Patent Application
20140230810
Provided are apparatuses and methods for crystallization. More particularly, apparatuses and methods are provided for efficiently mixing and cooling heavy massecuites.
Crystallization of solids from solution takes place from a supersaturated solution of the material, such as sucrose. Maintenance of supersaturation may be achieved by (i) removal of solvent by evaporation, or (ii) by cooling the solution. The first is standard practice in the sugar industry, although there are advantages to the general application of cooling crystallization. Supersaturated sugar solutions are viscous, and efficient cooling crystallization depends upon both good heat transfer and good mixing of the massecuite (mixture of crystals and the mother liquor). Up to now, equipment capable of meeting optimal heat transfer and mixing requirements has not been available.
Cooling crystallization has been used in the sugar industry for quite some time, but has not been the focus of much development. It has been considered of little importance where it is only applied to massecuites obtained via evaporative crystallization before their centrifugation, where it tends to be concentrated on the final crystallization stages; whereas front end exhaustion by cooling is given low priority, and little or no attention is paid on white sugar massecuite production. Historically, there have been limits to high purity massecuite exhaustion since, as the crystal content by mass increases, the massecuite becomes excessively viscous and leads to several mechanical problems: the massecuite cannot be removed from the pan and crystallizer in a reasonable time; the centrifugal cannot be properly and evenly loaded; the crystallizer cannot handle the viscous massecuite; and excessive sugar washing is required to reach sugar target purity, resulting in a purity rise across the centrifugal, which negates the benefits of additional crystallization. Additionally, traditional crystallizers have a tendency of having stagnant regions of massecuite that result in build-up of material in parts of the equipment.
Thus, there is a need in the art for apparatuses and methods for crystallization that are capable of increasing crystal content in massecuites in an effective and efficient manner while avoiding many of these downfalls.
In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to apparatuses for crystallization. An exemplary apparatus includes a vessel defining an interior having a lower portion and an upper portion, a draft tube positioned in the interior of the vessel and extending between the lower and upper portions. The draft tube can be spaced from the inner walls of the vessel, thereby defining an annular space between the draft tube and the side wall of the vessel. The apparatus can also include a rotor shaft extending along a central axis of the vessel, and a helical screw; the helical screw can be positioned in the interior of the vessel and within the draft tube and can be attached to and configured to rotate with the rotor shaft.
According to other embodiments, a multi-stage apparatus for crystallization is provided that includes a vessel having a central axis and defining an interior having a lower and upper portion. The apparatus can further include a first stage barrier wall that extends perpendicularly to the central axis of the vessel, the first stage barrier wall being positioned between the lower and upper portions and separating the vessel into a first stage and second stage, while providing at least one fluid passageway between the first stage and the second stage. A first draft tube can be positioned in the first stage of the vessel and can define an annular space between the first draft tube and the side wall of the vessel. Similarly, a second draft tube can be positioned in the second stage of the vessel and can define an annular space between the second draft tube and the side wall of the vessel. A rotor shaft can extend along the central axis of the vessel and can extend through the first and second stages of the vessel. A first helical screw can be positioned in the first stage of the vessel and within the first draft tube, and a second helical screw can be positioned in the second stage of the vessel within the second draft tube. The first and second helical screws can be attached to and configured to rotate with the rotor shaft. According to a further embodiment, the first and second helical screws can have opposite pitches.
According to yet other embodiments, a method for crystallization is provided that includes supplying process material to the vessel interior of apparatuses described herein, moving the process material through a central portion of the vessel, circulating at least a portion of the process material through an annular space in the vessel, and discharging at least a portion of the process material through an outlet of the vessel. For example, the method can include rotating the rotor shaft and helical screw to draw the process material from the lower portion to the upper portion of the vessel within the draft tube, circulating at least a portion of the process material through the annular space between the draft tube and side wall of the vessel, and discharging at least a portion of the process material through the outlet.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects of the invention and together with the description, serve to explain the principles of the invention.
The present invention may be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.
As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “stage” can include two or more such stages unless the context indicates otherwise.
Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
Reference will now be made in detail to the present preferred aspects of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.
Provided are apparatuses and methods for crystallization, which can be used, for example, as part of a sugar production system.
An optional exemplary system for sugar production that combines cooling crystallization with evaporative crystallization and backblending is shown in
TABLE 1 shows the theoretical results of a pure sucrose massecuite produced in a pan, assuming that the mother liquor is saturated at 70° C. TABLE 1 also shows the results of the same massecuite, under the following conditions: (1) the mother liquor is saturated at 70° C., followed by a cooling crystallization process, reaching a saturated mother liquor at 30° C.; (2) the massecuite is lubricated in the crystallizer with cold undiluted (green) saturated mother liquor at 30° C., as shown in
Referring now to
The vessel 120 comprises an inlet 9 for introduction of process material into the interior of the vessel. In one embodiment, the inlet can be positioned to be in direct fluid communication with the lower portion of the vessel. The vessel can also include an outlet 8 for discharge of the process material. The outlet can be positioned to be in direct fluid communication with the upper portion of the vessel, as shown in
In a further embodiment, the apparatus 100 comprises at least one draft tube, such as draft tubes 3, 3a, shown in
The apparatus 100 can also include a rotor shaft 16 that extends along the central axis y of the vessel, and at least one helical screw positioned in the interior of the vessel and within the draft tube. The helical screw is attached to and configured to rotate with the rotor shaft.
As shown in
A second stage barrier wall 6a can be provided to separate the second stage 2a from the upper portion 7 of the vessel. The second stage barrier wall 6a can be attached to and extend outwardly from the rotor shaft 16, as shown in
As described briefly above, the apparatus can include at least one helical screw that is attached to and configured to rotate with the rotor shaft. For example, in the multi-stage apparatus 100 shown in
In yet a further embodiment, in the case of a multi-stage apparatus, the first helical screw 4 and second helical screw 4a can have opposite pitches. As can be appreciated, when the rotor shaft rotates, the helical screws rotate in the same direction, but because of their opposing pitches, they effect fluid movement through the respective draft tubes in opposite directions from each other (as can be seen in
Each helical screw has a respective flight diameter that in one embodiment is less than the inner diameter of a respective draft tube. In one embodiment the flight diameter is only slightly less than the inner diameter of the draft tube, such that only a small clearance is provided between the draft tube and the helical screw outer edges. For example, and not meant to be limiting, the flight diameter can range from approximately 93% to approximately 95% of the inner diameter of the draft tube. In yet a further embodiment, the pitch of each helical screw can be approximately half of the flight diameter.
According to other embodiments, the apparatus comprises heat exchange means for supplying heat to, or extracting heat from, the vessel. Whether the heat exchange means supplies heat to or extracts heat from the vessel would depend upon the preferred operating temperature of the vessel with at least one heat exchange means inlet port 12, 12a and at least one heat exchange means outlet port 13, 13a. For example, a jacketed housing can be provided to surround at least a portion of an apparatus component. As shown in
In other embodiments, temperature sensing means can be provided for sensing the temperature within at least a portion of the interior of the vessel. As shown in
According to various embodiments, at least one lubricant inlet port 15, 15a can be provided to provide lubrication to each stage of the apparatus. The lubricant can be added to the process material or massecuite within the vessel in a technique known as backblending. This technique involves lubricating the massecuite with green runoff. Green runoff is primarily undiluted mother liquor, and white runoff is made up of some mother liquor together with washing from sugar plus centrifuge basket cleaning. In other embodiments, a liquid outlet port 10 can be provided to allow for the discharge of liquid from the apparatus.
Although described above with reference to a two-stage apparatus 100 in vertical arrangement, such as shown in
It has been found that apparatuses for crystallization as described herein closely approach perfect mixing, with the process material entering the stage rapidly assuming a final uniform temperature and composition. The exit stream of process material from the stage has approximately the same temperature and composition as the process material within the stage. Within each stage, there is no build-up of stagnant process material along the vessel side wall or stage barrier walls. As discussed above, several stages can be connected in series to obtain a close approach to plug flow with all the process material entering in the first stage having the same average residence time at the outlet of the last stage.
Methods are also provided for crystallization using the apparatuses described herein. The method can include supplying the process material to the interior of a vessel, and moving the process material through a central portion of the vessel. At least a portion of the process material can be circulated through an annular space in the vessel. At least a portion of the process material can also be discharged through an outlet. More specifically, with reference to the apparatuses described herein, a single-stage crystallizer (such as shown in
Thus, it can be appreciated that a multi-stage apparatus can be used to perform a method for crystallization. Again with reference to
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the apparatuses, systems and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., frequency measurements, etc.), but some errors and deviations should be accounted for.
A 1 ft3 batch crystallizer was manufactured, similar to the single-stage crystallizer described above. Three thermocouple probes were mounted at different levels within the vessel, at different radial distances from the rotor shaft, and on different sides of the vessel. These positions were selected to give as much information as possible about the mixing in the vessel. The process material selected was final massecuite, which is the material with the highest viscosity encountered in the sugar industry.
During the cooling and heating processes of the process material selected, the temperature at different points of the vessel were monitored over time and the probe outputs were recorded and plotted as shown in
Thermocouple-based mixing tests were performed on the same batch unit with the same process material. The technique was based on the addition of a portion of the process material that has a different temperature from the bulk. Data collected by the thermocouples were processed to obtain a characteristic mixing time for the system under investigation. The data was first normalized to eliminate the effect of different probe gains. The mixing time is defined as the time required for the normalized probed outputs to read and remain between 99% and 101% (i.e., ±1%) of the final equilibrium temperature.
With reference to
Exhaustion of massecuites of different purities was carried out in batch operation in two crystallizers, made according to the single-stage crystallizer described above, sized to 1 ft3 and 12 ft3. A known amount of massecuite was taken from the pan at the strike, and was placed into a respective crystallizer. For high purity massecuite, to remove the excessive crystal content that is achieved when the mass is cooled and exhausted; a fraction of undiluted mother liquor was added. The massecuite was preheated to the strike temperature, and then cooled down following a scheduled temperature profile. Undiluted mother liquor samples were obtained by withdrawing a small quantity from the crystallizer and separating the mother liquor through C-centrifugal screen mesh, and then massecuites and nutsches were analyzed. The results are shown below for the different kind of massecuites:
Results: Mill High Purity Massecuite (1 ft3 Crystallizer)
Results: Mill High Purity Massecuite (12 ft3 Crystallizer)
Results: Mill Medium Purity Massecuite (1 ft3 Crystallizer)
Results: Mill Medium Purity Massecuite (12 ft3 Crystallizer)
Results: Mill Low Purity Massecuite (1 ft3 Crystallizer)
Results: Mill Low Purity Massecuite (12 ft3 Crystallizer)
Results: Recovery House High Purity Massecuite (12 ft3 Crystallizer)
Results: Recovery House Medium Purity Massecuite (1 ft3 Crystallizer)
Results: Recovery House Low Purity Massecuite (1 ft3 Crystallizer)
The results show that with both crystallizers (1 ft3 and 12 ft3), very close to the limit crystal content for both high and low purity grade massecuites were achieved. There was significantly improved heat transfer and mixing, and the time required to achieve the desired purity drop was much shorter than in conventional crystallizers. It was determined that evaporative crystallization, combined with cooling crystallization and backblending, can be used to reduce and simplify the number of crystallization stages in a raw sugar boiling scheme.
Taking into account the experimental data shown above, material and energy balances were calculated for a raw sugar double Einwurf four boiling scheme and for a two boiling scheme combined with cooling crystallization and backblending on high and low purity massecuites. Results are shown below.
By comparing the results, the advantages of the two boiling system can be seen as follows:
Mass and energy balances were done for a four boiling white sugar scheme, and for a two boiling white sugar scheme with cooling crystallization and backblending.
TABLE 2 shows that if a two boiling white sugar scheme with cooling crystallization and backblending is compared with traditional four boiling white sugar scheme, the required pan capacity is 27% lower, the required centrifugal capacity is 3.9% lower, the required crystallizer capacity is 100% higher, and steam demand is 20% lower.
Materials and energy balances for the potential application of the “pure” cooling crystallization in white sugar production, using the apparatus for crystallization shown in
It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other aspects of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
This application is a non-provisional of U.S. Provisional Application No. 61/766,994 filed Feb. 20, 2013, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61766994 | Feb 2013 | US |
