Method of producing a hypohalogenated acid

A method of using an elongated, generally vertically extending cocurrent reactor vessel for the production of hypochlorous acid by the mixing and reaction of a liquid alkali metal hydroxide and a gaseous halogen is provided wherein an atomizer is mounted near the top of the reactor vessel to atomize the liquid alkali metal hydroxide into droplets in the vessel. The vessel has a spraying and reaction zone immediately beneath the atomizer and a drying zone beneath the spraying and reaction zone to produce a gaseous hypochlorous acid and a substantially dry solid salt by-product.

This invention relates generally to a reactor vessel and more specifically 
to a reactor vessel for the production of and the method of producing 
therein hypohalogenated acid by the mixing and reaction therein of an 
alkali metal hydroxide and a gaseous halogen. A preferred product acid is 
hypochlorous acid. 
Hypochlorous acid is used extensively in the preparation of chlorohydrin 
and chloramines. Chloroisocyanurates are typical examples. Dilute 
hypochlorous acid and large quantities of halogen have been used recently 
to produce hypohalites, such as sodium hypochlorite. Hypochlorous acid has 
been produced by several processes or techniques. 
One technique employs the process in which chlorine, steam and air are 
bubbled through an aqueous solution of an alkaline earth metal 
hypochlorite, such as calcium hypochlorite, to remove the resulting 
hypochlorous acid in vapor form. The hypochlorous acid is then condensed 
and stored for use. This process, however, produces a large volume of 
undesirable by-product, calcium chloride. 
Another process uses a low concentration of aqueous caustic solution to 
scrub chlorine gas. However, the solution has an available chlorine 
content of about only 5% and, because of the chloride ion content, the 
hypochlorous acid that is formed quickly decomposes, usually to chloric 
acid. 
Another related process prepares a solid mixture of alkali metal 
hypochlorite and alkali metal chloride by reacting chlorine gas with a 
spray of alkali metal hydroxide, while drying with a gas the reactants and 
product. Some cooling of the reacting chemicals and the drying gas may be 
done. The primary products of this process have very limited utility. 
A more recent process, which produces hypochlorous acid vapor, sprays 
aqueous alkali metal hydroxide in droplet form or solid alkali metal 
hydroxide particles into gaseous chlorine. This approach attempts to 
utilize droplet sizes to attain the maximum surface to volume ratio 
possible. Droplets having an average diameter of less than about 1000 
microns are employed. 
These previous processes, and the apparatus employed to produce these 
processes, have suffered from not achieving substantially complete 
reactions between the chlorine and the alkali metal hydroxide. A critical 
factor in determining the complete reaction is the droplet size of the 
alkali metal hydroxide. It is also desirable that any hypochlorous acid 
produced and any water present be readily vaporizable. The salt particles 
produced as by-products in any process should be dry to facilitate 
handling and to reduce the amount of moisture in the salt that exists as 
free water and which fuels the decomposition of the product hypochlorous 
acid to chlorate and chlorine. The most detrimental decomposition reaction 
occurs almost instantaneously between a pH of 4 and 7.5 and is driven by 
the chloride ion concentration that is present when the free water 
dissolves the salt, forming chloride ions in solution that react with and 
decompose the hypochlorous acid. In the presence of this free water, the 
product hypochlorous acid decomposition reaction occurs as follows: 
EQU 5HOCl.fwdarw.HClO.sub.3 +2Cl.sub.2 +2H.sub.2 O. 
Therefore, it is advantageous to dry the by-product salt particles as 
quickly as possible. The dryness of the salt is a direct function of the 
size of the alkali metal hydroxide particles sprayed and the heat 
introduced into the reactor. 
The by-product salt particles should be sized so that they readily separate 
from the gaseous product mixture of hypochlorous acid. Prior processes 
have utilized oversized alkali metal hydroxide droplets that result in the 
undesired reaction of hypochlorous acid and the oversized by-product salt 
particles to produce significant alkali metal chlorates. The presence of 
such alkali metal chlorates reflects reduced yields of the desired 
hypochlorous acid, while increasing the raw material and operating costs. 
The resulting oversized by-product salt particles also retain excessive 
moisture so that caking results and the caked mass adheres to the reactor 
surfaces. 
These problems are solved by the present invention via the method of 
employing a reactor vessel and the vessel s particular design wherein a 
reactor vessel for the production of hypochlorous acid is provided in 
which the mixing and reaction of an alkali metal hydroxide and a gaseous 
halogen occurs to produce a substantially dry solid salt by-product and 
the gaseous hypohalogenated acid. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process and a reactor 
vessel within which a liquid phase controlled reaction can occur to 
produce a hypohalogenated acid. 
It is another object of the present invention to provide a reactor vessel 
in which both a liquid-gas reaction and drying occur to produce a gaseous 
product and a by-product that is substantially dry. 
It is a feature of the present invention that an atomizer is employed to 
produce small droplets of an alkali metal hydroxide to ensure that the 
undesirable secondary reactions are minimized and that proper drying of 
the desired particles occurs. 
It is another feature of the present invention that the reactor vessel 
permits the rapidly sequential events of absorption of gaseous halogen 
into the atomized particles of alkali metal hydroxide and water 
evaporation to occur. 
It is still another feature of the present invention that the atomizer is 
located near the top of the reactor vessel. 
It is yet another feature of the present invention that a heated halogen 
gas mixture is fed into the top of the reactor vessel. 
It is a further feature of the present invention that the solid by-product 
alkali metal salt is dried as quickly as possible in the reactor to 
produce a substantially dry solid by-product. 
It is still a further feature of the present invention that the moisture 
content of the solid by-product is determined by the length of the reactor 
vessel in proportion to its diameter, in addition to the droplet size of 
the alkali metal hydroxide sprayed and the amount of heat introduced into 
the reactor vessel. 
It is yet a further feature of the present invention that the solid 
by-product with a substantially reduced moisture content contains less 
than about 5% by weight of water, preferably less than about 2% by weight 
of water and more preferably less than about 1% by weight of water. 
It is an advantage of the present invention that the use of oversized 
alkali metal hydroxide droplets is avoided and that undesirable secondary 
reactions are minimized. 
It is another advantage of the present invention that the manufacturing 
costs of the reactor and the production costs of the hypochlorous acid are 
substantially reduced. 
It is still another advantage of the present invention that the process to 
produce a hypohalogenated acid is substantially halide free. 
These and other objects, features and advantages are provided in an 
elongated and generally vertically positioned reactor vessel and the 
process of using that reactor vessel for the production of hypochlorous 
acid from the mixing and reaction of an alkali metal hydroxide and gaseous 
chlorine by the use of an atomizer for atomizing the alkali metal 
hydroxide. The atomizer is mounted within the reactor vessel above the 
spraying and reaction zone and the drying zone. The solid by-product 
alkali metal salt is substantially moisture free to substantially reduce 
or prevent undesired decomposition reactions of the product 
hypohalogenated acid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the reactor, indicated generally by the numeral 10, which 
reacts the liquid alkali metal hydroxide, such as caustic, supplied by 
feed line 11 with the gaseous halogen, such as chlorine, to produce the 
solid salt crystals and the gaseous product, such as HOCl. The HOCl is 
condensed to produce liquid hypochlorous acid which, for example, can be 
mixed with a lime slurry to produce calcium hypochlorite. 
Although the reactor will be discussed in terms of producing hypochlorous 
acid, it is to be understood that any halogen, including chlorine, 
bromine, fluorine or iodine could be employed to produce a hypohalogenated 
acid. Additional such acids include, for example, hypobromous or 
hypofluorous acid. Similarly, although the instant invention will be 
discussed primarily in terms of spraying caustic or sodium hydroxide 
droplets, it is to be understood that any suitable hydroxide could be 
employed, such as any of the alkali metal hydroxides or alkaline earth 
metal hydroxides, or mixtures thereof. It is also possible that the method 
of the instant invention could employ any suitable basic compound, 
including carbonates, in lieu of the hydroxide. 
Gaseous chlorine, along with some chlorine monoxide in the recycle system, 
is fed into reactor 10 via gas infeed 12 in the top 14. Top 14 is in the 
shape of an inverted funnel, that can be constructed of a suitable 
corrosion resistant material, such as titanium; coated titanium; an alloy 
of nickel, chrome, molybdenum, iron and other materials; tantalum; and 
lined carbon steel or lined fiberglass reinforced plastic. The lining can 
be a suitable polyfluoropolymer. 
Reactor vessel 15 has a perforated plate 16 at the top between the reactor 
top 14 and the vessel 15. The plate 16 is also made of a suitable 
corrosion resistant material, such as polytetrafluoroethylene or one of 
the above mentioned materials with respect to top 14, and serves to create 
a straight cocurrent flow path for the chlorine gas flowing down from the 
top 14. The fresh chlorine gas enters the reactor vessel 15 through feed 
line 20 at a temperature of about 40.degree. C. Ethylene 
chlorotrifluoroethylene has also been used as a construction material for 
reactor vessel 15. Vessel 15, similarly can be made from any suitable 
corrosion resistant material, such as carbon steel with a liner or coating 
of a suitable perfluoropolymer, such as that sold under the tradename 
TEFLON.RTM. PFA. 
Reactor vessel 15 has a generally elongated cylindrical central section 18 
which tapers to a conically shaped funnel bottom 19 to permit solid alkali 
metal halide salt, such as NaCl, product to discharge out through a 
standpipe, not shown, for further processing. Vessel 15 has a caustic feed 
line 11 that enters through its side and provides the caustic, which is 
heated to between about 80.degree. to about 110.degree. C. and more 
preferably between about 95.degree. to about 100.degree. C., to an 
atomizer nozzle 21. Nozzle 21 is mounted along the center line 22 of the 
vessel 15 about six (06) inches below the top of vessel 15. Nozzle 21 
creates caustic droplets of a desired size between about 50 to 200 microns 
which are of sufficient size to absorb virtually all of the gaseous 
chlorine feed while the chlorine and caustic react fast to produce the 
reaction product mixture of gaseous and solid products as shown in the 
equation: 
EQU NaOH+Cl.sub.2 .fwdarw.HOCl+NaCl 
The reaction occurs at a pH of about 4 to about 6 with a stoichiometric 
ratio of about 30 to 1 chlorine to caustic. 
The gaseous mixture in the reactor vessel 15 includes hypochlorous acid 
vapor that results from the almost instantaneous vaporization of the 
liquid phase hypochlorous acid, dichlorine monoxide, unreacted chlorine 
and water vapor. The gaseous mixture includes a high concentration of the 
dichlorine monoxide, which is the predominate chloroxy species present and 
which is present in eguilibrium with the hypochlorous acid vapor and the 
water vapor as expressed by the equation: 
EQU 2HOCl.fwdarw.Cl.sub.2 O+H.sub.2 O. 
The gaseous HOCl and water vapor are condensed by a condenser 32 of FIG. 1 
between about 0.degree. to about 10.degree. C., after exiting the reactor 
and the appropriate gas-solid salt by-product separation equipment (not 
shown), to recover a concentrated HOCl solution. The dichlorine monoxide 
dissolves in substantially all of the condensed water to significantly add 
to the concentrated HOCl solution. 
Recycled gases, such as chlorine and dichlorine monoxide, are exhausted 
from the vessel 15 through exhaust duct 24 and are fed back into reactor 
10 as a gaseous heated medium via a recirculation loop 30 at about 
140.degree. C., after passing through a heat exchanger 31 to achieve the 
necessary heat, when combined with the heat of reaction to evaporate the 
hypohalogenated acid, such as hypochlorous acid, and water phase to leave 
a dry sodium chloride or salt solid by-product. The desired reaction 
temperature ranges from about 80.degree. to about 100.degree. centigrade. 
The recycled gases are also used as reactant gases in the production of 
the hypohalogenated acid. 
The recycled gases, for example chlorine and dichlorine monoxide, enter the 
reactor vessel top 15 and disperse outwardly in the inverted funnel top 14 
and pass through the flow directing means or perforated plate 16 to enter 
the reactor vessel 15 in a generally vertical flow orientation. Fresh 
halogen gas, for example chlorine, is fed in through chlorine feed line 20 
through the reactor top 14 and is directed down over the nozzle or 
atomizer 21. 
Nozzle 21 may be a single fluid atomizer, a two fluid nozzle or a wheel 
atomizer dependent upon the viscosity and density of the alkali metal 
hydroxide being atomized and the amount of pressure to which the liquid is 
subjected. The materials of construction of the nozzle must be capable of 
withstanding the harshness of the environment within the reactor. 
The vessel 15 has an outlet or exhaust duct 24 at the bottom of the drying 
zone 26 just above the funnel or conically shaped bottom 19 to remove the 
product gas, the unreacted halogen gas and some by-product into the 
recirculation loop 30 as previously described. Outlet or exhaust duct 24 
exits through the side of vessel 15 generally horizontally and has an 
inlet 28 that is undercut such that the top overhangs or overlies and 
covers the bottom to preclude solid alkali metal chloride by-product, for 
example sodium chloride, from falling directly into it. The preferred 
shape of the inlet 28 is an undercut ellipsoid, as seen in FIG. 2. 
Alternately, and more preferably the product gas, the unreacted halogen 
gas and the by-product solid salt all exit through the funnel bottom 19 
into a common outlet pipe 23 for routing to a common gas-solid salt 
by-product separation apparatus, such as a baghouse (not shown). 
The vessel 15 has its central section 18 preferably cylindrically shaped, 
but it could also be polygonal, as appropriate. The cylindrical design has 
a desired diameter and length. The length extends from the top at the 
perforated plate 16 to the bottom of the drying zone 26, just above the 
funnel bottom 19, or alternately the length can be defined as the distance 
of the straight side portion of the cylinder or reactor vessel 15. The 
dimensions of the length and the diameter can be selected so that the 
length to diameter ratio, 1/d, can range from about 1 to 1 to about 5 to 
1. A preferred ratio is about 3.7 to 1, with the more preferred ratio 
being about 2.3 to 1. The larger the diameter of the reactor vessel 15 the 
slower is the rate of fall of the solid salt by-product particles, 
utilizing a fixed gas flow rate. 
The length of the reactor vessel 15 in proportion to its diameter helps 
determine the moisture content of the solid by-product salt particles, in 
combination with the size of the alkali metal hydroxide droplets sprayed 
into the vessel 15 from atomizer nozzle 21 and the amount of heat 
introduced into the vessel 15 to accomplish drying. Since the alkali metal 
hydroxide droplets react almost instantaneously with the halogen gas to 
form the solid salt by-product particles, the size of the reactor is 
determined by the time required to dry the resultant salt by-product 
particles. The constant rate drying time is negligible since a crust forms 
on the surface of the salt by-product particles almost instantaneously. 
Therefore, the reactor vessel 15 dimensions are determined assuming only a 
falling rate drying time by using the following derivation of the equation 
presented by W. E. Ranz and W. R. Marshall Jr. as part of graduate study 
at the University of Wisconsin presented in the March and April 1952 
editions of Chemical Engineering Progress: 
##EQU1## 
In this equation W' represents the moisture content of the solid 
by-product salt particle, t represents the falling time of the solid 
by-product salt particle, K.sub.d represents the thermal conductivity of 
the unreacted recycled chlorine gas used for drying, .DELTA.T represents 
the temperature difference between the solid by-product salt particle and 
recycled gases, .tau. represents the latent heat of vaporization of water, 
.rho..sub.s represents the density of the dry solid salt by-product, and 
D.sub.c represents the solid by-product salt product diameter at the point 
of evaporation. 
The equation is solved for the drying time, knowing the temperature of the 
gas and of the hydroxide as it comes into the reactor, the volume of gas 
that must pass through the reactor to dry the material in the reactor, as 
calculated from the material balance, and the heat that must be supplied 
by the recycled gas to evaporate the quantity of moisture in the 
by-product salt particles in the reactor. Using these factors the length 
and diameter of the reactor are sized to be able to handle the required 
flow rate and the time required for drying. 
In operation the halogen gas, for example chlorine, is fed into the reactor 
10 through feed line 20 and is directed generally vertically downward over 
nozzle 21. Recycled gases are fed in from the recirculation system via gas 
infeed 12 into the reactor top 14 and are directionalized by perforated 
plate 16 down into reactor vessel 15. Vessel 15 has an elongate 
cylindrical section 18 which has a spraying and drying zone 25 adjacent 
the top surrounding nozzle 21 and a drying zone 26 therebelow. 
The reacted gases exit the reactor 10 through outlet or exhaust duct 24 for 
processing and recirculation, as appropriate. The solid by-product alkali 
metal halide, such as sodium chloride, exits the vessel 15 through the 
conically shaped funnel bottom 19 for processing. Bottom 19 is connected 
by conventional flanging to outlet pipe 23 and then to other connecting 
pipes (not shown). 
The solid by-product alkali metal halide is dried as it passes down through 
the drying zone 26. When employed, the overhanging top portion of exhaust 
duct 24 prevents substantial quantities of the solid by-product from being 
drawn out through the undercut ellipsoid inlet 28 with the product HOCl 
gas and the recycle gases. The preferred water content of the solid 
by-product alkali metal halide is less than 5% by weight, preferably less 
than 2% and more preferably less than 1%. Low moisture or water contents 
such as these facilitate separating the solid by-product salt and product 
gas streams, while limiting the decomposition of the product 
hypohalogenated acid. 
The following example is presented to define and illustrate the advantages 
of the present invention more fully without any intention of limiting the 
invention thereby. All percentages are by weight, unless otherwise 
specified. 
EXAMPLE 1 
A plurality of NaCl salt samples were obtained from the reactor of FIG. 1 
utilizing isokinetic sampling. The reactor vessel was about 11 feet seven 
inches long with a vessel diameter of about 3 feet two inches. Each sample 
was analyzed for water content by gravimetric measurement. The salt sample 
was first weighed as taken from the reactor and then was dried for about 5 
minutes, such as by infrared heating. The dried sample was then weighed 
and the weight differential is the water content. 
A corresponding product HOCl sample was obtained from the condenser and was 
analyzed for percentage concentration. This is significant when 
determining the yield of HOCl from the process since the concentration is 
directly proportional to yield. The yield is defined as the percentage 
conversion of caustic to HOCl on a molar basis. An HOCl concentration of 
49% is believed to translate to a yield of about 85%, while a 
concentration of about 40% is believed to translate to a yield of about 
65%. 
The caustic atomization pressure of the atomizer within the reactor for all 
samples was maintained between about 975 and 1000 psig. The data 
corresponding to the samples taken is shown below in Table 1. The reactor 
temperature was increased to obtain a drier NaCl sample, although the most 
important controlling factor is the atomization size of the caustic 
droplets, since the drying rate is a function of the square of the size of 
the by-product salt particle that is formed by the drying of the caustic 
droplets. As seen from the data in Table 1, there is an inverse 
relationship between the moisture content of the solid by-product particle 
and the concentration of the product hypochlorous acid. 
TABLE 1 
______________________________________ 
HOCI CONCENTRATION VS. NaCl "DRYNESS" 
NACL REACTOR HOCl CONDENSER 
WT. % H2O TEMP (.degree.C.) 
WT. % TEMP (.degree.C.) 
______________________________________ 
2.09 84 45.4 6 
2.49 95 41.9 6 
0.95 93 51.4 5 
1.92 84 46.2 5 
1.89 94 47.1 4 
14.07 82 23.8 5 
1.90 95 45.9 2 
1.64 99 46.1 3 
1.65 99 45.7 7 
3.31 98 42.6 3 
3.66 98 43.3 8 
4.10 97 43.1 9 
8.43 86 38.6 9 
21.89 93 32.7 9 
20.03 98 31.9 7 
2.22 82 44.7 4 
______________________________________ 
The graphical plot depicted in FIG. 3 shows the correlation between the 
concentration of the product hypochlorous acid and the NaCl water content. 
Generally, the lower the water content of the solid by-product salt, the 
higher is the concentration of the product HOCl. The data points with the 
higher percent by weight water analyses were likely the result of caustic 
droplets being formed from a clogged or blocked spray nozzle. The center 
line on the plot represents the computed value of the HOCl concentration 
using the equation -0.89X+46.87=Y derived from the statistical analysis 
linear regression of the data in Table 1, while the two parallel lines 
above and below represent the statistical upper and lower limits of the 
HOCl concentration at 95% confidence. One random data point occurred with 
the NaCl percentage of water content of 14.07% which fell outside the 
lower limit of the computed HOCl concentration. 
While the preferred structure in which the principles of the present 
invention have been incorporated is shown and described above, it is to be 
understood that the invention is not to be limited to the particular 
details thus presented, but, in fact, widely different means may be 
employed in the practice of the broader aspects of this invention. For 
example, it is possible to practice the method of the instant invention in 
either a cocurrent reactor, such as is described herein, or a 
counter-current reactor such as that disclosed in patent application U.S. 
Ser. No. 254,559, filed Oct. 7, 1988 and assigned to the assignee of the 
present invention, which is hereinafter specifically incorporated by 
reference in pertinent part The scope of the appended claims is intended 
to encompass all obvious changes in the details, materials, and 
arrangement of parts which will occur to one of skill in the art upon a 
reading of the disclosure.