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Bubble point test [BP]
The bubble point procedure is a commonly used test method to characterize a membrane and monitor product consistency and quality. The bubble point itself is a determination of the minimum pressure (bubble point) at which a wetting liquid is pressed out of the pore system of a membrane while forming a steady bubble chain. The bubble point test procedure is also a common method of determining the maximum pore size.
Cross-flow Filtration
In cross-flow filtration, a fluid (feed) stream runs tangential to a membrane, establishing a pressure differential across the membrane. This causes some of the particles to pass through the membrane. Remaining particles continue to flow across the membrane, "cleaning it".
Dead-end filtration
In the dead-end filtration technique all the fluid passes through the membrane, and all particles larger than the pore size of the membrane are retained on its surface. Trapped particles will start to build up a "filter cake" on the surface of the membrane, which has an impact on the efficiency of the filtration process.
Downstream side
The filtrate side of the membrane.
Feed
The fluid/gas that has not passed through the membrane and has therefore not been filtered.
Filtration
A process of removing particles of different diameters from a fluid or a gas by passing it through a permeable material, such as a membrane.
Filtrate
The fluid/gas that has already passed through the membrane and has been filtered.
Flux
A term that is commonly used for the flow rate of water/liquid through membranes.
Fouling
Deposition of solid substances on the surface of the membrane. Fouling causes a decrease in both the amount of filtrate produced and the quality of the filtrate. The performance of fouled membranes can usually be restored with cleaning procedures.
Hydrophobic
Membranes characterized as hydrophobic repel water. They have little or no tendency to absorb water so that a droplet remains on the surface.
Hydrophilic
Hydrophilic membranes have an affinity for water. Their surface chemistry allows these materials to be wetted spontaneously.
Inner diameter [ID]
A value characterizing the geometry of a capillary or tubular membrane. The wall thickness and the outer diameter of a membrane are the other essential parameters for characterization of a capillary or tubular membrane.
Maximum Pore Size [PGmax]
A calculated value gained from the results of the bubble point test. The maximum pore size is theoretically the largest pore within the pore structure of a membrane.
Microfiltration
Microfiltration is a way of removing contaminants in the size range of 0.1 to 10.0 µm from fluids or gases by passage through a microporous medium such as a membrane. There are two techniques used in microfiltration: dead-end filtration, where microfiltration is widely used, and cross-flow filtration, using a tangential flow for the fluid being filtered.
Pore Size
The pore size is usually stated in micrometers (µm) and refers to the diameter of particles a membrane is likely to retain to a defined degree of efficiency.
Silt density index
A measure of the ability of water to foul a membrane or plug a filter.
Through-put
A very practical characterization technique for a microfiltration membrane. The intention is to evaluate the achievable filtrate output during the life-time of a filter: i.e. the total volume of a specific fluid that passes through the membrane before it must be replaced. The terms dirt holding capacity or high loading capacity are also used.
Trans-membrane flow [TMF]
A measurement of the transmembrane flow is carried out to document the membrane flow characteristics. The TMF is defined as the initial volume of liquid passing through the membrane wall within a given unit of time, related to surface area and pressure, and expressed in ml/min cm² bar.
Ultrafiltration
Ultrafiltration is a process similar to microfiltration. The main difference is the "tighter" retention behavior: the ultrafiltration membrane retains much smaller particles from the passage through the membrane than do microfiltration membranes. Typically the particle size is measured by molecular - weight, and ultrafiltration membranes have retention ranges from 1,000 to 1,000,000 molecular weight.
Upstream side
The feed side of the membrane.
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