Osmotic distillation process for concentrating a liquid containing sodium chloride

Abstract

Described is an osmotic distillation process for concentrating a liquid containing sodium chloride, and in particular a treatment process for used reaction water containing sodium chloride from the production of polymers.

Claims

1. An osmotic distillation process for concentrating an aqueous liquid containing sodium chloride, where the liquid has a concentration of up to 20% by weight of sodium chloride, which comprises at least the following steps, a) optionally prepurification of the liquid to remove organic secondary constituents, optionally down to a total content of the organic secondary constituents of 20 ppm, b) introducing the optionally prepurified liquid into a first evaporation zone which is separated from a diffusion zone adjoining the first zone by a hydrophobic first membrane which is permeable to water vapor, c) diffusion of water vapor from the liquid containing sodium chloride through the membrane into the diffusion zone, d) further diffusion of water vapor from the diffusion zone through a second hydrophobic membrane which is permeable to water vapor into a stripping zone adjoining the diffusion zone and absorption of the water vapor in a draw solution which is continuously replaced in the stripping zone.

2. The process as claimed in claim 1, wherein a concentrated alkaline metal hydroxide solution, having a concentration of alkaline metal hydroxide, of from 10% by weight to 50% by weight, is used as the draw solution.

3. The process as claimed in claim 1, wherein the aqueous liquid containing sodium chloride is production wastewater from a process for preparing polymers.

4. The process as claimed in claim 1, wherein the first membrane and the second membrane are, independently of one another, comprised of a hydrophobic polymer.

5. The process as claimed in claim 1, wherein the evaporation zone, diffusion zone, and stripping zone are maintained independently of one another at atmospheric pressure or reduced pressure.

6. The process as claimed in claim 1, wherein the evaporation zone, diffusion zone, and stripping zone are, independently of one another, maintained at a temperature of from 10 to 80 C.

7. The process as claimed in claim 6, wherein the temperature in the evaporation zone is greater than in the diffusion zone and the stripping zone.

8. The process as claimed in claim 1, wherein the liquid containing sodium chloride in the evaporation zone and the draw solution in the stripping zone are conveyed in countercurrent relative to one another past the respective membranes.

9. The process as claimed in claim 1, wherein the aqueous liquid containing sodium chloride has turbulent flow in the evaporation zone.

10. The process as claimed in claim 1, wherein the evaporation zone, diffusion zone, and stripping zone have been joined to one another by means of fusion bonding.

11. The process as claimed in claim 1, wherein the process steps b), c) and d) are carried out in a plurality of stages, with at least one further evaporation zone, diffusion zone, and stripping zone being employed.

12. The process as claimed in claim 1, wherein liquid exits from the first evaporation zone in a first arrangement made up of evaporation zone, diffusion zone, and stripping zone and is again subjected to process steps b) and c) in at least one downstream arrangement made up of evaporation zone, diffusion zone, and stripping zone and diluted draw solution exits from a second stripping zone in at least one downstream arrangement made up of evaporation zone, diffusion zone, and stripping zone is used for carrying out step d) in the first arrangement made up of evaporation zone, diffusion zone, and stripping zone.

13. The process as claimed in claim 12, wherein a concentrated aqueous liquid containing sodium chloride exits from the evaporation zone of the first osmotic distillation arrangement and is subjected to heat exchange.

14. The process as claimed in claim 12, wherein a draw solution exits from the stripping zone of the first arrangement and is subjected to heat exchange.

Description

(1) The invention is illustrated below, with the aid of the figures, by the examples which do not, however, constitute a restriction of the invention.

(2) The figures show:

(3) FIG. 1 a cross section through an OVD (osmotic vacuum distillation) arrangement according to the invention

(4) FIG. 2 a plan view of a possible connection of a plurality of OVD modules with heat exchangers arranged in between (example of feed solution and draw solution having the same temperature)

(5) FIG. 3 a plan view of a possible connection of a plurality of OVD modules with heat exchangers arranged in between (example of cold feed solution and hot draw solution)

(6) In the figures, the reference numerals have the following meanings: 1 draw solution chamber (stripping zone) 2 inlet for concentrated draw solution 3 outlet for dilute draw solution 4 feed chamber (evaporation zone) 5 inlet for feed solution 6 outlet for concentrated feed solution 7 vapor chamber (diffusion zone) 8 dam (for drainage) 9 first membrane for contact with draw solution 10 second membrane for contact with feed solution 11, 11a outlet for any draw solution passing through the membrane 9 12, 12a outlet for any feed solution passing through the membrane 10 13 vacuum line 14, 14a heat exchanger 15, 15a total OVD module according to the invention 16, 16a polypropylene separating film in the heat exchanger 14, 14a

EXAMPLES

Example 1

(7) A focus of the invention is the use of a novel arrangement for separation of materials, namely osmotic vacuum distillation (OVD module), for preventing mixing of liquids in the case of membrane wetting. The concept of the module arrangement is shown in FIG. 1. This novel arrangement makes it possible to separate the liquids (feed solution and draw solution) by means of two hydrophobic membranes 9 and 10 between which a vapor channel 7, which also serves to discharge any liquids which have intruded, is arranged.

(8) At the start of the process, the system pressure is reduced to a pressure of 20 mbar (20 hPa) using a vacuum pump via the conduit 13. Mass transfer can be significantly improved thereby, since the transport of water vapor can be hindered by the membrane pores and, in the vapor chamber 7, by any inert gasses. Feed solution (NaCl 10% by weight, temperature 50 C., water vapor partial pressure 110 mbar) is admitted via the inlet 5 into the feed chamber 4. Water vapor is conveyed through the membrane 10 having the active area of 0.5 m.sup.2 into the vapor chamber 7 (diffusion zone). The vapor flux through the membrane 10 is 3 kg/h.Math.m.sup.2. The concentrated feed solution (NaCl 10.1% by weight, temperature 41 C., water vapor partial pressure 70 mbar) leaves the feed chamber 4 via the outlet 6. The concentrated draw solution (NaOH 30% by weight, temperature 50 C., water vapor partial pressure 64 mbar) entering at the inlet 2 takes up water vapor which goes through the membrane 9 into the draw solution chamber 1 (stripping zone). The vapor flux through the membrane 9 is likewise 3 kg/h.Math.m.sup.2. The diluted draw solution (NaOH 29.6% by weight, temperature 60 C., water vapor partial pressure 100 mbar) leaves the draw solution chamber 1 via the outlet 3. A dam 8 in the vapor channel 7 additionally ensures that mixing of any feed solution and any draw solution which have gone through the membranes 9 and 10, respectively, is not possible. These are separately discharged via the lines 11 and 12 and optionally recirculated.

(9) A further advantage of the present arrangement is that two membranes 9 and 10 having different properties can be used, depending on the requirements which feed solution and draw solution have to meet. A membrane 9 having the following properties can advantageously be used: active layer: PTFE, layer thickness about 25 m, pore size 0.2 m, water intrusion pressure 3.5 bar; support layer: PP, layer thickness about 200 m.

(10) The latent heat of the water vapor cools the feed solution 6 as a result of evaporation and heats the draw solution 3 as a result of the condensation. However, this heat transport reduces the driving vapor pressure difference for mass transfer.

Example 2

(11) FIG. 2 depicts, by way of example, an arrangement having two coupled modules 15, 15a which are employed for a concentrated draw solution at 50 C. and a dilute feed solution at 50 C. The arrangement consists of two membrane modules 15, 15a and two heat exchangers 14, 14a. The heat exchangers 14, 14a are configured as frame-like inserts which have inlets and outlets for the liquids (feed solution and draw solution) and are closed at the side by the modules 15, 15a or by an end plate. Thin (about 25 m) polypropylene films 16 and 16a, which serve as heat transfer area, are applied to each of the frames of the heat exchangers 14, 14a (FIG. 2). The heat exchangers 14, 14a formed in this way are assembled alternately with the OVD membrane modules 15, 15a to form a block. The innovative concept of realizing all functions in one block offers advantages in respect of avoidance of additional external heat exchangers and piping. In the heat exchangers 14, 14a, heat exchange between the respective depleted draw solution and enriched feed solution occurs (heat recovery).

(12) The two solutions, draw solution 2 and feed solution 5, are conveyed in countercurrent. The system pressure is reduced to 20 mbar at the beginning of the process. The draw solution (NaOH, 30.00% by weight, temperature 50 C., 100 kg/h) enters the OVD module 15 via the inlet 2. The feed solution (NaCl, 9.85% by weight, temperature 50 C., 101.5 kg/h) enters a heat exchanger 14a via the inlet 5. The draw solution leaves module 15 and, after passing through heat exchanger 14, is introduced into the draw chamber of module 15a. Further depleted draw solution leaving module 15a is, after passing through the further heat exchanger 14a, discharged (conduit 3). In order to achieve optimal mass transfer between feed solution and draw solution, a plurality of the two-module arrangements described in this example 2 are connected in series, with the number of two-module arrangements used being in the order of 20 in the case of an increase in the concentration of the feed solution of 100 kg/h from 7% by weight to 20% by weight. The parameters relevant to the process can be seen in tables 1 and 2.

(13) TABLE-US-00001 TABLE 1 Parameters for membrane modules Module 15 Module 15a NaOH NaCl NaOH NaCl Flux [kg/hm.sup.2] 3 3 3 3 membrane area [m.sup.2] 0.5 0.5 0.5 0.5 Tin [ C.] 50.0 56.0 53.0 58.5 Tout [ C.] 59.9 47.4 62.5 50.0 Mass flow_in [kg/h] 100.0 100.0 101.5 101.5 Mass flow_out [kg/h] 101.5 98.5 103.0 100.0 Concentration_in [% by weight] 30.00 10.00 29.56 9.85 Concentration_out [% by weight] 29.56 10.15 29.13 10.00 Vapor pressure_in [mbar] 63 140 68 165 Vapor pressure_out [mbar] 100 95 120 108 Cp [J/kgK] 3590 4180 3590 4180

(14) TABLE-US-00002 TABLE 2 Parameters for heat exchangers Heat exchanger 14 Heat exchanger 14a NaOH NaCl NaOH NaCl Area [m.sup.2] 0.25 0.4 k [W/m.sup.2K] 800 800 Tin [ C.] 59.9 50.0 62.6 50.0 Tout [ C.] 53.0 56.0 52.6 58.5

Example 3

(15) The modular construction is very flexible and can easily be changed by altering the order of heat exchanger blocks and membrane blocks. FIG. 3 shows, by way of example, a modification of the arrangement of example 2, which is designed for a hot concentrated draw solution (NaOH, 30% by weight, temperature 70 C., 100 kg/h) and a dilute cold feed solution (NaCl, 9.85% by weight, temperature 40 C., 101.5 kg/h). In this case, the two solutions 2 and 5 are firstly fed into a heat exchanger 14 located upstream of the first module 15 in order to effect heat exchange. The feed solution which has been heated to 60 C. flows into the module 15a. The NaOH which has been cooled to 50 C. flows into the module 15. The two solutions are then conveyed in a manner analogous to example 2 in countercurrent through the modules 15, 15a and the heat exchanger 14.