Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1986-07-10
1987-09-29
Schwartz, Larry I.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
134 10, 134109, F26B 516
Patent
active
046961143
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the separation of solid materials from a liquid wetting the materials. In particular, it relates to the economical drying of solid materials by displacement of their wetting liquid with a second wetting liquid, the drying of which is more economical than the drying of the first liquid.
In many important industrial processes, it is necessary to separate solid material, before its further processing, from a liquid phase which wets the material. Commonly, the wetting liquid is water, which presents an especially expensive problem; because the boiling point and specific and latent heats of water are relatively high, its removal is ordinarily quite energy-intensive.
Prior methods have included filtration and the application of heat to dry the particles. But where direct drying is applied, the volume of combustion gases causes particle entrainment and environmental problems. Should the surface-area-to-mass ratio be great, as with finely divided particulate matter, the high cost of energy and environmental pollution of drying becomes a significant consideration, because water constitutes a substantial proportion of the mass of the wet solids. It is to this problem especially that the invention is directed.
If the water is washed from the solids with a solvent in which it is miscible, the solvent-wetted solids may be more economical to dry, but much energy cost may reappear with the necessity of separating the water from the solvent rinse.
SUMMARY OF THE INVENTION
According to the invention herein, a first liquid wetting a solid substance is displaced entirely by a second liquid immiscible with the first and more economical to remove.
Hereinafter, the first liquid will be referred to as "water" and the second as "solvent," for convenience. It must be understood that the process is more general. The second liquid should be immiscible with the first, and of course the solid should be essentially insoluble in it.
It will of course be recognized that in general all liquids will have some solubility in each other. Thus, by the term "immiscible" it is meant that the liquids under discussion form two well-defined phases when mixed in the proportions necessary for the practice of the invention, though each will have a (usually small) proportion of the other dissolved in it. By "solvent" or "displacing liquid" a displacing liquid is meant which may contain water or the original liquid to the limits of its solubility. In most cases, as will be seen in the description to follow of the preferred embodiment, it will be most energy efficient to use a displacing liquid such that the solubility of water in it is relatively low. However for particular applications, other considerations may outweigh energy efficiency, and the use of the invention is not limited to these cases.
If the solvent is chosen to have low specific and latent heats, with a low boiling point, compared to water, its evaporation by conventional means will require much less heat input, and the energy savings in drying the solid will be substantial.
One part of the preferred embodiment of the invention is the means for achieving the displacement of water by the solvent. This displacement would otherwise be difficult, as the bond between the solid surface and water is difficult to break, particularly when the solid surface is porous. Assuming that the solid is denser than water, as is usual in industrial applications, the solvent is chosen so that its boiling point is substantially lower than that of water, but its density is greater than that of water and preferably less than that of the solid. The wet solids are heated to a temperature which is less than the boiling point of water but considerably above the boiling point of the solvent. Introduction of the heated wet solids to the solvent causes a controlled but violent boiling action that causes the water surrounding the solids to be displaced by the solvent. Then a separation of phases takes place, yielding an upper layer of solvent-saturated water (ordinarily along with some entra
REFERENCES:
patent: 4599807 (1986-07-01), Wells et al.
Hochberg D. Peter
Schwartz Larry I.
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