Method for non-destructively compressing ozone gas

A method for non-destructive compression of a reactive gas such as ozone employs a liquid used as a compressant or sealant in a compressor where the liquid is substantially free of elements which decompose the gas. Provision is also made for extraction of heat of compression absorbed by the liquid of the compressant or sealant, for addition of chilled and pH adjusted liquid, and for addition of acid, if necessary.

BACKGROUND OF THE INVENTION 
This invention relates generally to gas handling in industrial processes 
and more particularly to compression of gases which are subject to thermal 
or catalyzed decomposition, such as ozone. 
Compression of gases generates heat of compression which goes to increase 
the temperature of the gas. Often this heat is removed from the compressed 
gas by passing it through an aftercooler which exchanges the heat with a 
reservoir of cool or refrigerated medium. Some gases are unstable and 
decompose due to the temperature increase attendant upon compression. 
Ozone gas is used in many industrial processes. It is produced in ozone 
generators from air or oxygen in concentrations of up to about 15% by 
weight of ozone in air or oxygen carrier gas. Ozone generators normally 
produce ozone at less than about 15 psig in order to minimize capital 
costs for the generation equipment and to maintain high ozone generation 
efficiency. 
Low pressure ozone is suitable for many processes, but many modern 
technologies require ozone at higher pressures. Also, many technologies 
require ozone at high concentrations in a variety of compatible carrier 
gases such as air, nitrogen, or oxygen. 
Compression of ozone by commonly available reciprocating compressors or 
centrifugal compressors results in adiabatic heating and consequent 
thermal decomposition of a fraction of the ozone. By cooling the working 
parts of such compressors, or by providing intercoolers between stages, 
and aftercoolers for the compressed gas, most of the thermal decomposition 
can be avoided. However, such measures greatly increase the cost of such 
compressors; and half-measures still result in too great a loss of ozone 
to be acceptable. 
One example of a technology which requires compressed ozone is bleaching of 
wood pulp at consistencies lower than about 45%. A typical system for 
bleaching low to medium consistency, i.e., up to about 18% consistency, 
may require an ozone pressure of between 30 and 200 psig for effectively 
contacting the ozone with the pulp or for effectively dissolving the ozone 
in a pulp bearing fluid. It is to be expected that an increasing number of 
processes will be and have been developed in a wide variety of technical 
fields which will require compressed ozone containing gases. 
A liquid ring compressor may be used for compression of thermally reactive 
gas. Such compressors employ a liquid to act as an internal sealant 
between the casing of the compressor and the rotating vanes. The liquid 
forms a ring, by centrifugal action, against the compression chamber wall; 
and, by alternately filling and emptying the spaces between the vanes of 
the radially offset impeller, the liquid ring also serves as the 
compressant for the gas being compressed. Heat of compression is quite 
readily transferred from the gas to the liquid permitting the liquid ring 
compressor to approximate isothermal operation. This suppresses the 
thermal decomposition of the compressed gas and causes a corresponding 
increase in vapor pressure of the liquid ring. A quantity of the liquid 
may be discharged with the compressed gas and must be replenished for 
continuing operation. 
In the case of ozone and some other gases, suppression of thermal 
decomposition is not alone sufficient to preserve the compressed ozone. 
Since ozone is very reactive, it is also subject to consumption and 
decomposition by reaction with the liquid of the liquid ring. Ozone even 
reacts with water in a variety of reactions which are well documented. One 
article dealing with these reactions is Weiss, J., "Reaction of Ozone With 
Water", Transactions of the Faraday Society, 31, 668 (1935), which is 
incorporated herein by reference. 
Since ozone is a costly gaseous reagent, any measurable loss is to be 
avoided. Thus, with an ever increasing technological demand for high 
pressure ozone, the need for a non-destructive compression process becomes 
more urgent. 
The foregoing illustrates limitations known to exist in present devices and 
methods. Thus, it is apparent that it would be advantageous to provide an 
alternative directed to overcoming one or more of the limitations set 
forth above. Accordingly, a suitable alternative is provided including 
features more fully disclosed hereinafter. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention, this is accomplished by providing a 
method for non-destructively compressing ozone gas including using a 
compressor having a liquid serving as an internal sealant and/or 
compressant, the liquid being a cooled liquid which is substantially free 
of reactants which promote decomposition of the ozone; feeding the ozone 
gas to an inlet of the compressor; and extraction of the compressed ozone 
gas from a discharge of the compressor for use in a subsequent process. 
The foregoing and other aspects will become apparent from the following 
detailed description of the invention when considered in conjunction with 
the accompanying drawing figures.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a schematic view of a compressor, 100, which uses a liquid as 
a sealant or compressant. Depending on the design of such a unit, this may 
have a cylindrical housing with rotor offset from center, or an elliptical 
housing with the rotor on center. In this liquid ring type compressor 100, 
the liquid is fed to the compressor through inlet pipe 115, and takes on 
the shape of the housing which forms a converging annulus which compresses 
the gas from gas inlet 210, to gas outlet 230, during each rotation of the 
rotor. In the case of a compressor 100 using a liquid, as a sealant only, 
fed to the compressor through inlet pipe 115; the gas is positively 
displaced through intermeshing petals and grooves, as the liquid sealant 
is used to prevent leakage between each compression petal/groove pair. 
Such compressors are commercially available from a number of sources. 
For compressing ozone according to the method of the present invention, the 
liquid sealant and/or compressant must be non-reactive with ozone. Where 
local water contains elements which may decompose ozone, the best 
selection for the sealant or compressant liquid may be deionized or 
demineralized water free of metal ions and other compounds which react 
with ozone. Ozone will react with even pure water through a variety of 
mechanisms. Typical reactions are: 
EQU O.sub.3 +OH.sup.- .fwdarw.O.sub.2.sup.- +HO.sup.0 
EQU O.sub.3 +HO.sub.2.sup.0 .fwdarw.20.sub.2 +OH.sup.0 
EQU O.sub.3 +OH.sup.0 .fwdarw.O.sub.2 +HO.sub.2.sup.0 
EQU HO.sub.2 +OH.sup.0 .fwdarw.O.sub.2 +H.sub.2 O 
Since these reactions proceed through unstable intermediate hydroxyl ions 
and polyoxides, the reaction with water will continue, and some of the 
water will be converted to oxygen. In order to suppress the reaction of 
ozone with any water, the pH of the water may be adjusted to a value lower 
than about 7, preferably to a range of 2 to 3, by addition of inorganic 
acid, such as H.sub.2 SO.sub.4, so for example. This reduces the 
decomposition of ozone in water. 
A small amount of the liquid in the liquid ring is discharged through 
discharge port 230 with the compressed ozone. It is necessary, therefore, 
to replenish the lost liquid periodically by metering a quantity of 
properly chilled and buffered deionized or demineralized water into the 
liquid supply 115. This process could be applied in a pulp bleaching 
operation where the slight amount of water will blend in with the liquor 
of the pulp slurry. 
FIG. 2 shows a system for practicing the method of the present invention 
which includes a liquid separation unit 40. Ozone gas is admitted through 
inlet port 210 to compressor 100, compressed, and discharged through 
discharge line 230 to gas separator 40 in which the compressed ozone and 
entrained water are separated. The compressed ozone is discharged through 
line 45 to be used in pulp bleaching or other process. Separated liquid is 
discharged through return line 50 through which it is returned to 
compressor 100 through liquid supply connection 118. Chilled and/or PH 
adjusted and/or buffered deionized or demineralized make-up water is added 
to return line 50 through valve 150, if necessary, to replace water which 
may be consumed by reactions within the system or lost in saturated vapor 
discharged from the compressor. 
FIG. 2a shows a system similar to that of FIG. 2, except for the addition 
of acid supply through valve 140, to control and maintain the pH of the 
recirculated water loop constant. 
FIG. 3 shows a system similar to that of FIG. 2, except for the addition of 
a cooling unit 46 for extracting heat from the water in return line 42 
from liquid separator 40. Again, chilled and/or pH adjusted and/or 
buffered deionized or demineralized make-up water is added to return water 
in line 50 through valve 150. This closed loop system can also be used for 
compressing reactive gases which require expensive non-reactive liquid for 
the liquid compressant or sealant since liquid loss is minimal. Its use 
for an ozone/water system even improves the economics of this relatively 
inexpensive process. 
FIG. 3a shows a system similar to that of FIG. 3, except for the addition 
of acid supply through valve 140, to control and maintain the pH of the 
recirculated water loop constant. 
Please note that FIGS. 2a and 3a show systems which will tolerate tap water 
of reasonable quality; because undesirable reactions are suppressed by 
lowering the pH by means of the acid supply valve provided. 
The embodiments of the process described with reference to the figures 
represent only four possible embodiments preferred for compressing ozone 
and other reactive gases. Depending upon the specific application, gas 
reactivity, gas cost, required gas pressure, and flow rate, the selection 
of embodiment is quite well defined. The process can significantly reduce 
loss of gas and improve efficiency of gas compression.