Abstract:
A method of continuous precipitation of insoluble solids formed by the reaction of a liquid solution and a reactant by hydrodynamic agitation of the solution and the reactant in a continuous hydrodynamic precipitation apparatus. The apparatus comprises vertically standing elongated tank means; liquid level control means for maintaining tank liquid at a predetermined level in the tank means; hydrodynamic agitator means in the tank means for receiving and agitating the solution, the reactant and recirculated tank liquid; conduit means for supplying predetermined quantities of solution and reactant to the agitator means; pressure pump means for recirculating at least a portion of the liquid under pressure from the bottom portion of the tank means to the hydrodynamic agitator means; whereby the solution, the reactant and the recirculated tank liquid are thoroughly mixed by the hydrodynamic agitator means; and means for removing at least a portion of the precipitated solids from the bottom portion of the tank means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process and apparatus for precipitating insoluble solids from a liquid solution and a reactant which react to form the solids and, more particularly to a process and apparatus for precipitating insoluble saccharate from an aqueous sucrose solution. 
     2. Prior Art 
     Precipitation of insoluble solids from a liquid solution by the addition of a reactant is commonly facilitated by means of mechanical agitation of the solution and the reactant. For example, in the sugar industry, the formation of insoluble saccharates is commonly accomplished by such a process. 
     In the common commercial processes of recovering sugar from sugar beets or the like, the beets are cut into thin slices (&#34;cossettes&#34;), the cossettes are extracted with hot water to produce a sucrose-containing diffusion juice, and then the diffusion juice is processed to produce crystalline sugar and a molasses solution. Additional crystalline sugar may be recovered from the molasses solution by the &#34;Steffen Process&#34; which comprises the steps of: (1) diluting the molasses solution with water to produce a solution containing about 6% sucrose, (2) adding finely powdered quicklime (CaO) to the solution with violent agitation to precipitate insoluble saccharate, (3) filtering the solution (about 90% of the sugar is recovered in the precipitate with about 10% portion remaining in the filtrate), (4) heating the filtrate to about 90° C to form additional precipitate (contains about 6.5% of the sugar originally present in the molasses solution, and (5) recovering the additional precipitate by settling and filtration. The precipitated saccharate may then be slurried in water and reprocessed for recovery of additional crystalline sugar. 
     In the prior art, various methods and apparatus have been used to facilitate the addition of quicklime to the molasses solution to ensure a uniform reaction by thorough mixing of the quicklime and the molasses solution by mechanical agitation. The prior art processes have also utilized mechanical cooling in the reaction chamber to dissipate the heat of dissolution of the quicklime in the solution and the heat of reaction of the CaO with the sucrose, to obtain precipitation of the saccharate. The prior art processes have further required a very dilute molasses solution (e.g., a maximum sucrose concentration of about 6%) for efficient processing. 
     SUMMARY OF THE INVENTION 
     A method and apparatus are provided for the continuous precipitation of insoluble solids from a liquid solution and a reactant by hydrodynamically agitating the solution and the reactant. Hydrodynamic agitation results in quick, complete and intimate contact of the solution and the reactant, thereby facilitating the precipitation process. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
    
    
     Referring to the accompanying drawing of a presently preferred and illustrative embodiment of the inventive concepts, 
     FIG. 1 is a schematic side elevational view, partly in section, of a vertically standing precipitator tank and associated apparatus; and 
     FIG. 2 is an enlarged side elevational view, partly in section, of a hydrodynamic agitator portion of the precipitator tank of FIG. 1. 
    
    
     Referring now to FIG. 1, a vertically standing precipitation tank 10 providing a process chamber 11 is shown to comprise an elongated cylindrical wall portion 12 having a cover plate 14 on the upper end and a downwardly inwardly converging conical wall portion 16 at the bottom end terminating in a reduced diameter cylindrical outlet portion 18 having a connecting flange 20. Various access openings and cover plates 22, 24, 26, 28 may be provided. 
     A hydrodynamic agitator assembly 30 is centrally coaxially mounted in tank 10 by suitable support means (not shown) with an upwardly opening upper end portion 32 located in an intermediate top portion of chamber 11 and a downwardly opening lower end portion 34 located in an intermediate bottom portion of chamber 11. 
     Conduit means 36 for supplying a reactant to the tank are located at the top portion of the tank and comprise a hopper 38, a supply conduit portion 40 extending through wall portion 12, and a discharge conduit portion 42 having a downwardly facing discharge opening 44 generally coaxial with agitator assembly 30 and located in upwardly spaced relationship to the upper end portion 32 thereof. Conduit means 36 may also comprise regulating means 37 for regulating the rate at which the reactant is supplied to the tank and may be an auger driven by a variable speed motor in the case where the reactant is in the form of a powdered solid. 
     Conduit means for supplying a liquid solution to the tank are located at the top of the tank and comprise a supply source 52 for supplying a regulated amount of the solution to the supply conduit 54, a supply conduit portion 54 extending through wall portion 12, and a discharge conduit portion 56 having a downwardly facing discharge opening 58 located in upwardly spaced relationship above the upper end portion 32 of the agitator assembly. 
     Conventional liquid level regulator means 60 are provided to maintain the level of liquid in process chamber 11 at 62 at the upper portion 32 of the agitator assembly and to maintain an atmospheric chamber in the process chamber above the liquid level which comprise a vertical conduit 64 connected to the bottom portion of the tank 10 through wall portion 12 at 66, a reverse bend conduit 67, a stand-pipe conduit 68, and a discharge conduit 69. 
     Pressurized circulation means for recirculation of at least a portion of the liquid in process chamber 11 comprise an inlet conduit means 70 extending through wall portion 12 for connection to an intermediate portion of the agitator assembly, a conventional recirculation pump means 72 suitably connected to the outlet portion 18 at the bottom of the tank, and conventional conduit means 74 connecting the pump means 72 to the inlet conduit means 70. Cooling means 140 are also provided for cooling the pressurized recirculation liquid prior to passage of the liquid into the agitator assembly. 
     Liquid deflector assembly means 120 may be provided in a downwardly spaced relationship beneath the agitator tank assembly to create a desired liquid flow pattern in process chamber 11. The deflector assembly means comprises a first upwardly facing conical wall portion 122 of minimum included angle, a second upwardly facing conical wall portion 124 of maximum included angle, and a lower radially extending flange portion 126 coaxially mounted relative to the agitator assembly. 
     Referring now to FIG. 2, the agitator assembly comprises an upper inlet tank means 80 having an upper cylindrical wall portion 82 of relatively large diameter connected to a lower cylindrical wall portion 84 of relatively small diameter by an intermediate downwardly inwardly converging conical wall portion 86. The relatively small diameter cylindrical wall portion 84 provides a relatively narrow first venturi-type liquid passage 130 between upper inlet tank means 80 and lower outlet tank means 92. An inlet opening screen may be provided by a ring member 88 suitably mounted on the top of tank means 80 with a plurality of circumferentially spaced vertically extending inlet slots 90 enabling flow of tank liquid from process chamber 11 into the upper portion of inlet tank means 80. The inlet slots are preferably provided with deflection means (not shown) to provide for tangential flow of the tank liquid thereby creating a vortex as the liquid flows into and through the inlet tank means. 
     The agitator assembly further comprises a lower outlet tank means 92 having an upper relatively small diameter cylindrical wall portion 94, a lower relatively large diameter cylindrical wall portion 96, a first intermediate downwardly outwardly diverging conical wall portion 98 connecting wall portions 94 and 96, and a second lower downwardly outwardly diverging conical wall portion 100 terminating in a radially extending flange portion 102. The inside diameter of wall portion 94 is larger than the outside diameter of wall portion 84 so as to provide an annular relatively narrow width second venturi-type liquid passage 104 therebetween with the bottom surface 106 of wall portion 84 terminating within the wall portion 94 somewhat more than one-half the distance from the lower end to the upper end of wall portion 94. The inside diameter of wall portion 96 is approximately the same as the inside diameter of wall portion 82. 
     Although the method and apparatus of the invention is deemed to have general applicability, it has been found to be particularly advantageous in the precipitation of saccharate from an aqueous sucrose-containing molasses solution by reacting the solution with quicklime (finely powdered CaO). It is in this context that the method of operation of the apparatus previously discussed is described. 
     In normal operation, the chamber 11 of tank 10 contains a mixture of an aqueous molasses solution, quicklime and precipitated solids (collectively termed &#34;tank liquid&#34;) with a liquid level maintained at 62 by liquid level regulating means 60 so that the top portion of the tank liquid is constantly flowing into inlet tank 80 through inlet slots 90. A uniform flow of tank liquid forming a vortex in the inlet tank, having an upper surface configuration generally illustrated at 85 in FIG. 2, is thereby obtained with the flow being directed generally radially inwardly into the central portion of chamber 130 provided by wall portion 84 to provide a central area of high activity of tank liquid interaction. In addition, regulated amounts of aqueous molasses solution and quicklime are continuously added to tank 80 through inlet conduits 42, 56 and discharge openings 44, 58 and are mixed with the tank liquid flowing through tank 80. Since the diameter of wall portion 84 is substantially smaller than the diameters of wall portions 82, 94 and 98, a pressure differential is created between inlet tank means 80 and outlet tank means 92 whereby a first venturi-type effect is obtained. 
     At the same time that the molasses solution, the quicklime and tank liquid flow through tank 80 as previously described, tank liquid is continuously added through manifold 110 under pressure of recirculation pump means 72. The relatively high pressure liquid in chamber 114 rapidly flows through the second venturi throat area provided by passage 104 into the area provided within wall portion 94 below wall portion 84 and then downwardly into the expansion area provided by conical wall portion 98 providing an area of relatively low hydrostatic pressure below passage 104. The mixture of tank liquid, incoming molasses solution, and additional quicklime are thereby drawn through the first venturi throat at relatively high velocity and immediately enter a zone of extremely high turbulence below the second venturi throat area and are very quickly and uniformly mixed and intimately contacted with the recirculated tank liquid entering the second venturi throat area from passage 104. A conical reaction chamber is provided by the conical wall portion 98 wherein the quicklime substantially completely reacts with the aqueous molasses solution, the tank liquid and the pressurized recirculation liquid to form insoluble solids comprising saccharate as the liquid flow expands downwardly through chambers of increasing area provided by wall portions 96, 98, 100. In the presently preferred embodiment, the ratio of volume of pressurized recirculation liquid from pump 72 to the volume of molasses solution, added at the top of tank 80 is between 5:1 and 10:1. The highly efficient interaction of the quicklime, the molasses solution and the tank liquid obtained by the invention permits the processing of more highly concentrated sucrose solutions than is possible with prior art systems. For example, the incoming molasses solution may contain up to about 10% by weight sucrose. 
     In the continuous precipitation process, a first portion of tank liquid flowing from lower outlet tank 92 internally recirulates upwardly as indicated by arrows 121, 131 to re-enter the upper inlet tank 80 while a second portion of the tank liquid flows downwardly to the recirculation pump 72 for pressurized re-entry into manifold chamber 114. During the process, a portion of the processed tank liquid, including the insoluble solids comprising saccharate, is drawn off through a discharge opening 140 for further processing in a conventional manner so as to maintain a constant liquid level in process chamber 11 as additional molasses solution and quicklime are added. Precipitated particles of insoluble saccharate settling toward the bottom of the tank are drawn off through the discharge opening along with the processed tank liquid or may additionally be drawn off from time to time through another suitable discharge opening (not shown) located toward the bottom of wall portion 12 or conical wall portion 16. 
     While inventive concepts have been disclosed hereinbefore in relating to a presently preferred and illustrative embodiment of the invention, it is contemplated that the inventive concepts may be variously otherwise employed and embodied in alternative structure. For example, although the above description contemplates continuous hydrodynamic precipitation, such precipitation may be carried out on a batchwise basis. In addition, although the inventive concepts provide particular advantageous results in the processing of sugar beet molasses, the inventive concepts may be applicable to other types of processes. Thus, it is intended that the appended claims be construed to cover alternative embodiments of the inventive concepts except insofar as excluded by the prior art.