Plug flow process for the production of chlorine dioxide

A plug flow process for the production of chlorine dioxide by reacting chloric acid and/or an alkali metal chlorate, optionally a mineral acid and a reducing agent in such proportions that chlorine dioxide is produced. Reactants are continuously fed to a plug flow reactor under conditions that chloride dioxide is produced as the reactants flow through the reactor. The process stream has an acidity between 2N and 11N. The process stream is subjected to superatmospheric pressure in the reactor sufficient to maintain the formed chlorine dioxide in solution. After removal of the chlorine dioxide from the process stream exiting the plug flow reactor, the process stream can optionally be fed to a second chlorine dioxide generator for further reaction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the art of chlorine dioxide production, and in 
particular, to the continuous production of chlorine dioxide in a plug 
flow reactor by reduction of an alkali metal chlorate or chloric acid with 
a reducing agent. In a preferred embodiment, the process of the invention 
uses hydrogen peroxide as the reducing agent. 
2. Description of the Prior Art 
Chlorine dioxide used in aqueous solution is of considerable commercial 
interest, mainly in pulp bleaching, but also in water purification, fat 
bleaching, removal of phenols from industrial wastes, etc. It is therefore 
desirable to provide processes in which chlorine dioxide can be 
efficiently produced. 
In existing processes for the production of chlorine dioxide, chlorine gas 
is often formed as a by-product, due to the use of chloride ions as a 
reducing agent. The basic chemical reaction involved in such processes is: 
EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 
O [1] 
The chlorate ions are provided by alkali metal chlorate, preferably sodium 
chlorate, the chloride ions by alkali metal chloride, preferably sodium 
chloride, or by hydrogen chloride, and the hydrogen ions are provided by 
mineral acids, generally sulfuric acid and/or hydrochloric acid. 
In the production of chlorine dioxide with chloride ions as the reducing 
agent according to reaction [1], half a mole of chlorine is produced for 
each mole of chlorine dioxide. This chlorine gas by-product has previously 
been used as such in paper mills as a bleaching agent in aqueous solution. 
However, increased environmental demands have resulted in a change-over to 
pure chlorine dioxide bleaching. To achieve pure chlorine dioxide 
bleaching there is an increasing demand for chlorine dioxide manufacturing 
processes which do not produce chlorine as a by-product. 
One known way of reducing the chlorine by-product is to use reducing agents 
which do not produce chlorine as a by-product. One example is in the 
so-called "Solvay" process, wherein alkali metal chlorate is reduced in an 
acid medium with methanol as the reducing agent. Another example is in the 
"Mathieson" process, in which chlorate is reduced with sulfur dioxide in a 
sulfuric acid-containing medium. These processes use methanol and sulfur 
dioxide, respectively, as indirect reducing agents, and hence the rate of 
reaction is very slow. In U.S. Pat. No. 4,081,520, an allegedly more 
effective "Solvay" process is described using a reduced pressure and a 
high acid normality in a single vessel reactor. 
The direct reducing agent in the case of methanol and sulfuric acid 
reactions is chloride ion reacting according to reaction [1]. The chlorine 
produced then reacts with methanol to regenerate chloride ions according 
to the reaction: 
EQU CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O.fwdarw.6Cl.sup.- +CO.sub.2 +6H.sup.+[ 2] 
or with sulfur dioxide according to the reaction: 
EQU Cl.sub.2 +SO.sub.2 +2H.sub.2 O.fwdarw.2HCl+H.sub.2 SO.sub.4[ 3] 
According to one prevalent theory holding that chloride ion must be 
present, it is often necessary to continuously add a small amount of 
chloride ion in order to obtain a steady production. Due to the continued 
presence of chloride ion, even with methanol or sulfur dioxide as the 
reducing agent, a certain amount of chlorine by-product is produced. 
According to U.S. Pat. No. 4,081,520, operating with methanol as reducing 
agent, the amount of chlorine by-product produced is decreased with 
increasing acid normality in the reaction medium. The reaction rate is 
also increased with increasing acid strength. At a low acid normality, the 
reaction is so slow that it is of no commercial interest. However, the 
drawback with a high acid strength in the reaction medium is, in addition 
to more corrosion in the equipment, the production of an acid salt in the 
form of sesquisulfate (Na.sub.3 H(SO.sub.4).sub.2) or bisulfate 
(NaHSO.sub.4). This occurs at an acid normality of from above about 5N to 
about 12N. An acid salt results in loss of acid in production and costs 
for neutralization of the salt. From about 2 N to about 5N acid normality, 
a neutral alkali metal salt (alkali metal sulfate) is formed. 
It is also known to speed up the reaction rate at low acidities by using 
catalysts both with chloride and methanol as the reducing agent. U.S. Pat. 
No. 3,563,702 discloses catalysts for chloride reduction. However, 
catalysts are expensive and thus increase the production costs. 
Another drawback with methanol as the reducing agent is the possible 
formation of chlorinated organic compounds, from by-products of methanol, 
in the downstream bleaching process. It is well known that the efficiency 
of the added methanol is lowered due to side reactions wherein 
formaldehyde and formic acid are formed. Also, some of the methanol leaves 
the reactor without having participated in the reduction reaction. The 
corresponding ether and ester are probably there as well. It could be 
expected that reactions can occur in the bleaching train with aldehyde, 
acid, ether and ester, resulting in chlorinated organic compounds. 
In U.S. Pat. Nos. 5,091,166 and 5,091,167, the draw-backs of using methanol 
as a reducing agent are addressed by substituting hydrogen peroxide for 
methanol. These patents disclose production of chlorine dioxide using a 
single vessel process under subatmospheric pressure. Alkali metal chlorate 
is reduced with hydrogen peroxide as the reducing agent in an aqueous 
reaction medium containing sulfuric acid. The reaction medium is 
maintained at its boiling point of between 50.degree. C. and 100.degree. 
C. such that water is evaporated therefrom, forming steam. A gaseous 
mixture containing the steam, produced chloride dioxide, and by-product 
oxygen is withdrawn from the vessel. 
In the reaction medium an alkali metal salt crystallizes and is removed. 
The type of salt crystallized is a function of the acid normality of the 
reaction medium. At an acid normality of between 2 and 4, a neutral sodium 
sulfate salt, for example, Na.sub.2 SO.sub.4, forms. At higher acid 
normalities, a sesquisulfate salt or a bisulfate is formed. 
While the processes disclosed in U.S. Pat. Nos. 5,091,166 and 5,091,167 are 
a great improvement over the prior art methanol processes, they are 
performed in a single vessel process (SVP.RTM.) reactor in which the 
generation and separation of chlorine dioxide are carried out in a single 
reaction vessel maintained at the boiling point of the reaction medium. 
Kinetically, the single reaction vessel functions as a constant flow 
stirred tank reactor ("CFSTR" or "CSTR"). There are indeed numerous 
advantages to this type of reaction vessel. By maintaining the reaction 
medium at its boiling point, the evolved chlorine dioxide is diluted with 
steam, thereby reducing explosion risk. Alkali metal salt concentration in 
the reaction medium is maintained at saturation, resulting in the alkali 
metal salt being continuously deposited and easily removed. 
On the other hand, single vessel processes require long residence times to 
obtain an acceptable rate of conversion. Long residence times require 
either a low flow rate through the reaction vessel (and subsequent low 
production rate) or a large vessel size. Production requirements, at least 
for large consumers of chlorine dioxide, dictate that residence times be 
maintained via a large reaction vessel. Paper pulp mills, for example, 
require between 15 and 60 tons per day (TPD) of chlorine dioxide, and this 
production level requires large scale equipment. 
A typical SVP.RTM. reaction vessel for 40 TPD of chlorine dioxide is about 
10 feet in diameter and has a volume of about 8800 gallons. Since chlorine 
dioxide is produced on-site (for safety reasons), the SVP.RTM. reaction 
vessel and related process equipment must be shipped to and installed at 
the location of use. Shipping costs are high due to the weight and bulk 
involved. Also, the size of the reaction vessel requires the commitment of 
considerable plant space. 
In addition to the costs and the space requirements of an initial 
installation, further costs are incurred if an upgrade in chlorine dioxide 
production capacity using SVP.RTM. technology is required. Such an upgrade 
would require removal of the existing generator vessel and replacement 
with an even larger vessel, essentially duplicating the initial 
installation costs. Alternatively, the upgrade would require the addition 
of a secondary SVP.RTM. generator having additional installation costs and 
space requirements. In either case, the upgrade would be expensive. 
The concept of plug flow reactors has heretofore been applied to various 
chemical processes and offers the advantage of small size with reasonable 
production rates. However, plug flow reactors were not believed feasible 
for producing chlorine dioxide, due to the relatively slow kinetics of 
uncatalyzed reaction schemes. Catalyzed systems were also deemed 
unsuitable for plug flow processes due to accumulation and clogging of the 
equipment by the solid phase catalyst. 
In very small scale processes, non-CSTR, continuous chlorine dioxide 
reactions have been used successfully. For example, in U.S. Pat. No. 
5,061,471, there is disclosed a process for continuous production of 
chlorine dioxide using alkali metal chlorate, sulfuric acid and sulfur 
dioxide as the reducing agent. This process is suitable for small scale 
chlorine dioxide applications such as treatment of drinking water, etc. 
This patent does not teach plug flow, however, since the concentration 
profile in the reactor is uniform, which approximates a CSTR. 
There is accordingly a need in the art for a chlorine dioxide process which 
has the advantages of low chlorine by-product generation and high 
production rate and which also has reduced installation and upgrading 
costs compared to processes using single vessel process generators. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a process for 
producing chlorine dioxide having reduced installation costs. 
It is another object of the invention to provide a process for producing 
chlorine dioxide, as above, whereby the space requirements of the chlorine 
dioxide plant are reduced compared to a single vessel process. 
It is still another object of the invention to provide a process for 
producing chlorine dioxide, as above, which can be used to upgrade the 
capacity of an existing chlorine dioxide plant with reduced installation 
costs and space requirements. 
It is yet another object of the invention to provide a process for 
producing chlorine dioxide, as above, having low chlorine by-product 
generation. 
These objects are achieved by a process for producing chlorine dioxide, 
wherein alkali metal chlorate, sulfuric acid and hydrogen peroxide as a 
reducing agent are fed to one or more reaction conduits which function as 
a plug flow reactor. Chlorine dioxide is formed as the process stream 
flows through the reactor and is recovered from the stream after it exits 
the reactors. Alkali metal salt is also recovered from the exiting process 
stream, preferably while still in solution in the reaction medium. 
The objects of the invention are also achieved by a process for producing 
chlorine dioxide, wherein chloric acid and hydrogen peroxide are fed to 
the plug flow reactor. One of the advantages of using chloric acid 
compared to alkali metal chlorate is that the formation of sulfate salt is 
eliminated. This in turn eliminates the need for a downstream salt cake 
filter. This process is particularly adapted for use as a "stand alone" 
chlorine dioxide generating plant. Optionally the chloric acid feed can 
contain alkali metal chlorate which is not reacted and thus comprises a 
"dead load" in the system. This reaction scheme allows the use of a mixed 
chloric acid/chlorate product produced by the partial electrolysis of 
alkali metal chlorate. 
The objects of the invention are also achieved by a method for increasing 
the production capacity of an existing single vessel chlorine dioxide 
generator by installing a plug flow reactor upstream from, and in series 
with, a single vessel generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides for a plug flow process having high 
production rate through the use of selected reactants and hydrogen 
peroxide as a reducing agent. Plug flow is defined as fluid flow through a 
conduit in an orderly manner with no element of fluid overtaking or mixing 
with any other element ahead or behind. The plug flow process of the 
invention can be used as a stand alone chlorine dioxide generator, or in 
series with one or more additional plug flow reactors, or with one or more 
single vessel process (SVP.RTM.) generators. In one preferred embodiment, 
the plug flow reactor is used to upgrade the production capacity of an 
existing SVP.RTM. installation. In this embodiment, a plug flow reactor 
(pfr) is installed upstream of and in series with an existing SVP.RTM.. In 
a highly preferred embodiment, existing SVP.RTM. installations using prior 
art technologies such as methanol, SO.sub.2 or Cl.sup.- based reactions, 
can be upgraded by conversion to hydrogen peroxide technology in 
conjunction with use of the plug flow reactor. 
In another embodiment, pure chloric acid is reduced with hydrogen peroxide 
in a pfr to form chlorine dioxide. This process is particularly adapted 
for a small scale, stand alone plug flow generator producing chlorine 
dioxide for use in water treatment and related applications wherein strict 
environmental regulations preclude discharge of by-products and unreacted 
starting materials such as chlorates, methanol, methanol derivatives, 
sulfur-based compounds, etc. 
A plug flow reactor suitable for use in the invention is illustrated in 
FIG. 1 and is generally indicated by the number 10. Hydrogen peroxide 
enters the reactor 10 through conduit 12, and a mixture of sulfuric acid 
and either or both chloric acid and alkali metal chlorate are fed through 
conduit 14. Mixing of the feeds from conduits 12 and 14 occurs in the 
distribution header 16 of the reactor 10, forming a reaction medium. The 
reaction medium enters a plurality of reaction tubes 18 wherein reaction 
takes place in a substantially plug flow manner. 
Flow through the tubes 18 is preferably in the turbulent flow region, but 
the level of turbulence is such that back mixing or forward mixing of the 
flow stream is minimized in accordance with plug flow requirements. Each 
tube functions as an individual pfr. The size of each tube and the flow 
velocity through the tubes is a function of the necessary residence time 
to convert a predetermined portion of the chlorate ion to chlorine 
dioxide. Typical tube velocities range from about 0.01 to about 1.0 
ft/sec, desirably from about 0.02 to about 0.3 ft/sec and are preferably 
around 0.02 ft/sec. In general, the reaction tubes 18 have a length of 
from about 5 to about 20 ft, desirably from about 10 ft to about 16 ft, 
and preferably about 12 ft. The inner diameter of the reaction tube 18 is 
between about 0.5 in and about 6 in, desirably between about 1 in and 3 in 
and preferably between about 1.5 and about 2.0 in. Chlorate conversion is 
typically in the range of 5-70%, desirably between about 20% and 50% and 
preferably about 40% of the incoming chlorate feed. 
After reaction, the reaction medium flows out of the reaction tubes into 
the shell 20 of the pfr. Gas--liquid separation occurs in the shell 20. 
Product chlorine dioxide gas flows out of the reactor 10 via conduit 22, 
and is recovered by well-known means. Spent reaction liquor flows out of 
the tubes 18 and collects in the bottom of the reactor shell 20. 
Thereafter, the liquor is pumped out of the shell via conduit 24 and into 
the SVP.RTM. generator (not shown). 
The pfr can be operated isothermally (constant temperature), or 
adiabatically (no energy input or lost). Because sulfuric acid is used in 
this embodiment, a heat of dilution is generated which is sufficient to 
operate the process adiabatically. Adiabatic operation has the advantage 
of simplifying the reactor design and conserving energy. Thus, the energy 
needed to drive the reaction in the pfr is provided exclusively by the 
heat of dilution for the acid feed. The pfr is operated in a 
non-crystallization mode, whereby the sulfate salt is not crystallized in 
the pfr. 
The pfr can operate over a range of pressures. Typically, the total 
pressure in the reactor can be between about 200 and about 760 mm Hg 
absolute. Desirably the total pressure is from about 290 to about 310 mm 
Hg absolute, with about 300 mm Hg preferred. Air can be added to the pfr, 
if necessary, to dilute the chlorine dioxide product gas. 
In a first preferred embodiment, the pfr reactor is used in series with an 
SVP.RTM. generator, and employs alkali metal chlorate. The chemical feeds 
are split between the pfr and the SVP.RTM.. Generally from about 50% to 
about 80% by weight of the alkali metal chlorate, from about 80% up to 
100% by weight of the sulfuric acid and from about 30% to about 80% by 
weight of the hydrogen peroxide is fed to the pfr, with the remainder of 
the chemicals fed directly to the SVP.RTM. generator. Desirably, the 
fraction of alkali metal chlorate fed to the pfr is between about 60% and 
70% and preferably is about 65% by weight. Desirably at least about 90% by 
weight of the total sulfuric acid is fed to the pfr. Hydrogen peroxide is 
fed to the pfr in an amount desirably between about 40% and about 70% by 
weight with about 50% by weight of the total peroxide being preferred. The 
purpose of using split feed is to maximize the acid concentration in the 
pfr which in turn maximizes chlorate conversion to chlorine dioxide. 
In this embodiment, the spent liquor exiting the pfr and pumped to the 
SVP.RTM. generator is still rich in reaction chemicals. In the SVP.RTM. 
generator the chlorate conversion to chlorine dioxide is at least about 
25% and can approach 100%. 
In a second preferred embodiment, a mixture of chloric acid and alkali 
metal chlorate is fed to the pfr. The reaction is divided such that 
chloric acid reacts in the pfr and the liquor exiting the pfr, containing 
the unreacted alkali metal chlorate, is fed to an SVP.RTM. generator. In 
this embodiment, no sulfuric acid is fed to the pfr, thus preventing 
reaction of the alkali metal chlorate in the pfr. The entire amount of 
sulfuric acid is instead fed to the SVP.RTM. generator for reaction of the 
alkali metal chlorate in this later stage. Between about 80% and 100% by 
weight of the hydrogen peroxide is fed to the pfr, and desirably at least 
90% by weight. 
This embodiment is especially advantageous for using the output from an 
electrolytic cell for producing chloric acid, which is operated for 
partial conversion of alkali metal chlorate to chloric acid, leaving a 
considerable amount of unreacted alkali metal chlorate in the output 
stream. Generally the feed stream of chloric acid/alkali metal chlorate 
has a concentration of chloric acid from about 0.5 to about 6M, preferably 
from about 2.0 to about 3.0M, while the amount of alkali metal chlorate in 
the feed is from about 2.0 to about 5.0M, and preferably from about 3.0 to 
about 4.0M. It is advantageous from a process standpoint to maximize the 
amount of chloric acid in the feed stream, but the upper limit of chloric 
acid concentration is determined by the economics of the electrolytic cell 
process. Alternatively, instead of using an SVP.RTM. generator to react 
the alkali metal chlorate "dead load" in the exit stream from the pfr, the 
exit stream can be recycled to the electrolytic cell for further 
conversion to chloric acid. 
FIG. 2 schematically illustrates a pfr reactor connected in series to an 
SVP.RTM. generator. This arrangement could be used for the first and 
second embodiments described above. 
As shown in FIG. 2, chemical feeds (chlorate and/or chloric acid, reducing 
agent and optionally sulfuric acid) are split between the pfr reactor and 
the SVP.RTM. to optimize reaction performance, as described above. A plug 
flow reactor 100, contains one or a plurality of reaction tubes (not 
shown). The chemical feed enters the pfr via conduits 112 and 114. 
Unreacted chemicals and chlorine dioxide exit via conduit 116 and are fed 
to an SVP.RTM. generator 118 where further reaction to approximately 100% 
conversion can occur. Sodium sulfate salt is crystallized in the SVP.RTM. 
generator 118. The solid sodium sulfate is thereafter removed via conduit 
120. 
In a third preferred embodiment, the feed to the pfr is pure or 
substantially pure chloric acid. In this embodiment, due to the high 
reactivity of chloric acid with hydrogen peroxide and the lack of a 
saltcake by-product, the pfr can function as a stand alone chlorine 
dioxide generator. Chloric acid concentration is at least about 0.5M and 
can be as high as about 6M. The limiting factor for chloric acid 
concentration is stability. Above about 6M, the chloric acid solution 
tends to decompose. Desirably the chloric acid concentration is at least 
about 2.0M and preferably at least about 3.0M. 
In general, the plug flow reactor of the invention can be either a single 
conduit or a plurality of conduits through which the process stream flows 
in parallel. 
While the invention is not to be limited thereto, kinetic studies have 
revealed that the rate expression for a chlorine dioxide process using 
hydrogen peroxide, sulfuric acid and alkali metal chlorate is: 
##EQU1## 
The rate expression for processes using either chloric acid alone or a 
mixture of chloric acid and "dead load" alkali metal chlorate is: 
##EQU2## 
Thus the reaction is essentially 6th order for alkali metal chlorate 
processes and 11.5 for chloric acid processes. 
The invention is illustrated by the following examples: 
EXAMPLE 1 
A 40 TPD SVP.RTM. operated with methanol, sulfuric acid and sodium chlorate 
is converted to a 55 TPD process by connecting a 15 TPD plug flow reactor 
in series to the SVP.RTM. generator. Hydrogen peroxide is used as the 
reducing agent for chlorate in a sulfuric acid medium in both the pfr and 
the SVP.RTM.. To minimize the pfr size, the reactant profiles in the 
reactor are optimized. This is achieved by splitting the total chemical 
feeds to the system between the pfr and the SVP.RTM. generator. For this 
example 70% by weight of the total chlorate feed and 100% by weight of the 
total acid feed are combined and directed into the pfr. Fifty percent of 
the total peroxide feed is also directed into the pfr. The pfr reactant 
feeds thoroughly mix in the short distribution header of the reactor. The 
mixed reactants then flow through the individual reaction tubes of the 
reactor. The calculated production rate in the pfr is as follows: 
##EQU3## 
To achieve these conversions, the residence time in each tube is 
approximately 15 minutes. The expected reactant concentrations and 
temperature profiles for this reactor are shown in FIG. 3. 
EXAMPLE 2 
A 40 TPD SVP.RTM. using alkali metal chlorate, sulfuric acid and methanol 
as the reducing agent, operated in the substantial absence of added 
chloride ion, i.e., without salt addition, is converted to a 60 TPD 
process by connecting a 20 TPD plug flow reactor in series to the SVP.RTM. 
generator. The feed to the pfr is a mixture of alkali metal chlorate and 
chloric acid which constitutes the output from an electrolytic cell. 
Hydrogen peroxide is used as the reducing agent both for chloric acid in 
the pfr, and for alkali metal chlorate in the SVP.RTM.. Sulfuric acid is 
fed only to the SVP.RTM. generator. In this example, 100% of an alkali 
metal chlorate--chloric acid feed are combined and directed into the pfr. 
The concentration of these species in the feed is 3.5M chlorate and 2.5M 
chloric acid. One hundred percent of the total peroxide feed is directed 
into the pfr through the center distributor conduit. The pfr reactant 
feeds thoroughly mix in the short distribution header of the reactor. The 
mixed reactants then flow through the individual reaction tubes of the 
reactor. Each reaction tube in the pfr is 1.5 inches in internal diameter 
and 1.5 ft long. The total number of tubes in the pfr is 8. Chlorate 
conversion is 35% of the incoming chlorate feed. The calculated production 
rate in the pfr reaction is shown below. 
##EQU4## 
To achieve these conversions, the residence time in each tube is 10 
minutes. Each tube is open-ended at the top of the reactor to allow for 
gas--liquid separation and liquid overflow. Product chlorine dioxide gas 
flows out of the top reactor head, and spent reaction liquor flows out of 
the tubes and collects in the bottom of the reactor shell. This liquor is 
rich in chlorate and peroxide and is pumped out of the shell, and into the 
SVP.RTM. generator. In this vessel the alkali metal chlorate is converted 
to chlorine dioxide (approximately 100% conversion). The pfr is operated 
isothermally, with the energy needed to drive the reaction in the pfr 
provided by steam. The pfr is operated in a non-crystallization mode at a 
pressure of 300 mm Hg. Air is added to the pfr to dilute the chlorine 
dioxide product gas. The expected reactant concentrations and temperature 
profiles for this reactor are shown in FIG. 4. 
EXAMPLE 3 
From an electrolytic cell, partially converted chlorate solution containing 
chloric acid is fed to a pfr as described above and reacted with hydrogen 
peroxide. Since no other acids or detrimental by-products are present, 
only the chloric acid reacts to form chlorine dioxide. Unreacted sodium 
chlorate is then combined with fresh alkali metal chlorate, recycled to 
the electrochemical cell, and further converted to chloric acid. This 
process is optimized to give good current efficiency and chemical 
stability by keeping the per pass conversion of chlorate low in the cell 
(i.e., 25% conversion of the total chlorate feed to the cell) and the per 
pass conversion of the chloric acid in the pfr high (i.e., 50% conversion 
of the total chloric acid feed to the pfr). 
EXAMPLE 4 
A 40 TPD SVP.RTM. using alkali metal chlorate, sulfuric acid and methanol 
as the reducing agent, operated in the substantial absence of added 
chloride ion, i.e., without salt addition, is replaced with a 40 TPD 
pfr--chloric acid cell system. For this example, 100% of the total 
chlorate--chloric acid feed is directed into the pfr. One hundred percent 
of the total peroxide feed is also directed into the pfr through the 
center distribution conduit. The pfr reactant feeds thoroughly mix in the 
short distribution header of the reactor. The mixed reactants then flow 
through the individual reaction tubes of the reactor. Flow through the 
tubes is in the laminar flow region. Each tube functions as an individual 
pfr, and has an internal diameter of 2 inches and a length of 2.5 ft. 
Chlorate conversion is 35% of the incoming chlorate feed. The calculated 
production rate in the pfr is shown below. 
##EQU5## 
To achieve these conversions, residence times in each tube are 10 minutes. 
The chloric acid portion of the feed is driven to almost complete 
conversion. Product chlorine dioxide gas flows out of the top reactor 
head, and is collected. Spent reaction liquor flows out of the tubes and 
collects in the bottom of the reactor shell. This liquor is still rich in 
chlorate but low in acidity and peroxide. It is then pumped out of the 
shell, and back to an electrochemical cell. The pfr is operated 
isothermally. The energy needed to drive the reaction in the pfr is 
provided by a suitable heating medium (i.e., steam or hot water). The pfr 
is operated at a pressure of about 300 mm Hg. Air is added to the reactor 
to dilute the chlorine dioxide product gas. 
Although the present invention has been described in connection with 
preferred embodiments of the invention, it will be appreciated by those 
skilled in the art that additions, substitutions, modifications and 
deletions not specifically described, may be made without departing from 
the spirit and scope of the invention as defined in the appended claims.