NZ230737A - Production of gaseous chlorine dioxide by atomisation of liquid solution of chlorine dioxide; treatment of foul air, wood pulp or flour - Google Patents

Description

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Complete Specification Filed: Class: <^Q.l &.U/Q3.;. ftbJi.i5)/.i.vJ &Z)£&r.is>kr\ £)3>\Zftjx*: Publication Oato: P.O. Journal. No: . ....!S».-.

NEW ZEALAND PATENTS ACT. 1953 No.: Oile: COMPLETE SPECIFICATION GAS CHLORXXB OXOZIDB CSNESATION ~3°CTI9By JC! We, ICI AOSTKALIA 0PK8ATX0KS PROPRIETARY LIMITED, a COOpany incorporated under tho laws of the State of Victoria, of 1 Nicholson Street, Melbourne 3001* Victoria, Australia, hereby declare the invention for which^T? we pray that a patent may be granted to roe/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - (followed by page 1A) ' ; • ~.VTy"' v w . , ( , , -1A. 23073? r Gas Chlorine Dioxide Generation This Invention relates to a process for preparation of gaseous chlorine dioxide.

Chlorine dioxide has a range of industrial uses for bleaching and sanitizing. Typically, because of Its explosive character, chlorine dioxide Is manufactured at the point of use.

Chlorine dioxide Is generally prepared In aqueous solution by a method Involving reduction of sodium chlorate which Is generally convenient In large scale operations or by oxidation of chlorite, a process generally favoured 1n small scale processes.

In order to prepare gaseous chlorine dioxide methods Involving bubbling a gas through a solution of chlorine dioxide have been reported.

We have found that chlorine dioxide gas can be more efficiently removed from solution by atomizing the chlorine dioxide solution.

Accordingly we provide a process for preparation of gaseous chlorine dioxide comprising atomizing an aqueous solution of chlorine dioxide.

Preferably the solution 1s atomized using a gaseous propellant which 1s preferably air.

By gaseous chlorine dioxide we mean chlorine dioxide 1n the form of the free gas as distinct from chlorine dioxide present 1n aqueous solution. &ocTm -v i£5&- v •* 230737 The temperature at which the solution 1s atomized 1s not narrowly critical but Is typically 1n the range of from 5 to 50°C. Atomlzatton at ambient temperature Is convenient and generally preferred.

Atomlzatlon may be carried out by a range of techniques Including using centrifugal disks, pressure nozzles, two fluid nozzles and electrostatic sprayers. Two fluid nozzles are generally preferred whereby, the solution Is broken up by Impingement with a high velocity stream of gas, preferably air, to provide droplets of, for example, average particle size 1n the range of from 1 to 150 microns.

O The concentration of dissolved chlorine dioxide In the solution Is typically from about 0.05 to 5g per litre. Preferably the concentration will be from about 2g to 5g of dissolved chlorine dioxide per litre of solution. Preferably the pH of the solution which Is atomized Is In the range of from 5 to 8 and more preferably from 6.5 to 7.

The nozzle used In atomlzatlon 1s preferably constructed of a corrosion resistant plastics material such as poly vinyl chloride.

The process of the Invention has a range of applications for example 1n bleaching of wood pulp or flour, 1n sanltlzatlon and In odour. * Odour emissions from sewage treatment plants and pump stations, h$% for many years been of major concern to municipal authorities. Local resident complaints regarding unacceptable sewage odours, have significantly Increased over recent years. There are various methods zap.: ■ ■ - , \ ^ I 230737 r> I ^ *—V . i ; : O i c i i of reducing or eliminating odour levels, however, many of these are only partially effective and are cost prohibitive. Therefore, controlling odour production In many sewage treatment systeas Is Impractical, the result of which presents problems at points of heavy turbulence or aeration of the raw sewage. The normal problea areas for sewage odours are pump stations and the Inlet areas of sewage treatment plants.

Eliminating sewage odours will always be a necessary part of this Industry, therefore any cost effective control method will have great application.

One embodiment of the Invention comprises treatment of foul air such as In a sewage treatment plant whereby foul air Is contacted with an atomized aqueous solution of chlorine dioxide.

Typically It 1s convenient to atomize the chlorine dioxide solution within an air space containing the foul air.

It has been found that treatment with chlorine dioxide In the manner 1s a most efficient method of eliminating, or at least significantly reducing, the presence of hydrogen sulphide, a toxic gas which Is the main contributor to the odour problem.

Typically 1n this embodiment the concentration of the solution to be atomized Is about 50%to 500 mg/L as chlorine dioxide. It 1s convenient to generate the chlorine dioxide at pH 5 to 7 to produce In the range of from 0.1 to lOg chlorine dioxide per hour.

Typically the dose rate of chlorine dioxide Is In a ratio to hydrogen sulphide of at least 0.5 aole chlorine dioxide per sole hydrogen sulphide and preferably In the range of from 0.5 aole to 1.5 aole per aole of hydrogen sulphide. A ratio of 0.9 aole chlorine dioxide per aole hydrogen sulphide Is preferred.

A water scrubber, optionally dosed with alkali to provide a pH of about 10 to 12, aay be used to ellalnate any remaining chlorine dioxide froa the treated air.

In a preferred eabodlaent of the Invention, which 1s particularly suited to flour treataent applications, the water alst produced by atoalzlng the chlorine dioxide solution 1s reaoved by suitable filtration aeans. He have found that by a process of atomlzatlon and filtration It Is possible to liberate at least 90% and generally at least 95% of dissolved chlorine dioxide Into the gaseous phase.

Accordingly we provide a process for preparation of gaseous chlorine dioxide comprising atoalzlng an aqueous solution of chlorine dioxide 1n to a gas stream and substantially removing the resulting liquid droplets by filtration without removing gaseous chlorine dioxide from the gaseous phase. A range of suitable filtration means may be used to remove the liquid droplets such as mist eliminators or water traps. The filtration means preferably comprises polypropylene-filters.

The said gas stream may be provided by the atomizing gas, that 1s the gas which 1s used as the propellant. •srssc, ' V c *7 O * — 5- / O ' The chlorine dioxide solution nay be atomized within a vessel containing an aqueous Mixture, which may for example be the residual salt solution remaining after removal of chlorine dioxide atomlzatlon of a solution. A portion of the atomized solution will tend to ^ condense and become Incorporated Into the aqueous mixture. The level of residual chlorine dioxide In the aqueous solution may be minimized by bubbling a gas through the aqueous mixture and Into the gas steam. • s ^ Chlorine Dioxide has been used for many years 1n treatment of flour, however, until recently only two main gaseous C102 generation systems have been used world wide. One Is the Dyox System designed by Wallace I Tlernan, which reacts sodium chlorite with gaseous chlorine, to generate Chlorine Dioxide which Is air stripped for Injection Into a flour agitator. The second process available 1s called the Hy-Kure process which uses a frozen Chlorine Dioxide hydrate which 1s stored at the site of use at low temperatures. When this hydrate 1s placed In water a solution of Chlorine Dioxide results. Chlorine Dioxide gas Is produced by passing the solution through a stripping tower after which 1t Is then fed to the flour agitator.

The present Invention allows efficient removal of gaseous chlorine dioxide to the gas phase for Imedlate injection Into the flour to be O treated.

Accordingly we provide a process for treatment of flour comprising: generating an aqueous solution of chlorine dioxide; atomizing the chlorine dioxide solution Into a gas stream; substantially removing the resulting liquid droplets from the gas stream by filtration to -JocTnsr sC*, provide gaseous chlorine dioxide and treating flour with the gaseous chlorine dioxide.

The chlorine dioxide solution to be atoalzed In accordance with the process of the Invention say be prepared by a range of techniques.

The well known chlorine dioxide process Involves reaction of chlorine In aqueous solution whereby chlorine hydrolysed to fora hypochlorous acid In accordance with the following:- <a) C12<gas) + H20 —> H0C1 + HC1 <b> H0C1 + HC1 + 2NaC102 —> 2C102 * 2 Ha CI + H20 The solution pH Is preferably In the range of from 3 to 3.5 for this generation technique.

An alternative generation technique comprises reaction of aolecular chlorine 1n aqueous solution with sodlua chlorite to produce chlorine dioxide gas.

In order to alnlalse undesirable side reactions It 1s generally preferred that the resulting chlorine dioxide solution has a concentration of froa 4 to 5 g/L although this solution aay be diluted prior to use.

Prior to reaction of sodlua chlorite with chlorine 1n aqueous solution It Is preferred that the solution pH be below 2 at which level about <c) Cl2<gas) + 2NaC102 —> 2C102 ♦ 2NaC1 50% of the chlorine Is In the form of hypochlorous acid. Hydrochloric acid or sulphuric acid Is generally used to adjust the solutions pH.

The Invention will now be further described with reference to the attached drawings.

In the attached drawings: Fresh water 1s fed through a standard conaerclally available water flow control and monitoring devices comprising water Inlet line strainer (1), water Inlet solenoid value (2), water pressure regulator, (3) water rotameter (4), and water flow alarm switches (5) via a non-return valve <6> Into a commercially available chlorlnator Injector (7).

The chlorine gas Is Injected through the Injector <7) via the chlorlnator <9) from the gas cylinder (8). The Injector (7) draws chlorine gas under vacuum and the resultant chlorine solution 1s preferably maintained at greater than 500 milligrams per litre as Clg. If necessary sulphuric acid or hydrochloric acid 1s added from a holding tank (Il> via a metering pump (10) to adjust the pH of the solution.

F1g 1 1s a schematic drawing of a plant which Incorporates equipment for producing free gaseous chlorine dioxide 1n accordance with the process of the Invention. v o -r *30CT(989 Into this solution is metered, from the reagent tank (14), a quantity of sodlua chlorite via metering pump (13), Just before the reaction chaaber (15) where aqueous chlorine dioxide solution Is produced.

Holding tanks (11) and (14) aay be equipped with level float switches (12).

Chlorine dioxide solution is transferred to a holding tank (18) on a high/low level control systea activated by a control float switch (21). A sight glass (16) and concentration alara (17) aay be Incorporated Into the transfer line for monitoring purposes. The holding tank (18) Is vented to end use process area.

Free gaseous chlorine dioxide 1s produced In accordance with the process of the Invention by transfer of chlorine dioxide froa the holding tank (18) to the air atoalzlng nozzles (26) positioned within a gas stripping tank (19) via a metering pump (23) and line filter (24). The transfer line Is regulated by a pulsation damper (25).

Compressed air 1s transferred froa a clean air supply to the nozzles (26) via air Isolating solenoid valve (27), air filter (28), air pressure underpressure switch (29) and air pressure regulator (30). The gas pressure Is generally maintained at about 300 kpa but may be selected depending on the nozzle type, solution transfer rate and other factors.

Spray from the nozzles (26> are directed downward Into the gas stripping tank (19) and a portion of the spray condenses and Is collected In the tank (19). Overflow froa the stripping tank (19) 1s r> transferred to a waste tank (20) which retains the residual aqueous salt Mixture before safe disposals.

The stripping tank (9) and optionally also the waste tank (20) are equipped with a coapressed air line (38) which bubbles air into the base of tanks (19) and (20). A gas streaa flow froa the tanks (19) and (20) to the alst eliminators (37) and (33) 1s provided by air froa the atoalzlng nozzles (26) and coopressed air line (38). The alst formed by atoalzatlon Is passed through secondary alst ellalnator (37) and prlaary alst eliminator <33) to reaove liquid particles and the resulting free gaseous chlorine dioxide aay be transferred to storage or Is preferably used Immediately In Its Intended application, for exaaple In flour bleaching.

A sight glass (34) aay be Incorporated Into the llae to allow viewing of the gas produced.

Although the above generator operating description 1s based on the utilization of sodlua chlorite and gaseous chlorine, this style of generator aay also utilise the generation technique whereby sodlua chlorite, sodlua hypochlorite, and hydrochloric acid are the chealcal reactants. The functional difference between this style of generation and the gas chlorine systea, as described above. Is that the gas chlorine Injector Is removed and hydrochloric acid 1s aetered into the dilution water after the non-return valve. Sodlua hypochlorite Is then aetered Into dilution water prior to a prlaary reactor, which Is a simple PVC packed coluan to proaote alxlng and convert the hypochlorite to hypochlorous acid. The sodlua chlorite solution Is Injected prior to the C102 reactor OTESS^V' • . * 7 - »■' Xf- - 10 - o *J A ** O »<- J < o f Free gaseous chlorine dioxide nay be thus generated In accordance with the above whereby an aqueous solution of chlorine dioxide 1s Initially generated by for exaaple reaction of equal voluaes of a 9% solution of hydrochloric acid and 7.5% solution of sodlua chlorite.

The above described process has application 1n a variety of treataents including chemical Maturing of flour products, odour control, gas bleaching of flour products, odour control, gas bleaching In various food Industries and cold rooa sterilization.

Eyaoplg 1 This Exaaple demonstrates the use of the present Invention In odour control.

Chlorine dioxide solution of concentration 200 ag/L as C102 and pH 5 to 6 was air atoalzed into a wet well of sewage treataent puap station using a dose rate of approxlaately 0.9 aole C102 per aole HgS.

The chlorine dioxide solution used was generated by reaction of gaseous chlorine and sodlua chlorite at a pH between 5 and 6 aole at a rate of 0.5g/hour of ClOg.

The ealsslon of odours was significantly reduced.

Exaaole 2 This Exaaple deaonstrates the use of the process of the present Invention to generate free gaseous chlorine dioxide for use 1n flour treataent.

The process described In hereinbefore with reference to F1g 1 was used 1n preparation of gaseous chlorine dioxide with the exception that the aqueous chlorine dioxide solution was prepared by reaction of 9% hydrochloric acid and 7.5% solution of sodium chlorite. An air pressure of 300 kPa was used to air atomize a chlorine dioxide solution of concentration of approximately 4g/L ClOg. Gaseous chlorine dioxide was generated at a rate of about 150g/hour and was used to treat 7.5 tonnes of flour per hour at a close rate of 20 ppm.

Claims (19)

WHAT 4jlWE CLAIM IS:

1. A process for preparation of gaseous chlorine dioxide the process comprising atoalzlng an aqueous solution of chlorine dioxide.

2. A process according to claim 1 wherein the solution Is atomized using at least one two-fluid nozzle whereby the solution is broken up by impingement with a high velocity stream of gas.

3. A process according to claim 2 wherein the gas is air.

4. A process according to claim 1 wherein the chlorine dioxide solution Is atomized to produce droplets of average size In the range of from 1 to 150 microns.

5. A process according to claim 1 wherein the chlorine dioxide solution has a concentration 1n the range of form 0.05 to 5 gram per litre.

6. A process according to claim 5 wherein the chlorine dioxide solution has a concentration in the range of from 2 to 5 grams per litre.

7. A process according to claim 1 wherein the gaseous chlorine dioxide Is used to treat foul air by contacting the foul air with the atomized solution.

8* A process according to claim 7 wherein the foul air comprises hydrogen sulphide and is treated using a dose rate of from 0.5 to 1.5 mole chlorine dioxide per mole hydrogen sulphide. -.a5£B5to- . .v , • O c - 13 - OQP^'> ** o u i 6 :

9. A process according to clala 7 for treating foul air In a sewage treatment plant comprising contacting the foul air with an atomized aqueous solution of chlorine dioxide and subsequently removing residual chlorine dioxide by passing the treated air through a water scrubber dosed with alkali to provide a pH 1n the range of from 10 to 12.

10. A process for preparation of gaseous chlorine dioxide according to clala 1 and coaprlslng atomizing an aqueous solution of chlorine dioxide Into a gas streaa and filtering the gas stream to remove liquid droplets without removing gaseous chlorine dioxide.

11. A process according to clala 10 wherein the gas stream Is filtered using polypropylene-filters.

12. A process according to clala 10 wherein at least 90% of the chlorine dioxide present In solution 1s retained 1n the filtered gas streaa.

13. A process according to clala 10 wherein at least 95% of the chlorine dioxide present In solution Is retained In the filtered gas steaa.

14 A process according to clala 10 wherein the chlorine dioxide solution coaprlses In the range of froa 2 to 5 graas per litre. "?rt" -v v. . m- 230737 ; o

15. A process according to clala 14 wherein the chlorine dioxide solution coaprlses 1n the range of froa 4 to 5 grans per litre of chlorine dioxide.

16. A process according to claim 10 wherein the chlorine dioxide solution Is atoalzed Into a gas stream within a vessel containing an aqueous alxture such that at least a portion of the liquid particles resulting froa atoalzatlon are condensed and thereby Incorporated In the aqueous alxture and wherein a gas Is bubbled through the aqueous alxture and Into the gas streaa.

17. A process according to clala 10 wherein the resulting gas streaa Is used In treataent of flour.

18. A process as defined in clain 1 for preparation of gaseous chlorine dioxide substantially as herein described with reference to any exavpl* thereof or to the accompanying drawing. O

19. Gaseous chlorine dioxide prepared in accordance with any of the processes defined in claiss 1 to 18. ICI An* CfeKMTOISjt* ac?. /""N V 'lwii Agtnt 1S87.rl*13