GB2039295A - Sugar production - Google Patents

Abstract

A saccharose solution is carbonated to produce sugar by continuous purification by a gas phase/liquid phase reaction, the saccharose solution being the liquid phase and CO2 being the gas phase, the CO2 gas as reaction gas being introduced into an elongate reaction vessel (1) disposed vertically, wherein in the upper third of the vessel (1) pretreated circulation liquid, pumped out of a lower part of the vessel (1) and constituting the liquid phase is introduced at a pressure of 150 to 300 kPa into a mixing device (6) contained in the upper third of the vessel (1) and reaction gas is introduced at a pressure of 30 to 50 kPa into the mixing device (6) to mix with the liquid, after which a twist is imparted to the resultant gas/liquid mixture which later is introduced into the lower third of the vessel (1) at a speed of 8 to 30 m/sec., so as to pass into a column of liquid constituting a main reaction zone (26), the direction of flow being deflected in a region below the zone (26) and large bubbles of gas being separated from the mixture, the rapid ascent of rising gas being braked by gas distribution grids (27) also serving to distribute the gas in the form of fine bubbles, a fresh solution of calcium hydroxide and saccharose and/or circulation liquid, as required, being added in the form of a wide jet, and in a tangential direction, to a rotating liquid reservoir level (16) in the upper third of the vessel (1). <IMAGE>

Description

SPECIFICATION Process and apparatus for the carbonation of saccharose solutions The invention relates to a process and apparatus for the carbonation of saccharose solutions for the production of sugar by continuous purification, preferably from sugar beet juice.

Numerous processes and apparatus are known for carbonation, also termed saturation, which cause precipitation of the colloidal impurities in the beet extract, these being bound by the calcium carbonate occurring in the neutralization reaction between calcium hydroxide and carbon dioxide. In Swiss Patent Specification 415301 it has been suggested that the carbon dioxide should be pressed into the lower part of the reaction vessel at a relatively high pressure via inlet apertures. Owing to the density difference between the column of liquid and the carbon dioxide the gas bubbles ascend in the reaction vessel and cause a carbonation reaction with the saccharose solution.In this known method the energy required for the dispersal of the gas bubbles, in order to ensure intensive exchange of material, is provided by imparting potential energy to the gaseous phase only, in order to increase the pressure. A drawback of the known technical methods is due to the limited dispersal undergone by the gas in the liquid and to the consequently inadequate degree to which the gas is utilized. This comparatively slight utilization of the gas results from the limits set to the possibility of obtaining in the gas phase a sufficient increase of pressure which can be converted at small gas outlet apertures into high kinetic inlet energy serving to produce small gas bubbles. A further drawback of the known technical methods resides in the risk of incrustation.

The formation of serious incrustations in the small gas outlet apertures necessitates expensive cleaning of the units at relatively frequent intervals and thus impedes continuous production cycles.

The purpose of the invention is to provide a process and an apparatus for the carbonation of saccharose solutions which will make it possible to complete a cycle of operations uninterruptedly, affording a high degree of protection and involving only moderate investment and operating costs.

The object of the invention is to introduce carbon dioxide in the form of fine bubbles into aqueous solutions of calcium hydroxide and saccharose, with adequate utilization of the gas, and also to ensure reliable operating stability by reducing the incrustations at the gas outlet apertures.

According to one aspect of the invention there is provided a process for the carbonation of a saccharose solution for the production of sugar by continuous purification by a gas phase/liquid phase reaction, the saccharose solution constituting the liquid phase and carbon dioxide gas constituting the gas phase, the carbon dioxide gas being introduced as the reaction gas into an elongate reaction vessel with its longitudinal axis disposed vertically, wherein in the upper third of the reaction vessel pretreated circulation liquid, pumped out of a lower part of the reaction vessel and constituting the liquid phase is introduced at a pressure of 150 to 300 kPa into a mixing device contained in the upper third of the reaction vessel and reaction gas is introduced at a pressure of 30 to 50 kPa into the said mixing device to mix with the pretreated circulation liquid, after which a twist is imparted to the resultant gas/liquid mixture, the said resultant gas/liquid mixture being introduced into the lower third of the reaction vessel at a speed of 8 to 30 m/sec., so as to pass into a column of liquid constituting a main reaction zone, the direction of flow being deflected in a region below the said main reaction zone and large bubbles of gas being separated from the said mixture, the rapid ascent of rising gas being braked by means of gas distribution grids which also serve to distribute the gas in the form of fine bubbles, a fresh solution of calcium hydroxide and saccharose and/or circulation liquid, as required, being added in the form of a wide jet, and in a tangential direction, to a rotating liquid reservoir level in the upper third of the reaction vessel. Preferably the saccharose solution comprises sugar beet juice.Generally the reaction vessel is cylindrical.

According to another aspect of the invention there is provided an apparatus for carrying out the process of the invention, comprising an elongate reaction vessel adapted when in use to be disposed so that its longitudinal axis is vertical in which disposition the upper third of the reaction vessel contains a distribution device of trapezoidal configuration, a mixing device for mixing liquid and gas phases, a circulation pressure pipe leading into the said mixing device vertically and a downwardly inclined gas supply pipe leading into the said mixing device, the exit end of the said circulation pressure pipe leading into a mixing chamber constituting a lower part of the said mixing device, the said mixing chamber tapering conically downwards and containing a downwardly extending pipe centrally disposed therein with a tapering end, swirl vanes or the like being fixed to the bottom end of the said centrally disposed pipe at an angle of 10 to 150 in relation to the main direction of flow therethrough, there being provided in the said mixing chamber a progressively narrowing annular gap for passage of gas from the exit of the gas supply pipe, the said annular gap leading downwardly to a centrally disposed mixing pipe extending into a lower part of the reaction vessel constituting the main reaction zone and being provided at the exit end of the said mixing pipe with a gas absorber, gas distribution grids above the said lower part of the reaction vessel being provided and in a region underneath the said lower part of the reaction vessel a gas separator is provided. Preferably the reaction vessel is cylindrical.

Advantageously the downwardly inclined gas supply pipe is disposed at an angle of 35 to 500 to the horizontal. Preferably the said gas absorber comprises U-sections offset at right angles relative to one another; preferably the said gas distribution grids comprise angular sections offset horizontally in respect of one another but at an angle of 90 , and preferably the gas separator comprises a conical baffle plate with a frustum-shaped casing attached to the bottom thereof, in an offset position.

Preferred forms of the apparatus will be described in detail as follows: In a widened zone situated in the upper third of a vertical cylindrical reaction vessel a circulation pressure pipe is situated on one side, together with a branch piece and underneath it a trapezoidally widened distribution device, which is connected to an inlet pipe for a fresh solution of calcium hydroxide and saccharose. A gas supply pipe, which is connected to a dosaging valve with a regulator and a pH-value regulator, terminates on the opposite side of the inlet. The circulation pressure pipe and the gas supply pipe combine to form, in the upper third of the reaction vessel, a central mixing device widening out in the form of a truncated cone.The mixing device is provided at its tapering lower end with a mixing pipe which passes centrally through two or more gas distribution grids, offset in respect of one another by 90 in the horizontal direction and consisting of parallel and preferably isosceles angular profiles and a gas absorber, of a number of sections, particularly U-sections, situated at right angles to one another, and which terminates in the main reaction zone of the reaction vessel, in the zone of the gas absorber. In the lower part of the reaction vessel the main reaction zone is delimited by a gas separator, constructed as a system consisting of a flat cone as a baffle plate and of a frustum-shaped casting positioned underneath it and offset in relation thereto in the vertical plane.Underneath the gas separator a circulation suction pipe, followed by a circulation pump, as well as an extraction pipe with filling-level regulators, is connected to the reaction vessel via pipe sockets. The gas supply pipe leads into the mixing apparatus at an angle of 35-50", terminating in a mixing chamber which tapers conically downwards and contains a central pipe tapering at the end. The mouth of the central pipe is provided with swirl vanes affixed to an angle of 10-15 in relation to the main direction of flow. By switching on a circulation pump the liquid solution of calcium hydroxide and saccharose is caused to flow through the circulation pressure pipe and also the reaction gas into the mixing chamber.The liquid phase and the flow of reaction gas which enter through an annular gap, provided concentrically in the mixing chamber and which continuously narrows down in the zone of the transition to the liquid phase, are caused to perform a twisting motion by means of swirl vanes, and this motion causes the mixture to be uniformly distributed throughout the whole of the space in the mixing pipe. After leaving the mixing chamber and the mixing pipe the mixed flow, at a high speed, enters the column of liquid in the main reaction zone, which is penetrated by the current of mixture, owing to the impulse imparted to the latter. The solid reaction products therefore remain in the liquid as particles and form a suspension with this latter.The flat cone of the baffle plate and the frustum-shaped casing of the gas separator prevent gas bubbles from entering the circulation suction pipe and thus being circulated. The degree of utilization of the reaction gas is improved by the gas separator, which is situated at the actual mouth of the mixing pipe, and also by the gas distribution grids. The addition of a fresh solution of calcium hydroxide and saccharose is effected through the inlet pipe and through the distribution device, widening in the form of a vane and positioned tangentially, and impinges, as a wide jet fanning out, on the rotating level of liquid in the upper chamber.The pH value in the main reaction zone is kept constant by ascertaining the said pH-value in the extraction pipe by means of a pH-value measuring device and by making the appropriate correction, by means of a regulator and a dosaging valve, in the addition of reaction gas. In order to reduce foaming still further and also to introduce crystal nuclei into the uppermost reaction zones, a small individual current of the circulation liquid may be applied, according to need and via the branch piece, to the level of the liquid.

By way of example a preferred form of apparatus embodying the invention and its use for carrying out the process of the invention will be described with reference to the accompanying diagrammatic drawings, wherein: Figure lisa schematic elevation of one form of apparatus embodying the invention, Figure2 is a longitudinal section to an enlarged scale through a mixing device as shown in Figure 1, Figure 3 is a lateral view of a distribution device as shown in Figure 1, Figure 4 is a plan view corresponding to Figure 3, Figure 5 is a plan view of a gas distribution grid as shown in Figure 1, and Figure 6 is a plan view of a gas absorber as shown in Figure 1.

In carrying out a process in accordance with the invention liquid phase is mixed, in the upper third of a vertically disposed reaction vessel 1, above liquid level 16, at a pressure of 150 to 300 kPa, with reaction gas, at a relatively low pressure of 30 to 50 kPa, and introduced through a central mixing pipe 9 of sufficient volume into the lower third of the reaction vessel 1 at a speed of 8 to 30 metres per second. A twist is imparted to the mixture of the liquid phase and the reaction gas while it is still in the mixing device 6. It has been found that when the reaction gas is introduced at a relatively slight excess pressure the mixture, owing to the impulse imparted to it, penetrates the column of liquid of main reaction zone 26, via the liquid phase and with considerable kinetic energy, as a result of which the reaction gas is utilized to a considerable degree. Underneath the main reaction zone 26 the direction of flow is deflected and gas bubbles are separated from the mixture. The degree of ulitization of the reaction gas is further improved by the provision of a gas absorber 18 at the mouth of the mixing pipe 9 and also by means of two or more gas distribution grids 27. A fresh solution of calcium hydroxide and saccharose is added, in a wide jet and in a tangential direction, to the rotating level 16 of the liquid. When the mixture is introduced with observing the technical parameters required by the invention incrustation by solid reaction products is largely prevented. The invention enables a rational carbonation process to be effected. The carbonation process takes place uniformly throughout the whole of the reaction vessel 1.The transfer of material is favourably affected by the high mixing intensity, which ensures the convective transport of fresh calcium ions to the boundary surface of the phase whenever required.

In the apparatus shown a circulation pressure pipe 5, a branch piece 28 and underneath a trapezoidally widened distribution device 14 are provided in the upper third of the vertically disposed reaction vessel 1, on one side, the said distribution device 14 being connected with an inlet pipe 2 for a fresh solution of calcium hydroxide and saccharose. A gas supply pipe 10, which is connected to a dosaging valve 22 with a regulator 23 and a pH-value regulator 19, terminates on the opposite side of the inlet.The circulation pressure pipe 5 and the gas supply pipe 10 combine to form, in the upper third of the reaction vessel 1, a central mixing device 6 widening out upwardly in the form of a truncated cone.The mixing device 6 is provided at its tapering lower end with the mixing pipe 9, which passes centrally through two or more gas distribution grids 27, offset in respect of one another by 90 in a horizontal direction and consisting of parallel and preferably isosceles angular profiles and gas absorber 18, of a number of sections, particularly U sections, situated at right angles to one another, and which terminates in the main reaction zone 26 of the reaction vessel 1, in the zone of the gas absorber 18. In the lower part of the reaction vessel 1 the main reaction zone 26 is delimited by a gas separator 17, constructed as a system consisting of a flat cone as a baffle plate 12 and of a frustum-shaped casing 15 positioned underneath and offset in relation thereto in a vertical plane.Underneath the gas separator 17 a circulation suction pipe 3, followed by a circulation pump 4, as well as an extraction pipe 25 with filling level regulators 24, and overflow pipe 13 is connected to the reaction vessel 1 via pipe sockets. The gas supply pipe 10 leads into the mixing device 6 at an angle of 35-50-, terminating in a mixing chamber 11 which tapers conically downwards and contains a central pipe 7 tapering at the end. The mouth of the central pipe 7 is provided with swirl vanes 8 affixed at an angle of 10 to 15 in relation to the main direction of flow. The mixing chamber 11 contains, in the zone of the transition from the gas supply pipe 10 to the liquid phase, an annular gap 29 which is positioned concentrically and which continuously narrows in the direction of the central pipe 7.By switching on the circulation pump 4 the liquid solution of calcium hydroxide and saccharose and also the reaction gas are caused to enter the mixing chamber 11. The liquid phase and thus the flow of reaction gas which enters through annular gap 29 provided concentrically in the mixing chamber 11 are caused to perform a twisting motion by means of swirl vanes 8, and this motion causes the mixture to be uniformly distributed throughout the whole of the space in the mixing pipe 9. After leaving the mixing chamber 11 and the mixing pipe 9 the mixed flow, at a high speed, enters the column of liquid in the main reaction zone 26, which is penetrated by the current of mixture, owing to the impulse imparted to the latter.The flat cone of baffle plate 12 and the frustum-shaped casing 15 of the gas separator 17 prevent gas bubbles from entering the circulation suction pipe 3 and thus circulation of the gas bubbles is prevented. The degree of utilization of the reaction gas is further improved by the gas absorber 18, which is situated at the mouth of the mixing pipe 9, and also by gas distribution grids 27. The addition of a fresh solution of calcium hydroxide and saccharose is effected through the inlet pipe 2 and through the tangentially positioned distribution device 14, widening in the form of a vane, and impinges, as a wide jet fanning out on the rotating level of liquid 16 in upper chamber 20 having an outlet pipe 21 for residual gas.The pH value of the main reaction zone 26 is kept constant by ascertaining the said pH value in the extraction pipe 25 by means of the pH value measuring apparatus 19 and by making the appropriate correction, by means of the regulator 23 and the dosaging valve 22, during the addition of reaction gas. In order to reduce foaming still further and also to introduce crystal nuclei into the uppermost reaction zones, a small individual current of the circulation liquid is applied, according to need and via the branch piece 28, to the level of the liquid.In addition to the reduction of the formation of foam in the upper chamber 20 the addition of the circulation liquid and of a fresh solution of calcium hydroxide and saccharose provide a further advantage, inasmuch as this inoculation process, with the distribution device 14, results in the introduction of solid particles around which further solid reaction products form, which can then be separated from the turbid liquid, in a subsequent separation step, with relative ease. The process provided by the invention makes it possible to dispense with costly apparatus for the introduction of a reaction gas in the form of fine bubbles for the purpose of intensive carbonation and to avoid or minimise the production losses otherwise caused by the usual removal of incrustation and by nozzles and perforated plates no longer capable of functioning. The illustrated apparatus embodying the invention, owing to its completely reliable and self-contained units, requiring no moving components or those which rotate or perform a reciprocal motion other than the means to circulate the reactants and to rotate the liquid level, ensures a very high degree of protection.

Claims (12)

1. A process for the carbonation of a saccharose solution for the production of sugar by continuous purification buy a gas phase,liquid phase reaction, the saccharose solution constituting the phase and carbon dioxide gas constituting the gas phase, the carbon dioxide gas being introduced as the reaction gas into an elongate reaction vessel with its longitudinal axis disposed vertically, wherein in the upper third of the reaction vessel pretreated circulation liquid, pumped out of a lower part of the reaction vessel and constituting the liquid phase is introduced at a pressure of 150 to 300 kPa into a mixing device contained in the upper third of the reaction vessel and reaction gas is introduced at a pressure of 30 to 50 kPa into the said mixing device to mix with the pretreated circulation liquid, after which a twist is imparted to the resultant gasiliquid mixture, the said resultant gasiliquid mixture being introduced into the lower third of the reaction vessel at a speed of 8 to 30 misec., so as to pass into a column of liquid constituting a main reaction zone, the direction of flow being deflected in a region below the said main reaction zone and large bubbles of gas being separated from the said mixture, the rapid ascent of rising gas being braked by means of gas distribution grids which also serve to distribute the gas in the form of fine bubbles, afresh solution of calcium hydroxide and saccharose andior circulation liquid, as required, being added in the form of a wide jet, and in a tangential direction, to a rotating liquid reservoir level in the upper third of the reaction vessel.

2. A process according to Claim 1, wherein the saccharose solution comprises sugar beet juice.

3. A process according to Claim 1 or Claim 2, wherein the reaction vessel is cylindrical.

4. A process according to Claim 1, substantially as herein described and with reference to the accompanying drawings.

5. Sugarwhich has been produced by the process claimed in any preceding claim.

6. An apparatus for carrying out the process claimed in Claim 1, comprising an elongate reaction vessel adapted when in use to be disposed so that its longitudinal axis is vertical in which disposition the upper third of the reaction vessel contains a distribution device of trapezoidal configuration, a mixing device for mixing liquid and gas phases, a circulation pressure pipe leading into the said mixing device vertically and a downwardly inclined gas supply pipe leading into the said mixing device, the exit end of the said circulation pressure pipe leading into a mixing chamber constituting a lower part of the said mixing device, the said mixing chamber tapering conically downwards and containing a downwardly extending pipe centrally disposed therein with a tapering end, swirl vanes or the like being fixed to the bottom end of the said centrally disposed pipe at an angle of 10 to 15- in relation to the main direction of flow therethrough, there being provided in the said mixing chamber a progressively narrowing annular gap for passage of gas from the exit of the gas supply pipe, the said annular gap leading downwardly to a centrally disposed mixing pipe extending into a lower part of the reaction vessel constituting the main reaction zone and being provided at the exit end of the said mixing pipe with a gas absorber, gas distribution grids above the said lower part of the reaction vessel being provided and in a region underneath the said lower part of the reaction vessel a gas separator is provided.

7. An apparatus according to Claim 6, wherein the reaction vessel is cylindrical.

8. An apparatus according to Claim 6 or Claim 7, wherein the downwardly inclined gas supply pipe is disposed at an angle of 35 to 50 to the horizontal.

9. An apparatus according to any one of Claims 6 to 8, wherein the said gas absorber comprises U-sections offset at right angles relative to one another.

10. An apparatus according to any one of Claims 7 to 9, wherein the said gas distribution grids comprise angular sections offset horizontally in respect of one another but an angle of 90".

11. An apparatus according to any one of Claims 7 to 10, wherein the said gas separator comprises a conical baffle plate with a frustum-shaped casing attached to the bottom thereof, in an offset position.

12. An apparatus for the production of sugar substantially as herein described and illustrated by the accompanying drawings.