colloidal particles is proposed. This process, called depth membrane filtration, yields two product streams: permeate, the liquid that passed through the semipermeable HF membranes, and filtrate, the liquid that was clarified by collection of the colloidal particles on the HF shells. A mathematical model in which convective transport and reversible adsorption account for the deposition of colloids onto the membrane surface is developed. Analytical expressions for the coefficients of adsorption and peptization are derived. The concentrations of suspended particles in the product water of an HF filter treating a dilute latex suspension in continuous-flow and batch operations were calculated. The calculations based on the values of the latex-solution properties and the HF operating and design parameters available in the literature demonstrated the feasibility of the process.