Water purification method

A water purification and/or collection method uses a water-specific reaction with an anhydrous salt to separate organic and inorganic contaminants in a water supply. A stream of water vapor containing the contaminates is passed over a desiccant bed of anhydrous salt. Only the water in the stream reacts to form hydrates while the contaminates pass by in an air stream for disposal elsewhere. The fully hydrated bed is then regenerated by heat to free the purified water which is condensed.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a method for water purification with a 
water-specific reaction, and, more particularly, to a method and apparatus 
for purification of water contaminated by organic and/or inorganic 
compounds (petroleum; nuclear, biological, chemical (NBC) agents; and 
other volatile compounds) by trapping water in an anhydrous salt and then 
isolating water by decomposing the hydrate and condensing the purified 
water vapor while, at the same time, regenerating the desiccant. 
In the past, it was known that water could be extracted from the atmosphere 
by using physical adsorbents such as silica gel or molecular sieves which 
are inert and have been used with no reported hazards. Practical apparatus 
can be constructed for extracting 200 gallons of water per hour at 
80.degree. F. and 10% relative humidity. Higher relative humidities yield 
higher water production, e.g. 700 gallons per hour at 40% RH and 
80.degree. F.; 1000 gallons per hour at 55% RH and 80.degree. F. It was 
not, however, considered that such devices could be used for water 
purification. 
Several methods are also known to remove water from the atmosphere. These 
include the use of refrigeration cycles and the passing of water vapor 
through a membrane. The most energy-efficient technique is, however, to 
use a desiccant to adsorb the water from the atmosphere, which would then 
be recycled to release the water and regenerate the desiccant. Known water 
purification methods depend on the quantity and nature of the contaminant. 
For example, if the level of contamination was more than that of treatment 
for microorganisms, such as chlorination, then some form of separation was 
used. Most known methods used for liquid-phase water purification require 
that the contaminant be removed from water. This type of separation of the 
impurity has been effected by filtration, adsorption, coagulation and 
sedimentation, or ion-exchange. 
Water which has been contaminated by volatile compounds is difficult to 
purify by conventional methods such as distillation, reverse osmosis or 
ultrafiltration. Adsorbents might be used to remove petroleum or other 
contaminants from water but they have limited selectivity and limited 
capacity. Therefore, the use of activated carbon or similar adsorbents to 
remove gross contamination would not be practical. Distillation will not 
separate a volatile component from water, since both would distill. 
Reverse osmosis and ultrafiltration will also have difficulty in removing 
petroleum or other volatile contaminants. None of these conventional 
methods would be effective for NBC (nuclear, biological or chemical) 
contamination. 
Using an adsorption principle, however, with a water-specific reaction, the 
present invention permits effective and efficient water purification by 
using air saturated with water vapor from the contaminated water source. 
Only water from the vapor stream is extracted by the desiccant. 
Contaminates in the water vapor pass through the system without reacting 
with a desiccant in the form of an anhydrous salt; pure water is obtained 
by dehydration while the air stream containing the contaminates exits the 
system. 
In carrying out this purification process, a presently contemplated 
embodiment of the present invention utilizes a desiccant bed comprised of 
a suitable anhydrous salt to effect water extraction at low relative 
humidities. A suitable salt must have a rate of adsorption which is much 
higher than other desiccants, must be non-toxic and should be recyclable 
many times. 
Another aspect of the system in accordance with the present invention is 
that it can operate at the minimum relative humidity and stay within the 
weight and size limitations with the use of a low pressure blower (i.e. 
about 20 inches of water pressure). Typically, such a blower powered by a 
Diesel engine will have a capacity of 150,000 CFM. This blower will be 
incorporated in a system having a weight of 14,500 lbs (not including 
desiccant), a desiccant consisting of anhydrous salt, a desiccant bed area 
of 75 ft.sup.2 and a 1 hour cycle period. 
As is well known, the water content of air is a function of temperature. As 
the temperature increases, the amount of water that air can hold 
increases. Relative humidity is the ratio of the quantity of water in air 
to the maximum quantity of water air can hold at a given temperature. 
Therefore, if the air temperature is increased, the total amount of water 
in air at a fixed relative humidity would be increased. The size of the 
system is controlled by the weight and size limitations of the hardware, 
but the amount of water that can be held by the system is determined by 
the quantity of the desiccant in the adsorption bed. The limits on 
collection for the high relative humidities would be the size of the bed, 
energy available, and recycle time. 
Thus, it has been found possible to construct a water collection device 
that will produce purified water by separating volatile organic and 
inorganic contaminants at 200 gal/hr from air with a relative humidity of 
10% and 80.degree. F. It would require two of these units to produce 
purified water at 200 gal/hr and 5% RH at 80.degree. F. If the relative 
humidity is raised to 40% and 80.degree. F., the volume of purified water 
collected per hour will increase to 750 gallons, and at 55% RH and 
80.degree. F. the collection rate would be 1000 gal/hr. The system has the 
additional advantage of being simple, rugged and self-contained. 
The following hydrates have been identified as particularly suitable for 
the water purification apparatus and method of the present invention based 
on low toxicity, low dehydration temperature, and high percent of water in 
the hydrated salt: cupric sulfate pentahydrate; sodium acetate trihydrate; 
cadmium sulfate octahydrate; ferrous sulfate heptahydrate; cobalt chloride 
hexahydrate; aluminum sulfate octadecahydrate; and zinc sulfate 
heptahydrate. 
For a desiccant-anhydrous salt to perform properly in the present 
invention, the decomposition temperature of the hydrate should be above 
the atmospheric boiling point of water. This would permit distillation of 
the water from the desiccant to a condenser at atmospheric pressure. In 
addition, if the desiccant loses water below the boiling point of water, 
then the water will dissolve part or all of the salt and thus lead to 
caking of the desiccant. 
In a process embodying the present invention, the first step causes the 
contaminated water vapor to saturate the air at the inlet of the system 
which contains an anhydrous salt-desiccant bed. The saturated air is then 
forced through the bed, but only the water reacts with the salt to form a 
hydrate. The contaminates will pass through the system and have no 
interaction with the bed. The second step is to heat and decompose the 
hydrate by removing (condensing) the water to provide a source of purified 
water in a simple yet effective manner. 
In one presently contemplated embodiment of a system in accordance with the 
present invention, three desiccant beds are used. Air is forced into a 
manifold where it is valved through one of the three desiccant beds. The 
exit air, which is dehydrated, it is passed into the second bed, which is 
initially hot from a previous regeneration step. This second bed is cooled 
for use as the next bed to hydrate. A third bed is in the process of being 
regenerated by distilling the trapped water through the condenser. 
Finally, water is collected in a receiver. At the end of this cycle, the 
first bed is hydrated, the second bed is cooled, and the third bed is hot 
but dehydrated. There are three steps in the process for hydrating and 
dehydrating the desiccant beds. Heat from the Diesel engine exhaust may be 
used to dehydrate the beds, and air-cooled condensers are used to collect 
the water vapor.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIG. 1A, a contaminated water source 1 which has been 
contaminated with volatile organic or inorganic compounds, is heated and 
mixed in the form of vapor with air supplied to a desiccant bed 2 
containing anhydrous salt. The bed 2 interacts with only the water in the 
vapor/contaminate mixture and converts the salt to a hydrate. The 
remaining air which is contaminated with volatile compounds is exited from 
the system. 
In the second basic step of the process, as shown in FIG. 1B, the hydrated 
salt in the bed 2 is heated by a heat source HS after the bed has been 
completely converted to hydrate. As a result of heating the bed 2, water 
vapor is removed to a condenser 3 where pure water is distilled by 
condensation. 
In a more practical form of the system of the present invention, as shown 
schematically in FIG. 2, three desiccant beds 11, 12, 13, are provided 
each containing a heating coil 14, 15, 16. Contaminated air and water 
vapor flow through a supply line 10 through a first valve 1 into the first 
bed 11 while valves 2 and 3 are closed. A hydrate is thus formed as 
described above, with the contaminated air exiting from the bed through a 
line 24, via open valves 21, 22 to the second bed 12 while valve 23 is 
closed to prevent the stream from flowing from the outlet of the first 
desiccant bed 11 to the outlet of the third desiccant bed 13. During this 
part of the process, valves 4 and 6 are closed while valve 5 at the outlet 
of the second desiccant bed 12 is open. The air flow cools the second 
desiccant bed and the third desiccant bed is heated by the associated 
heating coil 16, while heating coils 14 and 15 are off, to decompose the 
hydrate that has been formed in a previous cycle and thereby regenerate 
the desiccant bed. Valve 20 is open to a condenser 26, while valves 18 and 
19 are closed, during the heating cycle so that water vapor can pass from 
the dehydrated third bed 13 to the condenser 26 where the purified water 
is formed. The condenser 26 is joined selectively to the other two beds 
11, 12 through a common line 25 and the respective valves 18, 19. 
The above-described process is repeated for the next cycle in which the 
second desiccant bed 12 is hydrated while the third desiccant bed 13 is 
cooled and the first desiccant bed 11 is dehydrated. The valves 1, 2, 3, 
4, 5, 6, 18, 19, 20, 21, 22 and 23 are adjusted accordingly to control the 
flow to effect this process as well as the third cycle in which the third 
bed 13 is hydrated, the second bed is dehydrated and the first bed 11 is 
cooled. 
It will thus be appreciated that the invention avoids the problems which 
have been encountered with known distillation techniques which cannot 
separate volatile components. The present invention also provides a 
solution to the problem caused by the volatility of soluble components in 
contaminated water inasmuch as the present invention uses only the water 
vapor from the contaminated source. Consequently, the present invention is 
particularly useful with petroleum or other volatile organic contaminants, 
although it is also effective for NBC products as previously mentioned. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. For example, it is 
within the scope of the present invention to use several different types 
of anhydrous salts, one particular salt contemplated is magnesium sulfate. 
Biological agents which have been removed from the contaminated supply can 
be destroyed with a biocide placed in the bed or, where applicable, by the 
heat from the dehydration process itself. Therefore, the spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.