Patent Application
20120020866
The invention pertains to the processing of a stream of liquid chlorine containing nitrogen trichloride from a chlorine production process, for example a chloralkali production process.
In the industrial production of chlorine, a small amount of by-product nitrogen trichloride (NCl3) is produced. In a chloralkali production process, the amount formed is proportional to the amount of ammonia present in the salt fed to the process. Nitrogen follows the product chlorine leaving the chloralkali cell house. It is often removed from the product chlorine through an absorption step, for example in a chlorine scrubber, prior to chlorine compression and liquefaction. In the scrubbing step, nitrogen trichloride is absorbed into fresh, clean product chlorine and pushed down the scrubber and into a holding tank, referred to as the nitrogen trichloride decomposer, containing carbon tetrachloride or sometimes chloroform. In the decomposer, the solvent is maintained at a temperature above the boiling point of the chlorine. When the liquid chlorine contacts the warm solvent, it flashes back into the chlorine scrubber while nitrogen trichloride is absorbed by the solvent. Conditions in the decomposer are selected so that nitrogen trichloride slowly and safely decomposes. In time, tars and other impurities build up in the solvent, and the solvent must be periodically replaced, generating a waste stream that must be disposed of.
For reasons of both regulatory constraints and product quality (i.e. to reduce organic content in the final product chlorine), it is desirable to avoid the use of solvents such as carbon tetrachloride and chloroform in the chlorine production train.
A method of disposing of nitrogen trichloride without using carbon tetrachloride or chloroform solvents is described in U.S. Pat. No. 3,568,409, Ferguson et al., in which gas chlorine from the drying tower is contacted with hydrochloric acid upstream of the compression and liquefaction steps. However, the process produces an acidic waste stream that must be disposed of or used elsewhere.
It would be desirable to provide an improved process for disposing of the nitrogen trichloride removed from the chlorine stream without using organic solvents such as carbon tetrachloride or chloroform, and without generating a waste stream.
The invention provides a method of processing a stream comprising liquid chlorine containing nitrogen trichloride contaminant from the chlorine production train of a chlorine production process. The liquid stream is vaporized to produce a stream comprising chlorine gas and nitrogen trichloride gas formed by decomposition of the nitrogen trichloride. That gas stream is processed by destroying the nitrogen trichloride gas, without the use of solvents or liquid chemicals, and producing only one stream, comprising chlorine gas with nitrogen gas, which stream is recycled to the chlorine production train.
The invention further provides an apparatus for processing a stream comprising liquid chlorine containing nitrogen trichloride from a chlorine production train. The apparatus includes a chlorine vaporizer having an inlet for the liquid stream and a boiling zone downstream of the inlet. The apparatus has means for destroying nitrogen trichloride gas downstream of the boiling zone, and means for recycling the resulting stream comprising chlorine gas and nitrogen gas from the nitrogen-destroying means to the chlorine production train.
These and other features of the invention will be apparent from the following description and drawings of particular embodiments.
In the following description and in the drawings, corresponding and like elements are referred to by the same reference characters.
As illustrated in
The vaporizer 20 produces a stream 40 comprising chlorine gas with nitrogen trichloride gas. This stream is routed to one or more unit operations for the destruction of nitrogen trichloride. The gas leaving the nitrogen trichloride destruction step, i.e. chlorine gas and nitrogen gas, is recycled back to the chlorine production train of the chloralkali process. The process avoids the generation of a waste stream or the addition of other chemicals or solvents to deal with the nitrogen trichloride. The step of destroying the nitrogen trichloride can be carried out in various ways. As illustrated in the embodiment of
The gas stream 52 leaving the catalytic bed, comprising chlorine gas and nitrogen gas, is recycled back to the chlorine production train 24.
As an alternative to using a catalytic bed, the gas mixture evaporated in the vaporizer may be introduced into a superheater, which may be part of or separate from the vaporizer unit. In the embodiment of
The gas stream 52 leaving the superheater, comprising chlorine gas and nitrogen gas, is recycled back to the chlorine production train 24 of the chloralkali process.
Optionally, the process may use both a superheater and a catalytic bed to destroy the nitrogen trichloride. The catalytic bed may be within a superheater zone of the vaporizer, rather than being a separate unit.
Optionally, the gas leaving the nitrogen trichloride destruction step, e.g. the superheater 30 or catalytic bed 54, may be routed to a temperature conditioning step 56 before being recycled back to the chlorine production train (stream 60), as shown in
Although the invention has been described in terms of various embodiments, it is not intended that the invention be limited to these embodiments. Various modifications within the scope of the invention will be apparent to those skilled in the art. The scope of the invention is defined by the claims that follow.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2010/000468 | 3/6/2010 | WO | 00 | 9/29/2011 |
