Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1998-12-22
2002-01-01
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C023S30200R, C423S206100, C423S427000
Reexamination Certificate
active
06334989
ABSTRACT:
The invention relates to a process for producing anhydrous soda (sodium carbonate, Na
2
CO
3
) with a bulk density of between 1300 and 1600 kg/m
3
, generally by means of crystallization of soda, optionally after or during calcination of sodium bicarbonate, from a composition comprising an aqueous medium, sodium carbonate, optionally sodium bicarbonate, and a water-miscible organic solvent.
Such a process is described by D. A. Weingaertner, et.al. in
Ind. Eng. Chem. Res
., Vol. 30, No.3, 1991, pp. 490-501. More specifically, it is disclosed that a hot, dry, and (partially) water-miscible solvent is added to an aqueous medium containing sodium carbonate to extract water, The extraction of water from the medium causes (part of) the soda to crystallize. The soda is filtered off, the wet solvent is recycled into dry solvent, and the filtrate is recycled to the feed. To ensure that anhydrous soda is produced, the temperature is kept above the monohydrate transition temperature of 109° C. as long as the soda is in contact with water. Because this temperature is above the boiling point of the composition at atmospheric pressure, the whole process is conducted under positive pressure. Before the pressure is released, the soda crystals are washed with hot solvent to further remove water. Additionally, the crystals are dried in such a fashion that all liquid adhering to them is removed before the temperature drops below 109° C., to eliminate all possibility of crystal hydration.
This process, if conducted at temperatures below 140° C., requires the use of large amounts of solvent, because at such temperatures the organic phase contains a maximum of 35% of water. To avoid the use of such large amounts of solvent, the temperature is raised to 180° C. and higher. The pressure in the crystallization, washing, and filtration equipment will, consequently, be high.
The use of large amounts of solvent during the crystallization is undesired from both an economical and a handling point of view. A high temperature during crystallization is undesired because of the energy requirements associated therewith. High pressure in the crystallizer and work-up section is undesired because it requires the use of complicated equipment and makes control and handling of the process more difficult, Also, the washing step is undesired because it increases the overall processing time and requires the use of extra solvent, with the associated disadvantages.
Therefore, there is a need for a process for making anhydrous soda with a high bulk density that does not require the use of large amounts of solvent, high temperatures, high pressures, nor, preferably, washing steps.
During our investigations it was found that all of the undesired process conditions and/or process steps are related to the underlying problem that there is no feasible process for crystallizing and/or handling anhydrous soda in an aqueous medium at atmospheric pressure. In conventional processes, the boiling point (at atmospheric pressure) of the aqueous medium from which the soda is precipitated is lower than the monohydrate transition temperature. Further study led us to a new and surprisingly simple process for making anhydrous soda with a high bulk density wherein the crystals can be treated and processed at atmospheric pressure and in the presence of water, without monohydrate being formed.
The new process is characterized in that at least part of the process, when both sodium carbonate and water are present, is conducted at atmospheric pressure while using from 1 to 50 percent by weight, based on the weight of the total composition, of at least one water-miscible organic solvent to increase the boiling point of the composition to above the monohydrate transition temperature of soda in said composition.
It is noted that GB-A-2 024 187 discloses that in an aqueous medium comprising soda, 10 to 22% by weight of sodium chloride can be used to increase the boiling point of the medium and to lower the monohydrate transition temperature, so that the boiling point temperature becomes higher than the transition temperature. However, the use of salt is highly undesired because it contaminates the anhydrous soda and because the hot, sodium chloride-containing solution is highly corrosive, leading to numerous problems in commercial installations. GB-A-2 024 187 does not suggest using water-miscible solvents instead of sodium chloride.
It is noted that U.S. Pat. No. 4,183,901 discloses a process to make anhydrous soda wherein 25 to 5000 parts per million by weight of an additive selected from the group consisting of specific maleic-acid-alkanol telomers, sodium tripolyphosphate, benzoic acid, phenol, and mixtures thereof are used. Said additives reduce the temperature of transition of sodium carbonate monohydrate to crystalline anhydrous sodium carbonate and are preferably used in such small proportions that no detectable change in boiling point is observed.
For the sake of good order, it is mentioned that the person skilled in the art will understand that the solvent which is used in the new process must fulfill a number of requirements if its use is to be allowed, especially in commercial processes. Besides meeting requirements in respect of safety, environmental, and economical concerns, the solvent preferably also does not adversely influence: a) the formation and growth of soda crystals by acting as a nucleation or growth inhibitor, b) the solubility of the soda in the aqueous phase, and c) the viscosity of the crystallizing mixture. Furthermore, the solvents preferably d) have a sufficiently high boiling point, e) are acceptable in trace amounts in soda, because such a contamination of the soda can hardly be avoided, and f) are stable under wet calcining conditions of up to 150° C. By “stable under wet calcining conditions” is meant that the solvent, under such conditions, essentially does not hydrolyze, does not react with other compounds in the composition, and does not undergo cyclization or ring-opening reactions. In a process wherein crystallization takes place during wet calcination, such a crystallization/calcination step generally precedes one or more further steps which can be conducted at atmospheric pressures and in the presence of water, without soda monohydrate being formed.
Without wishing to be restricted to such a theory, Applicant believes that by using a solvent in the process according to the invention, the monohydrate transition temperature of the soda is lowered. At the same time, the boiling point of the aqueous medium will depend on various parameters such as the types and concentrations of the compounds in it. Increasing the amount of appropriate solvent in the aqueous medium increases the boiling point, In order to avoid the necessity of using large amounts of solvent, which is required when the monohydrate transition temperature has to be lowered too much, the boiling point of the solvent preferably is above 80° C. More preferably, the boiling point of the solvent is above 95° C., while it is most preferred to use a solvent with a boiling point above 100° C. If the solvent forms an azeotrope with one or more of the other compounds in the aqueous medium, the boiling point of the azeotrope is to be substituted for the boiling point of the solvent.
In a further preferred embodiment of the process according to the present invention, the mother liquor comprising water, solvent, soda, and contaminants which remains after crystallization of the soda is recycled to the feed of the soda process. Such existing or desired recycle streams in the total process (e.g., the filtrate of the crystals) may require that (part of) the solvent be recycled to the calcination step of the process. Hence, it is foreseen that the solvent is present during the calcination step. If such recycle streams are indeed present, certain contaminants that are present in the feed can build up, since, ideally, only anhydrous soda is removed. To remove such contaminants, preferably (part of) the recycle stream is purged or further treated. In the latter case,
Geertman Robert Michael
Meijer Johannes Albertus Maria
Oosterhof Harald
Van Rosmalen Gerda Maria
Witkamp Geert-Jan
Fitch Even Tabin & Flannery
Griffin Steven P.
Medina Maribel
Technische Universiteit Delft
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