给排水专业英语Unit13

Unit13

Membrane Filtration represents a very efficient and economical way of separating components that are suspended or dissolved in a liquid. The membrane plays the role of a selective barrier that allows certain compounds to pass through and retains other compounds in the liquid. The liquid passes through the semipermeable membranes which represents the permeameter, which of it containing the retained constituents is the retentate (also known as concentrate).

Membranes are classified by solute exclusion size thatt is sometimes referred to as pore size. A Reerse Osmosis (RO) or Hyperfiltration Membrane rejects solute as small as 0.0001 μm,which is in the ionic or molecular

size range. However, a Nanofiltration (NF) membrane rejects solute as small as 0.001 μm,which is also in the ionic and molecular size of 0.01 μm and 0.10 μm,respectiely. These membranes reject colloidal particles, bacteria, and suspended solids by size exclusion.

The development of membrane materials has played an integral part in the development of industrial-scale membrane separation applications. The range of available membrane materials used in water and wastewater treatment is quite bored,and the principal types of membranes used are polypropylene, cellulose acetate, aromatic polyamides, and thin-film composite. The properties of membrane, such as selectivity, permeability, mechanical stability, chemical resistance, and thermal stability, are dependent on the type of material. Recently, ceramic materials made by sintering inorganic materials, such as Aluminum Oxide, or a Carbon Nanocomposite formulation, have been considered for MF applications. Common NF and RO membranes materials include aromatic polyamide and polyamide for spiral-wound configurations, and cellulose triacetate for RO hollow fine fiber membranes.

According to the driving force of the operation, the membrane processes can be

classified as:

(1) Pressure gradient driven operations: such as , RO, NF, UF, MF, gas separation(GS), or partial

pressure driven processes, such as pervaporation(PV).

(2) Concentration gradient driven operations, such as dialysis and osmosis. (3) Electrical potential gradient driven operations, such as elctrodialysis(ED). (4) Temperature gradient driven operations, such as membrane distillation(MD).

Membrane modules are the core elements in membrane-based separation and purification system. Typical membrane modules are classified into four types: plate and frame, spiral wound, tubular and hollow fiber. Plate and frame

In the plate and frame module,membranes are attched to both sides of a rigid plate.The

plate may be constructed of solid plastic with grooved channels on the surface,porous fiberglass materials.This type of module is simple and the membrane replacement is easy. But because of its complexity,it is very expensive to operate for large-scale operations. Spiral wound module

This consists of large consecutive layers of membrane and support material

rolled up around a tube. The membrane are glued along three edges with the fourth side left open which is attached to a perforated pipe. Spiral system are usually staged with three to six membrane element becomes the feed to the following element,

and so on, for each element within the pressure tube. The feed flows axial through the cylindrical module parallel to the central pipe and the permeate flows are radically towards the central pipe. The spiral configuration makes better use of space and is less expensive. However, extensive pretreatment is necessary because it is more sensitive to pollution.

In this type of module,the membrane is either cast on the inner surface of ,or placed

within, a porous tube and sealed into place .The feed solution always flows though the centre of the tubes and the permeameter is collected in the tubular housing .Generally it is used for viscous or bad quality fluids and can be cleaned either mechanically or hydraulically with ease .The system is not very compact and has a high cost per membrane area installed when compared with either the spiral or the hollow fine fiber for water treatment applications.

Hollow fiber module consists of a large number of hol

low fibers assembled together in a module.The ends of the fibers are epoxy sealed to form a sheet-like permeate tube.The feed solution flows through the open cores of the fibers and then the permeate is collected in the cartridge area surrounding the fibers.The fitration can be carried out either 'inside-out'(the feed solution passes through the bore of the fiber and the permeameter is collected on the outside of the fiber) or 'outside-in'(the feed solution enters the shell side of the fibers and the permeate passes into the fiber bore).The characteristics of this configuration are its extremely high packing density and self-supporting.

Typical operation modes for membrane filtration processes are dead-end and cross

flow. In dead-end filtration the direction of the fluid flow is perpendicular to the membrane surface so that the retained particles and other components accumulate and deposit on the membrane surface. In cross-flow filtration the feed flow is parallel to the membrane surface, retentate is removed from the same side further downstream , whereas the permeate flow is tracked on the other side. Moving the feed flow tangentially to the membrane surface can result in much higher permeation fluxes as the stream continuously removes retained material. Comparing to cross-flow, (1)the dead-end membranes are relatively easy to fabricate which reduces the cost of the separation process, (2)the dead-end membrane separation process is easy to implement and the process is usually cheaper than cross-flow membrane filtration, and (3) a dead-end filtration is the extensive membrane fouling and concentration polarization.

Membrane fouling refers to a long term flux decline caused by the deposition of

retained Particles onto the membrane surface and/or within the pores of the membrane,deteriorating the performance of the membrane.membrane fouling is a widespread problem limiting the performance and application of synthetic membrane proocesses and,as such,is an important consideration in the design and operation of a membrane system because cleaning frequency,pretreatment requirement,operaing condition,cost,and performance are affected by membrane fouling.fouling of the membrane can occur in three general forms:(1)a buildup of the constituents in the feedwater on the membrane surface,(2)the formation of chemical pprecipitates due to the chemistry of the feedwater,and(3)the damage to the membrane due tothe presence of chemical substances that can react with the membrane or

biological agents that can colonize the membrane.in general,there are two approaches tocontrol membrane fouling(1)pretreatment of the feedwater,and(2)physical and chemical cleaning of the membranes.

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