Screw press

Screw presses may be used in the diffusion process for sugarcane, wherein the liquids for the diffusion of sugar solutions and fresh makeup water are extracted from die cut cane chips by the single or multistage unit. 

Some odier products that can be liandled by continuous screw presses are  

Presses are available with many types of casings, designed to suit the characteristics of the material to be pressed, such as  

heavy 1-in.-thick carbon or stainless steel slatted casing  

3/16-in.-thick naval alloy brass drilled screens, tapered for self-cleaning  

The expressive forces are high so that screw presses are rugged machines, and the shell can only have fairly coarse perforations. Accordingly, the liquid produced in a screw press will almost certainly need a stage of filtration after the press. 

A similar function to that of the screw press, i.e. the expression of liquids contained in natural products, can be achieved by means of rollers, or vertically mounted discs (rotating on axes slightly inclined down from the horizontal so that there is a pinch zone at the bottom of Ihe rotation) - but neither is a filtration prcxress. 

Depending on the furnish quality as well as on the requirements of the finished product, the tasks of a dispersing system vary widely. They are e.g. 

Dispersing is used in secondary fiber processing. It is located at the point of water loop separation where the suspension is dewatered up to a consistency of about 25 to 35 %. Often bleaching is done in combination with dispersing. For high quality demands two dispersing steps may be applied in the process. 

During dispersing high shear forces are applied to the debris particles to be dispersed and also to the fibers. The shear forces have to exceed the strength of these debris particles in order to reduce their size. Hence stock consistency must be about 24 to 30 % to ensure the transfer of the required amount of dispersing energy, and the temperature has to be elevated to reduce their strength. 

A dispersing system consists of three major process steps (Fig. 4.50)  

The fiUings of a disk disperger (Fig. 4.53) usually have intermeshing teeth or sometimes refiner-type bars. The rotor/stator element spacing is 1 mm or less, peripheral speed is about 50 to 100 m s . The specific energy demand is 50 to 80 kWh in certain cases up to 150 kWh r. Disk dispergers are always operated at elevated temperatures otherwise the loss in freeness would be considerable. This effect may have use for HC refining of recycled fibers (see Section 4.2.5). 

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Mechanical Pressing. Historically, the first large commercial production of oils from seeds and nuts was carried out using labor-intensive hydraulic presses. These were gradually replaced by more efficient mechanical and screw presses. Solvent extraction was developed for extraction of seeds having low oil content. For seeds and nuts having higher oil content, a combination of a screw press followed by solvent extraction is a common commercial practice (prepress—solvent extraction). 

Screw presses consist of a single or double screw conveyor that has perforated walls. The soHds being conveyed are squeezed due to a gradual reduction in the pitch or diameter of the screw. Screw presses are used for dewatering of rough organic materials. 

Other designs squee2e the cake between two permeable belts or between a screw conveyor of diminishing diameter, or pitch, and its permeable enclosure. The available filters which use mechanical compression can be classified into four principal categories, ie, membrane plate presses, tube presses, belt presses, and screw presses. 

Membrane plate and tube presses are dealt with here belt and screw presses are included in the discussion of continuous pressure filters. 

Continuous Compression Filters. The variable chamber principle appHed to batch filtration, as described before, can also be used continuously in belt presses and screw presses. 

The Vepex process developed in Hungary (Table 9) involves disintegration of plant materials followed by double screw pressing to maximize juice production. Green chloroplastic protein is removed by direct steam-injection heat treatment at 82°C with the addition of flocculents and centrifugation. The white protein fraction is separated from the chlorophyU-free process juice by direct steam injection at 80°C, followed by centrifugation and drying (94). 

Screw presses have been used extensively in North America, but are not popular in Europe. The mash is added at the top of a vertical rotating screw. As the mash moves down it is compressed in the taper and juice flows out through slatted conical walls. Compression of the pomace blocks channels for juice flow and iuhibits juice extraction. Extensive use of press aids is necessary for efficient operation. 

Cranberry juice, too acidic to be consumed as a 100% juice drink, has been sold since 1929 as cranberry juice cocktail. Juice extraction usually involves pressing the juice from thawed cranberries in a tapered screw press, which affords a 60—64% juice yield. The juice is diluted with two volumes of water and sugar is added to raise the °Brix to 15 to produce a juice cocktail. Under the Federal Food, Dmg and Cosmetic Act, cranberry juice cocktail must contain not less than 25% single-strength cranberry juice with soluble soHds content of 14—16 °Brix, vitamin C content of 30—60 mg/177 mL (6 02), and... 

Screw-pressed oil is aUowed to stand to settle out suspended soUds, filtered through plate filter presses, and then pumped to storage. The oil-rich solvent (miscella) from the solvent-extraction process is filtered or clarified, and most of the solvent is removed in a long tube evaporator. FinaUy, the concentrated oil passes through a stripping column where sparging steam is injected to remove the residual solvent. A metric ton of cottonseed yields ca 91... 

Drying and Solids Separation. Separation of the polymer gel from the meth an o1 /methyl acetate Hquid is an important step, accompHshed by using standard pieces of equipment such as filters, screw presses, or centrifuges. 

Screw presses (Fig. 2) do not produce a clear liquid product. Frequendy, the product is further filtered in a filter press to give a clear Hquid product. Press aids are added to feed materials containing fine particles or particles that can deform and plug the slots in the edge of a screw press. Typical press aids include sawdust, rice hulls, perlite, and diatomaceous earth (see Diatomite). A vertical screw press is a continuous press that has been used for dewatering sewage sludge (2). 

Fig. 2. Cross section of screw press used for fmit juice (32). 1, Hopper 2, perforated sheets 3, main shaft 4, perforated cage 5, draining cylinder 6, cone ...

Elastomer and Polymer Processing Systems, Bulletin MPR 76,1976, Pre-Press, Bulletin FO 175 and French Dual Cage Screw Press, Bulletin OP8130,1981, The French Oil Mill Machinery Co., Piqua, Ohio. 

Screw Presses Figures 18-155 and 18-156 show screw presses. The rotating screw shown in Fig. 18-156 is of constant pitch and has a constant diameter of about 0.3 m (12 in). Pressure to squeeze juice from fruit placed in the press comes from restricting the opening at the end of the barrel with a hydraulically adjustable cone and by making the spindle of the screw thicker toward the discharge end. 

FIG. 18-155 Cross section of screw press used for fruit juice ( 32j (Ij hopper, [1] perforated sheets, (oj nuiin. shaft, (4.) perforated ca e, (.5] driiinin cylinder, (Bj cone, i ] hv draiilic cvlinder, (8j driiinin cvlinder oil, (9j earliox. (Courtcsij of the h ivnch Oil MW Mnehinenj Co.)... 

Screw presses were traditionally used for seeds and fruits that had to be mechanically ruptured to release the liquid in the seeds or cells... 

Spindel-31, n. spindle oil. -presse, /. screw press. -wa(a)ge, /. hydrometer, -zelle, /. fusiform cell. 

Presses are of two basic types hydraulic batch presses and screw presses. Hydraulic presses are used for extracting fruit juices, and screw presses for dewatering materials such as paper pulp, rubbish and manure. The equipment used is described in the handbooks Perry et al. (1997). 

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