WO2009043413A1

WO2009043413A1 - Process and plant for recovering water from air

Info

Publication number: WO2009043413A1
Application number: PCT/EP2008/006975
Authority: WO
Inventors: Wolf-Christoph Rauser
Original assignee: Outotec Oyj
Priority date: 2007-10-02
Filing date: 2008-08-26
Publication date: 2009-04-09
Also published as: CL2008002831A1; DE102007047319A1; PE20090929A1

Abstract

When recovering water from air, the air is brought in contact with sulfuric acid having a concentration ≥ 60 wt-%, so that water condenses on the surface of the acid. The sulfuric acid thus diluted is fortified to a concentration > 60 wt-%, wherein the steam released is condensed by cooling. The condensate obtained is discharged.

Description

Process and Plant for Recovering Water from Air

This invention relates to the recovery of water from gas, in particular ambient air.

In many regions of the earth, lack of water represents an increasing problem. Although in some parts of the world water is regarded as a renewable resource, the constantly growing demand cannot be satisfied due to the pollution of the environment or an insufficient water infrastructure. The lack of water constitutes an increasing risk for the industry worldwide, which for instance in mining has started to recycle as much water as possible. However, this leads to a deteriorated quality of the repeatedly recycled water, whose content of salt and noxious substances is increased by evaporation.

Usual water recovery processes require the availability of crude water in water-bearing layers or sea water. Independent of the technology used, the plants for the treatment of water should be located as close as possible to the crude water source, in order to minimize the conduit or pump costs. In addition, the high salt content of sea water leads to a great loading of the conduits and therefore to high maintenance costs.

Before desalting crude water, the same must be purified and liberated from solids, immiscible liquids, hardly soluble salts etc., since accumulations of these materials on the desalting surfaces distinctly deteriorate the efficiency. In addition, the service life of the desalting plants is reduced without an adequate pretreatment of the crude water, which reduces the operating times and further increases the maintenance costs.

The process for desalting sea water mostly used at present is the distillation/condensation, which is employed in more than 70% of all desalting plants. In high- temperature distillation, temperatures above 95°C are employed. Mechanical systems operate at temperatures below the boiling point and utilize different processes for evaporating crude water. In addition, membrane techniques such as electrodialysis or reverse osmosis are known. Electrodialysis is an electrochemically operated membrane process, in which ion-exchange membranes are used in combination with an electric potential difference, in order to separate ionic species from uncharged solvents or impurities. If in such electrodialysis plants the polarity regularly is reversed, so that the ions flow into the opposite direction, the anions are recirculated through the membrane and promote the break-up of deposits on the surface of the membrane.

In reverse osmosis, the solution (sea water) is pressed through a semipermeable membrane under high pressure, in order to overcome the osmotic pressure. This semipermeable membrane acts like a filter and only lets pass certain ions and molecules. Thus, a separation of the original solution is obtained. By means of the membrane filter, salts, bacteria, viruses, lime and poisons such as heavy metals can be retained.

In these known processes, however, considerable waste streams are produced. During the pretreatment, the solids and the chemicals added to precipitate the same form flakes, which must be removed from the crude water by clarification. The sewage sludge is dewatered and dumped. Beside the solid waste, liquid waste can also be obtained during filtration. During the evaporation in the distillation/condensation system, a concentrated brine is obtained, which can increase the salt content of the ground water when discharge into the sea is not possible. Membrane desalting is the most efficient process for producing drinking water, but most of the pretreatment waste is produced thereby. In addition, a brine of low concentration and the used membrane modules are obtained here.

It also has already been proposed to recover water from air. DE 197 34 887 C2, for instance, describes an apparatus for recovering water from air by means of a hygroscopic absorption material which is intermittently charged with humid air and is exposed to a heat source. The air traverses a container, which includes a highly porous substrate, e.g. silica gel, whose pores are impregnated with a selective water-absorbing material such as calcium chloride, lithium bromide or sodium sulfate. In the apparatus known from DE 10 2004 026 334 A1 , water is recovered from atmospheric air by means of a flowable adsorbent or absorbent, in particular a brine solution with a hygroscopic salt.

There are also known processes for drying air, in which drying the air is effected by means of absorption on solid or liquid absorbents. DE 100 14 792 A1 describes the drying of air in ventilation means for fuel tanks by means of vapor permeation membranes and sorbents. As liquid absorbents, lithium chloride solutions, sulfuric acid and glycols are proposed. The regeneration during operation of the absorption driers is effected by thermal cycling or pressure swing methods or by replacement of the used materials.

It is the object of the invention to provide for the economic recovery of water without large waste streams.

This object substantially is solved with the invention by a process for recovering water from gas, in particular air, in which the air is brought in contact with sulfuric acid having a concentration > 60 wt-%, so that water is condensed and absorbed on the surface of the acid, wherein the sulfuric acid thus diluted is fortified to a concentration > 60 wt-%, wherein the vapor released is condensed by cooling and the condensate is discharged. By means of the invention, the water thus is extracted from the ambient air, so that no crude water must be provided. In addition, there are no large waste streams as compared to the desalting processes. Rather, it is even possible to recover water from waste gases of chemical or metallurgical processes.

In accordance with a preferred aspect of the invention, the air is brought in contact with sulfuric acid having an (original) concentration of 65 to 80 wt-% and in particular about 70 to 75 wt-%, wherein the air preferably is guided through a drying tower (e.g. packed tower) in counterflow with sulfuric acid trickling down.

For recovering the water from the acid, the same is fortified to its original concentration. In accordance with the invention, fortifying the dilute sulfuric acid is effected by vacuum evaporation, and at the above-mentioned concentrations a low or technical vacuum is provided in the range from 10 to 90000 N/m², preferably 100 to 10000 N/m², particularly preferably 2000 to 2500 N/m².

It can also be provided that a part of the dilute sulfuric acid is supplied to a sulfuric acid plant, so that the supply of water required for dilution of the sulfuric acid can be reduced there. This dilute sulfuric acid also can wholly or partly be supplied to other plants, which require such sulfuric acid directly or for dilution or fortification. Before being delivered to other parts of the plant, this sulfuric acid can also be prepurified.

In accordance with another aspect of the invention, the gas, in particular air, is passed through a humidification means, before it is brought in contact with sulfuric acid. In most cases, ambient air is below its state of saturation, so that it can additionally be humidified in the humidification means. Part of the water evaporates into the air stream, and at the same time a cooling of the water is effected. This cool water then can be used for condensation of the steam from the vacuum unit.

In accordance with a development of the invention, this humidification means provides for purifying contaminated water. For this purpose, the gas, preferably air, likewise can be cleaned before humidification, in order to reduce impurities of the water.

In accordance with the invention, a partial stream of the condensate can be branched off after the condenser and be supplied to the humidification means, so that an external water supply can be omitted.

In accordance with a development of the invention, the humid gas can originate from a plant in which humid gas is generated. Examples for this include the drying of biomass, leaching residues and crystallization products, drying and/or calcination of aluminum hydroxide or other dehydrating substances, combustion of biomass, hydrocarbons, steam or other materials containing hydrogen. After a first step for recovering water from these gases, e.g. by condensation according to EP 1 063 472 or AU 2005 237179, this process furthermore can be used for the more complete recovery of water from the waste gas. This invention also comprises a plant for recovering water from gas, in particular air, which is suitable for performing the process described above and includes a drying tower in which air is passed through concentrated sulfuric acid, an evaporator to which the sulfuric acid diluted by water condensed out of air is supplied, and a condenser in which steam discharged from the evaporator is condensed.

In one aspect of the invention, a humidification means is provided upstream of the drying tower, in which the air is humidified by means of cooling water. In accordance with a development of this invention, the water outlet of the humidification means is connected with the water inlet of the condenser, in order to utilize the water for the cooling and condensation of the steam emerging from the evaporator.

Developments, advantages and possible applications of the invention can be taken from the following description of an embodiment and the drawing. All features de- scribed and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

In the drawing:

Fig. 1 shows a diagram of the water content of air over sulfuric acid at 40^cC in dependence on the acid concentration,

Fig. 2 shows a diagram of the H₂SO₄ vapor pressure over sulfuric acid at 40⁰C in dependence on the acid concentration, and

Fig. 3 schematically shows a flow diagram of the process of the invention.

Ambient air usually has a water content of 15 to 30 g H₂O/Nm³. When this ambient air is brought in contact with concentrated acid, steam condenses on the acid surface, on which the saturated steam pressure is lower than the steam partial pressure in the gas volume, and is absorbed by the acid. After sufficient contact with the acid, the residual water content of air theoretically corresponds to the partial pressure of the steam over the acid at the existing temperature and concentration. Fig. 1 shows that with acid concentrations above 60 wt-% a significant drying effect is achieved, with which only a water content < 10 g/Nm³ is left in the air.

The recovery of water from the acid is effected by fortification of the acid to its original concentration. As can be taken from Fig. 2, the acid content of the gas phase is neg- ligeably small at acid concentrations of less than 75 wt-% H₂SO₄ due to the low H₂SO₄ vapor pressure. Thus, at an acid concentration below 75 wt-% H₂SO₄, substantially only water is evaporated.

From the marginal conditions illustrated in Figures 1 and 2, it can be taken that the acid concentration for an optimum recovery of water from the ambient air lies in the range from 65 to 75 wt-%. If a higher acid concentration is chosen, which would extract more water from the air, this would lead to the fact that too much impurities, in particular sulfur trioxide (SO₃), would be contained in the vapors produced in the evaporator and an additional air cleaning would have to be provided.

In accordance with the invention, the fortification of the sulfuric acid is effected by vacuum evaporation. The dilute sulfuric acid is maintained at boiling temperature by indirect heating under a vacuum. A fortification of the acid to 70 wt-% at 70^cC requires a vacuum of about 2200 N/m², which must be regarded as a low or technical vacuum and therefore can be provided at reasonable cost.

The vapors produced during evaporation are condensed in a cooling system, so that except for negligeable quantities of inert gases desorbed from the acid no waste gas streams are obtained. The concentrated acid is recirculated to the drying tower, while the vapor condensate is discharged for further use and treatment.

The energy required for evaporation of water usually is provided in the form of low- pressure vapor. Since the heat of condensation is, however, directly transferred into the acid and heat losses are negligeable, it is possible to keep the amount of heat which must be supplied from outside distinctly smaller than in the conventional processes. The vacuum generation unit actually consumes most of the energy. By using multistage evaporators and/or reducing the applied vacuum, the amount of the required steam or hot water can distinctly be reduced.

The efficiency of the plant is further improved by using a combined saturation/cooling tower upstream of the drying tower. The supplied air, which in most cases is below its state of saturation, passes through a water-irrigated cell, before it enters the drying tower. Part of the water evaporates into the air stream, whereby the water is cooled. The cold water then is used to condense the steam out of the vacuum unit.

Instead of a conventional cooling tower with closed circuit, it would also be possible to use waste gases which are passed through the plant and then are discharged. The waste gas which passes through the saturator evaporates water into the acid, which finally is recovered as a pure condensate.

The combination with the saturation/cooling tower reduces the stream of air required per ton of water produced.

Fig. 3 shows a flow diagram of the process.

Via an air supply conduit 1 , ambient air is supplied to a cooling tower 2, in which it is humidified by cooling water, which is supplied through a conduit 3 and is guided in counterflow with the air, preferably up to saturation. At the same time, a cooling of the cooling water is achieved thereby.

The air emerging from the top of the cooling tower 2 is supplied from below via a conduit 4 to a drying tower 5 filled with suitable filler material (packing), in which the air is sprinkled with sulfuric acid having a concentration of about 70 wt-%, which is supplied via a conduit 6. Most of the water contained in the air is absorbed by the sulfuric acid and dilutes the same. The dried air is discharged from the drying tower 5 via a dis- charge conduit 7.

The sulfuric acid diluted by the water condensed out of the air is supplied via a conduit 8 to a vacuum evaporator 9, in which at a low vacuum of e.g. 2200 N/m² substantially only water is expelled from the sulfuric acid and evaporated, until the sulfuric acid again is fortified to its original concentration of about 70 wt-%.

The steam is supplied via a conduit 10 to a condenser 11 , in which it is condensed by means of the circulating cooling water, which was withdrawn from the cooling tower 2 via the conduit 12.

In the acid circuit, a cleaning means for the sulfuric acid, which is not shown in Fig. 3, can also be incorporated, in order to liberate the sulfuric acid from dissolved or particu- late impurities.

The condensate thus obtained is withdrawn via a conduit 13. A partial stream of the condensate can be supplied to the cooling tower 2 via a conduit 14, in order to promote the humidification and saturation of the ambient air.

Example

Process data:

ambient pressure: 101.3 kN/m² air supply: 1.5 MNm³/h air temperature: 25°C relative humidity: 61% water content: 16 g H₂O/Nm³ of air

The acid dilution and water condensation leads to a distinct release of heat in the drying zone. Thus, the temperature of the acid emerging from the drying tower is a function of the flow of acid through the tower.

acid flow, in: 650 m³/h acid concentration, in: 70 wt-% acid flow, out: 678 m³/h acid concentration, out: 68,6 wt-% Assuming a tower efficiency of 95%, the air leaves the drying tower with a remaining water content of 2.3 g/Nm³. The amount of water produced with such a system is 500 t/d.

List of Reference Numerals:

1 air supply conduit

2 cooling tower

3 conduit

4 conduit

5 drying tower

6 conduit

7 discharge conduit

8 conduit

9 vacuum evaporator

10 conduit

11 condenser

12 conduit

13 conduit

14 conduit

Claims

Claims:

1. A process for recovering water from gas, in particular air, wherein the gas is brought in contact with sulfuric acid having a concentration > 60 wt-%, so that steam condenses on the surface of the acid, wherein the sulfuric acid thus diluted is fortified to a concentration > 60 wt-%, wherein the steam released is condensed by cooling, and wherein the condensate is discharged.

2. The process according to claim 1 , characterized in that the gas is brought in contact with sulfuric acid having a concentration of 65 to 75 wt-%.

3. The process according to claim 1 or 2, characterized in that the gas is guided through a drying tower in counterflow with sulfuric acid trickling down.

4. The process according to any of the preceding claims, characterized in that the dilute sulfuric acid is fortified to a concentration of 65 to 75 wt-%.

5. The process according to any of the preceding claims, characterized in that the dilute sulfuric acid is fortified by evaporation.

6. The process according to claim 5, characterized in that evaporation is effected under a vacuum, in particular at a pressure of < 90000 N/m².

7. The process according to any of the preceding claims, characterized in that the dilute sulfuric acid is at least partly supplied to a sulfuric acid plant.

8. The process according to any of the preceding claims, characterized in that the gas is passed through a humidification means, before it is brought in contact with the sulfuric acid.

9. The process according to any of the preceding claims, characterized in that water from the humidification means is used for condensing the steam.

10. The process according to claim 8 or 9, characterized in that a partial stream of the condensate is branched off after the condenser and supplied to the humidification means.

11. A plant for recovering water from gas, in particular from air, and for performing a process according to any of the preceding claims, comprising a drying tower (5), in which the gas is passed through concentrated sulfuric acid, an evaporator (9), to which the sulfuric acid diluted by water condensed out of the gas is supplied, and a condenser (11 ), in which steam discharged from the evaporator is condensed.

12. The plant according to claim 11 , characterized in that upstream of the drying tower (5) a humidification means (2) is provided, in which the gas is humidified by means of cooling water.

13. The plant according to claim 11 or 12, characterized in that the water outlet of the humidification means (2) is connected with the water inlet of the condenser (1 1 ).