DE4244823C2 - Values and water recovery from aq. process liq. - Google Patents

Abstract

Recovery of values and water from aq. process liqs., esp. from electroplating and electroless plating plants, anodising and conversion coating plants, pickling baths, polishing baths, residual refining electrolytes, ion exchanger regenerates and rinsing baths, is carried out by (i) contacting the liq. with a liq.-tight, vapour permeable, microporous membrane which is not wetted by the liq.: and (ii) allowing water vapour and other opt. present volatile matter to pass as gases through the micropores to the other side of the membrane for condensation at reduced temp.

Description

The invention relates to a device for performing membrane distillation. In the Membrane distillation is the liquid to be distilled from the distillate through a liquid dense but vapor-permeable, microporous membrane separated. Point the two through Membrane-separated liquids, usually due to a temperature difference, below different pressures, steam is transported along the steam pressure gradient.

Devices for carrying out the membrane distillation were mainly used to obtain Drinking water developed from sea water or saline groundwater. From the US PS 3340186 is known to separate units for membrane distillation in a plate stack summarize or the channels for the warm, salty water and for the cooler De Stillat spirally wound into a cylinder. In DE-PS 31 40 614 is also one Device for membrane distillation specified in the form of a stack of plates.

From US-PS 4545862 and DE-PS 38 24 829 is also a spiral to a cylinder wound device for membrane distillation known in which a heat recovery is possible without an external heat exchanger. These devices contain three different ones Channels: a channel for the hot, distilled by the microporous membrane de liquid, a distillate channel and a channel with vapor and liquid-tight walls for the cold liquid to be heated. When distilling through the micropores of the mem bran is heat from the hot liquid to the distillate and from the distillate to the cold transfer heating liquid. The liquid warmed by this becomes after the outlet further heated from the cylinder by external heating and then in by the mikropo red membrane directed channel.

DE 25 29 977 B2 and DE 22 34 289 B2 also include devices with wound, tubular membranes for performing reverse osmosis and Ultrafiltration known.

From the article "Membrane Distillation" by K. Schneider and K. J. van Gassel in the magazine Chemical engineering technology Volume 56, year 1984, volume 7, pages 514 to 521 is also be Knows not to use flat membranes for membrane distillation but capillary membranes the. This article covers the following main areas of application for membrane distillation called:

• - Seawater desalination using waste heat from diesel engines and power plants as well as solar energy,  
• - Concentration of product and waste solutions as well as the combination of concentrates production of demineralized water for industrial purposes,
• - Boiler feed water generation in power plants with a low-temperature condensate system.

The types of devices known for the production of drinking water from sea water Membrane distillation can also be used for other applications, e.g. B. for recycling winnung from aqueous process liquids z. B. use of surface treatment plants be det.

For the production of electrolytically (galvanically) deposited metal layers, without current (Chemically) deposited metal layers as well as anodically and chemically generated cones The product to be treated passes through a series of treatment baths contain aqueous process liquids. Common to all of these processes is pre-treatment development for the production of a pure product surface. The pretreatment usually includes egg degreasing and pickling.

After coating, an aftertreatment often follows, e.g. B. passivation, creating a conversion layer or seal. Between the different Treatment stages, the goods are rinsed to prevent carryover of the process fluids to avoid the subsequent treatment baths.

The last rinsing process must be carried out with deionized water (deionized water) Avoid stains on the surface of the goods after drying. Often, too because last rinsing between different treatment stages, especially before and after the coating baths, carried out with deionized water.

The distillate produced by membrane distillation is suitable for rinsing purposes as a replacement for Demineralized water, because apart from water the distillate contains at most volatile substances, which according to the Drying the washed goods can not cause stains on the surface. Since the Membrane distillation up to high concentrations close to the saturation concentration is feasible, the concentration in the concentrate obtained by membrane distillation be increased to the extent that the concentrates, if necessary, are returned to the process can be led. Membrane distillation allows both the aqueous solutions from the treatment baths as well as the rinse water from these treatment baths downstream sink are treated.

Due to different areas of application and different operating conditions, there are other re requirements for the materials used. For reasons of corrosion resistance, it is e.g. B. advantageous for the vapor and liquid-tight walls of the channel to be heated  Mende liquid films made of a corrosion-resistant plastic, for. B. a fully or partially fluo Plastic, such as PVDF, and no metallic foil, although metallic foils do would be advantageous against the greater thermal conductivity.

However, the known devices for membrane distillation can only be used when temperatures of about 70 to a maximum of 80 ° C in the device for membrane distillation nowhere to be exceeded. The known devices contain glued or Places potted with synthetic resin. Establishing a permanent link between the non-wettable plastics PP, PVDF and in particular PTFE with the sealing compound particularly difficult and only after a suitable, activate the surface of these plastics pretreatment possible. The risk of pestilence increases with increasing temperature due to the connection between the casting compound and the cast plastic different coefficients of thermal expansion, so that the liquid to be treated can pass into the distillate channel along with any material detachments that may occur.

When seawater is desalinated, the temperature generated in the heating device is approx. 70 ° C limited, otherwise there is a risk that precipitated substances such. B. plaster or Deposit lime in the desalination device and especially on the membrane.

This restriction is usually not given for other areas of application.

It is advantageous to carry out membrane distillation at higher temperatures. As the temperature increases, the vapor pressure rises ever faster. A predetermined temperature difference between the process liquid to be treated on one side of the non-wettable microporous membrane and the distillate on the other side therefore leads to a greater vapor pressure difference at higher temperatures and thus to a higher distillate output than at low temperatures. At higher temperatures, the proportion of non-condensable gases in the micropores of the membrane 20 is also lower. This also improves the separation performance of the membrane, since the proportion of heat losses due to heat conduction via the membrane decreases. In membrane distillation devices with internal heat recovery, operation at higher temperatures also leads to an increase in the proportion of heat recovered. The ratio of the externally supplied heat to the recovered heat corresponds to the ratio between the external temperature increase of the liquid to the temperature increase between the entry of the cold liquid into the device for membrane distillation and the entry into the external heating device.

The vapor pressure of water is approx. 300 mbar at 70 ° C and approx. 850 mbar at 95 ° C. A Heating of the aqueous process liquid in the heating device to 95 ° C instead of 70 ° C  therefore has a significantly higher distillate output. When choosing suitable ones Materials and a pressure-resistant design of the device for membrane distillation can Liquid in the heating device can be heated up to 120 ° C or even up to 130 ° C. At The steam pressure is almost 2 bar at 120 ° C.

Another disadvantage of the known devices for membrane distillation is that that in the event of a defect, in particular a defect in the microporous membrane, in the mei In most cases, neither repair nor shutdown of the defective area is possible that in these cases the entire device becomes unusable.

The object of the invention is therefore to construct devices for membrane distillation in such a way that the risk of total failure is significantly reduced.

Another object of the invention is also to provide devices for membrane distillation construct that the membrane distillation at high temperatures above about 80 ° C. can be carried out.

These tasks are characterized by a device with the specified in claim 1 drawing features met.

The device according to the invention is special for operation at high temperatures suitable because of the use of hoses within the membrane device distillation no gluing, welding or casting is required. It's just verb of the manifolds with the hoses connected in parallel. These Connections can be made accessible from the outside, so that they are critical Any damage that may occur can be repaired.

Another advantage is the fact that in the event of a hose defect inside the Device for membrane distillation the defective tube from the outside clamped or ver can be closed. In such a case, the distillate output is reduced accordingly, however, the device is not completely unusable.

If temperatures higher than approx. 90 ° C occur in the device for membrane distillation can, the device for membrane distillation should only consist of temperature-resistant and cor corrosion-resistant plastics e.g. B. partially or fully halogenated plastics such as PVDF or PTFE be built up: often as a backing for the microporous membrane and as PP material used should be replaced with one of these plastics.  

The membrane distillation should be carried out at temperatures at which the steam pressure of the process liquid to be concentrated is greater than the atmospheric pressure, then the device must be pressure-resistant, and the distillate should be approximately below the same ben pressure are like the process liquid, so that the pressure difference on both sides of the microporous membrane at no point a predetermined limit of z. B. 50 kPa exceeds.

In Fig. 1 illustrates an apparatus for membrane distillation with internal heat recovery embodiment. Fig. 2 shows a detail of this device.

The aqueous process liquid to be treated enters the device for membrane stillation at ( 30 ) and is distributed over several tubes with thin, liquid and gas impermeable walls ( 29 ). Through these hoses, the aqueous process liquid flows with heat absorption from the condensing distillate on spirals inwards and upwards. At the upper outlet from the device, the aqueous process liquid is collected in an entire line and further heated in a heating device ( 25 ). After this heating, the aqueous process liquid in the interior of the device for membrane distillation is distributed over several tubes with liquid-tight but gas-permeable microporous walls ( 20 ). Through these hoses, the aqueous process liquid flows with cooling due to the evaporation of part of the process liquid spirally downwards and outwards and leaves the device for membrane distillation at ( 31 ) via a manifold. The distillate formed condenses between the tubes with the gas-permeable walls ( 20 ) and the tubes with the gas- and liquid-tight walls, which can be removed at ( 32 ). The temperature in the device increases from bottom to top. This is an advantage for the most complete possible heat recovery, since gravity in this case does not lead to stratification in the distillate channel.

The condensing in the intermediate space between the tubes distillate is derived at the coldest point down and the device shown in Fig. 1 exits at (32).

The innermost tube is wound onto a truncated cone ( 36 ) with a ramp ( 37 ) which is guided around the outside of the truncated cone at a uniform slope. The width and pitch of the ramp ( 37 ) correspond to the outside diameter of the hoses used. For clarification, a top view of the ramp is shown in FIG. 2. To improve the heat transfer, it is advantageous not to use the ramp made of solid material but z. B. as a grid or ladder-like, as shown in Fig. 2. This winding ensures that each membrane tube with the process solution to be concentrated in the interior of the device for membrane distillation, as shown in Fig. 1, is surrounded by 4 film tubes and, conversely, each film tube by 4 membrane tubes.

The hoses shown in Fig. 1 can be compressed for larger diameters. If this danger exists, the hoses can be extruded via a plastic spiral or a similar device, by means of which the circular hose cross section is guaranteed over the full length of the hose. Such a spiral also improves the heat transfer to the hose wall. This is not necessary for small diameter hoses.

It is possible to wind up the hoses more densely than shown in Fig. 1, e.g. B. so that the centers of the tube cross sections of the different layers are not as shown in Fig. 1 Darge are superimposed but are shifted from layer to layer by the hose radius. The width and pitch must be reduced accordingly.

If, when using tubes with a small diameter, the distillate flow is impermissibly impeded, then a less dense winding of the tubes with additional auxiliary devices, not shown in FIG. 1, can also be achieved without problems.

In the FIG. 1 is a schematic representation. The number of hoses shown in FIG. 1, which are connected in parallel, serve only as an example. The exact number depends on the intended operating parameters, the degree of heat recovery desired, the hose diameter and the prices for hoses.

In the device for membrane distillation with integrated heat recovery shown in Fig. 1, a turbulent flow and thus a good heat transfer to the film or tube wall ( 29 ) or the microporous membrane ( 20 ) can be realized in the hoses. In contrast, there is only a low flow in the distillate space and thus poor heat transfer to the membrane ( 20 ). Since the backing of the microporous membrane immobilizes a liquid layer of appropriate thickness, it is more favorable to install the microporous membrane so that the supporting fabric faces the distillate space than vice versa. The arrangement is also advantageous if there is a lower pressure on the distillate side than in the channels with the process liquid to be concentrated or when pressure surges in the channels with the process liquid to be concentrated cannot be completely prevented.

Claims (8)

1. Apparatus for carrying out the membrane distillation of aqueous solutions with internal heat recovery, characterized in that the channels for the heated liquid to be concentrated ( 21 ) are formed by hoses with a preferably circular cross section, the walls of which are made of a liquid-tight but vapor-permeable gene, Microporous membrane ( 20 ) are formed so that the channels for the liquid to be heated ( 29 ) are formed by hoses with a preferably circular cross-section, the walls of which consist of liquid and gas-tight films, that the hoses are wound in several horizontally superimposed layers and within of a common housing are arranged such that the membrane tubing be adjacent tubes are film tubes and that the neighboring tubes are th tubes are membrane tubes. 2. Device according to claim 1, characterized in that all hose ends in the lowest and the highest horizontal position are arranged. 3. Apparatus according to claim 1 or 2, characterized in that the hoses on occurs in or at the outlet from the device for membrane distillation individually clamped or can be closed. 4. The device according to claim 1, characterized in that the supporting tissue of the micropo eyed membrane facing the distillate. 5. The device according to claim 1, characterized in that the microporous membrane and their support fabric is made of the same material. 6. The device according to at least one of the preceding claims, characterized net that the connections for the cold liquids in the lower area and the connections for the warm liquids in the upper area of the device.   7. Use of the devices according to at least one of the preceding claims Treatment of aqueous process solutions, in particular for the treatment of aqueous Process solutions from treatment baths or the downstream of these baths Rinse baths of surface treatment plants for galvanic or chemical Deposition of metal layers or for the production of conversion layers. 8. Use of the devices according to at least one of claims 1-6 for treatment of aqueous solutions with a temperature of at least 80 ° C.