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Flow through porous media - II
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INTRODUCTION

 

A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Knudsen diffusion and solution diffusion are prominent mechanisms. Knudsen diffusion is a means of diffusion that occurs in a long pore with a narrow diameter (2–50nm) because molecules frequently collide with the pore wall.The transport of gases through dense (nonporous) polymer membrane occurs by a solution diffusion mechanism.The gas dissolves in the polymer at the high pressure side of the membranes , diffuses through the polymer phase, and desorbs or evaporates at the low pressure side.

 

Membranes can be of various thickness, with homogeneous or heterogeneous structure. Membrane can also be classified according to their pore diameter. According to mesoporous (2nm < dp < 50nm) and macroporous (dp > 50nm). Membranes can be neutral or charged, and particles transport can be active or passive . The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Membranes can be generally classified into three groups:inorganic ,polymeric or biological membranes . These three types of membranes differ significantly in their structure and functionality.

 

Polymeric membranes are membranes that take the form of polymeric interphases, which can selectively transfer certain chemical species over others. There are several mechanisms that could be deployed in their functioning.Knudsen diffusion and solution diffusion are prominent mechanisms. Polymeric membranes are of particular importance in gas separation applications. Key industrial applications include: oxygen -nitrogen separation, removal of organics andor biomembrane is an enclosing or separating membrane that acts as a selective barrier, within or around a cell .

 

Reverse osmosis (RO) is a filtration method that removes many types of large molecules and from solutions by applying pressure to the solution when it is on one side of a selective membrane . The result is that the is retained on the pressurized side of the membrane and the puresolvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.Reverse osmosis is most commonly known for its use in drinking water purification fromand other substances from the water molecules. The process is similar to membrane filtration . However, there are key differences between reverse osmosis and filtration . The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate. Membrane filtration has a number of benefits over the existing water purification techniques: It is a process that can take place while temperatures are low. This is mainly important because it enables the treatment of heat-sensitive matter. That is why these applications are widely used for food production. It is a process with low energy cost. Most of the energy that is required is used to pump liquids through the membrane. The total amount of energy that is used is minor, compared to alternative techniques, such as evaporation.The process can easily be expanded.

 

Process management of membrane filtration systems - Membrane filtration systems can be managed in eitherdead-end flow or cross-flow . The purpose of the optimisation of the membrane techniques is the achievement of the highest possible production for a long period of time, with acceptable pollution levels.The choice for a certain kind of membrane system is determined by a great number of aspects, such as costs, risks of plugging of the membranes, packing density and cleaning opportunities. Membranes are never applied as one flat plate, because this large surface often results in high investing costs. That is why systems are built densely to enable a large membrane surface to be put in the smallest possible volume. Membranes are implemented in several types of modules. There are two main types, called the tubular membrane system and the plate & frame membrane system . Tubular membrane systems are divided up in tubular, capillary and hollow fiber membranes. Plate & frame membranes are divided up in spiral membranes and pillow-shaped membranes.

 


Membrane fouling - During membrane filtration processes membrane fouling is inevitable, even with a sufficient pre-treatment. The types and amounts of fouling are dependent on many different factors, such as feed water quality, membrane type, membrane materials and process design and control. Particles, biofouling and scaling are the three main types of fouling on a membrane. These contaminants cause that a higher workload is required, to be able to guarantee a continuous capacity of the membranes. At a certain point the pressure will rise so much that it is no longer economically and technically accountable.

 

Membrane cleaning - There are a number of cleaning techniques for the removal of membrane fouling. These techniques are forward flushing, backward flushing, air flushing and chemical cleaning , and any combination of the methods.

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