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Unformatted text preview: EXTRACTION and SUGAR INDUSTRY APPLICATIONS EXTRACTION 1LEACHING(SOLID EXTRACTION) a) GENERAL INFORMATION b) FACTORS INFLUENCING THE RATE OF EXTRACTION c) LEACHNG EQUPMENT 2 LIQUIDLIQUID EXTRACTION a) EXTRACTION PROCESS b) CLASSIFICATION OF EXTRACTION EQUIPMENT STAGEWISE EQUIPMENT FOR EXTRACTION DIFFERENTIAL CONTACT EQUIPMENT FOR EXTRACTION EXTRACTION IN SUGAR INDUSTRY APPLICATIONS 1 THE FXEDBED OR ROBERT DFFUSON BATTERY 2 CONTNUOUS DFFUSON BATTERES EXTRACTION Extraction is the method of removing one constituent from a solid or liquid by means of a liquid solvent.Extraction techniques fall into two categories.The first is called leaching or solid extraction and the is second called liquid extraction. LEACHING (SOLID EXTRACTION) A-GENERAL INFORMATION Leaching is concerned with the extraction of soluble constituent from a solid by means of a solvent.The process may be used either for the production of a concentrated solution of a valuable solid material,or in order to remove an insoluble solid ,such as a pigment ,from a soluble material with which it is contaminated. The selection of the equipment for an extraction process is influenced by the factors which are responsible for limiting the extraction rate.There are four important factors to be considered: Particle size: The smaller the size ,the greater is the interfacial area between the solid and liquid,and therefore the higher is the rate of transfer of material and the smaller is the distance the solute must diffuse within the solid. Solvent:The liquid chosen should be a good selective solvent and its viscosity should be sufficiently low for it to circulate freely. viscosity should be sufficiently low for it to circulate freely. Temperature:In most cases,the solubility of the material which is being extracted will increase with temperature to give a higher rate of extraction.Further ,the diffusion coefficient will be expected to increase with rise in temperature and this will also improve the rate of extraction. Agitation of the fluid:Agitation of the solvent is important because this increases the eddy diffusion and therefore the transfer of material from the surface of particles to tha bulk of the solution. B-FACTORS INFLUENCING THE RATE OF EXTRACTION LEACHNG EQUPMENT When the solids form an open ,permeable mass throughout the leaching operation ,solvent may be percolated through an unagitated bed of solids.With impermeable solids or materials that dissintegrate during leaching,the solids are dispersed into the solvent and are later separated from it.Both methods may be either batch or continuous. Leaching by percolation through stationary solid beds Stationary solidbed leaching is done in a tank with a perforated false bottom to support the solids and permit drainage of the solvent.Solids are loaded into the tank,sprayed with solvent until their solute content is reduced to the economical minimum,and excavated. In some cases the rate of the solution is so rapid that one passage of solvent through the material is sufficient ,but countercurrent flow of solvent through a battery of tanks is more common.In this method, fresh solvent is fed to the tank containing the solid that is most nearly extracted;it flows through the several tanks in series and is finally withdrawn from the tank that has been freshly charged.such a series of tanks is called an extraction battery. Moving-bed leaching In the machines that are used for this type of leaching, the solids are moved through the solvent with little or no agitation.The bollman extractor (figure a) contains a bucket elevator in a closed casing.There are perforations in the bottom of each bucket.At the top righthand corner of the machine ,the buckets are loaded with flaky solids such as soybeans and are sprayed with appropriate amounts of half miscella as they travel downward.Half miscella is the intermediate solvent containing some extracted oil and some small solid particles.As solids and solvent flow cocurrently down the righthand side of the machine ,the solvent extracts more oil from beans. Simultaneously the fine solids are filtered out of the solvent, so that clean full miscella can be pumped from the right hand sump at the bottom of the casing.As the partially extracted beans rise through the left side of the machine ,a stream of pure solvent percolates countercurrently through them.It collects in the lefthand sump and is pumped to the half miscella storage tank.Fully extracted beans are dumped from the buckets at the top of the elevator into a hopper from which they are removed by paddle conveyors. Bollman extractor In the Rotocel extractor,illusrated in figure b, a horizontal basket is divided into walled compartments with a floor that is permeable to the liquid.The basket rotates slowly about a vertical axis.Solids are admitted to each compartment at the feed point;the compartments then successively pass a number of solvent sprays, a drainage section, and a discharge point at which the floor of the compartment opens to discharge the extracted solids.The empty compartment moves to the feed to point to receive its next load of solids.To give countercurrent extraction, fresh solvent is fed only to the last compartment before the discharge point, and the solids in each preceeding compartment are washed with the effluent from the succeeding one. Rotocel extractor Dispersed solid leaching Solids that form impermeable beds, either before or during leaching , are treated by dispersing them in the solvent by mechanical agitation in a tank or flow mixer.The leached residue is then separate from the strong solution by settling or filtration. Small quantities can be leached batchwise in this way in an agitated vessel with a bottom drawoff for settled residue. LIQUID-LIQUID EXTRACTION The separation of the components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble is known as liquidliquid extraction. In this operation, it is essential that the liquidmixture feed and solvent are at least partially if not completely immiscible and, in essence, three stages are involved: 1Bringing the feed mixture and the solvent into intimate contact, 2Seperation of the resulting two phases, 3Removal and recovery of the solvent from each phase. It is possible to combine stages 1 and 2 into a single piece of equipment such as a column which is then operated continuously. Such an operation is known as differential contacting. Liquidliquid extraction is also carried out in stagewise equipment, the prime example being a mixersettler unit in which the main features are the mixing of the two liquid phases by agitation, following by settling in a separate vessel by gravity. Important applications of liquidliquid extraction include the separation of aromatics from kerosenebased fuel oils to improve their burning qualities and the separation of aromatics from paraffin and naphthenic compounds to improve the temperatureviscosity characteristics of lubricating oils. It may also be used to obtain, for example, relatively pure compounds such as benzene, toluene, and xylene from catalytically produced reformates in the oil industry, in the production of anhydrous acetic acid in the extraction of phenol from coal tar liquors, and in the metallurgical and biotechnology industries. EXTRACTION PROCESSES All liquidliquid extraction operations, may be carried out either as a batch or continuous process. In the singlestage batch process illustrated in the figure, the solvent and solution are mixed together and then allowed to separate into the two phases the extract E containing the required solute in the added solvent and the raffinate R, the weaker solution with some associated solvent. With this simple arrangement mixing and seperation occur in the same vessel. Fig.singlestage batch extraction A continuous twostage operation is shown in figure, where the mixers and separators are shown as separate vessels. Fig.Multiplecontact system with fresh solvent CLASSIFICATION OF EXTRACTION EQUIPMENT Essentially there are two types of design by which effective multistage operation may be obtained: 1Stagewise contactors, in which equipment includes a series of physical stages in which the phases are mixed and separated, and; 2Differential contactors, in which the phase are continuously brought into contact with complete phase separation only at the exits from the unit. STAGE-WISE EQUIPMENT FOR EXTRACTION The mixer settler In the mixersettler, the solution and solvent are mixed by some form of agitator in the mixer, and then transferred to the settler where the two phases separate to give an extract and a raffinate. In the settler the separation is often gravitycontrolled, and the liquid densities and the form of the dispersion are important parameters. Combined mixer-settler units Recent work has emphasised the need to consider the combined mixersettler operation.Thus WARWICK and SCUFFHAM give details of a design, shown in the figure in which the two operations are effected in the one combined unit. The impeller has swept back vanes with double shrouds, and the two phases meet in the draught tube. A baffle on the top of the agitator reduces air intake and a baffle on the inlet to the settler is important in controlling the flow pattern.This arrangement gives a good performance and is mechanically neat. Figure.Mixersettler The segmented mixersettler.In segmented mixersettler specially designed KnitMesh pads are used to speed up the rate of coalescence. The centrally situated mixer is designed to give the required hold up, and the mixer is pumped at the required rate to the settler which is formed in segments around the mixer, each fed by individual pipework.
Figure.Segmented mixersettler Kuhni have recently developed a mixersettler column which is a series of mixersettlers in the form of a column. The unit consists of a number of stages installed one on the top of another, each hydraulically separated, and each with a mixing and settling zone as shown in the figure. Fig.Khni mixersettles column Baffle-plate columns These are simple cylindrical columns provided with baffles to direct the flow of the dispersed phase, as shown in the figure. The efficiency of each plate is very low, though since the baffles can be positioned very close together at 75150 mm, it is possible to obtain several theoretical stages in a reasonable height. Figure.Baffleplate column The Scheibel column One of the problems with perforated plate and indeed packed columns is that redispersion of the liquids after each stage is very poor. To overcome this, SCHEIBEL and KARR introduced a unit, shown in the figure, in which a series of agitators is mounted on a central rotating shaft. Between the agitators is fitted a wire mesh section which successfully breaks up any emulsions. Figure.Scheibel column DIFFERENTIAL CONTACT EQUIPMENT FOR EXTRACTION Spray columns Two methods of operating spray columns are shown in next figure. Either the light or heavy phase may be dispersed. In the former case (a) the light phase enters from a distributor at the bottom of the column and the droplets rise through the heavier phase, finally coalescing to form a liquidliquid interface at the top of the tower. Alternatively the heavier phase may be dispersed, in which case interface is held at the bottom of the tower as shown in (b). Although spray towers are simple in construction, they are inefficient because considerable recirculation of the continuous phase takes place. As a result true countercurrent flow is not maintained and up to 6 m may be required for the height of one theoretical stage. Figure.Spray towers Packed columns The packing increasing the interfacial area, and considerably increases mass transfer rates compared with those obtained with spray columns because of the continuous coalescence and breakup of the drops. Packed columns are unsuitable for use with dirty liquids, suspensions, or high viscosity liquids. They have proved to be satisfactory in the petroleum industry. Rotary annular columns and rotary disccolumns With these columns mechanical energy is provided to form the dispersed phase. The equipment is particularly suitable for installations where a moderate number of stages is required, and where the throughput is considerable. A well dispersed system is obtained with this arrangement. The figure shows a rotary annular column. Figure.Rotary annular column Pulsed columns In order to prevent coalescence of the dispersed drops, VAN DUCK and others have devised methods of providing the whole of the continuous phase with a pulsed motion. This may be done, either by some mechanical device, or by the introduction of compressed air. The pulsation markedly improves performance of packed columns. There are advantages in using gauzetype packings since the pulsation operation often breaks ceramic rings.Pulsed packed columns have been used in the nuclear industry. Centrifugal extraction If separation is difficult in a mixersettler unit, a centrifugal extractor may be used in which the mixing and the separation stages are contained in the same unit which operates as a differential contactor. In the Podbielniak contactor,the heavy phase is driven outwards by centrifugal force and the light phase is displaced inwards. Referring to the next figure, the heavy phases enters at D, passes to J and is driven out at B. The light phase enters at A and is displaced inwards towards to shaft and leaves at C. The two liquids intermix in zone E where they are flowing countercurrently through the perforated concentric elements are separated in the spaces between. In zones F and G the perforated elements are surfaces on which the small droplets of entrained liquid can coalesce, the large drops then being driven out by centrifugal force. Figure.Podbielniak contactor The AlfaLaval contactor shown in the figure, has a vertical spindle and the rotor is fitted with concentric cylindrical inserts with helical wings forming a series of spiral passages. The two phases are fed into the bottom, the light phase being led to the periphery from which it flows inwards along the spiral, with the heavy phase flowing countercurrently. High shear forces are thus generated giving high extraction rates. Fig.Working principle of Alfa Laval centrifugal extractor EXTRACTION IN SUGAR INDUSTRY APPLICATIONS Extraction is needed for sucrose extraction from beets and cane. The processing starts by slicing the beets into thin chips. The slicing is done with sharp knives which cut a V selection slice 4 to 5 mm thickness to increase the surface area of the beet to make it easier to extract the sugar. The extraction takes place in diffusers. The two well known diffusers for sucrose extraction are The fixedbed or Robert diffusion battery and Continuous diffusion batteries or Silver continuous diffuser. Beets are washed and seperated from any remaining beet leaves before processing. THE FXED-BED OR ROBERT DFFUSON BATTERY This was developed primarily in the beetsugar industry, but is also used for the extraction of tanning extracts from tanbark, for the extraction of certain pharmaceuticals from barks and seeds, and similar processes. It consists of a row of vessels filled with the material to be extracted and through which water flows in series.The piping is so arranged that the fresh water comes in contact with the most nearly extracted material, and the strongest solution leaves from contact with the fresh material.Since each cell is filled and discharged completely , one at a time, each cell in the battery changes its position in the cycle,and therefore the piping must be so arranged that water can be fed to any cell, and the thick liquor drawn off from any cell, as circumstances may dictate.The arrangement of valves and piping became standardized in the beet industry and is generally found an all forms of diffusion battery. Figure shows that is a diagrammatic illustration of the principle of a diffusion battery.For every vessel or cell there is a heater, because the diffusion process takes place more rapidly at higher temperatures.Two main headers are necessary .One handles water and the other handles solution;and for every cell there must be three valves.In figure shows that the valves that are open are shown as circles and the valves that are closed are shown in solid black. Fig.Diagram of diffusion battery Consider figure; Cell 1 is nearly exhausted and the cell 3 has just been charged.The space between the cossettes in cell 3 is therefore filled with air.Water is introduced into cell 1 and flows down through cell 2, and up through its heater.It would not be convenient to pass the solution down through cell 3 because of the air which would be entrapped;and the charge is cold, therefore additional heating is desirable.Consequently, the liquid flows from the heater of the cell 2 through the solution line, down through the heater of cell 3,and up through cell 3. A vent at the top of this cell discharges air.When liquid appears at this vent, the valves are quickly changed to the position shown in figure.Liquid now flows down through cell 3, up through its heater, and out ti the process.The operation shown in figure continued until cell 1 is completely extracted.By this time another cell to the right of those shown has been filled, cell 1 is dumped, water is introduced to cell 2, and the process continued.In a diffusion battery for beet cossettes there may be from 10 to 15 cells. CONTNUOUS DFFUSON BATTERES The next figure shows the Silver Continuous diffuser.The figure shows only three units; but actually the battery consists of 20 to 24 units arranged in two tiers, one above the other.The battery consists essentially of a series of closed troughs A,A',A'', each provided with a helical screw B.Cossettes are intoduced into the battery through chute C and are carried together with the liquid in the direction indicated by the arrows.At the end of the first trough is a Wheel D with inclined perforated buckets on the inside.It is so arranged that the screw B discharges the cossettes into this wheel, where they are picked up by the buckets;drained free from juice lifted, and discharged through chute E which takes them into the second trough A. Here the helix carries them in the opposite direction discharges them from this to another wheel which in turn forwards them to another trough A'', and so on until they are exhausted and leave the battery. leave the battery. Silver continuous diffuser:A,A',A'', extraction trougs; B,conveyor for moving cossettes; C, feed chute; D, transfer Wheel; E, transfer chute for chips. ...
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- Spring '08