The benefits of membrane technology in the process industry

There is still strong growth potential for applications involving membranes in the production environment. The production process itself also includes a large number of separation steps, such as distillation, filtration or precipitation that can be replaced by membrane techniques.

Benefits:

• Saves energy
• Saves raw materials
• Enhanced quality of the product

During the past few years, we have predominantly witnessed a broadening of the applications of aqueous streams to include more challenging chemical process streams, such as solvents and strong acids.

Conventional membrane technology for process streams

Membrane technology is a fully fledged separation technology that can be used for a broad swathe of applications. The technology is commercially available and forms a dynamic and growing field for research, in which innovative membrane processes are continually being developed and commercialised.

The following are the most renowned systems in use within the industry that offer separation capabilities within a specified range of component size or component shape:

• Pressure-driven micro (MF)
• Ultra- (UF)
• Nano- (NF)
• Reverse osmosis (RO) filtrations

Recovering raw materials

Membrane technology is highly energy-efficient. It also offers a great deal of potential in the case of “zero-waste” processes, by enabling concentrate streams that are produced during water purification to be re-used. The reason for this is that those concentrate streams sometimes contain unused raw materials that can be re-used within the production process itself.

RO membranes

A classic example from the metals sector is the recovery of salts from the rinsing baths used for surface treatment. In this case, filtration takes place using RO membranes that separate the water from the saline stream, resulting in a pure water stream. This can be diverted back to the rinsing tanks, whilst the highly concentrated salt stream can be re-used in the plating bath. Using this system enables companies to achieve savings – on the one hand, by reducing the cost of waste treatment and on the other hand, thanks to the recycling of raw materials.

UF membranes

Membrane filtration can also be used in the valorisation of industrial waste streams by recycling raw materials. One example of this involves the use of UF for the recovery of sizing agents in the textile industry. Polyvinyl alcohol (PVA) is applied to the threads as a sizing agent before weaving begins. The woven thread must then be desized before it can be bleached or dyed. The desizing agent containing diluted PVA can be treated using membrane filtration. To achieve this, water and low-molecular contaminants such as salts and oil pass through the membrane, thereby enabling the concentrated PVA stream to be re-used.

NF membranes

In addition to constituents with a molecular weight greater than 200 daltons, NF membranes can also capture multivalent ions as a result of a membrane charging effect, while monovalent ions are able to pass freely. This phenomenon is potentially beneficial in the recovery of metals from ores, in which NaCl-rich waste streams can occur during the washing stages. Those streams contain heavy metals and sulphates and can be treated using a system of NF membranes to achieve streams of pure NaCl. By means of evaporation, the salt can ultimately be recovered for use as road salt.

Purification and fractionation of process streams using diafiltration

As well as concentrating the constituents of process streams, membranes can also be used in the purification and fractionation of process streams as well. The diafiltration process makes use of membranes that are porous enough to allow certain (macro) molecules or salts to pass through a UF or NF membrane, while capturing the constituents desired (proteins, biomolecules, etc.). By continually adding water, the smaller particles are washed out, as it were.

That process is frequently used in the food industry. The most renowned example is the desalination of whey. By using a UF membrane and adding water, the whey is converted into a protein fraction, from which lactose and salts have been removed. In a subsequent step, lactose is then separated from the salts by means of an NF membrane. Another example is the recovery of raw materials using an NF membrane when producing aspartame from aqueous streams that also contain dissolved salt.

Membrane filtration for the chemical and pharmaceutical industries

Conventional polymer membranes cannot be used in the majority of process flows in the chemical industry, due to the aggressive nature of those process flows (which involve substances such as solvents, acids or bases). Initially, ceramic membranes were suitable for such uses, however the high cost of the membranes required for large process flows meant that the level of investment required was not justified.

During the past ten years, pH-stable and chemically resistant polymer membranes have come onto the market. These broadened the potential for membranes to be used within processes in the chemical industry. The pH-stable membranes can be used across a broad pH-range from 0-14, or in highly acidic flows consisting of no more than 37 % HCl or 20 % H2SO4. The solvent-stable membranes are resistant to substances such as alcohols and ethers and can even resist the most aggressive types of polymers, such as DMF and NMP. The fact that chemical processes primarily require low-molecular components to be separated means that the applications for this type of membrane typically lie within the NF range.

Recovery of acids or bases

In the process industry, cleansing processes are responsible for a major proportion of the chemicals consumed and they also result in waste streams that are highly alkaline. Within the metal sector, sodium hydroxide or sulphuric acid are used as solvents during metal treatment, thereby creating waste streams made up of acids or bases contaminated with metals. Streams of that type can then be fed through a membrane system consisting of pH-stable NF membranes.

Not only does this make it possible to achieve up to 95 % filtration and cleansing, but it also ensures that the contaminants are brought together in a more concentrated form. Once purified, the waste streams can be directly re-used or further concentrated by means of evaporation.

Efficient use of raw materials within solvent streams

The availability of solvent-resistant nanofiltration membranes (SRNF) makes it possible to carry out new and innovative processes that form an alternative to conventional thermal separation processes.

For example, the amount of energy required to increase the concentration of a component from a solvent stream is less than 10% of the cost of using evaporation or distillation processes.

The solvent can also be recovered immediately, without using of a condensation step. SRNF can be used in many applications within the production chain. The less onerous conditions required are also advantageous when separating thermolabile constituents, which are frequently dissolved in solvents with high boiling points, such as NMP. Those components frequently occur in the pharmaceutical or fine chemical industries in the form of intermediates or final products and can partly be lost at high temperatures.

Implementing membrane separation devices in production processes makes it possible for raw materials to be converted into final products more efficiently.

When producing high-quality constituents, diafiltration using SRNF membranes is one of the techniques of choice when it comes to removing contaminants from product streams, during the synthesis of polymer specialties used in medical applications for example. The products must not contain any unreacted monomer or oligomer molecules that would reduce their quality. The molecules can be flushed out using SRNF by adding a solvent containing the dissolved polymer.

A further application of diafiltration is solvent-switching, which is currently carried out in pharmaceutical production processes. The solvent is gradually replaced by continually diluting it using the new solvent. The concentration of the product remains the same; it can be increased by combining it with a subsequent concentration step. What is more, the fractionation of end-products can also be achieved using diafiltration.

Combining solvent filtration with (bio)chemical reactions

In the chemical sector, membrane filtration techniques can be combined with reaction steps during the synthesis process. A particularly promising application is the recovery of homogeneous catalysts from reaction mixtures. The majority of these are precious metal-based, as a result of which the costs can increase sharply if any losses occur during the process. One of the important reactions that makes use of the catalysts is hydroformulation. During the process, olefins are converted into aldehydes using hydrogen and carbon monoxide (syngas). The most significant problem within this sensitive catalysis system lies in the separation of the reaction product. The only alternative technology available to separate the homogeneous rhodium catalyst is SRNF.

A relatively new membrane process is known as organophilic pervaporation (OPV) using solvent-resistant membranes that can be used in the recovery of volatile organic components from diluted aqueous streams. One example of this is batch-based butanol fermentation, in which microbial cells produce acetone-butanol-ethanol (ABE) until toxic concentrations are reached. By linking this to an OPV system, the organic products can be continually removed. That way, the fermentation process is not interrupted and production is therefore higher.