Choosing the Right Membrane

When process engineers need to separate effluent streams, clarify or fractionate and where they demand reliable and repeatable performance, membrane filtration systems are often their first choice. At its most basic level membrane filtration involves separating a single flow stream into two separate streams, one more concentrated than the other, by passing it through a membrane filter. These streams can then either undergo further processing, or in the case of a waste-stream be diverted to an appropriate outlet.

Choosing the right membrane for each application is vital and membrane filtration specialists such as PCI Membranes can help engineers make the right decision. For many applications, these companies can provide membrane filtration units as a standard design, requiring minimal testing. In other situations, design engineers may need to extensively test the process on site. PCI's has the experience and expertise in-house to provide the solution to filtration problems across a wide variety of industrial sectors.

The nature of the process fluid is one of the main deciding factors when choosing which membrane is best to use. Knowing the dissolved solids content, the molecular weight of the material, the nature and loading of any suspended material will direct the engineer towards the correct membrane configuration and geometry. The pH and temperature of the inflowing stream are also important in making the final decision.

The filtration spectrum starts at the smallest molecular level with Reverse Osmosis (RO); this provides the finest degree of separation, followed by Nanofiltration, Ultrafiltration and Microfiltration. Between them, these processes separate particles, which differ in size by anything from a few Angstroms (A, 10-10 metres) up to a few microns (ľ, 10-6 metres).

The internal pressures range from 30-50 bar in high-pressure systems down to 1-5 bar in low-pressure microfiltration units. The filtration process relies on this pressure forcing the liquid through a physical barrier - a membrane. Separating the suspended and dissolved material in the incoming feed produces the desired concentrated end product. By choosing the correct configuration, specific sized particles can be isolated or allowed to permeate through the membrane, according to the membrane type.

Reverse osmosis uses a tight membrane that retains most dissolved species such as molecules and salts. The pressure in this system must exceed the osmotic pressure to force the liquid fraction across the semi-permeable membrane. The food industry often uses this membrane system for processing fruit juices, and tea, coffee and sugar solutions. It is also often used for treating effluent streams. Legislative drivers in Europe are also encouraging the increased use of such technology in treating landfill leachate.

Nanofiltration bridges the gap between RO and Ultrafiltration; it is often used for desalting and concentration duties in the textile industry. A wide range of industrial applications use Ultrafiltration, as its versatile nature lends itself to separating flows as diverse as effluent streams from dye-houses and pulp mills to juice clarification. At the higher end of the spectrum as we move towards Microfiltration, ceramic membranes are often used to provide a wide range of pore sizes in the food and drink, pharmaceutical and chemical industries as well as for separating wastewater effluents.

Membrane filtration technology has developed both in the way membranes are packaged and in the type of material used. The result is a wide range of module configurations and membrane geometries, which are suited to a variety of applications. Polymeric membranes account for biggest proportion of installed membranes currently in use. Several different polymers are used to suit the molecular weight cut off required, or achieve the desired resistance to fouling or performance when contacted with a specific process fluid. Common polymers include; polysuphone and polyethersuphone which are used for the full range of UF membranes. PVDF is often used for open UF membranes, whilst polyamide is used as the thin film membrane layer in NF and RO membranes. Cellulose acetate, the first polymer widely used for membranes, is still used in some applications where it exhibits superior fouling characteristics, but its use is limited due to its tendency to hydrolyse in alkaline conditions. Membranes can be configured in tubular, spiral, flat sheet or hollow fibre arrangements.

Tubular membranes have several advantages. They can handle viscous liquids with high levels of suspended solids and can be chemically or mechanically cleaned. The tubular polymeric membranes are housed in modules of stainless steel or plastic.

Spirals, as the name suggests, consist of tightly packed filter material sandwiched between mesh spacers, and wrapped in a small tube. The high packing density significantly increases its surface area compared with tubular membranes. Spiral membranes require careful prefiltration to avoid blocking if suspended solids are present. However, developments in spacer designs are helping to increase the number of applications to which spirals are suited. The spacers in spirals provide the channels through which the process fluid flow as it traverses the membranes surface, and they have a major influence on the ability to resist blocking from solids, and the pressure drop along the length of the element. Developments include increasing the channel height from about 0.7mm to over 2mm, and changes to the geometric form of the spacer to give smoother flow channels. Polymeric spiral membranes are generally used when a high throughput is required, while polymeric tubular membranes, which can often be mechanically cleaned, are more suited for low maintenance operations, for highly viscous products, or fluids with suspended material.

Hostile environments, high levels of solvents, wide pH range and other process considerations may dictate the use of ceramic membranes. This technology, normally adopted for Ultrafiltration and Microfiltration applications, typically uses an alumina or zirconia coating that is applied to the inside surface of the ceramic support. Whilst the use of a ceramic membrane is sometimes the only viable proposition, the capital cost is much higher than conventional polymeric membranes, but in most cases a longer operational lifetime can be expected.

Before deciding which membrane system is right for the application, it is important to do some preliminary financial assessments to see if installing such a plant is a viable proposition. The design requirements must be set such as:

A membrane system company, such as PCI Membranes, will then conduct a short pilot trial in its own laboratory, using a sample of the process stream to narrow down the choice of membranes. Typical laboratory trials will involve simple batch runs, either concentrating the retentate, or diafiltering to purify the test the test fluid. Often a selection of membranes will be tested simultaneously to identify the best membrane for the application. After selecting the most appropriate membrane, a second test may be carried out with the emphasis on the separation and flux in order to confirm the feasibility of the application. Such trials normally take only two or three days to carry out, plus some time to analyse samples and results. However, as they use a limited quantity of process fluid, it is wise to follow these initial tests with a larger pilot rig set up at the factory site. This will allow design data to be collected, including information on fouling and cleaning, on a pilot system that can mimic the potential full scale plant.

At this stage it is important to collect as much useful data as possible because the final design parameters will be based on this information. Will the system be used on a batch or continuous basis? How long can the plant be taken off line for cleaning? What are the key criteria for judging success? These trials typically last for 2-3 weeks but can take longer if the process fluid is variable. A well-designed testing procedure will save time and effort later. Once the test rig has been set up, measurements can be taken of the degree of fouling,i the permeation rate (flux), pressure drop and retention levels as concentration increases, as well as the effectiveness of the cleaning regime, and the quality of the end product. Design engineers can estimate the probable membrane life at this stage. This can then be taken into account when considering the whole life costs of the system.

Cleaning membranes is an important part of optimising a system's performance. The type and frequency of cleaning is a function of both the membrane and the process fluid. In the food industry for example, it is common practice to clean daily, whilst in certain water applications, cleaning may only be necessary after three months, or even less frequently. The most widely applied cleaning technique is to circulate a solution of appropriate chemical around the plant at low pressure to remove the soil from the membrane surface. Acids are used to remove mineral foulants, caustic detergent, or enzyme detergents are used to remove proteinacious soil, whilst oxidising agents such as sodium hypochlorite are used to remove organic fouling. Not all these regimes can be used on all membranes, for example, polyamide membranes have little or no tolerance to oxidising agents, and cellulose acetate membranes have are sensitive to alkaline conditions.

However, in some cases a standard membrane is not suitable for the application in question. At this stage a new configuration of membrane system may be needed. Legislative requirements or market forces, in these days of increased environmental awareness, often drives the need for managers to consider alternative technologies for treating plant effluents. In some cases membrane technology is appropriate. This was the case at a Swedish paper mill. In order for the company to attain the Swan Mark, a sign of environmental excellence, the mill needed to reduce the COD in its bleach effluent by 50%. The bleach effluent came from two sources, a hard wood line, and a soft wood line. Both streams had COD levels of 9 to 12 g/litre, pH values of 9 to 10.5, and the temperature was around 70°C. The suspended solids content of each stream varied with the hardwood having up to four times the content of the soft wood, which itself had up to 30 g/litre. The high suspended material content of the process fluid indicated that an open channel membrane geometry would be required, leading to the choice of PCI Membranes tubular system. This choice was subsequently verified when the pilot plant blocked up due to over concentration, but was recovered without damage to the membranes. Initial tests, carried out by PCI's showed that although the target of a 50% reduction in the COD with a 98% reduction in volume was achievable on the soft wood with a polyethersulphone membrane from PCI's existing commercial range of membranes, excessive fouling was encountered with the hard wood effluent. A large ultrafiltration plant was ultimately installed and commissioned by PCI Membranes. It continues to effectively reduce the paper mill's COD releases without any problems. 

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