Membrane filtration as a means of purification or concentration relies on the principle that when a liquid is passed across or through a physical barrier, particles whose size is larger than the pores in the membrane are retained on one side of the barrier, while the remaining liquid is allowed to pass through. This principle holds true for coarse filters, such as metal sieves, as well as much finer materials which are capable of selective filtration and concentration of molecules of different sizes.
Over the past 15 years membrane filtration, which uses a polymer or similar material with an extremely small pore size, has become increasingly popular in industrial processes where reliable and repeatable purification or concentration are required. The technology is now being applied widely in dye manufacturing and textile colouring. This article sets out to explain how membrane filtration works and where it can be used successfully in the dye industry.
Filters of any type serve two purposes. They retain some material and allow the remainder to flow through to the next stage in a process. When making a cup of tea using tea leaves, the strainer collects the leaves (known as the retentate or concentrate) and allows the liquid which has been flavoured and coloured by the leaves (the permeate or filtrate) to flow through. Sometimes, as is the case when making tea, the retentate is an unwanted waste material and the permeate is the product of value. However, other applications rely on the efficient collection of the retentate. When panning for gold the prospector obviously wants to keep the small particles of metal but has no interest in the permeate water.
Membrane filtration works in exactly the same way. The characteristics which differentiate it from other forms of filtration are the relative size of particles being filtered and the driving force (usually pressure or a chemical concentration gradient) which allows the process to work.
There are four basic membrane filtration processes. These are, in ascending order of particle size:- reverse osmosis, nanofiltration, ultrafiltration and microfiltration.
Reverse osmosis is used to remove charged ions by applying a pressure, which exceeds the osmotic pressure of a solution, across a semi permeable membrane. This technique is often used to produce ultrapure water for laboratories and process. Further up the scale, nanofiltration is capable of separating particles of a few nanometres in diameter. While large ions and organic molecules are retained, smaller ions can pass through. In nanofiltration, osmotic pressure is still the major resistance to solvent flow through the membrane. Once the pores in the membrane are large enough that the material can be considered to be porous, the technique is referred to as ultrafiltration. This can be used to separate particles, typically of between 0.05 ľm and 0.15 ľm. Ultrafiltration is widely used in industry for process purification and concentration. The coarsest type of membrane filtration, which can handle particles of up to a few microns in diameter, is known as microfiltration. For each of these processes a variety of membrane materials and filter configurations are available, depending upon throughput, the nature of the process stream and the requirements of the manufacturing plant.
In the dye industry, membrane filtration has a number of possible applications. The largest of these is dye desalting, and concentration of the finished product, which is most commonly applied to reactive dyes, but can also be used on other products such as sulphur dye and direct dyes.
Membranes are also used with the many types of dye which are applied aqueously. Here the technology can be utilised for concentration and purification. Membrane filtration is used widely in a variety of process industries, not only for product purification but also for material recovery from waste and for the treatment of effluent prior to final disposal. In dye manufacture and application, like most other processes, there is a continual search for production methods which will improve product yield and reduce manufacturing costs. Membrane filtration now plays an important part in achieving both of these objectives.
In dye manufacture there are four main uses for membrane filtration. These are: improvements in the quality of the finished product, increased yield, savings in raw materials, or recovery of product from waste, and increased dryer capacity
PCI Membranes, who manufacture membranes and build filtration plants, have installed a number of dye facilities throughout Europe and Asia. Significant investment has taken place recently in South East Asia, with multinational chemical companies building plants to satisfy both local needs and the demand for product in the west. In many instances, the dyes manufactured in South East Asia will differ, according to their final market.
Many western dye manufacturers have succeeded in reducing the production costs of their plants in South East Asia. This has been brought about by improvements in manufacturing efficiency and product distribution.
In one instance in Thailand, at Matangi Dyestuffs Industries, output from the dye plant is being increased significantly by the use of nanofiltration. This increase puts tremendous pressure on the process plant and efficient, reliable concentration of dye for the spray drying facility is of paramount importance.
In this case, nanofiltration units are being installed by PCI Membranes which will increase efficiency of the existing spray dryers by increasing the feed concentration to the dryer from the current level of approximately 7 – 10% total solids right up to 25% total solids. By running the spray dryers continuously and using nanofiltration membrane technology to concentrate the dye prior to spraying it is envisaged that 170% increase in production volume will be achievable.
Elsewhere in Asia, at the Everlight company, one of Taiwan’s major dye producers, PCI has recently installed plants for dye desalting and the company is currently examining the feasibility of using nanofiltration as a method for treating dye effluent.
Because of the wide variety of chemical compositions of dyes, often due to a manufacturers own molecular construction of the dye carrier it is usually necessary to run pilot studies before finally selecting a particular membrane configuration. These studies, carried out at PCI’s research and development laboratories in Whitchurch, Hampshire involve the testing of various filter membranes and housing configurations. Site trials are then carried out for process development and optimisation, prior to scale up.
Dye desalting and purification, the process by which impurities are removed to improve the quality of the product, is currently one of the largest applications for nanofiltration technology, especially for reactive dyes. Most dyes fall within a molecular weight ranging from 600 to 900 although some variations do occur.
Dye manufacturers are now actively pursuing the desalting of the finished dye prior to spray drying and membrane filtration provides the ideal method for salt removal. In the opinion of many dye producers, desalting improves the product quality. It also makes spray drying more efficient, because the granulation of the dye takes place without the production of dust.
Another benefit of nanofiltration for desalting can be found in the manufacture of liquid dyes. These can become unstable during storage and, in some cases, the salt can precipitate out into the holding vessels. Desalting will help to stabilise these products.
Nanofiltration, using equipment such as PCI’s B1 module, a tubular filtration system is now being applied to dye desalting. The membrane material used in this type of filter module will vary according to the dye being treated. A number of membranes can be selected, depending upon the separation required.
Although tubular membrane filtration is favoured by many manufacturers of dyestuffs, an alternative geometry is available. Spiral membranes use similar material to those of tubular construction but in this case it is spirally wound. The choice of tubular or spiral configuration, either of which can be supplied by PCI, is almost entirely dependant on the feed material and conditions of operation. If the dye is in solution, and is likely to remain soluble during desalting, there is an advantage to using spiral elements. These provide a higher throughput because of the larger surface area of membrane available in a filtration module. However, if solids are present in the feed material, or if precipitation of solids is likely, then tubular membranes are recommended.
In many industries, membrane filtration systems are used to recover quantities of the finished product from the waste stream, thereby improving final production yields. Whilst this is sometimes feasible during dye production, where a reactive dye is concerned the reaction has often occurred by the time the dye has reached the effluent stage of the process. If this is the case, membrane filtration can still be used effectively, but to concentrate waste material prior to final effluent discharge rather than as part of a product recovery programme.
During the process of dying a material, large quantities of water are used. In many dye plants it could be viable to recover and reuse the process water, rather than to discharge it to waste. Depending upon the process economics of a particular plant, membrane filtration can be employed to remove impurities and purify the water ready for reuse in the dying procedure. Nanofiltration and/or reverse osmosis would normally be used at this part of the production process.
To summarise, membrane filtration technology provides dye manufacturers and users with a number of benefits connected with improved product quality and reduced process costs. Membrane filtration, used during the processing of liquid dyes, is useful for desalting and concentration. Tubular filtration systems such as those developed by PCI, are used to increase the stability or strength of the finished product. Once the dye has been passed through the membrane, the resulting dye either remains in liquid form or can be spray dried to form a powder. By reducing the amount of inorganic material present in grain based dyes, smaller amounts of the dye are required to provide colouring to natural fibres and man made fabrics. Transport and storage costs can also be reduced, which have a significant impact on large users of dyestuffs.
For liquid dyes, the solubility is improved by removing inorganic molecules. This ensures that the liquid dye remains stable once in solution.
As manufacturers and end users of dyes seek improvements in quality, process efficiency and profitability, membrane filtration technology will become even more significant in the dying and colouring industry.