Process and plant for the recovery of water from humid air

A process and a plant for the recovery of water from humid air are disclosed. Humid ambient air flows through an adsorber in which an adsorbing agent removes moisture from the air by adsorption. Subsequently, air heated by an air heater and delivered by a fan flows through the adsorber in order to recover by desorption the moisture from the moistened adsorbing agent. Thereafter, the water vapor-containing air is precooled in a counterflow heat exchanger and subsequently conducted into a condenser in which heat is extracted from it by means of a heat pump so that the water vapor condenses and is available in the form of liquid water. The air cooled and dried in the condenser is utilized in the counterflow heat exchanger for precooling the air fed into the condenser. The heat given off by the heat pump is supplied to the air heater in order to be utilized therein for heating the air flowing through the air heater.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention is concerned with a process for the recovery of water 
from humid air and with a plant for carrying out the process. 
(2) Description of the Prior Art 
Processes for drying air and other gases are known. According to these 
processes the water vapor is either condensed by means of refrigerating 
machines or it is adsorbed by adsorbing agents such as molecular sieves, 
activated carbon, or silica gel. 
In a known process for recovering water from the atmosphere the water vapor 
contained in the atmosphere is condensed on plastic sheets cooled by 
nightly radiation and the drops of water formed are collected. The yield 
of this process is very low. 
SUMMARY OF THE INVENTION 
It is one object of the present invention to provide an advantageous and 
highly effective, novel process for recovering water from the atmosphere 
or humid air which process increases considerably the amounts of water 
recovered as compared with those recovered by the known processes. 
Another object of the present invention is to provide a suitable system or 
plant for carrying out said process, said system or plant making use of a 
highly effective adsorbing agent and of a condensing means. 
Other objects of the present invention and advantageous features thereof 
will become apparent as the description proceeds. 
In principle the present invention is based on the following process steps: 
The humid air is contacted with an adsorbing agent which is capable of 
extracting and adsorbing the moisture from the air. The 
moisture-containing adsorbing agent is then contacted with dry air which 
recovers and desorbs the moisture from the adsorbing agent. Thereupon the 
air containing the moisture desorbed from the adsorbing agent is supplied 
to a condensing means in which heat is extracted from the humid air so 
that the water vapor contained therein is condensed. Finally the water 
formed by condensation of the water vapor is collected. 
According to the process of the present invention the air contacted with 
the adsorbing agent loses and transfers a portion of its moisture content 
to the adsorbing agent with a change of state which is nearly isenthalpic 
and with a simultaneous rise in temperature. The entropy formed by mixing 
the dry air and the moisture is removed from the resulting humid air in 
the condenser. The desorption step proceeds in a nearly isenthalpic manner 
while the air temperature decreases with increasing absolute and relative 
humidity. Thus the process according to the present invention is highly 
advantageous insofar as the yield of recovered water is substantially 
higher than with the known process. 
According to a further embodiment of the present invention, the heat 
withdrawn from the humid air in the condenser is supplied to the dry air 
before the dry air is brought into contact with the adsorbing agent. In 
this embodiment the dry air is heated to a temperature between about 
50.degree. C. to 60.degree. C. so that its power of absorption or 
receptivity for the moisture contained in the adsorbing agent is 
considerably increased. 
According to another embodiment of the present invention, the air cooled in 
the condenser is utilized for precooling the air entering the condenser. 
Due to such precooling of said air its enthalpy is decreased so that the 
temperature of the air is lowered and/or the water vapor contained in the 
air condenses. 
According to still another embodiment of the present invention the humid 
air, prior to contacting the adsorbing agent, is cooled to such a 
temperature that its relative humidity corresponds approximately to the 
saturation value. 
According to a further embodiment of the present invention, when carrying 
out the process in arid and semi-arid zones in which considerable 
differences in the day-time and the night temperatures are encountered, 
the humid air is derived during the night-time from those layers of air 
which are close to the ground and are saturated with water vapor. Such 
humid night-time air is conducted into the adsorbing agent while the 
withdrawal of the moisture from the adsorbing agent is effected during the 
day-time. 
According to an especially advantageous embodiment of the present invention 
there is provided a highly effective system or plant for carrying out the 
process of the present invention, said system or plant being characterized 
by comprising an air heater, at least one adsorber, a heat exchanger, a 
condenser, a heat pump, and a pipe line system which interconnects the air 
heater with the adsorber, the adsorber with the heat exchanger, the heat 
exchanger with the condenser, the condenser with the heat pump, and the 
heat pump with the air heater. 
Particularly useful adsorbing agents are specific types of silica gel which 
have wide pores and a low heat of wetting, and which are capable of 
desorbing or giving off the adsorbed water to a large extent at a 
temperature of about 30.degree. C. or even lower. Depending upon the 
proportions of the silica gel employed an adsorption of water in the 
adsorber up to 75 g./100 g. of gel can be achieved if the relative 
humidity in the adsorber does not substantially decrease below 50-60%. 
In order to operate the system or plant according to the present invention 
near the sea shore where constant air temperatures and a higher absolute 
humidity prevail, the system or plant advantageously comprises at least 
two adsorbers which enable a combination of at least two cycles of 
operation by alternately charging and regenerating the two adsorbers, i.e. 
by charging and regenerating one adsorber simultaneously with regenerating 
and charging the other adsorber. In this case excess heat is expediently 
dissipated into the sea the temperature of which is generally lower than 
that of the air. 
According to another embodiment of the present invention a multiple stage 
heat pump is employed. The heat pump is driven by a motor or is an 
adsorber pump operated by solar energy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The system or plant for carrying out the process of recovering water from 
the atmosphere according to the present invention comprises an air heater 
1 which is connected via fan 2 to an adsorber 3 containing silica gel as 
adsorbing agent. 
Adsorber 3 is connected to a first inlet of counterflow heat exchanger 4, 
the second inlet and the first outlet of which are connected with 
condenser 5 and the second outlet of which leads to the open air. 
Condenser 5 is connected to heat pump 6. It has an outlet for the water 
obtained on condensation. Heat pump 6 has a motor drive W which is 
represented by a dotted arrow and is connected to air heater 1. 
When the plant is in operation, humid air L1 flows through adsorber 3 in 
which the adsorbing agent withdraws the moisture from the air L1 and 
adsorbs it. The humid air flowing through adsorber 3 was already cooled 
before entering said adsorber 3 by contacting the ground cooled by the 
nightly radiation of heat. After the adsorbing agent is charged with 
moisture from the cooled air L1, fan 2 delivers hot air from air heater 1 
to adsorber 3. The hot air withdraws the moisture from the adsrobing agent 
and adsorbs it. The moisture-charged air L1b is then conducted into 
counterflow heat exchanger 4 in which it gives off heat to the cool air 
emanating from condenser 5 and flowing thereafter into the open through 
outlet L2b. The thus precooled air L2a is conducted into condenser 5 in 
which heat Q1 is extracted from it by heat pump 6 so that the water vapor 
contained in the air is condensed. The resulting drops of water 
precipitate on the condenser walls and are collected. The water may be 
recovered from the condenser via the outlet L2d, shown by the dash-dotted 
arrow. The air L2 cooled in condenser 5 flows back into heat exchanger 4 
in which it is utilized for precooling the moist air L1b passing through 
adsorber 3. The heat indicated by Q2 as it is given off by heat pump 6 is 
supplied to air heater 1 and is utilized therein for heating the air L2b 
flowing through said air heater 1. The air L2b flowing through said air 
heater 1 is drawn from the atmosphere. In FIG. 1 the air flow from air 
heater 1 to condenser 5 and from condenser 5 to counterflow heat exchanger 
4 is designated by L2c, L1b, L2a, and L2 and the direction of flow of this 
air flow is indicated by the arrows drawn between said air heater, 
adsorber, heat exchanger, and condenser. 
In between the fan 2 and adsorber 3 may be positioned a cooling means 10 
wherein the moist air is cooled such that its relative humidity 
approximates the saturation value. 
FIG. 2 shows the total change of state of the air flowing from air heater 1 
to condenser 5. The enthalpy values H and the entropy values S are plotted 
respectively along the ordinate and the abscissa of the graph. 
The sections of the curve marked with numerals represent the change of 
state of the air in the various units 1, 2, 3, 4, 5, and 6 of the system 
or plant. Said numerals are identical with the numerals of the units of 
the system or plant. The changes of state of the air in adsorber 3 do not 
proceed in a completely isenthalpic manner because heat of wetting is 
supplied to and extracted from the air in adsorber 3. 
Of course, many changes and variations in the apparatus and the like used 
in the system or plant according to the present invention may be made by 
those skilled in the art in accordance with the principles set forth 
therein and in the claims annexed hereto.