RU2488421C1 - Concentration of liquid solutions - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to separation of fluids by evaporation. Proposed method comprises evaporation of solvent from solution film formed on rotary drum inner surface. Solution is fed inside drum to allow displacement of solution toward unloading side. Solvent vapors are collected and condensed. Heat power consumed for solvent evaporation, drum rpm and level of solution inside the drum are selected provided concentrated product is discharged in the form of fluid. Flow rate of solution fed into drum is to be calculated by mathematical tools.

EFFECT: sufficient quality of concentrated product and solvent condensed vapors provided in continuous operation without drop in efficiency in wide range of concentrations of solutions.

11 cl, 1 dwg

Description

The invention relates to the field of separation of liquid media, and more specifically, to evaporation of solvents from solutions and to increase the concentration of solutions, including radiation and chemically hazardous substances, in particular liquid radioactive waste, liquid toxic waste from chemical industries.

A known method of concentrating liquid solutions in rotary film evaporators (film evaporators with a rotary stirrer) (Udyma PG, a training manual for the course design, installation and operation of heat and mass transfer units. Film evaporators. M., MPEI, 1985, p. 52), consisting in the fact that the solution is fed to the inner surface of the heated case and distributed over it with a thin layer of blades (scrapers), mounted on a central rotating rotor. The rotor blades create vortex structures in the solution layer that intensify heat transfer processes.

The rotor-film evaporator is a stationary heated tube with a rotor with blades placed inside it. The solution supplied to the pipe flows in the form of a film on its inner surface. The concentrated product is removed through the drain fitting into the finished product collector, and the steam obtained from the solution is discharged through the outlet fitting into the condensing device.

This method is used, for example, for the concentration of food products (milk, whey, tomato paste, fruit juices, etc.).

This method allows you to effectively carry out the process of concentration of highly mineralized liquids in a continuous mode without the growth of deposits on heated surfaces.

The disadvantage of this method is the unsolved problem of drip-aerosol entrainment, which sharply reduces the degree of purity of the condensable vapor of the solvent, which in turn leads to loss of product, and also significantly complicates the possibility of using the method for concentrating radiation and chemically hazardous substances. This drawback is due to the high circulation rates of the solvent vapor, which is created by the blades of an extended rotor that is in contact with the liquid film and rotates at a high angular velocity, which leads to the release of a large amount of dissolved microparticles (aerosols) together with the solvent vapor stream.

In addition, the method is effective in the formation of films of small thickness, which requires "fine tuning" of the rotor-film evaporator due to the need to withstand small gaps between the blades of the rotating rotor and the heated inner surface of the housing.

The closest analogue, coinciding with the claimed invention for the most significant features, is a method for concentrating liquid solutions (Laboratory instruments and equipment made of glass and porcelain. Reference publication. M., Chemistry, 1988, p. 211) by evaporation of the solvent from a thin film of solution formed on the inner surface of a rotating evaporation chamber (rotational evaporation). The method consists in the fact that a certain amount of a liquid solution is loaded into the evaporation chamber having the shape of a flask through a loading opening, the flask is rotated at an angle to the horizontal, while the bottom of the flask is immersed in hot liquid (water, oil, etc.). Heat is transferred from the hot liquid through the glass wall of the flask to the solution, heating it. During rotation, a solution film is formed on the inner surface of the flask, from which the solvent evaporates. The inner cavity of the flask is connected to a condenser made in the form of a tube with flowing cold water, in which solvent vapor is collected for condensation. Since steam is converted into liquid on the heat-exchanging surface of the condenser, a vacuum is created in the inner cavity of the flask, which is compensated by further evaporation from the solution film. During evaporation, heat is removed from the film itself and the adjacent glass wall, which is replenished by periodic immersion of the outer surface of the bulb wall in a hot liquid. After reaching a predetermined level of concentration of the solution, the rotation of the flask is stopped and the concentrated product is discharged through the same hole through which the solution was charged.

Due to the formation of a film on the inner surface of the flask, the evaporation area increases, while the process of vaporization is intensified. When selecting temperature conditions, a sufficiently high degree of purity of the condensable solvent vapor is provided.

The method is used in the chemical, petrochemical, pharmaceutical, medical and food industries, allows distillation of thermally unstable substances in mild temperature conditions, distillation of a mixture of high-boiling substances that cannot be separated by conventional distillation, degassing of liquids, evaporation of liquids, distillation of easily foaming substances and etc.

The disadvantage of this method is that the concentration process is periodic, due to the need to stop the rotation of the flask to unload the concentrated product and load a new portion of the solution. In the process of evaporation, the level of the solution in the flask gradually decreases, and, consequently, the surface of the film decreases, which leads to a decrease in the evaporation efficiency and ultimately a decrease in the productivity of the process. In addition, a sedimentary deposit forms on the inner surface of the flask, which is removed by periodic washing, which increases the volume of secondary waste, the disposal of which requires additional costs and complicates the process, especially when concentrating radiation and chemically hazardous substances.

The inventive method of concentrating liquid solutions eliminates these disadvantages and provides the required quality indicators of the concentrated product and the condensable vapor of the solvent in continuous operation without reducing performance in a wide range of concentrations of solutions, while during operation no shutdowns are required for periodic washing of equipment from deposits, which reduces secondary waste.

The technical effect is achieved by the fact that the method of concentrating liquid solutions involves heating the solution, evaporating the solvent from the solution film, collecting solvent vapor for condensation, unloading the concentrated product. Moreover, the film of the solution is formed on the inner surface of the rotating drum of a cylindrical or other form of at least one of the solution that is fed into the drum, while the solution is moved to the discharge side. The heat power spent on evaporation of the solvent, the rotation speed of the drum and the level of the solution inside the drum are selected from the condition of providing unloading of the concentrated product in the form of a liquid, and the flow rate of solution G supplied to the drum is determined from the ratio

, где $$\text{G}=\frac{\text{Q}}{{\text{[c}}_{\text{p}}\left(\text{t '}-\text{t}\right)+\text{r]}\cdot {\text{[w}}_{\text{2}}-{\text{w}}_{\text{one}}\text{]}}\text{,}\text{[kgf / s]}$$

where

Q - thermal power spent on the evaporation of the solvent, [W];

c _p is the heat capacity of the heated solution, [J / (kg · deg)];

t 'is the temperature of the solvent on the vapor-liquid saturation line, [° C];

t is the temperature of the feed solution, [° C];

r is the heat of vaporization of the solvent, [J / kg];

w ₂ is the concentration of solute in the unloaded product, [kg / kg];

w ₁ is the concentration of solute in the feed solution, [kg / kg].

The solution is heated to a temperature below its boiling point, mainly by heating the inner surface of the drum.

The thermal power spent on the evaporation of the solvent is determined from the ratio

, где $$\text{Q}={\text{q}}_{\text{F}}\text{π}\overline{\text{D}}\text{Lk}\text{,}\text{[W]}$$

where

q _F is the average density of the heat flux supplied to the inner surface of the drum, [W / m];

- средний диаметр внутренней поверхности барабана, [м]; $$\overline{D}$$

- the average diameter of the inner surface of the drum, [m];

L is the length of the inner surface of the drum, [m];

k is a coefficient taking into account the effect of deviation of the shape of the inner surface from a cylindrical view.

When concentrating radiation and chemically hazardous substances, in particular liquid radioactive waste, liquid toxic chemical waste, the thermal power spent on evaporating the solvent, the rotation speed of the drum and the level of the solution inside the drum are selected from the condition that the concentrated product is discharged in the form of a liquid, excluding drainage solution films.

The collection of solvent vapor and their condensation can be carried out in the inner cavity of the drum.

The heating of the solution, the evaporation of the solvent from the film of the solution, the discharge of the concentrated product is carried out mainly with a rotating drum.

When the drum rotates, its inner surface is additionally cleaned of deposits in a continuous or batch mode, cleaning is carried out mainly under the level of the solution.

Additionally create a vacuum in the inner cavity of the drum.

The drum can be placed in the housing, inside which create a vacuum.

The supply of the solution inside the drum, the movement of the solution in the discharge direction and the condition for unloading the concentrated product in the form of a liquid provide for the creation of a solution flow in the lower part of the drum. When the drum rotates, its inner surface is periodically immersed in the solution and a thin film of the solution is formed on it. The heating of the solution is carried out mainly by heating the inner surface of the drum. In this case, the film warms up due to contact with a heating surface. Due to the small thickness of the film, both the film itself and its surface in contact with the vapor-gas medium in the drum cavity are rapidly heated, which leads to an increase in the pressure of saturated solvent vapors at the interface and their exit (diffusion) into the drum cavity. In this case, the evaporation area remains constant, which ensures the concentration process without compromising productivity.

The productivity of the installation can be improved by increasing the evaporation area when using several drums, combined among themselves in a group, and forming a system of concentration of liquid solutions. The drums can be arranged, for example, in series, parallel or concentric.

As the solvent evaporates, the concentration of the solution flowing in the drum increases and reaches a maximum in the zone of product discharge from the drum. The intensive evaporation process is carried out continuously.

The established relationship between the process parameters allows the calculation of the flow rate of the solution supplied to the drum with known (measured) parameters (concentration, temperature, physical and chemical properties) to ensure the specified output parameters of the concentrated product in a wide range of solutions concentration changes.

Heating the solution to a temperature below its boiling point eliminates the volumetric and surface boiling of the solution, thereby eliminating drip entrainment of the solution and allowing to provide the required quality indicators of condensed vapors without loss of product.

When concentrating radiation and chemically hazardous substances, in particular liquid radioactive waste, liquid toxic chemical waste, by selecting process parameters, the drying of the solution film on the inner surface of the drum is additionally excluded, thereby preventing the formation and entrainment of dusty aerosols into the vapor condensation zone, which is not only increases the degree of purity of solvent vapor, but also increases the safety of the process, eliminates the harmful effects on personnel and the environment.

The most rational in terms of design and energy costs is heating the solution by heating the inner surface of the drum. In this case, the thermal power spent on the evaporation of the solvent is determined from the ratio taking into account the design features of the drum. The solution can be heated by other methods, in particular, by supplying heated gas into the drum, which can also be used to remove solvent vapors if they are condensed outside the drum.

To ensure the compactness of installations that implement the inventive method of concentration, the collection of solvent vapor and their condensation can be carried out in the inner cavity of the drum. Such a constructive solution is especially advisable when creating mobile installations for concentrating liquid solutions.

The heating of the solution, the evaporation of the solvent from the film of the solution, the unloading of the concentrated product is carried out mainly with a rotating drum, which ensures the continuity of the process of concentration of liquid solutions.

To form and hold a thin film of solution on the inner surface of the drum, this surface is wettable, which is ensured by the use of appropriate materials and surface treatment.

To achieve the required purity of the inner heating surface of the drum, it can additionally be cleaned of deposits in continuous or batch mode. Cleaning is carried out mainly under the level of the solution, which contributes to the most rapid destruction and dissolution of deposits, respectively, significantly reduces the time of forced shutdowns for washing and washing equipment, and also minimizes the amount of secondary waste.

A negative pressure system (reduced pressure relative to atmospheric pressure) can be connected to the inner cavity of the drum to intensify evaporation and prevent the release of gaseous products into the room.

The drum can be placed in the housing, inside which create a vacuum. In this case, the housing can also be used as a supporting structure for the drum, heating elements, inlet and outlet pipes and other elements of the concentration unit.

The graphic image is presented in the form of a scheme for the installation of concentration of liquid solutions, where 1 is a drum; 2 - rotation drive; 3 - heating element; 4 - case; 5 - device for cleaning from deposits; 6 - a system of dosed supply of a solution; 7 - a system for unloading and collecting concentrated product; 8 - a system for collecting, condensing vapors and condensate drain; 9 - a system for creating a vacuum; 10 - solution level.

The inventive method is implemented as follows.

The liquid solution is fed into the rotating drum (1) by the dosed solution supply system (6) and the solution is moved to the discharge side under the influence of gravitational forces. In this case, a solution flow is formed in the lower part of the drum (1). The movement of the solution can be further accelerated, for example, by tilting the drum (1) towards the discharge side.

Measure the parameters of the feed solution: the concentration of the dissolved substance is w ₁ [kg / kg]; temperature - t [° C]; determine the properties of the solution: heat capacity - c _p [J / (kg-deg)]; the temperature of the solvent on the vapor-liquid saturation line is t '[° C]; the heat of vaporization of the solvent is r [J / kg].

The rotation of the drum (1) is carried out by a rotation drive (2). The rotation speed of the drum, the level of the solution inside the drum (10), and the thermal power spent on the evaporation of the solvent are chosen experimentally from the condition of ensuring the discharge of the concentrated product in the form of a liquid with the required concentration. Unloading is provided by the system for unloading and collecting concentrated product (7). When unloading control the specified concentration of solute in the unloaded product - w ₂ [kg / kg]. The solution is supplied and the concentrated product is unloaded mainly from opposite ends of the drum (1). Unloading can be performed both at the end of the drum (1), where the maximum concentration of the product is achieved, and at several points along the generatrix of the drum, which makes it possible to obtain a solution of various concentrations without changing the process parameters.

In the above scheme, the heating of the solution is carried out by heating the inner surface of the drum (1) with a heating element (3) located outside the drum. The drum may be heated by electric heating elements or from a steam jacket into which heating steam is supplied. When using several drums, the concentration of solutions in which is carried out with different temperature conditions, in some drums layout schemes, solvent vapors removed from the inner cavity of another drum can be used as heating steam for one drum.

Solvent vapors from the inner cavity of the drum (1) enter the system for collecting, condensing vapors and condensate drain (8); in the above diagram, this system is placed outside the drum. The collection of vapors and their condensation can be carried out inside the drum (1), which will ensure greater compactness of the installation.

During the concentration process, the required cleanliness of the inner surface of the drum (1) is maintained with a deposit cleaner (5), preferably placed below the level of the solution, in order to obtain greater cleaning efficiency.

To intensify evaporation and prevent the release of gaseous products into the installation room, the process is carried out under reduced pressure in the inner cavity of the drum (1), which is provided with a vacuum generation system (9). The vacuum can be created either directly inside the drum (1), or inside a sealed enclosure (4) in which the drum (1) is placed.

The installation is equipped with instruments for monitoring the main operating parameters, as well as controls for regulating and maintaining the concentration process (not shown in the diagram).

The inventive method can be carried out the concentration of liquid radioactive waste (LRW) of a nuclear power plant (NPP) in a wide range of operating parameters - from the primary evaporation of LRW from the waste water collection tanks to various concentrations to produce bottoms (BO), concentrated bottoms (BWC) up to to obtain a concentrated salt melt.

When evaporating drain water, LRW is fed into the drum with the concentration (in mass fractions) of salts (w ₁ ) 0.005-0.01 kg / kg, at a temperature (t) of 25 ° C. The heat capacity of the heated solution (c _p ) is 4181 J / (kg · deg), the temperature of the water (solvent) on the vapor-liquid saturation line (t ') is 100 ° C, and the heat of vaporization of water (r) is 2256000 J / kg.

The evaporation process is carried out in a cylindrical drum with a diameter of the inner surface of 0.8 m and a length of the inner (working) surface of 4 m.

The drum is heated by external heat emitters mounted on the housing. As heat emitters, standard ceramic emitters of the LFFE type (large filled flat element) with a size of 245 × 95 mm and a power of 300 - 1400 W are used. The main surface heat transfer temperature is 596 ° C, the wavelength is 3.35 nm. The number of heating elements is 50 pieces. The average density of the heat flux supplied to the inner surface of the drum (q _F ) is 6.9 kW / m ² .

The thermal power spent on the evaporation of the solvent is determined from the ratio

$$\text{Q}={\text{q}}_{\text{F}}\text{π}\overline{\text{D}}\text{Lk}=\text{6}\text{,9}\times \text{3}\text{,fourteen}\times \text{0}\text{,8}\times \text{four}\times \text{one}\approx \text{70}\text{[kW]}$$

Such thermal power provides heating of the LRW solution to a temperature below its boiling point, which eliminates the drop entrainment of radionuclides and allows to provide the required quality indicators of condensed vapors.

For this process, the rotational speed of the drum is 15-30 rpm, and the solution level inside the drum is maintained at least 30 mm, which, at the installed heat output, also eliminates the drainage of the LRW film on the inner surface of the drum and, therefore, excludes the formation of aerosols.

Due to the absence of boiling and the formation of aerosols, a high coefficient of vapor condensate purification is achieved (up to K = 10 ⁶ ).

The evaporation process is carried out during discharge in the housing and, accordingly, in the inner cavity of the drum 0.02 ÷ 0.1 MPa.

Evaporation of LRW is carried out to obtain a still bottom with a concentration (in mass fractions) of salts during unloading (w ₂ ) of 0.16 kg / kg.

The flow rate of solution G supplied to the drum is determined from the ratio

$$\text{G}=\frac{\text{Q}}{\left[{\text{from}}_{\text{p}}\left(\text{t '}-\text{t}\right)+\text{r}\right]\cdot \left[\text{one}-\frac{{\text{w}}_{\text{one}}}{{\text{w}}_{\text{2}}}\right]}=\frac{\text{70,000}}{\left[\text{4181 (100}-\text{25)}+\text{2256000}\right]\cdot \left[\text{one}-\frac{\text{0}\text{01}}{\text{0}\text{,16}}\right]}\approx \text{0}\text{, 0291}\text{[kg / s]}$$

The mass flow rate of water vapor in this case will be 0.0272 kg / s, and the mass flow rate of the concentrated product is 0.0018 kg / s.

The annual flow rate of the feed solution is 781 m ³ per year.

When the bottling process is completed at the same installation, LRW is fed into the drum with a salt concentration (w,) of 0.16 kg / kg, at a temperature (t) of 25 ° C. The heat capacity of the heated solution (C _p ) 4110 J / (kg · deg). To obtain a concentrate with a salt content of 0.45 kg / kg, the flow rate of the solution supplied to the drum is set and maintained at the level of 0.0424 kg / s.

The mass flow rate of water vapor in this case will be 0.0273 kg / s, and the mass flow rate of one stripped off product is 0.0151 kg / s.

The annual productivity of the flow rate of the supplied solution is 1102 m ³ per year.

Upon deep evaporation of KCO before salt melt at the same installation, LRW with a salt concentration (w ₁ ) of 0.45 kg / kg, at a temperature (t) of 25 ° C is fed into the drum. The heat capacity of the heated solution (c _p ) 3890 J / (kg · deg). To obtain a salt melt with a salt concentration of 1.0 kg / kg, the flow rate of the solution supplied to the drum is set and maintained at 0.050 kg / s.

The annual productivity of the flow rate of the supplied solution is 1117 m ³ per year.

The annual volume of bottoms residue processing is commensurate with that of the deep evaporation units UGU-500 operating at nuclear power plants.

The washing of the drum is carried out once a year before the scheduled preventive repair (NDP). The volume of secondary LRW is 0.2% of the evaporation volume.

It is possible to create a system for concentrating LRW at nuclear power plants from a group of interconnected plants.

Thus, the method of concentration of liquid solutions allows to provide the required quality indicators of the concentrated product and condensable solvent vapor in continuous operation without reducing performance in a wide range of solution concentrations at a low secondary LRW.

In addition, the method does not require sophisticated equipment and expensive materials, which ensures its low cost. Also, the method does not require a staff of maintenance personnel, which in addition to process control constantly performs maintenance, repair and washing of equipment from deposits, which reduces operating costs, and in the case of concentration of radiation-hazardous substances, reduces dose loads on personnel.

Claims (11)

1. A method of concentrating liquid solutions, including heating the solution, evaporating the solvent from the solution film, collecting solvent vapor for condensation, unloading the concentrated product, characterized in that the solution film is formed on the inner surface of the rotating drum of a cylindrical or other shape of at least one of solution, which is fed into the drum, providing the solution moving in the direction of discharge, while the thermal power spent on the evaporation of the solvent, the rotation speed b Rab and solution level within the drum are selected from the conditions for ensuring discharge of the concentrated product in liquid form, and the flow rate supplied to the drum G solution is determined from the relation
$$\text{G}=\frac{\text{Q}}{{\text{[c}}_{\text{p}}\left(\text{t '}-\text{t}\right)+\text{r]}\cdot {\text{[w}}_{\text{2}}-{\text{w}}_{\text{one}}\text{]}}\text{,}\text{[kgf / s]}\text{,}$$

where Q is the thermal power spent on the evaporation of the solvent, W;
c _p is the heat capacity of the heated solution, J / (kg · deg);
t 'is the temperature of the solvent on the vapor-liquid saturation line, ° C;
t is the temperature of the feed solution, ° C;
r is the heat of vaporization of the solvent, J / kg;
w ₂ is the concentration of solute in the unloaded product, kg / kg;
w ₁ is the concentration of solute in the feed solution, kg / kg 2. The method according to claim 1, characterized in that the solution is heated to a temperature below its boiling point. 3. The method according to claim 1, characterized in that the solution is heated mainly by heating the inner surface of the drum. 4. The method according to claim 3, characterized in that the thermal power spent on the evaporation of the solvent is determined from the ratio
$$\text{Q}={\text{q}}_{\text{F}}\text{π}\overline{\text{D}}\text{Lk}\text{,}\text{[W]}$$

,
where q _F is the average density of the heat flux supplied to the inner surface of the drum, W / m ² ;
$$\overline{D}$$

- the average diameter of the inner surface of the drum, m;
L is the length of the inner surface of the drum, m;
k is a coefficient taking into account the effect of deviation of the shape of the inner surface from a cylindrical view. 5. The method according to claim 1, characterized in that when concentrating radiation and chemically hazardous substances, in particular liquid radioactive waste, liquid toxic chemical waste, the thermal power spent on evaporation of the solvent, the rotation speed of the drum and the level of the solution inside the drum are selected from conditions for providing unloading of the concentrated product in the form of a liquid, excluding the drying of the solution film. 6. The method according to claim 1, characterized in that the collection of solvent vapor and their condensation is carried out in the inner cavity of the drum. 7. The method according to claim 1, characterized in that the heating of the solution, the evaporation of the solvent from the film of the solution, the unloading of the concentrated product is carried out mainly with a rotating drum. 8. The method according to claim 1, characterized in that when the drum is rotated, its inner surface is further cleaned of deposits in continuous or batch mode. 9. The method according to claim 8, characterized in that they clean the inner surface of the drum from deposits under the level of the solution. 10. The method according to claim 1, characterized in that it further creates a vacuum in the inner cavity of the drum. 11. The method according to claim 1, characterized in that they additionally place the drum in the housing, inside which create a vacuum.

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