Overview: Comparison With Other Solid-Liquid Separation Technologies

Provided by RCAI

Rotary vacuum filtration (RVF) with a precoat (and other systems that require filter aid) and centrifuges historically have been used for many of the solid-liquid separation process steps required in pharmaceutical and food processing. Recently, these industries have been evaluating and utilizing membrane based technologies used in applications where RVFs and centrifuges were employed. The reasons why a membrane based technology, such as the PallSep system, may be preferable to the RVF and to the centrifuge are described below.

Membrane Based Systems Versus RVFs and Other Systems That Require Filter Aid

Rotary vacuum filtration systems, pressure leaf systems and belt filters are examples of solid-liquid separation systems that are usually operated with a filter aid. The filter aid can be body fed or recirculated to create a precoat on the filtration surface (for example, a cloth material). Filter aid based systems can provide a dry cake.

There are many drawbacks to using a filter aid such as diatomaceous earth (DE). The utilization of a filter aid requires a supply of the filter aid, since most filter aids are not indefinitely renewable. Thus, the cost of the filter aid, the cost of handling the filter aid and the cost of the disposal of the filter aid are part of the operating cost. For some facilities, the cost and labor associated with the disposal of the filter aid can be considerable. For example, DE needs to be removed from the system (usually with water) and this slurry then needs to be handled as a waste. Landfills are becoming a problem in some areas and the slurry often needs to be further treated (water removed) before it can be moved.

A very important consideration when a filter aid is used is that the solid material collected with the filter aid cannot be separated from the filter aid. If this solid material has value and could be sold as a product (for example if there is nutritional value in solids they often can be sold for animal feed), then the use of a filter aid makes utilizing this product an impossibility.

There are also operational difficulties when using a filter aid. DE, for example, will provide only one level of filtration, whereas with membrane filtration different micron ratings will provide different effluent qualities as required for a specific application. Since membrane filters can provide a finer filtration than DE, the processing further downstream can be easier. Additionally, diafiltration, which is often desirable for optimal product recovery, can often be difficult in a filter aid based separation technology.

In many operations, the filter aid is used as a precoat, and a recirculation period is generally needed to establish the precoat and provide filtrate clarity. Membrane systems can provide the required clarity from the beginning to the end of the operation.

Cleaning a filter aid based system after a process mission can be very labor intensive. There is direct operator contact with the DE and process fluid. Membrane systems provide a contained system.

Membrane Based Systems Versus Centrifuge Systems

There are many different types of centrifuge systems available for solid-liquid separations. The principle of operation is the use of centrifugal force (G-force) to separate materials of different densities. Some examples include: basket centrifuges, super (high G-force) centrifuges, solid bowl decanter and disk stack centrifuges. Centrifuges can retain solids for manual removal, periodically eject the solids, or continuously discharge solids. Some centrifuge types are capable of providing a dry cake (95-99% solids). The dryness will depend upon the centrifuge type and the process fluid. Centrifuges can provide a compact footprint and some types are contained systems.

A centrifuge will provide only one level of separation, whereas with membrane filtration different micron ratings will provide different effluent qualities as required for a specific application. Since membrane filters can provide a finer separation than the centrifuge, processing further downstream can be simplified.

Cleaning the system after a process mission, depending upon the design of the centrifuge, can be very labor intensive and can involve direct operator contact with the process fluid. Membrane systems provide a contained system.

Centrifuges generally require a high maintenance cost in comparison to membrane based systems. Even though there are some distinct advantages to the use of a membrane based system such as a system over the traditional equipment (RVFs or centrifuges), it may not be desirable to replace all of the existing equipment with systems. The use of new membrane-based separations equipment is likely to be strongly considered for an expansion or a new process. It may prove to be judicious to use the membrane based system in conjunction with the traditional equipment. For example, if a pressure leaf filter with a DE coat is currently being used to concentrate to a non-pumpable solid, then a PallSep system could be used upstream of the pressure leaf filter to expand the capacity of the DE-based system. PallSep systems require that the fluid be pumpable. This solution has the advantage of increased capacity with provision of the required high solids cake and a cleaner filtrate (at least for the fluid processed by the PallSep).

PallSep Versus Traditional Crossflow Systems

In order to understand how a high shear device, such as the PallSep system, compares with traditional crossflow technologies, such as spiral wound elements, plate and frame systems and ceramic modules, a comparison of the PallSep and traditional crossflow systems will be made.
A core difference between the technologies is the purpose of the crossflow. For the traditional static crossflow devices, shear is generated by the feed velocity, whereas for the PallSep the feed velocity is used for distribution of the fluid on the upstream side of the membrane.

Since the shear in the PallSep system is generated by mechanical means, the shear rate is independent of feed velocity and hence the pumping rate. A lower pumping rate will save energy costs and can improve product yield for cases in which the material can be damaged by the action of high speed recirculation.

In some crossflow devices, such as spiral wound filters, high feed velocity for shear development means that a significant pressure gradient can develop from the inlet to outlet (feed to retentate). Such pressure gradients can impact the performance of the membrane and cause fouling. The PallSep VMF system can be operated in an essentially uniform transmembrane pressure (TMP) mode. This phenomenon is very valuable in microfiltration because differences in the TMP cause different parts of the membrane to experience different loadings, which can cause premature fouling in some areas of the membrane. Premature fouling of the membrane can lead to severe difficulties in subsequent membrane regeneration.

Small gap sizes are required in crossflow systems to achieve high velocity and thus a high shear. This is not the case in a PallSep VMF system. This independence of gap width promotes flexibility in choosing the appropriate gap width. This is particularly advantageous when the solid loading in the fluid is high.

Since the fluid velocity is not critical, the ratio of permeate to feed can be much better controlled in a system. In applications requiring concentration, it may not be possible to achieve the high crossflow rates required as the material becomes more concentrated. Futhermore, the energy required to maintain high crossflow is increased as the feed material becomes more concentrated. The PallSep VMF system, unlike conventional crossflow systems, does not have these restrictions, as shear is independent of crossflow velocity.

The shear is developed at the membrane surface. Thus the upstream fluid is exposed to the high shear in a localized region only. This prevents the excessive damage that can be caused to shear sensitive material during the high speed pumping of feed fluid that is required in conventional crossflow systems. System footprint and hold-up volume can be minimized with the PallSep VMF system. The PallSep system provides a significantly smaller footprint than plate and frame filtration systems. A small footprint for a system can be a benefit for applications in which space is limited. A low hold-up volume can mean that more of the fluid can be recovered. Due to the design concept for the PallSep VMF system, a large amount of membrane can be packed into a small volume. The hold-up volume is further reduced because a high flow recirculation loop is not necessary.

PallSep is a registered trademark of Pall Corporation