Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1990-09-24
1992-12-22
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 92, 34 15, 159 21, 203 88, F26B 506
Patent
active
051724871
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for drying of a substantially particle-free, syrup-like material and an assembly for carrying out said method.
BACKGROUND ART
It is often difficult to remove the remaining water when preparing products, such as sugars, sugar alcohols and similar sensitive as well as hygroscopic products in dry form, such as in powder form.
One of the reasons is the high content of compounds with hydrophilic groups in most of the above materials, rendering the products hygroscopic.
Another reason is the tendency of some of said materials to form supersaturated solutions, wherefrom it is difficult to precipitate and isolate solid products.
Supersaturated and other concentrated solutions are very often highly viscous. Consequently they are difficult to handle and have the tendency to stick to the apparatus.
All these properties further impede the removal of water, e.g. by evaporation or drying, since problems arise during the heating of the material. An equal distribution of heat is, for example, not ensured, thus risking local overheating. During the heating the material can be destroyed or deteriorate e.g. by burning, carmamelization, denaturation or another form of decomposition.
Several methods for drying products in order to remove the remaining water are known, such as spray drying, drum drying, freeze drying or flash drying.
For spray drying, the solution to be dried is fed into a chamber in the form of tiny drops. The falling drops are dried by means of hot air so that the drops are transformed into a dry powder before they reach the bottom of the chamber. Spray drying cannot be used if the solution has the tendency to remain liquid, either as an supersaturated solution or in form of a melt, during the drying process, where the temperature is usually about 60.degree. C., since the material accumulates on the walls of the spray drying chamber.
Drum drying is normally performed at temperatures about 100.degree. C. At this temperature many materials occur in form of a melt and thus they cannot be transformed into a solid product. During drum drying the product accumulates on the warm surfaces, causing overheating with subsequent destruction or deterioration of the material.
With conventional flash drying water is removed almost instantaneously from wet, solid particles, said particles being dispersed at high speed in a warm stream of gas. In flash drying the temperature of the drying air is above 100.degree. C., rendering this drying method unsuitable for drying heat-sensitive products.
It is evident that neither spray drying, drum drying nor flash drying are suitable for the preparation of solid, dry products, such as some sugars and some sugar alcohols, other heat-sensitive products and/or those difficult to crystallize.
The only known methods suitable for drying such materials are freeze drying and microwave vacuum drying. These methods are, however, expensive, since their operational costs are high, especially with regard to energy consumption and capital costs.
The German Offenlegungsschrift No. 34 07 374 discloses a method for preparing dried products from sucrose syrup. According to this method the pre-concentrated sucrose syrup with a dry matter content of at least 70% is heated for a short period, such as below 60 sec, to a very high temperature, and the warm material is expanded to a concentrated syrup with a dry matter content of at least 90%. This syrup is transformed into a dry, pourable product by sudden cooling and subsequent release of the remaining water during crystallization.
This method is limited to easily crystallizable materials with a positive enthalpy of crystallization, i.e. materials crystallizing during cooling. The method is consequently unsuitable for amorphous products and other materials difficult to crystallize. Furthermore the material is subjected to high temperatures of about 135.degree.-155.degree. C., thus rendering the method unsuitable for heat-sensitive materials.
U.S. Pat. No. 1.250.496 discloses a process for drying g
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Andersen Torben
Hansen Ole C.
Bennet Henry A.
Danisco A/S
Gomada Denise
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