Introduction to Membrane Systems


Advantages and Disadvantages, Challenges of Membrane Separations

 

Membrane processes have a number of pluses and minuses compared to alternative means of performing separations.

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The advantages include:

Bullet - ThumbsupBecause membrane processes can separate at the molecular scale up to a scale at which particles can actually be seen, this implies that a very large number of separation needs might actually be met by membrane processes.

Bullet - ThumbsupMembrane processes generally do not require a phase change to make a separation (with the exception of pervaporation). As a result, energy requirements will be low unless a great deal of energy needs to be expended to increase the pressure of a feed stream in order to drive the permeating component(s) across the membrane.

Bullet - ThumbsupMembrane processes present basically a very simple flowsheet. There are no moving parts (except for pumps or compressors), no complex control schemes, and little ancillary equipment compared to many other processes. As such, they can offer a simple, east-to-operate, low maintenance process option.

Bullet - ThumbsupMembranes can be produced with extremely high selectivities for the components to be separated. In general, the values of these selectivities are much higher than typical values for relative volatility for distillation operations.

Bullet - ThumbsupBecause of the fact that a very large number of polymers and inorganic media can be used as membranes, there can be a great deal of control over separation selectivities.

Bullet - ThumbsupMembrane processes are able to recover minor but valuable components from a main stream without substantial energy costs.

Bullet - ThumbsupMembrane processes are potentially better for the environment since the membrane approach require the use of relatively simple and non-harmful materials.

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The disadvantages include:

Bullet - StopsignMembrane processes seldom produce 2 pure products, that is, one of the 2 streams is almost always contaminated with a minor amount of a second component. In some cases, a product can only be concentrated as a retentate because of osmotic pressure problems. In other cases the permeate stream can contain significant amount of materials which one is trying to concentrate in the retentate because the membrane selectivity is not infinite.

Bullet - StopsignMembrane processes cannot be easily staged compared to processes such as distillation, and most often membrane processes have only one or sometimes two or three stages. This means that the membrane being used for a given separation must have much higher selectivities than would be necessary for relative volatilities in distillation. Thus the trade-off is often high selectivity/few stages for membrane processes versus low selectivity/many stages for other processes.

Bullet - StopsignMembranes can have chemical incompatibilities with process solutions. This is especially the case in typical chemical industry solutions which can contain high concentrations of various organic compounds. Against such solutions, many polymer-based membranes (which comprise the majority of membrane materials used today), can dissolve, or swell, or weaken to the extent that their lifetimes become unacceptably short or their selectivities become unacceptably low.

Bullet - StopsignMembrane modules often cannot operate at much above room temperature. This is again related to the fact that most membranes are polymer-based, and that a large fraction of these polymers do not maintain their physical integrity at much above 100 oC. This temperature limitation means that membrane processes in a number of cases cannot be made compatible with chemical processes conditions very easily.

Bullet - StopsignMembrane processes often do not scale up very well to accept massive stream sizes. Membrane processes typically consist of a number of membrane modules in parallel, which must be replicated over and over to scale to larger feed rates.

Bullet - StopsignMembrane processes can be saddled with major problems of fouling of the membranes while processing some type of feed streams. This fouling, especially if it is difficult to remove, can greatly restrict the permeation rate through the membranes and make them essentially unsuitable for such applications.

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Challenges of Membrane Processes

The following questions generally need to be considered when considering a membrane option:

Bullet - GlobeWhat is the most appropriate niche for membranes in the real, large-scale operating plant?

Bullet - GlobeHow effective would the membranes be in comparison with "traditional" separation methods such as distillation, absorption, etc?

 

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