Various process feed or process residue treatment processes for solid/liquid separation are known which require significant residence time, high pressure and high temperature. Generally, liquids must be separated from treated solids at those conditions. Conventional liquid/solid separation equipment is not satisfactory for the achievement of high liquids/solids separation rates and solids with low liquid content.
For example, in the pretreatment of lignocellulosic biomass, hydrolyzed hemicellulose sugars, toxins, inhibitors and/or other extractives must be squeezed from the solid biomass/cellulose fraction. It is difficult to effectively separate solids from liquid under the high heat and pressure required for cellulose pre-treatment.
Many biomass to ethanol processes generate a wet fiber slurry from which dissolved compounds and liquid must be separated at various process steps to isolate a solid fibrous portion. Solid/liquid separation is generally done by filtration and either in batch operation, with filter presses, or continuously by way of screw presses.
Solid/liquid separation is also necessary in many other commercial processes, such as food processing (oil extraction), reduction of waste stream volume in wet extraction processes, dewatering processes, suspended solids removal.
Commercial screw presses can be used to remove moisture from a solid/liquid slurry. However, the remaining de-liquefied solids cake generally contains only 40-50% solids. This level of separation may be satisfactory when the filtration step is followed by another dilution or treatment step, but not when maximum dewatering of the slurry is desired, the leftover moisture being predominantly water. This unsatisfactory low solids content is due to the relatively low maximum pressure conventional screw presses can handle, which is generally not more than about 100-150 psig of separation pressure. Commercial Modular Screw Devices (MSD's) combined with drainer screws can be used, which can run at higher pressures of up to 300 psi. However, their drawbacks are their inherent cost, complexity and continued filter cake limitation of no more than 50% solids content.
During solid/fluid separation, the amount of liquid remaining in the solids fraction (solids cake) is dependent on the amount of separating pressure applied, the thickness of the solids cake, and the porosity of the filter. The porosity of the filter is dependent on the number and size of the filter pores. A reduction in pressure, an increase in cake thickness or a decrease in porosity of the filter, will all lower the separation efficiency and result in a decrease in the degree of liquid/solid separation and in the ultimate degree of dryness of the solids fraction.
For a particular solids cake thickness and filter porosity, maximum separation is achieved at the highest separating pressure possible. For a particular solids cake thickness and separating pressure, maximum separation is dependent solely on the pore size of the filter.
High separating pressures unfortunately require strong filter media, which are able to withstand the separating pressure, making the process difficult and the required equipment very costly. When high separating pressures are required, the thickness of the filter media needs to be increased to withstand those pressures. However, to maintain the same overall porosity as a filter with the thinner filter media, thicker filter media require a larger pore size. This may create a problem, depending on the solids to be retained, since the acceptable pore size of the filter is limited by the size of the fibers and particles in the solids fraction, the clarity of the liquid fraction being limited solely by the pore size of the filter media. Pores that are too large allow a significant amount of suspended particles to collect in the liquid fraction, thereby reducing the liquid/solid separation efficiency. A higher porosity can also be achieved by providing a higher overall number of pores, but that either weakens the filter media and thus lowers the maximum operating pressure, or unduly enlarges the filter surface.
In order to achieve an acceptable balance between separation efficiency and filter size, conventional solid/liquid separation equipment is generally operated at less than desirable pressures and larger than desirable pore diameters. However, over time, that leads to the filter media becoming plugged with suspended solids, thereby not only reducing the separation efficiency, but eventually leading to failure of the filter. This is especially the case at the high pressures required for cellulose pre-treatment. Thus, a backwash flow of liquid is normally required to clear a blockage of the filter and restore the separation rate. Once a filter becomes plugged, it takes high pressure to backwash the filter media. This is particularly problematic when working with filter media operating at elevated pressures, for example above 1000 psig, or with a process that is to be continuous.
Conventional single, twin, or triple screw presses are known, but have unacceptable separating capabilities. U.S. Pat. No. 7,347,140 discloses a screw press with a perforated casing. Operating pressures of such a screw press are low, due to the low strength of the perforated casing and the relatively low porosity of the casing. U.S. Pat. No. 5,515,776 discloses a worm press and drainage perforations in the press jacket, which increase in cross-sectional area in flow direction of the drained liquid. U.S. Pat. No. 7,357,074 is directed to a screw press with a conical dewatering housing with a plurality of perforations for the drainage of water from bulk solids compressed in the press. Again, a perforated casing or jacket is used. As will be readily understood, the higher the number of perforations in the housing, the lower the pressure resistance of the housing. Moreover, drilling perforations in a housing or press jacket is associated with serious challenges when very small apertures are desired for the separation of fine solids. Thus, an improved filter media or dewatering module for a screw press is desired.