CA1195918A - Process for countercurrent crystallization with recirculation - Google Patents
Process for countercurrent crystallization with recirculationInfo
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- CA1195918A CA1195918A CA000381707A CA381707A CA1195918A CA 1195918 A CA1195918 A CA 1195918A CA 000381707 A CA000381707 A CA 000381707A CA 381707 A CA381707 A CA 381707A CA 1195918 A CA1195918 A CA 1195918A
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Abstract
Title:
Process for countercurrent crystallization with recirculation Abstract of the disclosure:
This invention concerns an improved process for separating one crystallizable component from a liquid containing a plurality of components, which comprises passing a stream of said liquid successively through at least part of a series of zones of successively lower temperatures including the zone of lowest temperature, so as to induce the formation of crystals of said one component in each of said zones, passing the crystals formed in each of said zones in a direction countercurrent to the direction of the stream of liquid through at least part of said zones, including the zone of highest temperature, separating the crystals present in the zone of highest temperature from their mother liquor and recovering at least part of these crystals, separating the crystals in the zone of lowest temperature from their mother liquor and recovering that mother liquor, and recirculating part of said crystals present in one zone, from that zone to a zone of lower temperature.
This invention also concerns an apparatus for carrying out the process.
Process for countercurrent crystallization with recirculation Abstract of the disclosure:
This invention concerns an improved process for separating one crystallizable component from a liquid containing a plurality of components, which comprises passing a stream of said liquid successively through at least part of a series of zones of successively lower temperatures including the zone of lowest temperature, so as to induce the formation of crystals of said one component in each of said zones, passing the crystals formed in each of said zones in a direction countercurrent to the direction of the stream of liquid through at least part of said zones, including the zone of highest temperature, separating the crystals present in the zone of highest temperature from their mother liquor and recovering at least part of these crystals, separating the crystals in the zone of lowest temperature from their mother liquor and recovering that mother liquor, and recirculating part of said crystals present in one zone, from that zone to a zone of lower temperature.
This invention also concerns an apparatus for carrying out the process.
Description
This invention relates to a process whereby a c.rystallizable component is separated from a liquid multi-component system by a process of crystalliza~ion, induced by progressively cooling said system. This process involves for example the concentration of fruit juice, vegetable juices and extracts (such as coffee extract), wine, beer and other materials which comprise aqueous solutions which can be concentrated by the ~ormation and removal of ice crystals. However, also inorganic compounds can be separated out from aqueous solu~ions. Likewise this process may involve the resolution of non-aqueous mixtures of organic compounds.
In continuous, or semi continuous crystallization processes of this kind a stream of the multicomponent liquid lS system is progressively passed through zones of progressively lower temperatures. In cocurrent processes the crystals formed follow the same direction as the liquid~ In counter-current processes the crystals formed axe forced in a direction opposite to the direction of the liquid. In co-current processes the crystals have to be separated fromthe mother liquox at a point of the last zone where this mother liquour is percentagewise richest in components not being the component to be crystallized out and separated. In contras~, in countercurrent processes the crystals are se-parated out from the suspension in the first zone, where the ,,1 ~
liquid is comparatively poor in components which do nothave to be crystallized out. It stands to reason that it is easier to obtain purity of the component which crystallizes and which has to be sep~rated outr in the last (counter-current) case than in the first (cocurrent) case. The con-tent of impurities adhering to the crystals will be less.
Moreover, in most cases the viscosity in the first zone will be lower, which facilitates the separatio~ o~ mother liquor ~rom the crystals.
To summarize: generally in cocurrent processes, crystals have to be separated out from a relatively viscous mother liquor, with a relatively high concentration of substances which are to be considered as impurities if they adhere to the crystals, while in countercurrent processes the reverse holds true.
Examples of countercurrent processes of this kind are to be ~ound in U.S.P. 3,283,522 ~Ganiaris), U~S.P. 3,645,699 (Brodie) and U.S.P. 4rl88r797 (Thijs~en~.
In this last patent the process described involves ireeze concentration, whereby the Gne component to be sepa~
rated out is ice, but an apparatus of the same construction can also advantageou~ly by used when said one component is e.g. a crystallizable organic compound.
The present invention provides a process for the separation of one crystallizable component from a liquid con~; n; ng a plurality of components comprisin~ passing a stream of said liquid successively through at least part of a series of zones of successively lower temperatures in-cluding the zone of lowest temperature, so as to induce the formation of crystals of said one component in each of said zones, passing the crystals formed in each of said zones in a direction countercurrent to the direction of the stream of liquid through at least part of said zones, including the zone of highest temperature, separating the crystals present in the zone of highest temperature from their mo-ther liq~or and reco~ering at least part of these crystals, and separating the crystals in the zone of lowest temperature from their mother liquor and recovering that mother liquor, character-ized by recirculating part of said crystals present in one zone from that zone to a zone of lower temperature~
The advantages of the process according to the present invention will be discussed hereinafter.
When speaking of "one component" we do not mean that this one component crys~allizing out should necessarily have a distinct chemical identity, and certainly not to be already in a state of high purity. In the case whereby we have to do with a freeze concentration process and conse-quently said one component is ice, the crystals can generally have a high purity. This, however, is e.g. not the case with many mixtures of organic compounds.
Occlusion of components not desired may occur in the crystals, and, all dependin~ on the phase diagram, also "mixed crystals" may crystallize out. In specific cases it is even possible that the product desired has the composition gl~
of a mixed crystal. In most cases however a chemically pure product is desired. During their passage from the zone of lowes~ to the zone of highest temperature recrystallization may occur as the crystals formed in the low temperature zones come in contact, successively in zones of higher tempera~
tures, with liquid with a lower content of t'non-desirables";
e~uilibrium is es-tablished between the crystals and the liquid and/or occluded and adhering impurities are gradually removed.
When speaking of "recovering at least part of these crystals" it does not mean that these have to be fed ollt in their totality from ~he apparatus as such. One method of feeding out is in several cases to melt the crystals at least partly aftPr they have been separated from the mother liquor. The melt or the melt with crystals still suspended in it can then be f~d out as a fluid. Part of the melt can be used as a reflux to "wash" the crystals, as e.g~ described in U.S.P. 2,854,494 (Thomas) and U.S.P. 3,645,699 (Brodie).
There are several methods by which the crystals can be induced to follow a path countercurrently to the direction of the stream li~uid. One such method is described in U.S.P.
3,645,699 ~Brodie). By this method the zones of different temperatures are not separa-te, but are, so to say, contiguous to one another. In U.SOP. 3,283,522 (Ganiaris), there are separate zones and each zone is provided with means to separate the crystals from the liquid and pass these on in a different direction. The same occus according to UOS~P.
5~
4,188,797 (Tnijssen).
From the above it follows that the term ~Izone~ has to be taken in its broadest sense. Though an advantageous effect can be observed by recirculating part of the crystals S present in any one of the zones from that zone to a zone of lower temperature, the best results are generally obtained by recirculation from the zone of highest to the zone or lowest temperature. Althou3h the countercurrent process can in principle be performed while the crystals of any one zone do not pass all to the subsequent zones o higher temperature in succession but e.g. are directed immediately to the zone of highest temperature, preferably the countercurrent process is operated in such a way that all the crystals pass success-ively through all of the zones with a temperature higher than that of said zone.
The liquid can be fed in the zone of highest temper-ature. This is to be preferred embodiment in those cases whereby the crystals easily crystallize out in pure con-dition to wit without occlusion of and not forming mixed crystals with undesired components e.g. when we have to do with ice crystals in a process of freeze concentration.
With many mixture of organic compounds the crystals formed are less pure. In those cases the purity can be in-creased by feeding the liquid not in the zone of highest temperature, but in a zone intermediate between the zone of highest and the zone of lowest temperature. The crystals of the zone wherein the liquid is fed can in these cases be brought in contact ("washed") with a melt of the crystals of the one component to be recovered. To this purpose the crystals are partly melted and used as a reflux. One example - is to be found in U.S.P. 3,645,699 (Brodie). In that specific case the melt is brought countercurrently in contact with the impure crystals 50 as to establish equilibrium between the two phases. Cooling in what is called the "refining"
section takes care that the melted portion again~ crystall-izes. The refining section described in this patent is com-bined with an adiabatically operated sectionr designated as"purifying section", which is also used for purposes of se-parating crystals and liquid by grafity. The refining section can of course also be combined with e.g. a wash column instead of a separation by grafity.
While the feed in the processes at issue will be generally liquid, it may also contain crystals, -thus being a slurryO
It stands to reason that the effect is the more pro-nounced the greater the recirculation factor for the crystals is. When the xecirculated crystals are first separated from the liquid in which they occur, there is in principle no other limit to the recirculation factor than the requirement that in an equilibrated process as much of the "desired"
component has to be remo~ed fxom, as is formed in, the pro-cess. Of course, for reasons of economy we shall not pushthe recirculation factor beyond what is necessary. Depending on circumstancesl such as the very nature of the product to be separated out, the concentration of the liquid~ the cost of energy, the capital outlay for instrumentation etc., we shall choose the recirculation such as to obtain an econo-mical optimization. In most cases recirculation factors of 1-25% are adequate. Generally a recirculation factor of
In continuous, or semi continuous crystallization processes of this kind a stream of the multicomponent liquid lS system is progressively passed through zones of progressively lower temperatures. In cocurrent processes the crystals formed follow the same direction as the liquid~ In counter-current processes the crystals formed axe forced in a direction opposite to the direction of the liquid. In co-current processes the crystals have to be separated fromthe mother liquox at a point of the last zone where this mother liquour is percentagewise richest in components not being the component to be crystallized out and separated. In contras~, in countercurrent processes the crystals are se-parated out from the suspension in the first zone, where the ,,1 ~
liquid is comparatively poor in components which do nothave to be crystallized out. It stands to reason that it is easier to obtain purity of the component which crystallizes and which has to be sep~rated outr in the last (counter-current) case than in the first (cocurrent) case. The con-tent of impurities adhering to the crystals will be less.
Moreover, in most cases the viscosity in the first zone will be lower, which facilitates the separatio~ o~ mother liquor ~rom the crystals.
To summarize: generally in cocurrent processes, crystals have to be separated out from a relatively viscous mother liquor, with a relatively high concentration of substances which are to be considered as impurities if they adhere to the crystals, while in countercurrent processes the reverse holds true.
Examples of countercurrent processes of this kind are to be ~ound in U.S.P. 3,283,522 ~Ganiaris), U~S.P. 3,645,699 (Brodie) and U.S.P. 4rl88r797 (Thijs~en~.
In this last patent the process described involves ireeze concentration, whereby the Gne component to be sepa~
rated out is ice, but an apparatus of the same construction can also advantageou~ly by used when said one component is e.g. a crystallizable organic compound.
The present invention provides a process for the separation of one crystallizable component from a liquid con~; n; ng a plurality of components comprisin~ passing a stream of said liquid successively through at least part of a series of zones of successively lower temperatures in-cluding the zone of lowest temperature, so as to induce the formation of crystals of said one component in each of said zones, passing the crystals formed in each of said zones in a direction countercurrent to the direction of the stream of liquid through at least part of said zones, including the zone of highest temperature, separating the crystals present in the zone of highest temperature from their mo-ther liq~or and reco~ering at least part of these crystals, and separating the crystals in the zone of lowest temperature from their mother liquor and recovering that mother liquor, character-ized by recirculating part of said crystals present in one zone from that zone to a zone of lower temperature~
The advantages of the process according to the present invention will be discussed hereinafter.
When speaking of "one component" we do not mean that this one component crys~allizing out should necessarily have a distinct chemical identity, and certainly not to be already in a state of high purity. In the case whereby we have to do with a freeze concentration process and conse-quently said one component is ice, the crystals can generally have a high purity. This, however, is e.g. not the case with many mixtures of organic compounds.
Occlusion of components not desired may occur in the crystals, and, all dependin~ on the phase diagram, also "mixed crystals" may crystallize out. In specific cases it is even possible that the product desired has the composition gl~
of a mixed crystal. In most cases however a chemically pure product is desired. During their passage from the zone of lowes~ to the zone of highest temperature recrystallization may occur as the crystals formed in the low temperature zones come in contact, successively in zones of higher tempera~
tures, with liquid with a lower content of t'non-desirables";
e~uilibrium is es-tablished between the crystals and the liquid and/or occluded and adhering impurities are gradually removed.
When speaking of "recovering at least part of these crystals" it does not mean that these have to be fed ollt in their totality from ~he apparatus as such. One method of feeding out is in several cases to melt the crystals at least partly aftPr they have been separated from the mother liquor. The melt or the melt with crystals still suspended in it can then be f~d out as a fluid. Part of the melt can be used as a reflux to "wash" the crystals, as e.g~ described in U.S.P. 2,854,494 (Thomas) and U.S.P. 3,645,699 (Brodie).
There are several methods by which the crystals can be induced to follow a path countercurrently to the direction of the stream li~uid. One such method is described in U.S.P.
3,645,699 ~Brodie). By this method the zones of different temperatures are not separa-te, but are, so to say, contiguous to one another. In U.SOP. 3,283,522 (Ganiaris), there are separate zones and each zone is provided with means to separate the crystals from the liquid and pass these on in a different direction. The same occus according to UOS~P.
5~
4,188,797 (Tnijssen).
From the above it follows that the term ~Izone~ has to be taken in its broadest sense. Though an advantageous effect can be observed by recirculating part of the crystals S present in any one of the zones from that zone to a zone of lower temperature, the best results are generally obtained by recirculation from the zone of highest to the zone or lowest temperature. Althou3h the countercurrent process can in principle be performed while the crystals of any one zone do not pass all to the subsequent zones o higher temperature in succession but e.g. are directed immediately to the zone of highest temperature, preferably the countercurrent process is operated in such a way that all the crystals pass success-ively through all of the zones with a temperature higher than that of said zone.
The liquid can be fed in the zone of highest temper-ature. This is to be preferred embodiment in those cases whereby the crystals easily crystallize out in pure con-dition to wit without occlusion of and not forming mixed crystals with undesired components e.g. when we have to do with ice crystals in a process of freeze concentration.
With many mixture of organic compounds the crystals formed are less pure. In those cases the purity can be in-creased by feeding the liquid not in the zone of highest temperature, but in a zone intermediate between the zone of highest and the zone of lowest temperature. The crystals of the zone wherein the liquid is fed can in these cases be brought in contact ("washed") with a melt of the crystals of the one component to be recovered. To this purpose the crystals are partly melted and used as a reflux. One example - is to be found in U.S.P. 3,645,699 (Brodie). In that specific case the melt is brought countercurrently in contact with the impure crystals 50 as to establish equilibrium between the two phases. Cooling in what is called the "refining"
section takes care that the melted portion again~ crystall-izes. The refining section described in this patent is com-bined with an adiabatically operated sectionr designated as"purifying section", which is also used for purposes of se-parating crystals and liquid by grafity. The refining section can of course also be combined with e.g. a wash column instead of a separation by grafity.
While the feed in the processes at issue will be generally liquid, it may also contain crystals, -thus being a slurryO
It stands to reason that the effect is the more pro-nounced the greater the recirculation factor for the crystals is. When the xecirculated crystals are first separated from the liquid in which they occur, there is in principle no other limit to the recirculation factor than the requirement that in an equilibrated process as much of the "desired"
component has to be remo~ed fxom, as is formed in, the pro-cess. Of course, for reasons of economy we shall not pushthe recirculation factor beyond what is necessary. Depending on circumstancesl such as the very nature of the product to be separated out, the concentration of the liquid~ the cost of energy, the capital outlay for instrumentation etc., we shall choose the recirculation such as to obtain an econo-mical optimization. In most cases recirculation factors of 1-25% are adequate. Generally a recirculation factor of
2-10~ will be preferred. While conveniently and preferen-tially the recirculation may be accomplished continuously, intermittent recirculation is not excluded.
According to the present invention there is also pro~
vided an apparatus for applying the process described above, being an apparatus of the type mentioned above provided with means to recirculate a predetermined quantity of crystals per unit of time from any one stage to a stage of lower temperature. The advantageous effect according to the in-vention may be understood by the following considerations.
In the zone of highest temperatur2 the crystals willhave the largest size, having had time to grow when passiny on from the zones of lower temperature to said zone of highest temperature and will have the highest purity. When a number of these crystals are,transferred to e.g. the zone o~ lowest temperature, in which a liquid is present with the highest content of non~desi-rables, they will have the follow-ing effect, At the cooling surface of the zone of lowest temperature nuclei are formed at a high rate, These nuclei are di5tributed throughout the liquid in that zone by the scraping and stirring mechanism~ Now, big crystals having a lower solubility than small crystals, the nuclei are not ~5~
~ 8 --stable in the presence o~ the recirculated crystals and most of these nuclei will dissolve again adding to the growth of the recirculated bigger crystals. This means that the crystals ultimately obtained in the zone in question will be bigger than without recirculation as according to the invention.
All this means that the crystals which we will ultimately get in the zone of highest temperature will be appreciably bigger than would have been the case without recirculation.
It has to be emphasized that separation processes, whatever they may be, are the more easy to operate the bigger the crystals are, and will result in a purer product.
First the bigger the crystals the smaller the surface per weight unit, and consequently the smaller the quantity of mother liquor which may tend to adhere to the crystals.
Very important also is the fact that, e.g. in packed bed wash columns, the pressure to be applied for a certain through~ut is very dependent on the crystal size, rising sharply as the crystals get smaller. 'rhis is especially of importance with crystals of many organic compounds, which, being relatively soft, are easily de~ormable by prsssure. This results in com~
pression of the crystal layer and thus in lower porosity o~
this layer.
There is one more reason why recirculation according to the invention will produce a purer product. As the nuclei at the cooling surfaces of the crystallizer are formed rapidly there is in many cases an appreciable tendency for ~ 9 --impurities to be occluded. When the nuclei redissolve due to the presence of the big recirculated crystals, the mate-rial present in those nuclei will settle on those recirculated crystals, but in a more orderly way than by the "shock cooling"
5 at the cold cooling surfaces. Accordingly ~here will be less tendency for occlusion of impurities.
It may be remarked here to avoid any misunderstanding that the notion of "desired" and "not desired", as used above, only means "desired" or "not desired" to crystallize and does not bear in mind the question whether the component crystallized out is the most valuable one or, in contrast, the less valuable one which by crystallization and subse-quent separation has to be eliminated.
Recirculation of the crystals can ~.g. be accomplished by withdrawing some of the liquid in the zone of highest temperature with the crystals suspended in it to the zone of lowest temperature. As the case may be a thickening may first take place in order to create a slurry with higher crystal content. This can e.g. be accomplished in a thicken-er as descxibed in U.S.P. 4,188,797 (Thijssen).
Many methods for the separation of crystals from their mother liquor have been proposed in continuous crystalli-zation processes. Centrifuges are well known, but wash columns have definite advantages in several instances and have also been described in several patents, see e.g.U.S.~.
2,854,494 (Thomas), U.S.P~ 3,587,859 (Probstein); U.S.P.
According to the present invention there is also pro~
vided an apparatus for applying the process described above, being an apparatus of the type mentioned above provided with means to recirculate a predetermined quantity of crystals per unit of time from any one stage to a stage of lower temperature. The advantageous effect according to the in-vention may be understood by the following considerations.
In the zone of highest temperatur2 the crystals willhave the largest size, having had time to grow when passiny on from the zones of lower temperature to said zone of highest temperature and will have the highest purity. When a number of these crystals are,transferred to e.g. the zone o~ lowest temperature, in which a liquid is present with the highest content of non~desi-rables, they will have the follow-ing effect, At the cooling surface of the zone of lowest temperature nuclei are formed at a high rate, These nuclei are di5tributed throughout the liquid in that zone by the scraping and stirring mechanism~ Now, big crystals having a lower solubility than small crystals, the nuclei are not ~5~
~ 8 --stable in the presence o~ the recirculated crystals and most of these nuclei will dissolve again adding to the growth of the recirculated bigger crystals. This means that the crystals ultimately obtained in the zone in question will be bigger than without recirculation as according to the invention.
All this means that the crystals which we will ultimately get in the zone of highest temperature will be appreciably bigger than would have been the case without recirculation.
It has to be emphasized that separation processes, whatever they may be, are the more easy to operate the bigger the crystals are, and will result in a purer product.
First the bigger the crystals the smaller the surface per weight unit, and consequently the smaller the quantity of mother liquor which may tend to adhere to the crystals.
Very important also is the fact that, e.g. in packed bed wash columns, the pressure to be applied for a certain through~ut is very dependent on the crystal size, rising sharply as the crystals get smaller. 'rhis is especially of importance with crystals of many organic compounds, which, being relatively soft, are easily de~ormable by prsssure. This results in com~
pression of the crystal layer and thus in lower porosity o~
this layer.
There is one more reason why recirculation according to the invention will produce a purer product. As the nuclei at the cooling surfaces of the crystallizer are formed rapidly there is in many cases an appreciable tendency for ~ 9 --impurities to be occluded. When the nuclei redissolve due to the presence of the big recirculated crystals, the mate-rial present in those nuclei will settle on those recirculated crystals, but in a more orderly way than by the "shock cooling"
5 at the cold cooling surfaces. Accordingly ~here will be less tendency for occlusion of impurities.
It may be remarked here to avoid any misunderstanding that the notion of "desired" and "not desired", as used above, only means "desired" or "not desired" to crystallize and does not bear in mind the question whether the component crystallized out is the most valuable one or, in contrast, the less valuable one which by crystallization and subse-quent separation has to be eliminated.
Recirculation of the crystals can ~.g. be accomplished by withdrawing some of the liquid in the zone of highest temperature with the crystals suspended in it to the zone of lowest temperature. As the case may be a thickening may first take place in order to create a slurry with higher crystal content. This can e.g. be accomplished in a thicken-er as descxibed in U.S.P. 4,188,797 (Thijssen).
Many methods for the separation of crystals from their mother liquor have been proposed in continuous crystalli-zation processes. Centrifuges are well known, but wash columns have definite advantages in several instances and have also been described in several patents, see e.g.U.S.~.
2,854,494 (Thomas), U.S.P~ 3,587,859 (Probstein); U.S.P.
3,872,009 ~Thijssen~ and British Patent Application 79,21808 s~
(Published British Patent Application No. 2023564A) (Thijssen)~
Wash columns are appropriate whenever a "washfront"
can be created at about mel~ing temperature with at the one side the melt of the one component to be separated out and at the other side the mother liquor trapped in between the (as the case may be agglomerated) crystals. This has been extensively discussed in British Patent Application No. 79,21808 (Published British Application No. 2023564A) (Thijssen) for aqueous solutions, whereby the one component to be separated out is ice, but wash columns of the same construction can also advantageously be used when said one component is e.g. an organic crystalli7able compound. In U.S.P. 3,645,69g tBrodie) separa~ion is accomplished by gravity; the crystals of the component to be separated out "falling" through a melt of that same component.
The process according to the present invention will now be further described with reference to the accompanying drawings, in which~-Fig. 1 is by way of example a schematic representation of a flow sheet fox a preerred embodiment o the invention, in cases wherein an extra refining section is not necessary;
e.g. for freeze concentration; and Fig. 2 is by way of example a schematic representation of a flow sheet in the case that-we have to do with amixture of organic compounds, wherein an extra 1I refining section"
is necessaryO
For details of the apparatus which can be used in these cases (that is, of course, without recirculation as according to -the present invention) we refer e.gO to U.S.P. 4,188,797 (Thijssen) and to the British Patent Appli-cation No. 79,21808 (Pu~lished British Paten~ Application No. 2023564A) (Thijssen).
Referring to Fig. 1 the four stages of the crystal-lizer are shown at 1, 2/3 and 4. Thickeners for crystal slurry appear at 5, 6 and 7, while the final wash column is denoted with 8. At 17 we see an apparatus which operates as thickener or wash column. For this we refer e.~. to Fig. 8 of U.S.P. 4jl88,797 (Thijss~n). When the apparatus 17 operates as a thickener, some mother liquor of stage 1 will be introduced in stage 4. It has however been found that generally this does not introduce a major influence on the efficiency of the opPration.
The liquid to be concentrated enters at 9 and is fed to stage 1 together with part of the return liquid 10 from wash column 8. Crystal slurry from sta~e 1 is fed at 11 to wash column 8. At 12 the component sepaxated out is withdrawn from the wash coll~mn 8, while at 13 the liquid separated from the crystals is partly returned to stage 1 at 10 and partly, at 14, fed to stage 2. A slurry of crystals is recirculated at 15 from stage 1. It is thicken-ed in 17 and led to stage 4, while the mother liquor isrecirculated to stage 1. For the thickening process see U.S.P. 4,188,797 (Thijssen). The crystals and mother liquor are led countercurrently through the various stages, as will be clear from the figure and as descrihed in detail in the application m~ntioned above. Mother li~uor with the lowest content of the crystallizable component is withdrawn at 16.
Further details or the embodiment depicted in Fig. 1 are given in Example 1 (see especially table 1, in which all the different streams of material are described).
Referring now to Figure 2, (101) to (106) are crystal-lizer compa~tments as described in U.S.P. 4,188,797 (Thijssen) while (107) is a thickener as described in the same patent.
At (108) a wasih column is shown of the constxuction as de-scribed in British Patent Application No. 79,21808 (Published British Patent Application No. 2023564A) (Thijssen). As de-scribed in detail in U.S.P. 4,188,797 (Thijssen) and also above when discussing Fig. 1, the slurry from each of the compartments is led to a thickener, the thickened slurry is passed on to the preceding compartment (of higher temperature), while the liquid leaving the thic~ener is partly recirculated to the same compartment and for the rest to the following compartment (of lower temperature), or finally reco~ered from the last compaxtment. For reasons of clari~y and sim-plicity in this case only the general direction of flow of the liquid and the crystals are shown, the direction of the liquid being indicated with dotted lines, the direction of the crystalis with drawn lines.
The feed enters at (109) in crystallizing compartment (102) and the liquid passes on to (103~ up to (106). Mother liquid is recovered at (110).
~5~1~
The compartment (101) functions as a refining section, by a prccess of recrystallization. The crystals arriving in this compartment from compartment (102) are here brought in contact with a melt of the one component which has to be separated out of the multi-component liquid system. The wash column (108) separates the crystals from their mother liquor.
Part of the crystals after melting are returned to the com-partment (101), while the rest is recovered at (111). A sus-pension of crystals (112) is withdrawn from compartment (102) and fed to the thickener (107). The liquid (113) leaving the thickener is returned to the compartment (102), while the thickened slurry of crystals (114) is passed on to the com-partment (106) of lowest temperature. Of course more than one refining compartment may be necessary, depending on the specific composition of the li~uid to be treated and the purity of the crystallized component desired. Likewise the number of crystallizer compartmen~s depends on the specific case at hand.
The present invention will now be fuxther illustrated by way of the following Examples:-Example 1 Red wine was treated in an apparatus and according tothe process described above with reference to Fig. 1. Use was made of a crystallizer with four compartments (1) - (4) with ~5 the ~mpn~ions which were mentioned in U.S.P. 4,188~797 (Thijssen~. A wash colu~n (8) was used as described in Bxitish Patent Application 7g,21808 (Published British Patent Appli-95g3~3 ~ 14 -cation No. ~023564A) (Thijssen) while the crystals were re-circulated as a thickened slurry ob-tained in a thickener (17), of the construction as described in U.S.P. 4,188,797 (Thijssen). The wine was threefold concentrated. Referring to Figure 1 the streams of material were as indicated in table 1.
In a reference experiment the process was run in the same way as described above:, but without recirculation of crystals. In this case the streams of material were indicated in table 2.
From these data it is cleax that only about 2.5% of the ice crystals finally fed out from the apparatus were re-circulated. The average size of the ice crystals being treated in the wash column increased by this small percentage of recirculation two fold, from 70 to 140 micrometer. This resulted in a decrease of the pressure drop in the wash column from 7 atm. to 1.5 atm.
Streams of m~terial with Streams o~ m~r;~l without 2.5~ recirc~llation of crystals recir~ll~tinn of crystals Nr.of consistency kg/hour Nr.of consistencykg~hour 5 stream stream 9 liquid 75.00 9 liquid 75O00 12 melted ice 50.00 12 melted ice 50.00 16 liquid 25.00 16 liquid 25.00 11 slurry 200.00 11 slurry 200.00 12 melted ice 50.00 12 melted ice 50,00 13 liquid 150.00 13 liquid 150.00 liquid 56.60 10 liquid 56.60 14 li~uid 93.40 14 liquid 93.40 slurry 157.00 20 sluxry 152.00 31 thick.slurry70.65 31 thick.slurry68.40 21 liquid 86.35 21 liguid 83.60 22 liquid 16.35 22 liguid 13.60 23 liquid 70.00 23 liquid 70.00 24 slurry 105.00 24 slurry 100.00 32 thick.51urry47.25 32 thick.slurry45.00 liquid 57.75 25 liquid 55.00 26 liquid 11.15 26 liquid 8.40 27 li~uid 46.60 27 liquid 46.60 28 slurry 53.00 28 slurry 48.00 33 ~lick.slurry23.85 33 thick.slurry21.60 29 liquid 29.15 29 liquid 26.40 liquid 4.15 30 liquid 1.40 16 liquid 25.00 16 liquid 25.00 slurry 5.00 18 thick.slurry2.25 19 liquid 2.75 ~S~
Example 2 The process as generally discussed in Fig. 2 to which we refer here will now be described as applied to the sepa-~ ration of p-xylene from a mixture containing it and other isometric xylenes.
Use is made of a crystallizer with six compartments (101) to (106) with the dimensions which were mentioned in U.S.P. 4,188,797 (Thijssen). A wash column (108) is used as described in British Patent Application No. 79,21808 (Published British Patent Application No. 2023564A) (Thijssen), while crystals are recirculated as a thickened slurry ob~
tained in a thickener (107) of the construction as described in U.S.P. 4,188,797. Throughout the apparatus there is a temperature profile from about -11C in compartment (101) to -65C in compartment (106). The purified p-xylene is recovered at (111) as a melt while the rest is returned to compartment (101). The ratio of recovexed to refluxed melt of p-xylene is ~:1. The temperature of the melt is about -~15C (the melting point of pure p-xylene is ~13,26C).
In table 3 the material balance around the total apparatus i~ given, with a recirculation ratio of crystals of 20~, which goes to say that 80% of the crystals present in compartment (102) are passed on to compartment (101) and 20~ is recirculated to compartment (106). In table 3 also the composition is given of the p-xylene ~ed out at (lll)o In table 4 the same data are given in the case ~hat no recirculation of crystals is applied. In both cases the same pressure drop in the wash column is maintained. Comparing table 3 and 4 it is seen that the product purity is increased from 99.5% to 99.9% while at the same time the throughout of the apparatus by the recirculation is increased with 40%.
T~BLE 3 M;lt~l-; ~1 }~1 ~n~e around tot~l d~ald~U~; with 20~ recir~~ n of crystals F~ed cryst~l 1; 7.~hl P cn r~nnPnt- m~ther liqu~r separated out kg/hour % kg/hour % kg/h~ur %
Ethyl benzene 18.18 19.55 0.001 0.01 18.1822.63 para xylene 19.34 20.80 12.669 99.92 6.67 8.31 methaxylene 43.68 46.96 0.007 0.55 43.6754.37 otho xylene 11.72 12.60 0.002 0002 11.7214.59 others 00.08 00.09 0.000 0.00 0.08 0.10 Total 93.00 100.00 12.679 100.00 80.32100.00 J
~ELE 4 r~t~r;~l h~l~n~e around total d~LCUd~US without 20% recLrclllAti~n of c~ystals Feed cryst~ Ahl~ cnmr~n~nt ~other liquor S~ydld~ed out kg~hour % kg/hour -~ kg/hour %
Et~yl ~enzene 12.89 19.54 0.01 0.12 12.89 22.62 para xylene 13~73 20.80 8.99 99.50 4.74 8.32 meta xylene 31.00 46.97 0.03 0.29 30.97 54.37 oth~ xylene 8.32 12.60 0.01 0.09 8.31 14.59 others 0.0~ 0.. 09 0.00 0.00 0.06 ~.lQ
Total 66.00 100.00 9.04 100.00 56.97 100.00
(Published British Patent Application No. 2023564A) (Thijssen)~
Wash columns are appropriate whenever a "washfront"
can be created at about mel~ing temperature with at the one side the melt of the one component to be separated out and at the other side the mother liquor trapped in between the (as the case may be agglomerated) crystals. This has been extensively discussed in British Patent Application No. 79,21808 (Published British Application No. 2023564A) (Thijssen) for aqueous solutions, whereby the one component to be separated out is ice, but wash columns of the same construction can also advantageously be used when said one component is e.g. an organic crystalli7able compound. In U.S.P. 3,645,69g tBrodie) separa~ion is accomplished by gravity; the crystals of the component to be separated out "falling" through a melt of that same component.
The process according to the present invention will now be further described with reference to the accompanying drawings, in which~-Fig. 1 is by way of example a schematic representation of a flow sheet fox a preerred embodiment o the invention, in cases wherein an extra refining section is not necessary;
e.g. for freeze concentration; and Fig. 2 is by way of example a schematic representation of a flow sheet in the case that-we have to do with amixture of organic compounds, wherein an extra 1I refining section"
is necessaryO
For details of the apparatus which can be used in these cases (that is, of course, without recirculation as according to -the present invention) we refer e.gO to U.S.P. 4,188,797 (Thijssen) and to the British Patent Appli-cation No. 79,21808 (Pu~lished British Paten~ Application No. 2023564A) (Thijssen).
Referring to Fig. 1 the four stages of the crystal-lizer are shown at 1, 2/3 and 4. Thickeners for crystal slurry appear at 5, 6 and 7, while the final wash column is denoted with 8. At 17 we see an apparatus which operates as thickener or wash column. For this we refer e.~. to Fig. 8 of U.S.P. 4jl88,797 (Thijss~n). When the apparatus 17 operates as a thickener, some mother liquor of stage 1 will be introduced in stage 4. It has however been found that generally this does not introduce a major influence on the efficiency of the opPration.
The liquid to be concentrated enters at 9 and is fed to stage 1 together with part of the return liquid 10 from wash column 8. Crystal slurry from sta~e 1 is fed at 11 to wash column 8. At 12 the component sepaxated out is withdrawn from the wash coll~mn 8, while at 13 the liquid separated from the crystals is partly returned to stage 1 at 10 and partly, at 14, fed to stage 2. A slurry of crystals is recirculated at 15 from stage 1. It is thicken-ed in 17 and led to stage 4, while the mother liquor isrecirculated to stage 1. For the thickening process see U.S.P. 4,188,797 (Thijssen). The crystals and mother liquor are led countercurrently through the various stages, as will be clear from the figure and as descrihed in detail in the application m~ntioned above. Mother li~uor with the lowest content of the crystallizable component is withdrawn at 16.
Further details or the embodiment depicted in Fig. 1 are given in Example 1 (see especially table 1, in which all the different streams of material are described).
Referring now to Figure 2, (101) to (106) are crystal-lizer compa~tments as described in U.S.P. 4,188,797 (Thijssen) while (107) is a thickener as described in the same patent.
At (108) a wasih column is shown of the constxuction as de-scribed in British Patent Application No. 79,21808 (Published British Patent Application No. 2023564A) (Thijssen). As de-scribed in detail in U.S.P. 4,188,797 (Thijssen) and also above when discussing Fig. 1, the slurry from each of the compartments is led to a thickener, the thickened slurry is passed on to the preceding compartment (of higher temperature), while the liquid leaving the thic~ener is partly recirculated to the same compartment and for the rest to the following compartment (of lower temperature), or finally reco~ered from the last compaxtment. For reasons of clari~y and sim-plicity in this case only the general direction of flow of the liquid and the crystals are shown, the direction of the liquid being indicated with dotted lines, the direction of the crystalis with drawn lines.
The feed enters at (109) in crystallizing compartment (102) and the liquid passes on to (103~ up to (106). Mother liquid is recovered at (110).
~5~1~
The compartment (101) functions as a refining section, by a prccess of recrystallization. The crystals arriving in this compartment from compartment (102) are here brought in contact with a melt of the one component which has to be separated out of the multi-component liquid system. The wash column (108) separates the crystals from their mother liquor.
Part of the crystals after melting are returned to the com-partment (101), while the rest is recovered at (111). A sus-pension of crystals (112) is withdrawn from compartment (102) and fed to the thickener (107). The liquid (113) leaving the thickener is returned to the compartment (102), while the thickened slurry of crystals (114) is passed on to the com-partment (106) of lowest temperature. Of course more than one refining compartment may be necessary, depending on the specific composition of the li~uid to be treated and the purity of the crystallized component desired. Likewise the number of crystallizer compartmen~s depends on the specific case at hand.
The present invention will now be fuxther illustrated by way of the following Examples:-Example 1 Red wine was treated in an apparatus and according tothe process described above with reference to Fig. 1. Use was made of a crystallizer with four compartments (1) - (4) with ~5 the ~mpn~ions which were mentioned in U.S.P. 4,188~797 (Thijssen~. A wash colu~n (8) was used as described in Bxitish Patent Application 7g,21808 (Published British Patent Appli-95g3~3 ~ 14 -cation No. ~023564A) (Thijssen) while the crystals were re-circulated as a thickened slurry ob-tained in a thickener (17), of the construction as described in U.S.P. 4,188,797 (Thijssen). The wine was threefold concentrated. Referring to Figure 1 the streams of material were as indicated in table 1.
In a reference experiment the process was run in the same way as described above:, but without recirculation of crystals. In this case the streams of material were indicated in table 2.
From these data it is cleax that only about 2.5% of the ice crystals finally fed out from the apparatus were re-circulated. The average size of the ice crystals being treated in the wash column increased by this small percentage of recirculation two fold, from 70 to 140 micrometer. This resulted in a decrease of the pressure drop in the wash column from 7 atm. to 1.5 atm.
Streams of m~terial with Streams o~ m~r;~l without 2.5~ recirc~llation of crystals recir~ll~tinn of crystals Nr.of consistency kg/hour Nr.of consistencykg~hour 5 stream stream 9 liquid 75.00 9 liquid 75O00 12 melted ice 50.00 12 melted ice 50.00 16 liquid 25.00 16 liquid 25.00 11 slurry 200.00 11 slurry 200.00 12 melted ice 50.00 12 melted ice 50,00 13 liquid 150.00 13 liquid 150.00 liquid 56.60 10 liquid 56.60 14 li~uid 93.40 14 liquid 93.40 slurry 157.00 20 sluxry 152.00 31 thick.slurry70.65 31 thick.slurry68.40 21 liquid 86.35 21 liguid 83.60 22 liquid 16.35 22 liguid 13.60 23 liquid 70.00 23 liquid 70.00 24 slurry 105.00 24 slurry 100.00 32 thick.51urry47.25 32 thick.slurry45.00 liquid 57.75 25 liquid 55.00 26 liquid 11.15 26 liquid 8.40 27 li~uid 46.60 27 liquid 46.60 28 slurry 53.00 28 slurry 48.00 33 ~lick.slurry23.85 33 thick.slurry21.60 29 liquid 29.15 29 liquid 26.40 liquid 4.15 30 liquid 1.40 16 liquid 25.00 16 liquid 25.00 slurry 5.00 18 thick.slurry2.25 19 liquid 2.75 ~S~
Example 2 The process as generally discussed in Fig. 2 to which we refer here will now be described as applied to the sepa-~ ration of p-xylene from a mixture containing it and other isometric xylenes.
Use is made of a crystallizer with six compartments (101) to (106) with the dimensions which were mentioned in U.S.P. 4,188,797 (Thijssen). A wash column (108) is used as described in British Patent Application No. 79,21808 (Published British Patent Application No. 2023564A) (Thijssen), while crystals are recirculated as a thickened slurry ob~
tained in a thickener (107) of the construction as described in U.S.P. 4,188,797. Throughout the apparatus there is a temperature profile from about -11C in compartment (101) to -65C in compartment (106). The purified p-xylene is recovered at (111) as a melt while the rest is returned to compartment (101). The ratio of recovexed to refluxed melt of p-xylene is ~:1. The temperature of the melt is about -~15C (the melting point of pure p-xylene is ~13,26C).
In table 3 the material balance around the total apparatus i~ given, with a recirculation ratio of crystals of 20~, which goes to say that 80% of the crystals present in compartment (102) are passed on to compartment (101) and 20~ is recirculated to compartment (106). In table 3 also the composition is given of the p-xylene ~ed out at (lll)o In table 4 the same data are given in the case ~hat no recirculation of crystals is applied. In both cases the same pressure drop in the wash column is maintained. Comparing table 3 and 4 it is seen that the product purity is increased from 99.5% to 99.9% while at the same time the throughout of the apparatus by the recirculation is increased with 40%.
T~BLE 3 M;lt~l-; ~1 }~1 ~n~e around tot~l d~ald~U~; with 20~ recir~~ n of crystals F~ed cryst~l 1; 7.~hl P cn r~nnPnt- m~ther liqu~r separated out kg/hour % kg/hour % kg/h~ur %
Ethyl benzene 18.18 19.55 0.001 0.01 18.1822.63 para xylene 19.34 20.80 12.669 99.92 6.67 8.31 methaxylene 43.68 46.96 0.007 0.55 43.6754.37 otho xylene 11.72 12.60 0.002 0002 11.7214.59 others 00.08 00.09 0.000 0.00 0.08 0.10 Total 93.00 100.00 12.679 100.00 80.32100.00 J
~ELE 4 r~t~r;~l h~l~n~e around total d~LCUd~US without 20% recLrclllAti~n of c~ystals Feed cryst~ Ahl~ cnmr~n~nt ~other liquor S~ydld~ed out kg~hour % kg/hour -~ kg/hour %
Et~yl ~enzene 12.89 19.54 0.01 0.12 12.89 22.62 para xylene 13~73 20.80 8.99 99.50 4.74 8.32 meta xylene 31.00 46.97 0.03 0.29 30.97 54.37 oth~ xylene 8.32 12.60 0.01 0.09 8.31 14.59 others 0.0~ 0.. 09 0.00 0.00 0.06 ~.lQ
Total 66.00 100.00 9.04 100.00 56.97 100.00
Claims (9)
1. A process for separating one crystal-lizable component not being water from a liquid containing a plurality of components, which comprises passing a stream of said liquid successively through at least part of a series of zones of successively lower temperatures including the zone of lowest temperature, so as to induce the formation of crystals of said one component in each of said zones, passing the crystals formed in each of said zones in a direction countercurrent to the direction of the stream of liquid through at least part of said zones, including the zone of highest temperature, separating the crystals present in the zone of highest temperature from their mother liquor and recovering at least part of these crystals, separating the crystals in the zone of lowest temperature from their mother li-quor and recovering that mother liquor, and recircu-lating part of said crystals present in one zone, from that zone to a zone of lower temperature, said process being characterized by recirculating by direct intro-duction into a zone a part of the crystals formed in a preceding zone other than the immediately preceding zone, said part having a mean crystal size larger than the mean crystal size of the crystals formed in said zone had said larger mean size crystals not been intro-duced into said zone.
2. A process a claimed in claim 1, wherein said crystals withdrawn from one zone are recirculated to the zone of lowest temperature.
3. A process as claimed in claim 2, wherein the recirculated crystals are passed from the zone of highest to the zone of lowest temperature.
4. A process as claimed in any of claims 1 to 3, wherein the crystals formed in each of said zones are passed countercurrently and successively through all of the zones with a temperature higher than that of said zone.
5. A process as claimed in any of claims 1 to 3 wherein said liquid is fed in the zone of highest temperature.
6. A process as claimed in any of claims 1 to 3 wherein said one crystallizable component to be separated out is an inorganic compound, and said liquid containing a plurality of components its an aqueous solution.
7. A process as claimed in claim 1 wherein said one crystallizable component to be separated out is an organic compound and said liquid containing a plurality of components contains at least one other organic compound.
8. A process as claimed in claim 7 wherein said liquid is fed in a zone intermediate between the zone of highest and the zone of lowest temperature and a melt of said one component is passed in a direction cocurrent with the direction of the stream of said liquid through the zone(s) with a temperature higher than that of the zone wherein said liquid is fed.
9. An apparatus for separating one crystallizable compo-nent from a liquid containing a plurality of components, which comprises means for passing a stream of said liquid successively through at least part of a series of zones of successively lower temperature including the zone of lowest temperature so as to induce the formation of crystals of said one component in each of said zones, means for passing the crystals formed in each of said zones in a direction counter-current to the direction of the stream of liquid through at least part of said zones, including the zone of highest tem-perature, means for separating the crystals present in the zone of highest temperature from their mother liquor and re-covering at least part of these crystals, means for separating the crystals in the zone of lowest temperature from their mother liquor and recovering that mother liquor, and means for recirculating part of said crystals present in the zone of highest temperature, to the zone of lowest temperature.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000381707A CA1195918A (en) | 1981-07-14 | 1981-07-14 | Process for countercurrent crystallization with recirculation |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000381707A CA1195918A (en) | 1981-07-14 | 1981-07-14 | Process for countercurrent crystallization with recirculation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1195918A true CA1195918A (en) | 1985-10-29 |
Family
ID=4120435
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000381707A Expired CA1195918A (en) | 1981-07-14 | 1981-07-14 | Process for countercurrent crystallization with recirculation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1195918A (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5536512A (en) | 1993-03-23 | 1996-07-16 | Labatt Brewing Company Limited | Improvements in production of fermented malt beverages |
| US5695795A (en) | 1993-03-23 | 1997-12-09 | Labatt Brewing Company Limited | Methods for chill-treating non-distilled malted barley beverages |
| US5869114A (en) | 1994-03-18 | 1999-02-09 | Labatt Brewing Company Limited | Production of fermented malt beverages |
| USRE36897E (en) | 1993-03-23 | 2000-10-03 | Labatt Brewing Company Limited | Methods for chill treating non-distilled malted barley beverages |
-
1981
- 1981-07-14 CA CA000381707A patent/CA1195918A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5536512A (en) | 1993-03-23 | 1996-07-16 | Labatt Brewing Company Limited | Improvements in production of fermented malt beverages |
| US5695795A (en) | 1993-03-23 | 1997-12-09 | Labatt Brewing Company Limited | Methods for chill-treating non-distilled malted barley beverages |
| USRE36897E (en) | 1993-03-23 | 2000-10-03 | Labatt Brewing Company Limited | Methods for chill treating non-distilled malted barley beverages |
| US5869114A (en) | 1994-03-18 | 1999-02-09 | Labatt Brewing Company Limited | Production of fermented malt beverages |
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