RU2077006C1 - Method of drying dissolved or suspended substances - Google Patents

Abstract

FIELD: food, chemical and light industries; drying of dissolved or suspended substances in various solvents. SUBSTANCE: dissolved substances are extracted from solutions by heating them to temperature 10 to 20 C below their boiling point or decomposition temperature; this stage is performed at atmosphere or increased pressure. Then solution (suspension) is additionally heated by 30 to 40 C and is subjected to vacuum treatment. Since dried substances are subjected to elevated temperature within short period of time, they are not subjected to thermal decomposition or destruction. EFFECT: enhanced efficiency. 1 dwg, 3 tbl

Description

 The invention relates to methods for drying solutions or suspensions, in particular, to methods for extracting substances from them in a dissolved or suspended state by removing solvents.

 Quite often, it is difficult to extract from solutions the compounds present in them, such as sugar, some salts, soluble polymers, higher alcohols, i.e. sensitive to the effects of elevated temperatures or hygroscopic substances. In addition, it causes difficulties in working with oversaturated and some highly concentrated solutions, which tend to stick to the walls of the equipment. During evaporation and drying, it is difficult to achieve a uniform temperature distribution throughout the volume of the solution, as a result of which there is a danger of local overheating. In these cases, there is a possibility of destruction of the recovered compounds, for example, carbonization, caramelization, etc. In such cases, the usual methods of drying products are not applicable.

 Given the characteristics of the products, special types of drying are used. Thus, numerous specially developed methods for removing solvents and extracting dry products are known, for example, spray drying, freeze drying and vacuum drying.

A known method of drying dispersed materials by heating and subsequent evacuation, and the heating cycle, the evacuation is repeated many times and the depth of the vacuum from the cycle of the cycle increase [1]
In technological processes, very often substances are at a temperature at which their thermal decomposition occurs. In this situation, one of the most important tasks is to reduce the residence time of a substance at a given temperature, which helps to improve the quality of the final product. Moreover, given that the decomposition rate is exponentially dependent on temperature, an increase in temperature should be accompanied by a reduction in the residence time of the substance at a given temperature.

The closest solution to the proposed one is the method of drying in apparatus, including evacuation, which is carried out in a vacuum chamber equipped with a heating device located inside it. This drying method does not exclude local overheating of the dried product and is complicated in technological design [2]
The challenge facing the developers of the proposed technical solution is to create a universal drying method that is practically suitable for working with any aqueous or organic solutions, as well as for drying suspensions. In this case, the quality of the product should not be deteriorated due to destruction or spoilage. The process should be feasible in relatively inexpensive equipment, and provide a soft and short-term effect on the product.

 The essence of the proposed technical solution consists in the following: the solution or suspension to be treated is subjected to 2-stage heating under atmospheric pressure, which is created by nitrogen or another gas inert to the solution. The pressure value is selected for each solution individually, depending on its viscosity, and should be sufficient for the solution to pass through dispenser 2, see drawing.

In the first stage, the solution (suspension) is heated to a temperature that is 10 20 ^o C below its boiling point or possible decomposition at atmospheric pressure. At this stage, the solution may remain for several hours (1 to 5 hours).

Then the solution passes to the second stage, where it is quickly heated for 5 120 s to a temperature 30-40 ^o C above its boiling point at atmospheric pressure. The pressure in this chamber is set spontaneously depending on its magnitude in stage I.

 From the second stage, the product is dosed into a vacuum chamber, the pressure in which is maintained at the level of 0.1 0.005 atm, working without supplying heat from outside, the temperature in which is set spontaneously due to the heat of the supplied product. Once in this chamber, the product “boils”, solvent is removed from it, which is discharged through the refrigerator 4.

 The dry product in the form of a powder is discharged through an air lock into a receiver 5.

 The drawing shows a General installation diagram.

The differences of the proposed technical solution from the prototype method are as follows:
the dried material for a very short time (not more than 2 minutes, depending on its physicochemical properties) is in conditions that are dangerous from the point of view of its thermal decomposition (chamber 2);
heating in chambers 1 and 2 is carried out under pressure from 1 to 5 atm, which is provided by the supply of inert gas;
the residence time in the vacuum chamber does not exceed 1 s;
The developed method has several advantages, namely:
allows you to dry substances that are dissolved in aqueous or organic solutions of almost any concentration (from 5 to 95%).

 it is possible to dry both dissolved and suspended substances.

 preliminary evaporation of solutions to a certain concentration is not required.

 The method is applicable for evaporation processes.

Technological design of the method is relatively simple. It is carried out on the installation depicted in the drawing, consisting of a tank 1, into which the initial solution is supplied. This tank is equipped with a heater, which can be made in any form, eliminating local overheating of the product. The dimensions of the tank 1 are calculated so that it is possible to maintain a pressure of about 6 atm in it, and the material is inert with respect to the working solution. From the tank 1, the solution is supplied to the metering tank 26, where the product quickly, for 5 120 s, is additionally heated to 30 40 ^o C, and dosed into the vacuum chamber 3, which works without heating. The pressure in the vacuum chamber is from 0.5 to 0.001 atm, set depending on the nature of the dried product and its requirements. The product dried in the chamber 3 is discharged to the receiver 4 through an air lock.

 A variety of products were dried: aqueous and organic solutions of various substances, polymer solutions, suspensions. Especially effective is the application of the developed method for drying thermolabile substances, the quality of which deteriorates under the influence of elevated temperatures, solutions of polymers, fruit and other juices, etc. The following are examples of the implementation of the proposed method of drying. It should be emphasized that the parameters of the drying processes were determined for each specific substance depending on its physicochemical properties, namely, boiling point, decomposition or degradation. For polymers whose molecular weight changes (decreases) when exposed to elevated temperatures and pressures, these parameters are also selected experimentally.

 The temperature and pressure in the dispenser 2, firstly, depend on the properties of the product, and secondly, must ensure a safe process, i.e. eliminate sudden “boiling”, which is accompanied by an unplanned increase in pressure in the apparatus.

The results of individual experiments are summarized in tables, where T ₁ and T _{2 are the} temperatures in the tank 1 and the dispenser 2, respectively, P _{1 is the} pressure in the tank 1, P _{2 is the} pressure in the vacuum chamber 3, t is the residence time of the solution in the tank 2 (heating to T ₂ ) .

Example 1. Drying a solution of a prepolymer of elastic foam polyimide (EPP) in isopropyl alcohol (IPA). In this example, the solvent is the organic compound IPA, the content of which in the solution is 75% of the target product polyfoam foam, its concentration of 25 wt. is a thermolabile product, the structure of which at elevated temperature changes in the direction of decreasing molecular weight. It is known that when the solution is kept for 1 h at a temperature of 100 ^o C the characteristics of foaming EPI deteriorate by 10 20%, which indicates its partial thermal decomposition.

 The purpose of the drying process is to achieve a concentration of EPI of 5 wt.

The process was carried out as follows: 50 l of the initial solution in vessel 1 was brought to a temperature of 50 ^° C over 3 hours. At the same time, nitrogen was maintained at a pressure of 1 to 3 atm (see table 1). Then, the solution was supplied to vessel 2, where it was heated to 20 25 ^° C for 20 120 s. The solution heated in this way entered the vacuum chamber, where it was dried at 0.01 0.2 atm. As a result of a series of experiments, the parameters and results of which are presented in table. 1, the required solvent content in the dry (finished) product is 5%
Example 2. Drying suspensions of an acrylic pentaerythritol copolymer (SAKAP) in a dichloromethane solution (SAKAP is a polymer that undergoes degradation when exposed to elevated temperatures. It also changes its properties under prolonged exposure to high vacuum or high pressure. When the suspension is kept for 2 hours at 120 ^o C there is a decrease in the molecular weight of SAKAP, which adversely affects its operational properties.

The drying step of this polymer is a complex technological problem and involves precipitation, filtration and drying itself. The technological content of the solvent in suspension of 90 wt. The final concentration should not exceed 5 wt. The suspension loaded into the tank 1 was subjected to slow heating for 2 hours to 50-60 ^° C. The temperature in the dispenser 2 and the pressure in the vacuum chamber were changed, as shown in table 2. Subject to the indicated parameters, the required results were obtained, while the quality of the products corresponded to the technical specifications.

Example 3. Drying a solution of natural anthocyanin dye (ACC) in absolute ethanol. Staying the solution for 1 h at 80 ^o C leads to a discoloration (darkening) of the dry matter. The first series of experiments was carried out with an initial concentration of ACC in the solvent of 70 wt. The solution was heated in a container 1 to a temperature of 50 ^o C, which does not cause thermal decomposition of the products under pressure. In the dispenser 2, the temperature is brought to 80 ^o C, the pressure in the vacuum chamber is 0.005 atm, the residence time in it is 1 s.

 The residual solvent content in the product is not more than 5 wt. The results are shown in table 3 (experiment 1). The second series of experiments was carried out in order to obtain a more concentrated solution (evaporation).

 The parameters of the process and its results are presented in table 3 (experiment 2).

Claims (1)

A method of drying dissolved or suspended substances, including pre-heating and subsequent evacuation, characterized in that the substance is heated to a temperature of 10 20 ^o C below its decomposition temperature under a pressure of 1 5 atm created by an inert gas, then the temperature is further raised in 5 120 s 30 40 ^o With, after which it is subjected to vacuum.

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