IL35793A - Continuous process for crystallization - Google Patents

Description

Continuous process for crystallization AMICARBON N.V.

The present invention relates to an improved process for continuously crystallizing a solute or a mixture of solutes from a solvent or crystallizing a solvent from a solution, wherein the solution is brought to crystallization by cooling and/or evaporation. More particularly, the invention relates to an improved, continuous ^crystallization process in which the solution is brought to crystallization by cooling and/or evaporation in a first zone where crystal nuclei form and then, in a second zone, crystals grow to the desired dimensions.

A two stage crystallization has been previously described (see British Patent specification no. 977,686) in which partial crystallization of the solution is accomplished by cooling the solution in a scraped heat exchanger and the partially crystallized suspension is then introduced and cqn-tinuously flows through a separate flow-through device where the crystals are allowed to grow to the desired size. The suspension flow in the flowthrough device is plug flow with residence time being from 1 - 10 hours or longer. In such a process mixing of the entering suspension of small crystals and the suspension of larger crystals leaving the device is at most only negligible. The size and amount of crystals obtained in the plug flow through the crystallizer flow-through device is comparable to the crystals obtained' with a batch-wise storage of the solution in a ripening vessel for the same period of time as the residence time in the flowthrough device . ' An increase in residence time in the flowthrough device produces a corresponding increase in the average size of the crystals produced. At a constant residence time, the size of crystals produced depend upon the size of the crystal i.e., the larger the crystal nuclei produced in the scraped heat-exchanger, the larger the crystals will be which are produced in the flowthrough device. The average size of the crystals produced by the scraped heat-exchanger increases as the , formation of actual nuclei in the heat-exchanger decreases . The formation of nuclei can be limited by keeping the heat flux, i.e., the removal of heat per unit time per unit surface of the heat-exchanger, at a small magnitude, as the small heat flux minimises the undercooling of the suspension at the heat-extracting surface, thereby limiting the formation, of . crystal nuclei. The reduction in heat flux requires a large increase in cooling surface area and thus the equipment becomes"large, bulky and expensive.

In general, in all prior art crystallization process- es, the formation of crystal nuclei is controlled at a limited value so that large crystals are obtained rather than an increased number of small crystals. For example, in a crystal- lizer in which the crystallization is effected from a supersaturated solution, the formation of nuclei increases as the degree of supersaturation increases. As the number of crystal nuclei forming increases, the number of crystals produced increases, however, the average size of the crystals produced decreases. To limit the number of crystal nuclei being produced the heat flux is maintained at a low value which in turn requires a large amount of heat-exchanger surface. The equipment becomes bulky , and expensive.

The present invention provides a crystallization process in which, contrary to the prior art experience, formation of increased numbers of crystal nuclei results in an increase of the average crystal size. an average particle size which is larger than the crystals produced by prior art processes in similar sized equipment. Thus equipment which is far less bulky and less expensive than the equipment required in the prior art processes can be utilized which will produce the same amount of crystals of an average size larger than produced in more bulky equipment using the prior art processes.

The process of the present invention is accomplished by forming an increased number of crystal nuclei of which a large percentage are redissolved. in the crystallization zone and recrystallized on larger crystals already present in the crystallization zone. It has been unexpectedly found that formation of a large number of the crystal nuclei in crystal-lization processes is advantageous for the final production of large crystals provided that the crystal nuclei are dissolved in the crystallization zone and recrystallized on the larger crystals present by rapidly mixing the crystal nuclei in a suspension of the larger crystal particles.

The actual phenomena of nuclei dissolution is well known in the field of physical chemistry. Very small crystals of the order of magnitude of 0.1 to 10 microns for the smallest dimension of the crystal possess an appreciably larger solubility than crystals whose smallest crystal-dimension amounts for instance to 100 microns and larger, in a suspension of large and small crystals, the concentration, or temperature of the liquid phase adjusts itself to a level between the equilibrium concentration or equilibrium temperature of a solution in contact with the small crystals and. the equilibrium concentration or equilibrium temperature of a solution in contact with the larger crystals. Thus, the small crystals tend to. dissolve and the large crystals tend. laboratory phenomena in physical chemistry. However, there has never been suggested a process which was capable of taking full advantage of this laboratory phenomena on an industrial scale, According to the present invention, a process has been found wherein the solution to be crystallized is first treated in a nuclei-formation zone (also referred to hereinafter as a precrystallization zone) where many small crystal nuclei are formed. These nuclei are subsequently introduced into a well-stirred crystallization zone and rapidly mixed with the contents thereof under substantially adiabatic conditions. The contents of the crystallization zone contain crystals of an average size at least 10 times larger than the crystal nuclei being mixed therewith. The nuclei tend to dissolve in the solution contained in the crystallization zone and recrystallize upon the larger particles. A portion of the crystal/solution suspension in the crystallization zone is continuously withdrawn and the crystals separated therefrom. The motherliquor may partly recycle, the remain-der is discharged as a product stream or as waste.

An important advantage of the present process is that the small nuclei desired can be formed in a simple manner by effecting a strong formation of nuclei in the nuclei formation zone within a short residence time.

This strong formation of nuclei may be realized in a simple way by, for instance, applying strong cooling of the solution and/or strong evaporation of the solvent in the nuclei formation zone. This is in distinct contrast to the prior art processes wherein the heat- or vapour flux is limited to a small value. The present process is far more effective and less costly in being able to utilize a high - - cover ice from a water solution according to the present inven-tion at a rate of 100-400 kg/m of cooling surface per hour in comparison to 10-50 kg/m 2 of cooling surface using the prior art process. The ice crystals obtained by the present process having the same average size as the crystals obtained by the prior art processes.

In practising the present invention, it is preferable to keep the residence time in the precrystallization zone and in the conduit connecting the precrystallization zone with the crystallization zone as short as possible. Preferably, the nuclei which are formed in the precrystallization zone should not be given the opportunity of growing to a dimension of more than 10 microns. This can be accomplished by a short residence time of the. nuclei in the precrystallization zone and in the conduit to the crystallization zone. : The residence time in the crystallization zone is ; considerable longer than that in the precrystallization zone which allows those nuclei which survive the dissolution phe-nomena to grow into sufficiently large product crystals.

The average residence time for the nuclei in the precrystallization zone and the time necessary for the nuclei to pass from the precrystallization zone to the crystalliz- . ation zone should not be^more than one minute and, preferably, only a few seconds. The ratio between this time and the average residence time in the crystallization zone should be at · most 1:100 and, preferably, 1:1000 to 1:5000. The average residence time in the crystallization zone is preferably between 30 minutes and 1 to 5 hours, however, this time can be shorter or longer as according to the ratio of residence time in the precrystallization zone and the crystallization zone In the process according to the present invention, as contrary to the prior art processes, an increase in the number of nuclei formed with small particle size results in an in-crease in the average size of the product crystals in the crystallization zone. This is demonstrated by comparing the results of crystallizing ice from a 10 % by weight saccharose solution according to this invention wherein the average size of the ice nuclei formed in the precrystallization zone is 8 microns and an experiment wherein the average size of the nuclei is 5 microns. In both experiments the nuclei were held for 3 hours in the crystallizing zone containing a 30 % by weight suspension of ice crystals in a saccharose solution of 30 % by weight. The 8 micron nuclei produced ice crystals having an average dimension of approximately 300 microns. The 5 micron nuclei produced ice crystals having an average dimension of approximately 500 microns.

The process of the present invention can be commenced by forming a limited number of nuclei in the precrystalliz-ation zone which after being transferred to the crystallization zone grow to larger crystals. When sufficiently large crystals have been formed in the crystallization zone, the solution feed to the precrystallization zone is increased to form the large number of nuclei which are transferred to the crystallization zone where they dissolve and recrystallize on the larger crystals. The process can be commenced also by adding crystals directly to the crystallization zone and start the precrystallization in its normal operation.

The crystallization zone is operated under substan-tially adiabatic conditions and thus must be separated from the precrystallization zone where heat and/or solvent is withdrawn by indirect cooling and/or evaporation. The solu- zone. The nuclei formation may occur spontaneously as a result of the supersaturation or can be generated by artifices such as addition of seed material or subjecting the solution to ultrasonic vibrations.

Preferably a portion of the solution coming from the crystal separation is recycled to the precrystallizer with the remaining discharged from the system To achieve that the effect of the crystallization process - i.e. the difference in concentration between the feed and the mother liquor after separation of the crystals -will be as large as possible, a portion of the suspension will normally be recycled in a continuous manner from the crystallization zone to the entrance of the precrystallization zone.

Since disintegration of the crystals - as would be ; liable to occur in the pump of the recirculation line - would have a harmful effect on the average size of. the crystals to be produced and tend to cause blockage of the precrystalliz-ation zone, said disintegration is avoided in that according to the process of the invention the end of the recirculation line debouching into the recrystallization zone is provided with separatory device to ensure that a substantially crystal-free suspension is fed back to the precrystallization zone.

The process will be more fully described with reference to the following drawings and examples: In the drawings : Figure 1 is a schematic representation of one preferred apparatus and mode of operation of the present process.

Figure 2 is a schematic representation of another preferred mode of accomplishing the process of this invention.

Referring now to Figure 1 , is a schematic re- scraper device 3, B represents a crystallizer provided with a stirring device 5 and C represents a separator which separates the crystals from the mother liquor. Solution to be crystal- lized is fed to the heat exchanger A^ through lines 1 and 2. Recycled liquor from the crystallizer B can be fed to the precrystallizer A^ through screen 11, pump 12 and line 14. Recycle mother liquor from the separator C can be recycled to the precrystallizer through line 8, pump 10 and line 13. The suspension of crystal nuclei formed in precrystallizer flows through line 4 to crystallizer B and is rapidly mixed with the contents of the crystallizer by stirrer 5. A suspension of crystals flow from the . crystallizer B through the line 6 to crystal separator C. Crystals are separated from the mother liquor and discharged through line 7. The mother liquor is discharged via lines 8 and 9 with a portion recycled to the precrystallizer A^.

An alternative arrangement of apparatus for performing the process of the present invention is shown in Figure 2. In Figure 2, A2 represents a one pass heat exchanger, D represents a vapor liquid separator. The remaining apparatus being the same as in Figure 1. The liquor leaving the heat • exchanger A2 flows to separator D through line 4. The nuclei- liquor suspension formed in the separator D flows to the crystallizer B through line 4a. Solvent vapor is drawn from . separator D through line 15 by appropriate means not shown in Figure 2. The remaining operation of the apparatus shown in Fig. 2 is similar to that for Fig, 1 described hereinbefore.

Example 1 A sugar solution of 10 % by weight was concentrated by formation of ice crystals using the apparatus and arrange- The 10 % by weight sugar solution was supplied to the scraped heat exchanger at a rate of 100 kg/hour along with 3058 kg/hour of crystal free . liquor withdrawn from the crys-tallizer and recirculated in line 14. The recycle liquor had a sugar concentration of approximately 30 % by weight. The heat-exchanger had an effective heat-exchange surface of 2 0.35 m , and the temperature difference between the solution and the cooling medium was kept at approximately 25 °C result-ing in a heat flux of about 25,000 kcal/m^/h. Ice nuclei were formed in the heat-exchanger at a rate of 68 kg/hour.

The residence time for the solution to pass through the heat-exchanger and to the crystallizer was approximately four seconds. The ice nuclei formed in the heat-exchanger were all of a size smaller than 5 microns. The suspension from the heat-exchanger was rapidly mixed with the contents of the crystallizer by intensive . agitation. The crystallizer was sized to hold approximately 800 liters of crystal-liquor suspension. As mentioned earlier, 3058 kg/hour of the crystal free liquor from the crystallizer was recirculated back to the heat-exchanger. The level in the crystallizer was kept constant by removing 266 kg/hour of ice crystal/liquor suspension from the crystallizer through line 6 to the separating device C. Ice crystals 66.6 kg/hour, were separated from the liquor and discharged through line 7. A 30 % by weight, concentrated sugar product flow of 33,4 kg/hour was discharged through line 9 and 166.8 kg/hour of the 30 % sugar solution was recycled to the heat-exchanger through line 13. The product crystals which were obtained had an average diameter of 0.6 mm and were virtually spherical in shape.

An aqueous solution of 50 % by weight MgS04 . 7 H20 was processed using the apparatus and arrangement schematically shown in Fig. 2. The 50 % by weight salt solution was intro-duced at a rate of 33 kg/hour and a temperature of approximately 20 °C to the heat-exchanger A2. A 60 % by weight crystal free salt solution is recycled at a rate of 300 kg/hour from the crystallizer and 83.5 kg/hour from separator C and introduced with the 50 % by weight feed solution to the heat-exchanger. The total flow of solution to the heat-exchanger had a temperature of approximately 35 °C.

The heat-exchanger had an effective heat-exchange surface of 0.03 m , and was supplied with low pressure steam which indirectly heated the feed solution to 41.5 °C , result- 2 ing in a heat flux of approximately 100,000 kcal/m hour. The heated solution coming from the heat-exchanger was fed to the vaporliquor separator D where the pressure was reduced to 30 mm Hg absolute from the atmospheric pressure in the heat-exchanger. Water vapor was discharged through line 15 which simultaneously increased the salt concentration of the remaining solution and reduced the temperature of the solution, and as a result, MgS04 . 7 H20 nuclei formed. The nuclei containing suspension from the vapor-liquor separator was fed to the crystallyzer and rapidly mixed with the contents of the crystallizer by violent agitation in the crystallizer.

The average residence time of the suspension in the vaporliquor separator was approximately 5 seconds.

The crystallizer had an effective volume of 200 liters. The average residence time for crystals in the crystallizer was about 3 hours. As mentioned earlier, 300 kg/hour of liquor are recycled from the crystallizer by way o pump an ne ac o e ea -exc anger. suspenson of crystals and liquor was continuously discharged from the crystallizer to the crystal separator C at a rate of 100 kg/hour. Crystals of MgSO^ . 7 i^O having an average size of 1 mm were withdrawn from the separator C at a rate of 16.5 kg/hour. The liquor from the separator C was returned via lines 8 and 13 to the heat-exchanger at a rate of 83.5 kg/hour. The liquor from the separator C contained approximately 60 % . by weight of the dissolved magnesium salt.

Claims (1)

1. CLAIM3 1. A continuous process for crystallizing a crystallizable solute from its solution in a crystallizer comprising a pre-orystalllzation zone and a crystallization zone connected to the former by a conduit, wherein, in the pre-crystalllzation zone, the solution is rapidly made supersaturated with respect to the solute to he crystallized and large amounts of small crystal nuclei are formed, and aftei a residence time of the crystal nuclei of less than one minute in the pre^crystallization zone and conduit, the solution with the nuclei is delivered into the crystallization zone where it is mixed, during a residence time of at least 100 times the residence time in the pre-crystallization zone and conduit, with larger crystals of an average size at least 10 times that of the small nuclei, the crystallization zone being maintained under substantially adiabatic conditions, for (causing at least part f the small nuclei to dissolve and to produce a crystal growth on the larger crystals. 1 23/. A process according to claim 2^ wherein the short residence time in the precrystallization zone is effected by continuously recirculating a part of the solution present in the recrystallization zone through the precrystallization zone, said solution being substantially freed from crystals before entering the precrystallization zone. 1 3i. A process as claimed in claim jl wherein the solution is rapidly cooled by indirect heat transfer in the precrystallization zone causing said solution to become a supersaturated solution of said component and to form said small crystal nuclei. 4 . A process as claimed in claim wherein the solution is rapidly evaporated in the precrystallization zone causing said solution to become a supersaturated solution of said component and to form said small crystal nuclei.

1 . A process according to claim , wherein the smallest crystal dimension of the nuclei formed in the precrystal-- lization zone is upon entering the crystallization zone not more than 10 microns. 1 (iff. A process according to claim / wherein the average residence time of the crystals in the crystallization zone is at least 30 minutes. ji. A process as claimed in claim 1 wherein the solution is an aqueous solution to be concentrated and the, crystalli- zable component is ice. For the Applicants CS. KjHKOLD COKN &ND PARTNERS