Method of monitoring and recovering oxygen-rich gas from ozone bleaching

A method and installation for bleaching a lignocellulosic pulp with ozone, while recovering an oxygen-rich gas which may be recycled or re-used. Bleaching of the pulp with ozone is preferably carried out under conditions in which ingress of air is prevented. In a multi-step bleaching process, an oxygen-rich vent gas typically containing at least 90%, by weight, of oxygen may be recovered from a second contactor for potential use in different oxygen-consuming operations in a pulp mill.

FIELD OF THE INVENTION 
This invention relates to a method and installation for bleaching a 
lignocellulosic pulp with ozone, with recovery of an oxygen-rich gas. 
BACKGROUND OF THE INVENTION 
Wood contains two main components, a fibrous carbohydrate, which is a 
cellulosic portion, and a non-fibrous component. The polymeric chains 
forming the fibrous cellulose portion of the wood are aligned with one 
another and form strong associated bonds with adjacent chains. The 
non-fibrous portion of the wood comprises a three-dimensional polymeric 
material known as lignin. The lignin bonds the cellulosic fibers, and is 
also distributed within the fibers themselves. For use in paper-making 
processes the wood is converted to pulp. 
Pulp comprises wood fibers capable of being slurried or suspended and then 
deposited upon a screen to form a sheet of paper. The methods employed to 
accomplish the pulping step involve physical or chemical treatment of the 
wood, or a combination of these two treatments, to alter the chemical form 
of the wood and to impart desired properties to the resulting product. 
There are two main types of pulping techniques: mechanical pulping and 
chemical pulping. In mechanical pulping, the wood is physically separated 
into individual fibers; in chemical pulping, the wood chips are digested 
with chemical solutions to solubilize a portion of the lignin and thus 
permit its removal. The commonly utilized chemical pulping processes are 
broadly classified as (a) the soda process, (b) the sulfite process, and 
(c) the Kraft process. The Kraft process is the most commonly used. 
The soda process employs sodium hydroxide as the active reagent to break 
down the lignin and to assist in its removal. The sulfite process is 
described in the Handbook for Pulp & Paper Technologists--Chapter 6: 
Sulfite Pulping (TAPPI, U.S.A.). 
The Kraft process, together with its numerous variations, is the principal 
chemical process utilized in paper manufacturing. The basic Kraft process, 
as described in the Handbook for Pulp and Paper Technologists--Chapter 7: 
Kraft Pulping (TAPPI, U.S.A.), involves digesting the wood chips in an 
aqueous solution of sodium hydroxide and sodium sulfide. This process is 
highly effective in pulping. The Kraft process produces a relatively 
high-strength pulp since its use entails diminished attack on the 
cellulose component of the wood. 
The modified Kraft techniques result in even less degradation of the 
polymeric structure of the cellulosic fibers during pulping, and, 
consequently, strength loss in the resultant paper product is diminished 
as compared to that occurring with the standard Kraft process. 
One such modified Kraft pulping process is "extended delignification", 
which encompasses a variety of modified Kraft techniques, such as adding 
the pulping chemicals in a specific defined sequence, or at different 
locations within the digester apparatus, or at different time periods, or 
with a removal and reinjection of cooling liquors in a prescribed 
sequence, so as to more effectively remove a greater amount of lignin 
while reducing the severity of the pulping liquor's chemical attack on the 
cellulosic fibers. Another modification of the Kraft process is the 
Kraft-AQ process, wherein a small amount of anthraquinone is added to the 
Kraft pulping liquor to accelerate delignification while limiting the 
attack upon the cellulosic fibers which comprise the wood. 
A variety of other extended delignification techniques are known and 
include Kamyr Modified Continuous Cooking as described by V. A. 
Kortelainen and E. A. Backlund in TAPPI, vol. 68 (ii), 70 (1985); Beloit 
Rapid Displacement Heating as reported by R. S. Grant in TAPPI, vol. 66 
(3), 120 (1983); and Sunds Cold Blow Cooking as reported by B. Pettersson 
and B. Ernerfeldt in Pulp and Paper, vol. 59 (11), 90 (1985). 
Digestion of the wood in the Kraft or a modified Kraft process produces a 
brownstock, which is a dark colored slurry of cellulose fibers. The dark 
color of the brownstock is attributable to the presence of residual lignin 
not removed during digestion and which has been chemically modified in 
pulping to form chromophoric groups. In order to lighten the color of the 
brownstock pulp, so as to make it suitable for white paper manufacture, it 
is necessary to remove the residual lignin by use of delignifying 
materials and by chemically converting any residual lignin into colorless 
compounds by bleaching or brightening. 
Prior to bleaching the pulp, the digested material is conventionally 
transferred to a separate blow tank after the chemical treatments involved 
in the pulping process are completed. Within the blow tank, the pressure 
developed during the initial chemical treatment of the lignocellulosic 
material is relieved, and the pulp material is separated into a fibrous 
mass. The resulting fibrous mass is then subjected to a series of washing 
steps to remove residual chemicals and the soluble materials, such as the 
lignin, which were separated from the fibrous materials in the pulping 
process. Frequently, the pulp also undergoes one or more screening steps 
designed to separate out the larger portions of undefibered wood for 
special processing, for example recooking or mechanical grinding. 
The black liquor residue obtained from the washing process is collected, 
concentrated, and is typically incinerated in an environmentally safe 
manner in a recovery boiler. 
The delignification and bleaching processes are conducted on the washed 
fibrous mass in a series of steps, using selected combinations of chemical 
reactants. In the prior art, various combinations of chemical treatments 
have been suggested. Furthermore, individual treatment steps have been 
rearranged in an almost limitless number of combinations and permutations. 
Therefore, in order to simplify the explanation of the various bleaching 
processes and systems, the use of letter codes is conventionally employed 
in combination to describe the particular chemical reactants employed and 
the sequence of the steps of the process. 
The letter codes conventionally employed are as follows: 
______________________________________ 
C = Chlorination Reaction with elemental 
chlorine in acidic medium. 
E = Alkaline Extraction 
Dissolution of reaction 
products with NaOH. 
E.sub.o = Oxidative Alkaline Extraction 
Dissolution of reaction 
products with NaOH and oxygen. 
D = Chlorine Dioxide 
Reaction with ClO.sub.2 in 
acidic medium. 
P = Peroxide Reaction with peroxides 
in alkaline medium. 
O = Oxygen Reaction with elemental 
oxygen in alkaline medium. 
Z = Ozone Reaction with ozone. 
C/D = Admixtures of chlorine and 
chlorine dioxide. 
H = Hypochlorite Reaction with hypochlorite 
in an alkaline solution. 
______________________________________ 
Conventionally, delignification and bleaching of wood pulp has been carried 
out with elemental chlorine. 
Although elemental chlorine is a very effective bleaching agent, the 
effluents from chlorine bleaching processes contain large amounts of 
chlorides produced as the by-product of these processes. These chlorides 
readily corrode processing equipment, thus requiring use of costly 
materials in the construction of such mills. Further, the build-up of 
chlorides within the mill precludes recycling the washer filtrate after a 
chlorination stage in a closed system operation without employing recovery 
systems requiring extensive, and therefore expensive, modifications. In 
addition, concern about the potential environmental effects of chlorinated 
organics in effluents, which some authorities believe to be toxic to 
humans and animals, has caused significant changes in government 
requirements and permits for bleach mills which include standards that may 
be impossible to meet with conventional chlorine bleaching technology. 
To avoid these disadvantages, the paper industry has attempted to reduce or 
eliminate the use of elemental chlorine and chlorine-containing compounds 
from multi-stage bleaching processes for lignocellulosic pulps. 
Complicating these efforts is the requirement of high levels of pulp 
brightness for many of the applications for which such pulp is to be used. 
In this connection, efforts have been made to develop a bleaching process 
in which chlorine-containing agents are replaced, for example, by oxygen 
for the purpose of bleaching the pulp. The use of oxygen does permit the 
recycling of effluent from this stage for recovery and does permit a 
substantial reduction in the amount of elemental chlorine used. 
The use of oxygen, however, is often not a completely satisfactory solution 
to the problems encountered with elemental chlorine. Oxygen is not as 
selective a delignification agent as elemental chlorine, and the Kappa 
number of the pulp, using conventional oxygen delignification methods, can 
be reduced only a limited amount before there is an unacceptable attack on 
the cellulosic fibers. Also, after oxygen delignification, the remaining 
lignin has heretofore typically been removed by chlorine bleaching methods 
to obtain a fully-bleached pulp. Although such a process uses reduced 
amounts of chlorine, concerns associated with the use of chlorine still 
persist. 
To eliminate the need for chlorine bleaching agents, the removal of such 
remaining lignin with the use of ozone in the bleaching of chemical pulp 
has previously been attempted. Although ozone may initially appear to be 
an ideal material for bleaching lignocellulosic materials, the highly 
oxidative properties of ozone and its relative high cost have heretofore 
limited the development of satisfactory ozone bleaching processes for 
lignocellulosic materials. 
Since the delignifying capabilities of ozone were first recognized, there 
has been substantial and continuous work by numerous persons in the field 
to develop a commercially suitable method using ozone in the bleaching of 
lignocellulosic materials. 
The bleaching of pulp using ozone has been studied and reported during the 
last two decades by Singh, R. P. et al., Advances in Ozone Bleaching Part 
2. Bleaching of softwood Kraft pulps with oxygen and ozone combination 
TAPPI oxygen, Delignification Symposium, San Francisco, Calif., 1984, 
Liebergott, N., Bleach Plant of the year 2000. TAPPI Pulping Conference, 
Hollywood, Fla., 1985, and Soteland, N., Bleaching of Chemical Pulps with 
Oxygen and Ozone, Pulp and Paper Mag. Can. 75 (1974). 
The initial studies were carried out under laboratory conditions on either 
high consistency or low consistency pulp slurries because it was easier to 
contact the gas with the pulp under these conditions. 
More recently, laboratory studies have been carried out on medium 
consistency ozone bleaching by Laxen, T. et al, Medium consistency ozone 
bleaching, Paperi Ja, Peru 72 (1190: 5), and pilot scale operations are 
currently underway or being operated at Lenzing AG in Austria, Peter, W. 
et al, Experience with Medium Consistency--Ozone Bleaching Prototype in 
the Mill, Non-Chlorine Bleaching Proceedings, March 1992, Hilton, Head, 
S.C. 
Union Camp Patent Specification WO 91/18145, published Nov. 28, 1991, 
describes processes for delignifying and bleaching lignocellulosic pulp 
without the use of elemental chlorine, in which a high consistency pulp is 
partially delignified to a Kappa No. typically of up to 10 and a viscosity 
typically of more than 13 cps, for example, with oxygen, whereafter 
delignification is completed with ozone to a Kappa No. typically of up to 
5 and a viscosity typically of more than about 10 cps. The ozone 
delignification is carried out under conditions in which the Ph, 
temperature and consistency of the pulp, as well as the pulp particle size 
and density, are controlled to facilitate penetration of a majority of the 
pulp particles by ozone, with substantially uniform delignification and 
bleaching throughout a majority of the particles to form a bleached pulp. 
These studies have produced a body of knowledge concerning the effects of 
ozone on the bleaching of pulp and how the properties of the pulp are 
affected. However, there exists a need for a commercially advantageous 
scheme for using and recovering vent gases resulting from contacting an 
ozone-oxygen mixture with a pulp slurry. 
SUMMARY OF THE INVENTION 
It is known that to make ozone treatment of pulp effective, at least about 
five kilograms of ozone per ton of pulp is necessary. However, available 
medium consistency mixers can only accommodate up to about 35 percent gas 
volume. 
In order to increase the effective ozone concentration in the mixing stage, 
certain embodiments of the present invention include the elevation of 
pressure in the ozone/oxygen mixture to increase the ozone concentration 
therein for use in the process of the present invention. 
In accordance with the present invention, it has been found that bleaching 
of pulp with a gas comprising a mixture of ozone and oxygen can be carried 
out with production of an oxygen-rich vent gas which can then be employed 
in different operations having an oxygen requirement. 
Thus, the oxygen-rich gas can be recycled back to an ozone generator to 
generate a fresh mixture of ozone and oxygen for further pulp bleaching. 
Alternatively, depending on ultimate vent gas quality, it can be used in 
waste water treatment or in different operations carried out in a pulp 
mill, for example in oxygen delignification, white liquor oxidation, black 
liquor oxidation, lime kiln enrichment, lime mud oxidation, generation of 
polysulfides from white liquor or oxygen extraction processes. 
Significantly, it has been found that treating lignocellulosic pulp with an 
ozone mixture in a plurality of mixing units results in a commercial 
process having many advantages. It has been determined that vent gas from 
ozone mixing units downstream of a first ozone mixing unit is a superior 
quality gas relative to vent gas from the initial ozone mixer, and also 
superior to those vent gas mixtures resulting from pulp bleaching 
processes prior to the present invention. 
Among other factors, and without limiting the present invention to any 
particular theory of operation, it is believed the advantages obtained are 
due, in part, to a sparging effect which occurs in the initial ozone 
mixing unit. Additionally, with a plurality of mixing units, deaeration of 
entrained air in the pulp and degassing of carbon dioxide produced in 
prior acidification steps take place in the initial ozone contactor, prior 
to downstream ozone contacting steps. 
Thus, in the multiple stage embodiments, the vent gas from the initial 
ozone mixer is usable in other mill processes such as for example 
delignification, E.sub.o, and lime kiln processes. In accordance with the 
present invention, vent gas from the second and other downstream ozone 
mixing units may be recycled back to the bleaching process itself, with 
very little treatment required prior to reuse. Gas quality from the second 
and other downstream mixing units is sufficiently high that it is only 
preferable to subject the gas to be recycled to an ozone destruct unit and 
also preferably to a dryer before recycle to the ozone generator for 
re-use in the bleaching process. The simplicity of recycle steps with the 
present invention is of much advantage in commercial application. 
Accordingly, in a preferred embodiment of the present invention, a 
lignocellulosic pulp is bleached in a process comprising the steps of: 
generating ozone from a feed gas comprising oxygen; contacting at least a 
portion of the lignocellulosic pulp with a mixture comprising oxygen and 
ozone in a first contactor to produce a treated pulp and an effluent gas; 
contacting at least a portion of the treated pulp with a mixture 
comprising oxygen and ozone in a second contactor; recovering a bleached 
pulp, and; recovering an oxygen-rich gas having an impurity concentration 
sufficiently low to allow re-use of the recovered oxygen-rich gas. Among 
other factors, recycle of the effluent gas from the second contactor is 
made possible in accordance with the present invention by the treatment of 
the pulp in a first contactor with a mixture comprising oxygen and ozone. 
The vent gas from one or both ozone contactors may be monitored for flow 
and oxygen content, and flow and quality of the oxygen and 
ozone-containing feed mixture may be adjusted according to requirements 
for vent gas re-use. In this manner of control, beneficial results may be 
maximized. 
Additional embodiments of the above invention include the contacting of the 
pulp with a quantity of oxygen-containing liquid, preferably hydrogen 
peroxide, prior to contacting the pulp with an ozone and oxygen mixture in 
a second contactor, or including the O.sub.2 -containing liquid in one of 
both of the contactors themselves. Recycled pulps have appeared to benefit 
from hydrogen peroxide addition. 
Another embodiment of the present invention further provides a process for 
the production of an oxygen enriched gas for feed to the ozone generator, 
which gas has an O.sub.2 content of less than about 95%, most preferably 
about 93%, and is produced in an adsorption unit. When the pulp is treated 
in a first contactor with an ozone mixture, further bleaching with ozone 
in a second contactor has been found to produce a recoverable gas having a 
substantially high enough O.sub.2 level to enable its commercially 
valuable use in oxygen bleaching and in other processes. 
In accordance with this aspect of the process of the present invention, 
ozone is generated from a gas mixture preferably produced in a vacuum 
swing adsorption unit and having a reduced oxygen purity, to produce an 
oxygen/ozone mixture. A portion of the oxygen/ozone mixture is contacted 
with lignocellulosic pulp in a first contactor, and at least a portion of 
the treated pulp then contacted with another portion of the oxygen/ozone 
mixture in a second contactor. The oxygen content in the vent gas 
recovered from the second contactor is typically greater than about 85%, 
preferably about 90% oxygen, thus enabling the use of the recovered gas 
mixture from the second contactor in other application such as for example 
oxygen delignification, oxygen reinforced alkaline extraction, white 
liquor oxidation, lime kiln enrichment, and polysulfide generation. 
In order to maximize the oxygen content in the recovered vent gas from 
downstream bleaching units, it may be advantageous to provide for 
deaeration of the pulp, or to adjust the ozone to pulp ratio between the 
plurality of bleaching units, as further described below. 
In accordance with another aspect of the invention, there is provided a 
method of bleaching a lignocellulosic pulp with ozone, with recovery of an 
oxygen-rich gas comprising: 
(a) providing a washed, aqueous cellulosic pulp comprising cellulosic 
material dispersed in an aqueous vehicle, 
(b) mixing said aqueous pulp with ozone-containing oxygen to disperse the 
ozone in the aqueous vehicle, 
(c) maintaining contact between the dispersed ozone and the cellulosic 
material for a time sufficient to permit bleaching of said cellulosic 
material by said ozone, with liberation of by-product oxygen from said 
ozone, 
(d) recovering a bleached pulp, and 
(e) recovering an oxygen-rich gas, 
said steps of mixing (b) and maintaining contact (c) being carried out 
under a condition such that ingress of air is substantially prevented. 
In particular, the oxygen-rich gas recovered in (e) may be fed to an oxygen 
consuming process either directly, possibly after pretreatment steps, or 
after storage. 
In accordance with another aspect of the invention, there is provided an 
installation for bleaching cellulosic pulp with ozone, with recovery of an 
oxygen-rich gas comprising: 
(a) an ozone generator for generating a supply of ozone-containing oxygen, 
(b) a pulp mixer having means to disperse gas in an aqueous pulp, 
(c) a first conduit for delivery under pressure of an ozone-containing 
oxygen to said pulp mixer, from said ozone generator, 
(d) a retention housing communicating with said pulp mixer, and adapted to 
receive aqueous pulp containing dispersed ozone from said pulp mixer, said 
retention housing providing a flow path for the aqueous pulp sufficient to 
provide a contact time for bleaching of the dispersed pulp by the 
dispersed ozone, with liberation of by-product oxygen from said ozone, 
(e) vent means for removal of vent gases from said retention housing, said 
vent gases comprising oxygen from said ozone-containing oxygen, and 
liberated by-product oxygen, 
(f) a gas recovery line communicating with said vent means for recovery of 
an oxygen-rich gas, and 
(g) a recovery line for flow of bleached pulp from said retention housing, 
said pulp mixer and said retention housing being sealed against ingress of 
air.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
i) Pulp Type 
The above embodiments of the present invention can be employed with 
different lignocellulosic pulps including chemical Kraft pulp, which has 
been subjected to oxygen delignification, chemical sulfite pulps, 
chemi-thermomechanical pulps and pulps of recycled fibers or reslushed 
waste paper or paperboard, or mixtures thereof. 
The pulp may be a medium consistency pulp, by which is to be understood a 
pulp having a solids concentration in water of about 8 to 12%, typically 
about 10%, by weight; a high consistency pulp, by which is to be 
understood a pulp having a solids concentration in water of about 20 to 
40%, by weight; or a low consistency pulp, by which is to be understood a 
pulp having a solids concentration in water of about 0.5 to 3%, by weight. 
The invention may also be employed with pulps having a concentration or 
consistency intermediate those indicated above. However, for the 
concentrations indicated above, equipment is readily available for 
production and handling or pumping the pulps. 
Thus, low consistency pulps may be pumped by an ordinary centrifugal pump, 
and the pulps can be obtained without press rolls, using deckers and 
filters. Medium consistency pulps require special pumps which are 
available, and high consistency pulps are obtained by use of presses. 
The pulps which may be processed in accordance with the present invention 
comprise cellulosic material dispersed in an aqueous vehicle. 
In the case of Kraft pulp and chemical sulfite pulp, the pulp stock is 
suitably subjected to an oxygen delignification followed by washing, prior 
to bleaching with ozone. The washing removes solubilized lignin so that 
the Kappa number for oxygen delignified hardwood pulp is in the range 
5-15, especially 9-11, and for oxygen delignified softwood pulp is in the 
range 7-16, preferably 11-13. The pulp may also comprise a mixture of 
hardwood and softwood, especially in the case of recycled fiber pulp. 
In order to achieve these Kappa levels, efficient washing of the 
delignified pulp is necessary, and in order to achieve this, two stage 
washing may be necessary. 
The washed pulp is suitably acidified to a Ph of 1 to 5, preferably 2 to 4, 
whereafter the acidified pulp is subjected to bleaching with ozone. 
Suitable acids include sulphuric acid and oxalic acid. 
ii) Ozone Bleaching 
The ozone is suitably generated at the site of the pulp mill in an ozone 
generator, so that freshly generated ozone is delivered directly to the 
ozone bleaching section of the mill. 
The ozone is preferably generated from dry, high purity oxygen employing 
known procedures. In particular, the oxygen preferably has a dryness 
determined as a dewpoint of less than -70.degree. C. and is preferably 
substantially free of other gases, such as carbon dioxide, carbon monoxide 
and gaseous hydrocarbons. However, oxygen from a more economical source, 
such as a vacuum swing adsorption unit, may be used. 
The ozone generator typically generates ozone from oxygen to produce an 
ozone-containing oxygen which comprises 1 to 20%, preferably 5 to 15%, 
preferably about 6 to 10%, by weight, ozone, with the balance being oxygen 
and residual amounts of nitrogen and argon. 
The bleaching with ozone is conducted in one or more bleaching units to 
which the ozone-containing oxygen is fed at a pressure of about 5 to 20, 
preferably 9 to 15, preferably about 12 atmospheres absolute. Each 
bleaching unit includes a pulp mixer and a retention tube. 
The gas mixture comprising O.sub.2 /O.sub.3 and the aqueous pulp are both 
fed to a pulp mixer of a bleaching unit. Ozone is dispersed in the aqueous 
vehicle of the pulp by agitation in the pulp mixer, and, in particular, is 
dispersed as a plurality of fine bubbles or small voids which represent 
coalesced groups of fine bubbles. The gas pressure is selected to ensure 
that a plurality of discrete fine bubbles or small voids of ozone is 
dispersed in the aqueous vehicle; in this way the contact area between the 
ozone bubbles or voids and the cellulosic material is maximized. 
If the pressure is too high, the voids become large, and the contact area 
between the ozone and the cellulosic material is reduced, so that ozone is 
not consumed and bleaching efficiency is lowered. 
After dispersion of the ozone in the aqueous pulp, the pulp is fed into the 
retention tube which provides a flow path for the flowing pulp sufficient 
to provide a contact or exposure time between the dispersed ozone and the 
cellulosic material to consume the ozone in the bleaching of the pulp, 
with liberation of oxygen. 
Suitably, the flow of pulp and the length of the retention tube are 
correlated to provide a contact time depending on the charge of ozone. For 
example, a charge of 3 kg of ozone/ton of pulp typically will require a 
contact time of the order of 60 seconds, which may be achieved by use of a 
retention tube having a length of 10 ft. and a flow rate of aqueous pulp 
in the tube of 10 ft./min. A higher charge of ozone will require a larger 
contact time, thus a 5 kg/ton charge of ozone may require about 120 
seconds. This is to allow adequate time for reaction and thereby achieve 
efficient use of the ozone. 
In the case in which a plurality of bleaching units is employed, the 
bleached aqueous pulp from a first upstream bleaching unit flows from the 
retention tube of that first unit to the pulp mixer of an adjacent 
downstream bleaching unit; ozone-containing oxygen is fed to the pulp 
mixer of such downstream bleaching unit, and thereafter bleaching is 
effected in the same manner as described above. 
The variation in application rates of gases in the mixing stages is a 
function of the desired ozone application rate, the purities and gas 
volumes required for other mill processes to which vent gases are 
transported, and recycle requirement of vent gas to the bleaching units in 
accordance with the present invention. 
It is preferred, when employing a plurality of bleaching units, to maintain 
the ratio of ozone to pulp in the first stage at a predetermined level 
which is less than the ratio of ozone to pulp in the second contactor. 
Preferably, the ratio in the first contactor is between about thirty 
percent and about seventy percent of the ratio of ozone to pulp in the 
second contactor. Most preferably, the ratio in the first stage is about 
one-half of the ratio in the second stage to most advantageously recover 
an oxygen-rich gas which is low enough in contamination to be re-usable in 
other processes. 
In another embodiment and in accordance with the present invention, it is 
possible to use a produced oxygen having a lower purity for generating 
ozone to feed the bleaching units. The produced oxygen in this embodiment 
is preferably from a pressure or vacuum swing adsorption unit, although it 
may be from any source. Because gas impurities in the pulp are purged with 
the vent gas from the first stage contactor in the bleaching process, the 
resulting oxygen concentration of the effluent oxygen-containing gas from 
the downstream bleaching units will typically be higher than the purity of 
the upstream vent gas. This aspect is another advantage of the process. 
In another embodiment of the present invention, an oxygen-containing 
liquid, preferably hydrogen peroxide, is contacted with the pulp at least 
prior to the contacting of the pulp with the ozone in the second 
contactor. The aqueous mixture in the second contactor may optionally also 
contain an oxygen-containing liquid such as hydrogen peroxide. 
In each of the above embodiments of the process of the present invention, 
the bleaching units are preferably gas-tightly sealed to prevent ingress 
of air into the units and escape of ozone from the units. 
iii) Vent Gases 
Each of the bleaching units has a vent means for removal of vent gases 
developed during the bleaching in the retention tube. 
Preferably the vent is suitably sealed to prevent ingress of air or escape 
of vent gases and may operate under pressure or vacuum. The vent gases 
comprise oxygen from the ozone-containing oxygen employed in the bleaching 
as well as by-product oxygen formed from the ozone during bleaching. 
The vent gases further may contain unused ozone as well as air, carbon 
dioxide, carbon monoxide and gaseous hydrocarbon; although as described 
above, the content of these latter gases is dependent on their content in 
the aqueous pulp entering the bleaching unit. Vent gases from bleaching 
units downstream of the first contacting with ozone/oxygen mixture, when 
the process is practiced in accordance with the present invention, will be 
sufficiently pure to allow recycle of the vent gas. 
Oxygen delignification of pulp is carried out under highly alkaline 
conditions, employing caustic soda, at a temperature of about 100.degree. 
C., a pressure of about 100 psig and a resident time of about one hour, 
resulting in the formation of carbon dioxide and carbon monoxide. The 
carbon dioxide is absorbed in the caustic soda and forms sodium carbonate, 
and some of the carbon monoxide dissolves. 
When the pulp is washed, most of the sodium carbonate is removed, but some 
remains in the pulp. Thus, when the pulp is acidified prior to the ozone 
bleaching, carbon dioxide is liberated by decomposition of the sodium 
carbonate, and some of this remains in solution along with dissolved 
carbon monoxide. When this acidified pulp is mixed in a high shear mixer 
with an ozone-containing oxygen, the carbon dioxide and carbon monoxide 
gases are stripped from solution, and are then present in the vent gases 
that leave the pulp after the ozone has reacted. The liberation of these 
undesirable gases is particularly the case in the initial bleaching unit. 
Air may also be entrained by the pulp in the course of oxygen 
delignification or subsequent washing. This air is stripped out along with 
carbon dioxide and carbon monoxide and is the source of argon and nitrogen 
in the vent gas. 
It is possible to remove at least some of the carbon dioxide and air by 
passing the pulp slurry through a vacuum degassing device or through use 
of a pump prior to the bleaching units. This will remove air along with 
some carbon dioxide and hydrocarbons. A pump equipped with vacuum 
degassing can, for example, reduce the air content of a pulp from 10-20% 
down to 3-5%, by volume, at ambient temperature and pressure. 
It has also been found that the concentration of ozone in the vent gases 
depends on process conditions of the pulp, for instance the type of pulp 
(softwood or hardwood), the quantity of ozone added, and the pressure at 
which the addition and mixing are carried out. The effectiveness of the 
mixer and residence time of the gas in contact with the pulp will also 
have an effect, as will the COD of the aqueous pulp. 
In one example, it was found that if 6% W/W concentration of ozone in 
oxygen was mixed with hardwood pulp at a pressure of about 150 psig for a 
gas-pulp contact time of about 60 seconds, and the ozone charge was 0.3% 
on pulp, about 99% of the ozone was absorbed and the residual ozone was 
0.1% W/W or less. When this was passed through an ozone destruction unit, 
the ozone concentration was reduced to 0.001 ppm ozone. 
In another example, 6% ozone W/W concentration in oxygen was dispersed in a 
medium consistency hardwood pulp at a pressure of about 150 psig for a 
residence time of about 60 seconds and an ozone charge of 0.6% on pulp. 
Not all of the ozone was consumed, and the residual ozone was found to be 
1.8% W/W. When the vent gas passed through the ozone destruction unit, the 
ozone was reduced to a concentration of 0.001 ppm. 
The total hydrocarbon concentration is found generally to be less than 80 
ppm. This can be reduced to zero by passing the vent gas mixture 
containing oxygen across a heated catalyst. The oxygen present, along with 
the catalyst, will oxidize the hydrocarbons to CO.sub.2. The CO.sub.2 can 
be removed from the vent gases by absorption on a molecular sieve. 
If all of these steps are followed, a gas of high concentration in oxygen 
can be produced as shown for an ozone charge of 0.3% in Table I and an 
ozone charge of 0.6% in Table II. 
______________________________________ 
Process conditions for 0.3% ozone charge on medium consistency 
______________________________________ 
pulp 
Ozone concentration 6% w/w in O.sub.2 gas 
Pressure at Pulp/Gas Mixer 
150 psig 
Temperature Pulp 35.degree. C. 
Consistency 8-12% 
Type Pulp Hardwood 
Ph 2-4 
______________________________________ 
TABLE I 
______________________________________ 
Vent gas composition at different Process Steps 
Gas Composition 
O.sub.2 % 
Ar + N.sub.2 % 
CO.sub.2 % 
H/C 
Process Steps 
V/V V/V V/V ppm O.sub.3 % 
______________________________________ 
Without 89.3 8.6 2.0 35 0.1% W/W 
degassing 
With degassing 
94.2 4.6 1.1 35 0.1% W/W 
After O.sub.3 
94.2 4.6 1.1 35 0.001 ppm 
Destruct 
After removal 
94.2 4.6 1.1 0 0.001 ppm 
Hydrocarbon 
After removal 
95.3 4.7 0 0 0.001 ppm 
CO.sub.2 
______________________________________ 
If the ozone charge is doubled to 0.6% on pulp, the vent gas composition is 
as shown in Table II. 
______________________________________ 
Process conditions for 0.3% ozone charge on medium consistency 
______________________________________ 
pulp 
Ozone concentration 6% w/w in O.sub.2 gas 
Pressure at Pulp/Gas Mixer 
150 psig 
Temperature Pulp 35.degree. C. 
Consistency 8-12% 
Type Pulp Hardwood 
Ph 2-4 
______________________________________ 
TABLE II 
______________________________________ 
Vent gas composition at different Process Steps 
Gas Composition 
O.sub.2 % 
Ar + N.sub.2 % 
CO.sub.2 % 
H/C 
Process Steps 
V/V V/V V/V ppm O.sub.3 % 
______________________________________ 
Without 91.1 4.5 1.8 45 0.6% W/W 
degassing 
With degassing 
96.9 1.0 0.4 45 1.7% W/W 
After O.sub.3 
98.6 1.0 0.4 45 0.001 ppm 
Destruct 
After removal 
98.6 1.0 0.4 0 0.001 ppm 
Hydrocarbon 
After removal 
98.8 1.0 0 0 0.001 ppm 
CO.sub.2 
______________________________________ 
It can be seen from Tables I and II that the vent gases contain a high 
concentration of oxygen, some argon and nitrogen, carbon dioxide, small 
concentrations of hydrocarbons and some ozone residual; employing ozone 
destruction and procedures for removal of hydrocarbons and carbon dioxide 
provides an oxygen-rich vent gas which can be employed in processes which 
have an oxygen requirement. 
Thus, the oxygen-rich vent gas can be employed in such processes as: 
oxygen delignification, 
white liquor oxidation, 
black liquor oxidation, 
polysulfide formation from white liquor, 
lime kiln air enrichment, 
oxygen extraction processes, 
lime mud oxidation, 
ozone generation. 
For some of these processes the presence of residual amounts of other 
gases, for example nitrogen or carbon monoxide, is not a critical factor, 
and, therefore, post treatments for removal of other gases can be selected 
with regard to the intended use of the oxygen. In the case where the 
oxygen is employed for ozone generation, other gases should be minimized, 
and the oxygen should be dried to remove water. 
Likewise, for some processes the presence of residual amounts of ozone is 
not a critical factor, and the oxygen gas containing ozone can be used 
directly. In other cases, in particular ozone generation and polysulfide 
formation, where the oxidizing effect of ozone is too strong, the residual 
ozone should be destroyed. 
When the residual ozone is to be destroyed, this can be achieved in known 
ozone destruction units in which the ozone is converted to oxygen. One 
such destruction unit involves passing the vent gas containing residual 
ozone through a catalytic bed of alumina doped with manganese oxide and 
cupric oxide; another known destruction unit comprises a tube housing an 
electric heating element, and the vent gas containing ozone is fed along 
the tube. 
With further reference to FIG. 1, installation 10 includes a source of 
oxygen-delignified pulp 12, a washer 14, a bleaching unit 16, an ozone 
generator 18, an ozone destruction unit 20 and a bleached pulp line 22. 
The source 12 as shown includes a source 24 of oxygen-delignified softwood 
pulp and a source 26 of oxygen-delignified hardwood pulp. Feed line 28 
communicates source 12 with washer 14. The feed line 28 has branch lines 
27 and 29 communicating with the sources 24 and 26, respectively, and 
lines 27 and 29 are valve-controlled so that a feed of softwood pulp only, 
or of hardwood pulp only, or a mixture of the pulps, is fed along feed 
line 28. 
Washer 14 includes a conveyor belt 30 and an outlet 32 for washings. A 
screw conveyor 34 communicates with an outlet end of conveyor belt 30 in 
washer 14, and a downstream end of screw conveyor 34 communicates with a 
medium consistency pump 36 via a pulp line 33. Acid line 38 communicates 
with pulp line 33. Pulp conduit 35 communicates pump 36 with bleaching 
unit 16. Bleaching unit 16 includes a pulp mixer 40, for example a high 
shear mixer, and a retention tube 42. 
The retention tube 42 communicates with a vent gas tank 44 which has a vent 
line 46. Bleach pulp line 22 also communicates with vent gas tank 44. 
A vent gas blower 48 is disposed in vent line 46, and a pump 50 is disposed 
in bleached pulp line 22. An oxygen outlet line 52 for oxygen-rich gas 
communicates with oxygen destruction unit 20. Ozone generator 18 includes 
an oxygen inlet 54 and a pressurized ozone line 56. A compressor 58 is 
disposed in line 56. 
With further reference to FIG. 2, there is shown a preferred installation 
60 which comprises the same major components as installation 10 of FIG. 1. 
Insofar as common units are employed in installation 60 and installation 
10, the same element numbers are used in FIG. 2. 
Installation 60 includes a plurality of bleaching units. More especially, 
installation 60 includes an upstream bleaching unit 70 and a downstream 
bleaching unit 72. Upstream bleaching unit 70 preferably comprises an 
ozone destruction unit 74, and downstream bleaching unit 72 preferably 
comprises an ozone destruction unit 76. 
Bleaching unit 70 includes a pulp mixer 78 and a retention tube 80. A pulp 
outlet line 81 communicates retention tube 80 with vent gas tank 82. A 
vent line 84 communicates vent gas tank 82 with ozone destruction unit 74. 
A pulp line 86 communicates vent gas tank 82 with a pulp mixer 88 of the 
downstream bleaching unit 72. 
Downstream bleaching unit 72 includes the aforementioned pulp mixer 88 and 
a retention tube 90. A pulp outlet line 91 communicates retention tube 90 
with a vent gas tank 92. A vent line 94 communicates vent gas tank 92 with 
ozone destruction unit 76; a pump 96 is disposed in vent line 94. Bleached 
pulp line 98 communicates with vent gas tank 92, and a pump 100 is 
disposed in line 98. 
Generally, the purity of the oxygen entering the system from an oxygen 
source 17 is a function of the required purity of gas for the other mill 
processes utilizing oxygen-containing gas and the amount of entrained 
gases in the lignocellulosic pulp. 
Although the oxygen in the feed mixture may be the product of a cryogenic 
air separation process or derived from a tank containing industrial grade 
purity oxygen in bulk, in certain embodiments, the oxygen source 17 is an 
absorption unit, either VSA, PSA, or VPSA, where the product oxygen purity 
is less than other oxygen production processes such as with a cryogenic 
production process. Further, a radial design VSA system may be preferred, 
of the type described for example in U.S. Pat. No. 4,541,851, issued Sep. 
17, 1985 to Busquain et al.; or U.S. Pat. No. 5,232,479, issued Aug. 3, 
1993 to Poteau et al., which are incorporated by reference herein. 
The ozone generator 18 in FIG. 2 has an ozone line 102 with ozone branch 
lines 104 and 106. Ozone branch line 104 feeds ozone to upstream bleaching 
unit 70, and ozone branch line 106 feeds ozone to downstream bleaching 
unit 72. In the preferred embodiments, the ratio of ozone to pulp in the 
upstream bleaching unit 70 is controlled to be less than, preferably 
between about thirty and about seventy percent, most preferably about half 
of the ratio of ozone to pulp in the downstream bleaching unit 72. 
In certain embodiments, an oxygen-containing liquid, preferably hydrogen 
peroxide, may be added to the first stage mixer, or the downstream mixers, 
or both. 
The process of the invention is further described with reference to the 
installation 10 illustrated in FIG. 1. 
A medium consistency oxygen-delignified pulp is fed from source 12 along 
feed line 28 to washer 14. The pulp is washed in washer 14 to remove 
solubilized lignin developed during the oxygen delignification. In the 
case of a hardwood pulp from source 26, the washing is typically carried 
out to achieve a kappa number of about 9 to 11, and in the case of a 
softwood pulp from source 24, the washing is typically carried out to 
achieve a kappa number of 11 to 13. In washer 14 the pulp is carried on 
conveyor belt 30 and is typically subjected to a spray of water, which 
water passes through the belt taking with it solubilized lignin which is 
removed by washings through outlet 32. 
The washed medium consistency pulp is then conveyed from washer 14 by screw 
conveyor 34 and fed through pulp line 33 into medium consistency pump 36. 
Acid is introduced through line 38 into pulp line 33 to acidify the pulp, 
typically to a Ph of 2 to 4. 
Ozone is generated from oxygen in ozone generator 18 and is delivered as a 
mixture of ozone and oxygen, under a pressure typically of about 12 
atmospheres, along line 56 to pulp conduit 35, and thence into pulp mixer 
40. 
In pulp mixer 40, the aqueous pulp and the ozone are agitated to disperse 
ozone in the aqueous vehicle of the pulp. 
In the continuous process illustrated in FIG. 1, the aqueous pulp having 
ozone dispersed in the aqueous vehicle exits mixer 40 and flows through 
retention tube 42, which is elongate and allows a contact time between the 
dispersed ozone and the cellulosic material of the aqueous pulp to permit 
consumption of the ozone in the bleaching of the pulp. 
The bleached pulp from retention tube 42 flows from retention tube 42 to 
vent gas tank 44. Vent gases exit vent gas tank 44 through vent line 46 
and are pumped by blower 48 into ozone destruction unit 20. Residual ozone 
is destroyed, particularly by conversion to oxygen in unit 20. The 
bleached pulp is pumped from vent gas tank 44 along line 22 by pump 50 
and, from there, passes to the appropriate paper-making operation. 
The process is preferably carried out so that entry of air into bleaching 
unit 16 is avoided by appropriate sealing of the unit. Furthermore, in the 
preferred embodiment, the pulp entering mixer 40 is subjected to vacuum 
degassing prior to entry of the ozone, to reduce the content of air and 
other gases which may be present as a result of the preliminary steps 
carried out on the pulp. These gases include air entrained in the oxygen 
delignification or in the washing steps and carbon dioxide generated from 
sodium carbonate in the pulp by the addition of acid at line 38. 
In this way, the presence of gases other than oxygen from the oxygen ozone 
mix, and oxygen produced as a by-product from the ozone, are minimized in 
the vent gases, and an oxygen-rich vent gas is produced and can be 
delivered directly from line 52 to an installation having an oxygen 
requirement, after the residual ozone is converted to oxygen in oxygen 
destruction unit 20. 
The oxygen-rich vent gas in line 52 can be further purified, if required, 
depending on the area of use to which the gas is to be put. For example, 
if the gas is to be recycled to the ozone generator 18 to manufacture an 
ozone containing oxygen gas, it is appropriate to dry the gas to remove 
water which interferes with ozone generation. 
The pretreatment of the pulp is essentially as described for FIG. 1. 
The ozone/oxygen mixture is introduced through branch line 104 into the 
aqueous pulp entering pulp mixer 78, and the ozone is dispersed in the 
aqueous vehicle of the pulp as described in connection with FIG. 1. The 
ozone is then allowed to react with the cellulosic material in the aqueous 
pulp in retention tube 80, and a bleached pulp, together with oxygen and 
other gases, flows from retention tube 80 along pulp outlet line 81 into 
vent gas tank 82. Vent gases in vent gas tank 82 flow along vent line 84 
to ozone destruction unit 74 in which the residual, unused ozone is 
converted to oxygen. Bleached aqueous pulp flows from vent gas tank 82 
along pulp line 86 to pulp mixer 88, and ozone is introduced into this 
aqueous pulp through branch line 106. The ozone is dispersed in the 
aqueous vehicle of the aqueous pulp in mixer 88 as described for mixer 40 
in FIG. 1. The aqueous pulp then flows into retention tube 90, which 
allows for reaction between the ozone and the pulp to effect further 
bleaching of the pulp, and a bleached pulp and residual gases flow from 
retention tube 90 along pulp outlet line 91 to vent gas tank 92. Vent 
gases in vent gas tank 92 are pumped by pump 96 along vent line 94 to 
ozone destruction unit 76. Bleached aqueous pulp in vent gas tank 92 is 
pumped by pump 100 along bleached pulp line 98 and thereafter passes to a 
paper-making process. 
In this embodiment, even if the entrained air and other gases are not 
removed from the aqueous pulp prior to the upstream bleaching unit 70, 
most of such gases will exit in the vent gas stream in vent line 84. In 
such case, provided downstream bleaching unit 72 is sealed to prevent 
ingress of air, the vent gas recovered from ozone destruction unit 76 will 
be of high oxygen content and will be ready for use in most oxygen 
requiring processes. Such gas will, however, need to be dried if it is to 
be employed for ozone generation in ozone generator 18. 
Similarly, if the installation 16 includes additional bleaching units of 
the form of 70 and 72, all of the downstream units will produce an 
oxygen-rich gas directly, provided the units are adequately sealed to 
prevent ingress of air, since the air entrained in the pulp, and other 
gases formed in the pulp in the preliminary stages, will essentially form 
part of the vent gas from the first upstream bleaching unit 70. 
Consequently, by using an installation 60 with a plurality of bleaching 
units, it is possible to form an oxygen-rich gas which requires no 
post-treatment for many purposes, from the downstream bleaching units. In 
such case, the only post-treatment necessary would be removal of water, if 
the oxygen were to be cycled to the ozone generator 18. For other process 
uses, removal of water would not be required, and the oxygen-rich gas 
could be fed directly to the oxygen-consuming process. 
EXAMPLES 
Example 1 (Comparison) 
A process was carried out in accordance with the invention, following the 
procedure described in conjunction with FIG. 1. 
In a first operation no attempt was made to seal the bleaching unit 16 
against ingress of air, and no steps were taken to remove gases entrained 
in the pulp in the preliminary stages. 
An analysis of the vent gases in line 84, prior to the ozone destruction 
unit 74, revealed a gas mixture of the following composition: 
______________________________________ 
Oxygen 42.0-54.0% V/V 
Carbon Dioxide 0.7-1.4% V/V 
Hydrocarbons 10-15 ppm 
Residual Ozone 0.01-1.0% W/W 
Argon and Nitrogen Balance 
______________________________________ 
At first, the balance of gases, mainly argon and nitrogen, was thought to 
be due to air entrapped in the pulp in the preliminary stages. However, on 
further investigation it was found to be due mainly to air leaking into 
the vent gases. 
Example 2 
The procedure of Example 1 was repeated, but the bleaching unit 16 was 
sealed, and air leaks eliminated. The oxygen content increased, and the 
argon and nitrogen contents decreased correspondingly. A typical analysis 
before the ozone destruction unit was found to be: 
______________________________________ 
Oxygen 90-95% V/V 
Carbon Dioxide 1-3% V/V 
Hydrocarbons 20-80 ppm 
Residual Ozone 0.01-1.6% W/W 
Argon and Nitrogen Balance 
______________________________________ 
When the gas analysis was taken after the ozone destruction, the residual 
ozone fell to the range of 0.001 ppm, and the gas composition was as 
follows: 
______________________________________ 
Oxygen 90-95% V/V 
Carbon Dioxide 1-3% V/V 
Hydrocarbons 20-80 ppm 
Residual Ozone 0.001 ppm 
Argon and Nitrogen Balance 
______________________________________ 
It was expected that in the ozone bleaching the vent gases would contain a 
concentration of carbon dioxide and carbon monoxide similar to that 
measured in the vent gases from an Oxygen Delignification Process. 
However, it was found that their compositions were quite different than 
expected as can be seen from Table III: 
TABLE III 
______________________________________ 
Comparison of Carbon Dioxide and Carbon Monoxide 
concentration in Vent Gases from Ozone Bleaching and Oxygen 
Delignification Processes 
Process CO.sub.2 % V/V 
CO as CH.sub.4 ppm 
______________________________________ 
Ozone Bleaching 1-3 20-80 
Oxygen Delignification 
0.1-10% 3,000-15,000 
______________________________________ 
Example 3 
The procedure described with reference to FIG. 2 was followed. 
In this example medium consistency pulp from the washers of an Oxygen 
Delignification Process was pumped into the pulp mixer 78 operating at a 
pressure of 150 psig. Here, a gas mixture containing 6% w/w of ozone in 
oxygen was pumped into the pulp mixer 78, where it was dispersed in the 
aqueous pulp. From the mixer 78, the gas-pulp mixture flowed into the 
retention tube 80, where it resided for about 60 seconds to allow the 
ozone to be absorbed and react with the cellulosic material. 
The oxygen which does not react and residual unreacted ozone were then 
vented and the pulp flowed into the second mixer 88. Here, more ozone was 
added to the pulp and dispersed. The gas-pulp mixture then passed into a 
retention tube 90, where the ozone was consumed and the excess gases 
vented. 
Hence, in this process there are two sources of vent gas. The composition 
of the vent gases of the two sources is different because in the upstream 
mixer 78 stripping of the gases in the pulp, mainly air and carbon 
dioxide, is substantially complete, so that the gases vented after the 
downstream mixer 88 contain substantially no air or carbon dioxide. The 
composition of the vent gases from the two stages of mixing are shown in 
Tables IV and V. 
______________________________________ 
Process Conditions 
______________________________________ 
Ozone concentration 6% w/w in O.sub.2 gas 
Pressure at Pulp/Gas Mixer 
150 psig 
Temperature Pulp 35.degree. C. 
Consistency 8-12% 
Type Pulp Hardwood 
Ph 2-4 
______________________________________ 
TABLE IV 
______________________________________ 
Vent gas composition from Stage 1 Mixer after 
addition of 0.5% ozone charge on pulp. 
Gas Composition 
O.sub.2 % 
Ar + N.sub.2 % 
CO.sub.2 % 
H/C O.sub.3 % 
Process Steps 
V/V V/V V/V ppm W/W 
______________________________________ 
Without 91.0 5.9 1.9 40 1.1% W/W 
degassing 
With degassing 
94.4 3.4 0.8 40 0.2% W/W 
After O.sub.3 
95.8 3.4 0.8 40 0.001 ppm 
Destruct 
After removal 
95.8 3.4 0.8 0 0.001 ppm 
Hydrocarbon 
After removal 
96.6 3.4 0 0 0.001 ppm 
CO.sub.2 
______________________________________ 
Table IV shows that the vent gas leaving the first stage mixer, and after 
passing through the ozone destruction system, contains a high 
concentration of oxygen. This gas, without ozone destruction, could be 
directly used in other processes requiring oxygen such as: 
Oxygen Delignification 
White Liquor Oxidation 
Black Liquor Oxidation 
Lime Kiln Enrichment 
Oxygen Extraction 
Lime Mud Oxidation 
This gas, after ozone destruction, could be used in: 
Polysulfide Formation for White Liquor 
Ozone Generation 
Table V shows that the vent gas composition leaving the second stage mixer 
contains a high concentration of oxygen, and after passing through the 
ozone destruction system and a dryer, it could be recycled back to the 
ozone generator 18 or used in another process where oxygen is required. 
TABLE V 
______________________________________ 
Vent gas composition from Stage II Mixer after 
addition of 0.5% ozone charge on pulp. 
Gas Composition 
O.sub.2 % 
Ar + N.sub.2 % 
CO.sub.2 % 
H/C O.sub.3 % 
Process Steps 
V/V V/V V/V ppm W/W 
______________________________________ 
Without 98.2 0.6 1.0 20 1.1% W/W 
degassing 
With degassing 
-- -- -- -- -- 
After O.sub.3 
99.3 0.6 0.1 20 0.001 ppm 
Destruct 
After removal 
99.3 0.6 0.1 0 0.001 ppm 
Hydrocarbon 
After removal 
99.4 0.6 0 0 0.001 ppm 
CO.sub.2 
______________________________________