Continuous Pressure and Vacuum Filtration and Washing of Organic Cellulose and Biomass Products
The processing of organic, cellulose-containing materials has been around for many years as engineers have developed clever chemistry or cultivated natural conversion processes to make products with characteristics, which were not naturally available. Age-old examples are paper, linen or in the field of 'conversion' to alcohol and methane.
Currently, due to high oil prices along with the passage of the Renewable Fuels Standard (RFS) included as part of H.R.6, the Energy Independence & Security Act signed in December, 2007 calling for significant increase in elhanol usage nationwide from both grain based and cellulosic based sources, there is tremendous interest in developing efficient, cost-effective manufacturing processes to produce energy from cellulosic feed stocks.
Further, even if the main focus is not on energy production, well-established processes are being changed so that what used to be by-products are now becoming, if not primary products, at least valuable additional products. In addition, the combined effects of an excess of waste and a diminishing supply of basic raw materials focuses the attention of research workers on treating 'waste' as a potential raw material for further processing, thus opening a whole new field of bio-processing, bio-energy and specialty chemicals.
This article will use actual process data to highlight the choice of pressure or vacuum filtration and cake washing techniques for these newly developed bioprocesses.
BIOMASS
The word biomass is perhaps misleading since il has different meanings in different industries. Any suitable plant or the products of plants which contain cellulose can be labelled 'biomass.' Plants include, but are not limited to, trees, bushes, agricultural plants, weeds, vines, straw, flowers, kelp, algae and mixtures thereof. Commercial and agricultural waste products are also included such as stalks, paper, cotton, bagasses, corn cobs, corn stovers, starch, sugar cane, etc. Plants that have been partially decomposed, such as humus, peat, certain soft brown coal etc., may also fall into this category.
Nevertheless, in this article, biomass is defined as a mass of organic solids saturated by, or suspended in, a liquid, which may or may not be organic, and of which it is required that the liquid phase is separated, or replaced by other liquids to enable further processing. It is the filtration / separation in this process stage which is the focus of the discussion. In this article biomass does not mean digested sludge from sewage works.
Regardless of the type of biomass, most bioprocesses rely on reactions of solids and liquids, which later require to be separated. Because no liquid / solid separation process is 100% efficient, il is inevitable that some of the liquid phase will stay embedded in the solid phase and therefore require either mass transfer or evaporation.
The physical form in which the solid content can present itself is almost as varying as there are processes. It may be in the form of finely ground organic matter, il may be granules, fibres, chips or strips. Il may be that it has been chemically digested and is amorphous or it may have the original structure. The liquid phase may be water-thin or very viscous. The temperature of the mixture may vary as well as the flow characteristics of the mixture itself may be free-flowing or barely pumpable. Finally, Ihe percent solids and quantity may be large or moderate.
They have, however, one thing in common in that the solids do not like to give up their liquid content very easily and recovering or removal of the process liquors usually involves washing or leaching.
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