Production plant for polymer clean-up under construction
Water has obviously been used over a long period for many processes and this is also true to a smaller extent for superheated water. The current interest is to use water for processes that have previously been done in different and more polluting ways and these uses have recently been patented.
Superheated water has been used in the food industry for cooking a little above 100°C and the final extractions of instant coffee are sometimes up to 120°C. The processing of wood pulp is carried out sometimes above 100°C as has the hydrolysis of starch to polysaccharides out at high temperatures for some time. Re-crystallizations in water contained in sealed tubes above 100°C have been carried out for 100 years or more. Superheated water has also been used for waste treatment by the so-called wet-air oxidation process (a similar process has also been carried out in supercritical water). Chemical reactions have also been carried out in superheated water and this work has been thoroughly reviewed recently.
In the context of the recent use of water for more environmentally friendly processes, extraction has attracted the most interest. Extraction of plant materials to produce flavours and fragrances and valuable compounds has been carried out, for example the superheated water extraction of biomass for a cosmetic product and the production of indigo from woad. Extraction has been carried out for the removal of metal ions from a polymer and the removal of organic compounds from a polymer, even though the polymer is water soluble, by exploiting phase behaviour.
Clothing dyed with our indigo
There has been interest in the recycling of polymers by de-polymerisation, regeneration of rubber by de-vulcanisation and the clean-up of contaminated land.
superheated water extraction
A BBSRC LINK funded project Phytoderm to investigate the extraction and efficacy of polysaccharides from Marsh Mallow Root for skincare applications proved to be very effective. The superheated water extract from hydroponically grown root has now been commercialised by Boots plc for use in skincare and suncream formulations. The roots of a plant are extracted with liquid water at 150°C to give a solution containing a little more than 1.3% by weight of solids, which is then used in the formulation of a sun cream. The plant was used as a traditional remedy for sunburn and the sun cream is also very effective.
production of indigo from woad
The traditional production of indigo from indigoferous plants such as woad, is a long-winded and labour-intensive process. It is still carried out in some parts of the world. Then, in the nineteenth century, indigo was synthesised and most of it is now produced synthetically. In recent years more emphasis has been placed on greener and more sustainable processes, so we have looked for a method, with a number of European partners, of producing indigo sustainably from plants more quickly and efficiently.
In the process developed, the indigo precursors are extracted from woad using hot water. The extract is filtered and treated with air in alkaline conditions (from lime) to produce the insoluble indigo. This is allowed to settle and filtered off. In the traditional process an enzyme present in the plant converts the precursors to indigo in shallow pools with agitation over a period of 3 days, whereas the new process takes 10 minutes.
The small production plant now being used in Italy is shown above. It can process 5 tons of woad per day which will give up to 20 kg of indigo. The indigo produced has been used in a number of fashion shows, such as one in Finland, with an example shown below.
removal of metal ions from a polymer
A process that we carry out is to remove metals from a polymer so that the amounts of 16 metals are all below 1 ppm. The process is carried out at 180°C at around 10 bar. The metal ions are soluble in water, but the reason that a high temperature is used is that under theses conditions water is much less polar. It therefore is able to swell and plasticize the polymer so that the metals can more easily diffuse out of it.
A schematic diagram of the plant is shown, with temperatures at each point of the system. Water is supplied at 15°C and is first heated by the water coming out of the cell. It then passes to a second heat exchanger where it is heated by oil to 180°C before passing through the cell. On leaving the cell, the water is cooled by the incoming water.
Superheated water is not energy intensive. It takes only about one fifth of the energy to heat water from 15°C to 180°C as it does to convert water at 15°C to steam at 100°C. Further, heat can be recycled. By using the arrangement shown 82% of the heat is recovered and recycled.
The process is a low volume and high value and only 200 kg are processed in one batch. The cell has a volume of 700 L and the flow rate of water is 16 L per minute. 5 m3 of water are required for each batch of material. The plant is constructed of stainless steel.
A picture of the plant before cladding is shown above with the vessel, hopper, cold-water tank and ion exchange cylinders (for purifying the water) labelled. The heat exchangers are behind the bottom of the vessel and the oil heater is behind the product-collecting bath. A large compressor is required to dry the product after extraction.
removal of organic compounds from a polymer
Polymers which are water-soluble at all compositions at room temperature, because of hydrogen-bonding, can, depending on composition, separate into two phases as the temperature is raised. As the temperature is raised further, however, the mixtures become one phase again at all compositions. This situation is shown schematically in the diagram above. For example, at 200°C mixtures of a polymer and water with a polymer weight fraction of less than ~0.6 split into 2 phases one of nearly pure water and one containing 60% w/w of the polymer.
Impurities in the polymer, which have some water solubility at 200°C can be washed out with water at that temperature. The figure below shows schematically how this is done. A solution of polymer in water is put into the cell and heated to 200°C. Two phases are formed with the polymer-rich phase at the bottom. The right-hand valves are closed and water at 200°C pumped into the bottom of the cell and wastewater containing the impurities and a small amount of polymer passes out of the top. When the impurities have been removed to the specified level, the left-hand valves are closed and the right-hand valves opened. Nitrogen is then passed into the top of the cell and the product is pushed out of the bottom. The process is carried out at a pressure sufficient to maintain water as a liquid and heat transferred to or removed from the various streams as required, as shown in other applications. Finally water is evaporated from the product to give the purified polymer. This process can be made continuous.