DE102018002568A1 - Method and device for carrying out the method for concentration of solutions - Google Patents

Abstract

A method is presented, which is particularly suitable for the concentration of solutions, whereby the concentration of the solution in successive stages of evaporators and condensers are gradually increased. In the individual stages, solvent is evaporated with little expenditure of energy and discharged after its condensation. The recovered solvent may also be a product of the process or reused. Measures are shown which enable a high concentration of the solution and the crystallization in the HP plant.

Description

The present invention relates to the previous parent application: "Methods and apparatus for producing fresh water" AKZ 10 2017 006 726.0, filing date 14.2017, and extends the invention, especially with regard to larger plants for the concentration of process water, to crystallization of solid from the process water. The invention also has the task to open up opportunities for the highest possible recovery of large quantities of pure water with little effort. In doing so, the measures disclosed in the parent application are supplemented and expanded.

In the main application, measures are mainly taken to generate the largest possible amount of condensate in one stage of the evaporation-condensation process (humidification-dehumidification process, HD process) with as little externally supplied heat as possible. The present invention is concerned with new possibilities of the HD process, in particular for the stepwise concentration of the brine and for continuous operation with large condensate flow rates and optionally with crystallization. The possibilities arise from the inventive multi-stage and modular arrangement of the HD facilities, the continuous operation with low-temperature heat and the inclusion of crystallization in the HD system. Thus, disadvantages of the conventional technique can be overcome.

In the present description, the symbols and reference numerals used in the parent application are retained. New reference numbers are designated with numbers from 50 and listed in the list of reference numerals at the end of the description.

To simplify the description, a solution (brine) with salt concentration in the solvent water is assumed. However, solutions with other solutes and solvents are also meant when the solutes remain in the solution during evaporation.

Parts of substances in the solution that evaporate in the process but do not condense again can be expelled from the solution, so that just like the salts are not contained in the recovered solvent.

The HD process has been known for many decades. An overview of the development can be found in the conference paper by B. Seifert et al .: "The History of Humidification-Dehumidification Desalination Systems", October 2013, Conference: International Desalination Association World Congress on Desalination and Water Reuse At: Tianjin, China , Despite the advantages of the HD process, it has not yet been widely used. An improvement of the economy of the method is opened with the invention.

In the parent application it has been shown that a highly efficient use of the heat Q _ext supplied to the process takes place when the temperature increase Δt ₁₂ = t ₂ -t _{1 of} the raw water flow in the condenser 2 in comparison to the temperature increase Δt ₂₃ = t ₃ - t _{2 of} the raw water flow in the external heater 4 is great.

The ratio Δt ₁₂ / Δt ₂₃ is referred to in the parent application as effectiveness PR _HD and there in equation ( 1 ) Are defined. It can wall through the possibilities presented by the main application Active wall 3 be increased. The active wall provides for the targeted generation of air circulation with a variable over the height of the evaporator and the condenser air flow, which is adapted to the desired ratio of air flow G to the counterflow liquid flow L, so that a high efficiency PR _{HD is} achieved.

The optimal conditions for carrying out the method are dependent, in particular, on the running costs for the external supply of heat and on the expenditure on equipment. The invention makes it possible to adapt to the given framework conditions, above all by adapting the effectiveness PR _HD . An increase in effectiveness is mainly caused by a larger equipment cost. By modular enlargement of the apparatus height, the externally supplied heat is reduced by the counterflow operation.

A modular enlargement by superimposed and successively arranged, largely uniform modules is particularly advantageous. The modules of the HD system can be arranged and held in a frame structure, so that they can be completely or partially removed for maintenance and service from the frame structure. By bridging individual modules, therefore, their maintenance can be carried out without interrupting the overall operation. It can thus be created systems of almost any size and operated largely continuously.

The active wall between the evaporator and the condenser can also be designed with modules arranged one above the other in such a way that the optimum air flow is achieved in each module. For this purpose, it may be advantageous to install individually controllable fans and cross-flow openings for each module. The Active Wall allows manifold arrangements of evaporator and condenser because can be provided by the adjustable fans and cross-flow openings for optimum flow conditions largely independent of the position. For example, an evaporator can be arranged between two capacitors.

The optimal efficiency depends on the investment costs for additional modules and the resulting decrease in energy costs. With the help of the invention, the possibilities for an optimal adaptation to the basic conditions are extended.

In the HD method, it is known that heat sources with a low temperature (in particular below 100 ° C.) can also be used. As a result, the HD process can be advantageously connected to other processes in which low-temperature heat is generated. It is also very large systems can be operated advantageously, because often the corresponding external heat source is available inexpensively. The HD process can be adapted to the temperature level of the external heat source. For example, the HD process can also be operated with a heat source of 80 ° C by the temperatures t ₁ to t ₃ are chosen according to the dimensioning.

The low operating temperature and operation at atmospheric pressure allows the use of inexpensive and corrosion resistant materials for the apparatus components and of large simple walls for the housing of evaporator and condenser. Above all, plastics such as polypropylene are suitable as a material. With plastic components, the manufacturing costs can be reduced in large numbers in the direction of the material costs. The standardization of components in the modular design is therefore an effective measure to reduce investment costs.

The HD process can be used to concentrate solutions. The return of the brine from the evaporator 1 provides a way to get to ever higher concentration of brine. With this option mentioned in the parent application, the higher concentration of brine is achieved over time and the process must be stopped when a critical concentration is reached in order to prevent crystallization blockage of the plant's streams.

To avoid unwanted crystallization in the evaporator, the temperatures in the evaporator are advantageously adjusted so that a supersaturation of the solution is avoided. The crystallization can take place in a designated crystallizer, which can be installed in the HD system. This process control is applicable if the supersaturation can be achieved by lowering the temperature. For material systems in which the solubility of the solute is almost independent of the temperature (for example, NaCl-H ₂ O), an alternative of the invention in the description of 3 presented.

Thus, the benefits of the HD process are extended to producing very high concentrations of brine, up to nearly complete separation of solute and solvent.

• 1 ist Teil eines Anlagenschemas mit zwei HD-Stufen (HD-Apparaten),
• 2 zeigt die geometrische Konstruktion zur Ermittlung des von Stufe zu Stufe zunehmenden Salzgehalts S der in die Stufe eintretenden Sole.
• 3 ist Teil eines Anlagenschemas, bei dem eine hohe Konzentration der Sole erreicht wurde und Kristallisation auftritt.

Therefore, the method can also be used for tasks in the concentration of solutions for which other methods, such as reverse osmosis, are not suitable. For these new possibilities, the facilities and functions are based on 1 to 3 explained in simplified, schematic form.

• 1 is part of a plant scheme with two HD stages (HD equipment),
• 2 shows the geometric design for determining the increasing from stage to stage salinity S of entering the stage brine.
• 3 is part of a plant scheme in which a high concentration of brine has been achieved and crystallization occurs.

1 shows a simplified plant scheme with 2 stages, with the individual stages mainly from the evaporator 1 , the capacitor 2 and the external heating 4 consist of the continuous concentration of raw water or brine. The stages are distinguished in the description by indices (i, i + 1). The HD system can contain a large number of stages. In each stage, the concentration of brine entering the stage is increased by the withdrawal of salt-free condensate. The components are designated by reference numbers from the parent application.

The right in 1 shown stage is with the brine pump 50 supplied from the left stage. With a cooler 51 The brine can be cooled before entering the stage to ensure continuous operation and increase the amount of condensate. Each level is preferred with an active wall 3 with fan 13 and adjustable cross-flow openings 14 fitted.

$$\Delta {\text{S}}_{\text{i}}={\text{S}}_{\text{i}+1}-{\text{S}}_{\text{i}}$$

$${\text{M}}_{\text{i}}={\text{L}}_{\text{i}}-{\text{L}}_{\text{i}+1}.$$

wobei, wie üblich, angenommen wird, dass das ausgeschleuste Kondensat mit dem Mengenstrom M auch bei hoher Konzentration der Sole kein Salz enthält.From a mass balance for the brine flow L and its salt content S, the relationships for the stage i and the stage i + 1 following it emerge: $${\text{L}}_{\text{i}}*{\text{S}}_{\text{i}}={\text{L}}_{\text{i}+1}*{\text{S}}_{\text{i}+1}.$$

$$\Delta {\text{S}}_{\text{i}}={\text{S}}_{\text{i}+1}-{\text{S}}_{\text{i}}$$

$${\text{M}}_{\text{i}}={\text{L}}_{\text{i}}-{\text{L}}_{\text{i}+1},$$

whereby, as usual, it is assumed that the discharged condensate with the mass flow M contains no salt even at high concentration of the brine.

To a good approximation, in the HD process, the pure water stream M _i evaporated in the evaporator of stage i and recovered in the condenser can be determined from the simplified energy balance of the condenser: $${\text{M}}_{\text{i}}*\Delta {\text{H}}_{\text{v}}={\text{L}}_{\text{i}}*{\text{c}}_{\text{p}\text{, w}}*\Delta {\text{t}}_{12\text{i}}$$

The left-hand side of equation (6) is the heat flow released during the condensation of the condensate flow rate M _i (with the specific heat of condensation Δh _v ) which serves almost exclusively for preheating the raw water or the brine flow L _i by the temperature increase Δt _{12, i} , which is expressed as the right side of the equation. In this case, c _{p, w} denotes the specific heat of the brine. Equation (6) thus shows the relationship between the condensate flow M _i obtained in the condenser and the temperature increase Δt _{12, i of} the brine in the condenser before entering the external heater.

The ratio of the salt contents of the sols S _i / S _{i + 1} is given by equation ( 3 ) $${\text{S}}_{\text{i}}/{\text{S}}_{\text{i}+1}={\text{L}}_{\text{i}+1}/{\text{L}}_{\text{i}}$$

From the equations (6) and (7) follows the increase of the salt concentration ΔS _i according to equation (4) by the extraction of the condensate flow M _i $$\Delta {\text{S}}_{\text{i}}={\text{S}}_{\text{i}+1}/\left(\Delta {\text{H}}_{\text{v}}/{\text{c}}_{\text{p}\text{, L}}*\Delta {\text{t}}_{12\text{i}}\right)$$

2 shows the stage construction made possible by equation (8) in the enthalpy concentration diagram (hS diagram). A similar enthalpy-concentration diagram is shown in the chapter "Evaporation of salt solutions" on page 192 of the book by F. Bosnjakovic: Technical Thermodynamics, Part II, 4th Edition, Verlag Th. Steinkopff, Dresden and Leipzig (1965). In this / 70, however, no HD process, but the evaporation of a salt solution is shown.

The increase of the salt concentration ΔS _i in the stage i follows from the decrease of the water content of the brine L _i due to the obtained pure condensate flow M _i . The heat of condensation (= heat of vaporization) Δh _v is plotted on the ordinate (S = 0). In 2 this value is shortened for reasons of space. Also in the ordinate direction, the specific enthalpy increase Δh _i of the brine flow S _{i is} plotted by the preheating of the brine in the condenser. It is the product of the specific heat of the brine and the temperature increase in the condenser: $$\Delta {\text{H}}_{\text{i}}={\text{c}}_{\text{p}\text{, L}}*\Delta {\text{t}}_{12\text{i}}$$

The combination of the two ordinate values at S = 0 and at S _i yields two similar triangles whose abscissa values are the salt content S _{i + 1} (for the large triangle) and the salt content increase ΔS _i (for the small triangle). The graph shows the stepwise geometric determination of the salt contents in the successive stages, the condensate flow rates resulting from the realized in the respective stage temperature increase .DELTA.t _{12, i of} the brine in the capacitor according to equation (6).

An advantage of the invention is that the solvent obtained in the concentration of the solution can be used again for the original task, so that its procurement and disposal costs can be saved. But it is also another use of the pure solvent possible.

According to the invention, the brine successively passes through the process stages and their concentration is thereby increased stepwise. The concentration of the brine may approach the saturation concentration. This is generally higher the higher the temperature. Is in 2 accepted. It is then possible to exceed the saturation line by cooling, so that the formation of crystals is to be expected.

This production of crystallizate is in 2 shown on the right side of the diagram. When exceeding the solidification line 53 By cooling, the amount of crystals can arise, that of the position of the point M on the isotherms 54 of the crystallizer corresponds. The mass fraction of the crystals can be calculated from the mixing rule for the salt (on the right ordinate, ie at S = 1) and the saturated solution (on the solidification line) 53 ) are determined at the temperature in the crystallizer. Further recovery of crystals from the saturated solution can be achieved by further steps, by brine recycling and by the methods customary in crystallization technology.

In 3 is a stage drawn, which processes brine with high concentration. Under the evaporator 1 is a crystallizer 55 in which the crystals are kept in motion by the circulation flow while growing in the supersaturated solution. It is a circulation pump 59 drawn, which ensures the revolution of the solution. The supersaturation is due to the removal of solvent in the evaporator 1 and by cooling in the cooler 58 of the cooling crystallizer. There are a variety of crystallizer constructions known that allow continuous or batchwise crystallization and removal of the crystals.

In the 3 shown inventive arrangement of a crystallizer 55 directly under the evaporator 1 is particularly advantageous because it facilitates the transport of the supersaturated solution and the crystals.

In the event that the solubility of the solute in the solvent increases only slightly with the temperature is proposed according to the invention to make the internals in the evaporator so that crystallization in the evaporator can be allowed and the crystals are fed to the crystallizer below the evaporator. In the crystallizer, the further reduction of the supersaturation of the solution takes place. According to the invention for this case the internals 16 the evaporator designed so that a separation of the crystals of the internals can be done without interrupting the operation. The internals are preferably formed in the form of rotating brushes. The bristles may touch and separate the crystals adhering to the bristles so that they can fall into the crystallizer and be discharged. Other types of internals are also suitable, for example elastic packings which are oscillatingly compressed and relaxed, so that the adhering crystals come off. The rotation or oscillation can take place continuously or intermittently by drives.

Also, the crystallization can be carried out at temperatures that allow an advantageous use of plastic, with a small load from the operation occurs in the range of atmospheric pressure. The crystallizer can thereby be easily connected to the HD system.

From the crystallizer, the crystals can be discharged together with brine and mechanically separated from the approximate saturated solution. In 3 is a filter band for this separation 56 shown, so that the brine with the pump 57 into the HD process for further concentration at higher temperature can be attributed. The retained by the filter belt crystals is from the filter belt in the Kristallisatbehälter 60 promoted.

Most of the water to be removed was extracted from the solution by the multi-stage HD process with little effort (compared to evaporation processes). The externally per stage in external heater 4 supplied heat flow is in 2 not shown. It leads to a small increase in the enthalpy at a constant salt content S _{i of} the sols processed in the respective stage. The majority of the brine heating is done by the HD countercurrent process. The temperature increase Δt _{23, i} in the external heater applies here 4 : $$\Delta {\text{t}}_{23\text{i}}=\Delta {\text{t}}_{12\text{i}}/{\text{PR}}_{\text{HD}\text{i}}$$

Due to the countercurrent process of the HD process, therefore, only the heat flow reduced by the divisor PR _{HD is} externally supplied to the brine.

An advantageous arrangement may result in low salinity starting solutions if the HP process is preceded by reverse osmosis, which can be used in the region of low salt concentrations and works advantageously in this area.

By brine recirculation, which is described in the main application, the concentrated brine can be partially thickened at a sufficiently high temperature in the HD process.

To clean the internals, the evaporators can be provided with spraying, so that the internals can be cleaned with condensate during maintenance.

The process according to the invention can also be used to construct large plants for the concentration of solutions or for the recovery of solvents. The advantages of a modular design can be used. For example, very large systems can be created by series and parallel connection of modules with a height of 2 m, a width of 2 m and a depth of 2 m and operated continuously with a heat source of about 100 ° C.

On the other hand, the method is also suitable, for example, for the production of pure water for households, for example, for a system that is operated with a solar heating of 100 kWh / day and an efficiency of 7 and generates about 1 cubic meter of pure water per day.

LIST OF REFERENCE NUMBERS

1

Evaporator (humidifier)

2

Condenser (dehumidifier)

3

Active wall (installation according to the invention between evaporator and condenser)

4

External heating

5

raw water pump

13

Fan (fan) of Active Wall 3

14

Cross-flow openings of the active wall 3

15

Internals (e.g., tube bundles) of the capacitor

16

Internals (e.g., packages) of the evaporator 1

50

brine pump

51

brine cooler

52

Increase in salinity ΔS ₁ in the first stage

53

Saturation line of crystallization

54

Temperature line of the crystallizer

55

Crystallizer

56

filter belt

57

Brine pump at the filter belt

58

Cooler at the crystallizer

59

Circulation pump at the crystallizer

60

Kristallisatbehälter

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• B. Seifert et al .: "About the History of Humidification-Dehumidification Desalination Systems", October 2013, Conference: International Desalination Association World Congress on Desalination and Water Reuse At: Tianjin, China [0006]

Claims (10)

Process for concentrating solutions by using the evaporation-condensation process with at least two stages, each with evaporator, condenser, external heater, as well as with fan and cross-flow openings (active wall), to evaporate solvent and to condense the evaporated solvent and using the heat of condensation for preheating the solution, characterized in that the enriched by the evaporation solution of the following stage is fed to the further concentration of the solution. A method for concentration of solutions by the use of the evaporation-condensation process with several stages, each with evaporator, condenser, external heating, and preferably with fan and cross-flow openings (active wall), characterized in that the accumulation takes place in the stages the solution leads to crystallization, wherein the crystals are discharged via a crystallizer. Method according to Claim 2 , characterized in that the crystallization is carried out by cooling in the crystallizer. Method according to Claim 2 , characterized in that crystallization also occurs in the evaporator, which is equipped with internals, from which crystals are separated during operation. Device for carrying out the method according to one of Claims 1 to 4 , characterized in that systems are constructed of largely uniform modules, which are arranged side by side and übereinanderader and which are held in a frame structure. Device for carrying out the method according to Claim 3 or 4 , characterized in that the crystallizer is arranged under the evaporator. Device for carrying out the method according to Claim 4 , characterized in that brushes or elastic packings are used for the internals in the evaporator. Device for carrying out the method according to one of Claims 1 to 4 , characterized in that the modules are arranged in a frame structure and the internals of the modules can be completely or partially removed from the frame structure. Device according to one of the preceding claims, characterized in that an evaporator is arranged between two capacitors. Method according to one of Claims 1 to 4 , characterized in that the HD system is used together with a device for reverse osmosis, which is connected upstream of the HD system.

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