Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Reexamination Certificate
1999-06-10
2002-06-25
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
C423S24000R, C423S473000, C423S499400
Reexamination Certificate
active
06409981
ABSTRACT:
This invention relates to a scrubbing process and in particular to a process for scrubbing chlorine from a gaseous stream containing chlorine. An example of such a chlorine-containing gas stream is the vent gas from a plant producing chlorine: the vent gas often comprises oxygen, carbon dioxide and chlorine remaining as a result of incomplete condensation of the chlorine produced in the chlorine stream resulting from the electrolysis of brine. Other sources of chlorine-containing gas include reactor bleeds and off-gases resulting from the oxidation of chlorinated hydrocarbons; blow-down during chlorine tanker off-loading; pressure relief streams from chlorine processing plants; and primary brine dechlorination through air sparging. For environmental reasons it is desirable to remove essentially all of the chlorine from the gas stream before the latter is discharged to the atmosphere.
The chlorine removal is conventionally effected by scrubbing with an alkaline solution, often caustic soda solution, thereby forming a solution containing hypochlorite ions by the reaction
Cl
2
+2OH
−
→Cl
−
+OCl
−
+H
2
O
Thus one hypochlorite ion is formed for each molecule of chlorine scrubbed from the gas. Generally the resulting hypochlorite-containing liquor is treated to decompose the hypochlorite ions into oxygen gas and chloride ions before discharge of the liquor, i.e. according to the reaction
2OCl
−
→2Cl
−
+O
2−
It has been proposed in U.S. Pat. No. 4,297,333 to effect such decomposition of hypochlorite ions in a liquor obtained by the caustic scrubbing of chlorine from a tail gas from an electrolytic chlorine plant by passage of the hypochlorite-containing liquor through a bed of a nickel oxide containing catalyst.
The scrubbing operation is usually operated in a cyclic process: thus the chlorine-containing gas is passed through a scrubber, e.g. a contactor, of which a packed tower is the most common, through which is flowing an aqueous scrubbing liquor generally counter-current to the flow of gas. The scrubbing liquor scrubs the chlorine from the gas giving an effluent liquor laden with chloride and hypochlorite ions. This effluent liquor leaving the scrubber, e.g. tower, is then recycled, usually via a holding tank, pump, and one or more heat exchangers, e.g. to the top of the tower.
The process may be operated as a continuous process, in which case, fresh alkali solution is added, continuously or periodically, before recycle, and part of the circulating liquor is removed, continuously or periodically, as a purge stream. This purge stream, a hypochlorite solution generally containing some excess of alkali, may be recycled, after treatment, for example as aforesaid, to decompose the hypochlorite therein, e.g., to a chlorine production plant. Alternatively the purge may be taken as a hypochlorite product stream or discharged to drain, generally after destruction of the hypochlorite therein.
Alternatively the process may be operated as a batch or semi-continuous process. In such processes, a suitable reservoir, e.g. a holding tank, is provided in the circulation loop and initially this reservoir is charged with fresh alkali. The scrubbing operation is operated until the alkali concentration drops to a predetermined level and then some or all of the liquor in the circulation loop is discharged and replenished with fresh alkali. In some cases the system may be designed such that the scrubbing operation may be halted or switched to another scrubber during such discharge and replenishment of the liquor.
Usually the circulation rate, rate of addition (if any) of fresh alkali, and amount of purge (if any) are such that the circulating liquor has a maximum sodium hypochlorite ion content of the order of 12-15% by weight, although it may be lower as a result of natural decomposition of the hypochlorite or the use of more dilute alkali solutions. Lower hypochlorite concentrations may result where other acid gases such as carbon dioxide, sulphur oxides, and hydrogen chloride are also present in the gas stream and these are co-absorbed with the chlorine.
Attempts to operate with higher hypochlorite concentrations tend to result in the decomposition of hypochlorite with the formation of chlorates
3NaOCl→2NaCl+NaClO
3
Reaction 1
The formation of chlorates is generally undesirable since they tend to be explosive and very toxic. Furthermore the rate of reaction 1 is strongly affected by the pH and temperature as well as the hypochlorite concentration. When the alkali is exhausted, i.e. the system is over chlorinated, the rate of chlorate formation is greatly accelerated. Under these conditions hypochlorous acid may be formed by the reaction
NaOCl+Cl
2
+H
2
O→2HOCl+NaCl Reaction 2
The following reactions may then also occur
2HOCl+NaOCl→NaClO
3
+2HCl Reaction 3
HCl+NaOCl→HOCl+NaCl Reaction 4
The excess of chlorine thus favours reaction 2 and hence reaction 3. The rate of reaction 3 is much greater than that of reaction 1 and is strongly exothermic. The reaction thus has a runaway potential. This is normally avoided by providing for an excess of alkali and by cooling and/or by operating at a lower hypochlorite concentration.
In order to minimise the risk of chlorate formation while at the same time enable the system to cater for plant upsets producing an increased amount of chlorine in the gas being treated, it is desirable to operate at a lower circulating hypochlorite content. While this can be achieved by increasing the size of the holding tank and providing for significant increase in the circulation rate and amount of caustic soda added when such increases in the chlorine content occur, such modifications are not attractive economically. It is generally desirable to operate with a sufficient excess of caustic soda that increases in the chlorine content of the feed gas can be accommodated without providing for control of the amount of added caustic soda or of the circulation rate. In the case where changes in chlorine feed rate are due to emergency relief, these changes normally arise too fast for a control system to respond.
In the present invention this problem is overcome by providing for the catalytic decomposition of hypochlorite ions in at least part of the circulating liquor before it is recycled to the scrubber.
Accordingly the present invention provides a process for the scrubbing of chlorine from a chlorine-containing gas comprising contacting said gas with an aqueous feed liquor containing an excess of alkali over that required to react with the chlorine in the gas whereby said chlorine is scrubbed from the gas to provide an effluent liquor containing chloride and hypochlorite ions resulting from the reaction of said chlorine gas with said alkali, and passing at least part of said effluent liquor through a fixed bed of a catalyst for the decomposition of hypochlorite ions whereby hypochlorite ions in said at least part of the effluent liquor are decomposed to oxygen gas and chloride ions to give a treated liquor containing a decreased concentration of hypochlorite ions, characterised in that, for at least part of the time while said gas is being contacted with said aqueous feed liquor, at least part of said treated liquor is recycled as at least part of the aqueous feed liquor.
Thus in the invention, some of the hypochlorite produced from the reaction of chlorine with the alkali is decomposed to chloride ions and oxygen. As a result the standing concentration of hypochlorite in the circulating liquor can be decreased with consequent reduction in the risk of runaway reactions and chlorate formation. Further, as a result of the decreased risk of runaway reactions, it is possible to operate at higher temperatures, thereby reducing the need for cooling of the circulating liquor. Operation at higher temperatures is also beneficial as the rate of decomposition of hypochlorite is increased and so the volume of catalyst required
Hancock Frederick Ernest
Stitt Edmund Hugh
Imperial Chemical Industries PLC
Nguyen Ngoc-Yen
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