Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having growth from a solution comprising a solvent which is...
Reexamination Certificate
2002-11-18
2004-11-23
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having growth from a solution comprising a solvent which is...
C117S069000, C117S070000, C117S925000, C117S926000, C422S245100, C023S29300R
Reexamination Certificate
active
06821339
ABSTRACT:
The present invention relates to a process for the crystallization of substances having a narrow metastable supersaturation zone. The invention especially relates to a controlled crystallization of such compounds so as to provide a good crystal yield and a crystalline product having a uniform crystal structure and a narrow crystal size distribution.
The invention is useful for controlling the crystallization of substances for which hitherto no clear model for the crystallization is known and which hence are often crystallized in a very uncontrolled way. The invention is especially adapted to the crystallization of compounds with a narrow metastable supersaturation zone, such as betaine, potassium chloride, ammonium chloride, and the like.
Common sugar or sucrose is one of the most commonly crystallized compounds in industry and an enormous amount of work has been put into developing models and automation techniques for the crystallization of sucrose. It is well known in the art that there is a large number of parameters which influence the crystallization processes either separately or in intertwined combinations with each other.
The two principal steps of crystallization are the formation of crystal seeds (nucleation) and crystal growth. In most industrial processes crystallization is primarily based on crystal growth. The state of the art with respect to crystallization is represented, for instance, by Mathlouthi, M. and Reiser, P. (ed), Sucrose, Properties and Applications, Blackie Academic Professional, Suffolk, Great Britain, 1995, p.49 ff. This document explains the crystallization mechanism with respect to both nucleation and crystal growth. With respect to the industrial crystallization of sucrose the document states, for instance, that one must avoid concentrating the solution up to the nucleation zone, i.e. the zone whereat spontaneous nuclei formation readily occurs (p. 58); one must avoid the creation of an uncontrolled number of seeds (p. 59); and the crystallization should be carried out in the metastable zone not too close to the nucleation zone and the saturation curve (p. 60 to 61 and 63 to 64).
The metastable zone is the zone where spontaneous crystal formation will occur in the solution only if crystals are already present. The crystallization of sucrose is thus advantageously performed well within the metastable zone. Since sucrose, like most organic compounds, especially carbohydrates, is a compound which has a fairly broad metastable zone, this allows the crystallizer to monitor the supersaturation for selecting a suitable seeding point within said zone and for controlling the progress of the crystallization also within said zone.
A process for crystallizing organic compounds from their solutions having a high viscosity and supersaturation by spontaneous or active seeding and subsequent cooling is described in the same applicant's WO publication 9627029. Said invention aims at providing crystallization without any significant crystal growth.
Seeding of the solution is normally utilized in crystallization since the seeds introduced into the solution provide nucleation centers around which the actual crystal growth primarily occurs. Providing a multitude of seeds into the solution will minimize spontaneous nucleation and will result in a high crystal yield with a uniform structure and a narrow crystal size distribution.
Seeding of sucrose solutions forms one part of the common control techniques used in sucrose crystallization. Publications describing techniques for the seeding of sucrose solutions are, for instance,
Madsen, R. F., Vacuum pan automation and sugar house computer control, Paper presented at the 23rd Tech. Conf., British Sugar Corp., 1976, which describes means for measuring whether the massecuite in a pan has the right conditions for seeding. Such means comprise measuring the electrical conductivity, the rheology or the refractive index; DE 19535017, which describes cooling of a sucrose solution until a supersaturation of 1.2 to 1.3 has been obtained and subsequent introduction of air bubbles into said sucrose solution;
SU 798565 describes monitoring spontaneous crystal formation in a separate cooled circulation tube and seeding a sucrose solution within the metastable zone in response to a difference observed in the light passing through said tube and a reference tube. The crystal formation in the cooled tube starts between 3 and 10 minutes before any spontaneous nucleation occurs in the actual crystallizer.
In the prior art very few descriptions are found on how to control the seeding of other compounds than sucrose, i.e. non-sucrose compounds. Although some non-sucrose compounds may mimic sucrose in their nucleation and crystallization behaviour, it is evident that some others will behave very differently. It is not clear to those skilled in the art how a non-sucrose compound crystallization should be controlled.
The techniques for seeding sucrose solutions naturally take into account the need to seed the solution well within the rather broad metastable zone. For compounds with a narrow metastable zone the seeding and the subsequent crystallization is much more difficult to control since in most cases the measurements indicating that the saturation has reached the metastable zone are not exact enough for allowing time to seed before excess spontaneous seeding has taken place. The solution can be analyzed and monitored for parameters such as purity, concentration, density, viscosity, conductivity, boiling point elevation, refractometric index, etc. These parameters will indicate when the solution is getting close to saturation and thus close to the correct seeding point. However, for compounds with a narrow metastable zone monitoring these well known parameters has not been found to give an exact enough information for allowing prediction of the correct seeding point. In many cases the calibration accuracy of the instruments is not exact enough and the crystallization parameters vary from batch to batch so that predetermination of the seeding point based on these measurements is difficult if not impossible. Proper seeding in such cases requires extreme skills and apparatus-bound experience by the operator.
The result of an incorrect seeding is that the crystals obtained will most likely be non-uniform in structure and may have a wide crystal size distribution making the efficient separation of the crystals from the mother liquid difficult. An uneven size distribution also provides a product with poor final crystal quality. The crystal size distribution affects product parameters such as the free-flowing properties, hygroscopicity, bulk density, etc.
The present invention strives to overcome the problem of crystallization control of substances having a narrow metastable zone.
The present invention provides a means for controlling the seeding of non-sucrose compounds. It is especially adapted for controlling the seeding of non-sucrose compounds having a very narrow metastable supersaturation zone.
The present invention is defined in the appended claims, which are incorporated as such into the present specification. The present invention relates to a process for the crystallization of a substance having a narrow metastable supersaturation zone, wherein the saturation of a solution of said substance is gradually increased and said solution is seeded for the crystallization. The invention is characterized in that seeding is performed at a seeding point which is selected in response to a signal received from said process indicating imminent or initial spontaneous nucleation, said seeding initiating a controlled crystallization based on crystal growth around the nuclei provided by said seeding and enabling the production of a crystalline product having a uniform crystal structure and a narrow crystal size distribution.
The crystallization after seeding may be monitored in a conventional way or may be subjected to specific conditions required by the substance to be crystallized. Such crystallization conditions will be readily selected by thos
Eriksson Kristian
Nurmi Juha
Virtanen Jouko
Finnfeeds Finland Oy
Kunemund Robert
Scully Scott Murphy & Presser
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