Liquid chlorine scrubbing

Figure 80. Multistage reciprocating compressor for chlorine liquefaction at i MPa with cooling water at 15 C with liquid chlorine scrubbing...

A wash with concentrated hydrochloric acid removes the dangerously explosive nitrogen trichloride [209]. Scrubbing with liquid chlorine (see Fig. 80) mainly reduces the content of organic impurities and carbon dioxide, but it can also lower the bromine content. When the chlorine is cooled down to near its dew point, liquid chlorine scrubbing is often combined with compression by turbo or reciprocating compressors. 

High-pressure and liquid chlorine inventories (Melbourne only) specified by Orica to be contained within scrubbed buildings. 

The solution of chlorine in CCI4, now at a positive temperature, flows by its own pressure to a stripping column operating at 3-4 bar. A thermosiphon reboiler maintains the bottoms at about 130-135°C. The chlorine gas released in the stripper is scrubbed in the top section with liquid chlorine to remove traces of impurities and is then ready for use or recompression and another pass through liquefaction. The stripped solvent recycles to the absorber. The bottoms stream from the absorber and the stripper can be interchanged for heat economy. The lower part of the stripper and the tubes of its reboiler are Monel or Monel-clad. The rest of the absorber/stripper set is carbon steel. 

Chlorine boiled off in the stripper passes upward through a packed top section of the column where it is scrubbed and purified by liquid chlorine from the discharge knock-out drum. The stripper overhead stream, a mixture of chlorine and a small amount of inerts, is sent to the chlorine liquefaction system or recycled to the suction knock-out drum to maintain the stripper reflux [223], [224]. 

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

Sulphur may be freed from arsenic by treating it in the molten state with compounds, such as lime or sodium sulphide, which react to form compounds insoluble in the sulphur and which may be separated by settling and filtration 8 or the sulphur may be treated in the vapour or liquid state with chlorine or sulphur dichloride in excess, to form arsenic chloride, which may be removed by distillation and by scrubbing with air or an inert gas under reduced pressure.8 Other methods consist in... 

Figure 5.3 illustrates a process to manufacture acetylene from calcium carbide. The carbide is introduced by a screw conveyor into a perforated horizontal cylinder housed in a concentric envelope. Water is sprayed inside the internal shell. The acetylene formed passes upstream through the screw conveyor to a scrubbing tower, where, a new water spray carries off most, of the solids conveyed by the gas. The residual lime and carbide impurities are removed by a screw conveyor to a sludge receiver. The acetylene is cooled to — 10°C to condense most of the water. It is then purified by contact with dilute sulfuric acid in a liquid liquid absorber, and then with sodium hypochlorite prepared by the action of chlorine on caustic soda, to. remove impurities. The acetylene is then cooled to 0°C for the more complete separation of moisture. The final product nevertheless still contains 0.4 per cent by weight of water, which is suitable for most uses. More intensive dehydratation can be achieved by passage over silicagel. 

The final vent from the system should be regarded as a point of possible chlorine emission. It should be at a high level and away from personnel traffic and ventilation system intakes. Particularly when the low-pressure scrubbing device is a packed bed, booster fans may be added on the tail pipe. The piping arrangement around the fans then should avoid low points that could collect liquid. As with any vent manifold, the designer of a system serving several different units should take precautions to avoid mixtures in headers that may lead to undesirable side reactions. 

When chlorine is released to a scrubber, a liquid stream containing at least enough caustic to neutralize the gas must be available to meet it. It is the product of caustic flow rate and concentration that matters. If the flow rate drops too low, or if the concentration of the solution is depleted too much, there will be a release of chlorine to the atmosphere. As a rule of thumb, if the minimiun NaOH strength is taken to be 5%, the required rate of flow of scrubbing solution in tons per horn is equal to the production rate of chlorine in tons per day [85]. For concentrations other than 5% NaOH, the required flow rate is inversely proportional to the minimum strength. 

Processing of the chlorine through drying, mist elimination, compression, liquefaction, and liquid handling generally follows the practices recommended in Sections 9.1.4-9.1.8. In addition, all chlorine-generating plants must have vent scrubbers to prevent the release of the gas into the atmosphere (Section 9.1.10.3). Multiple scrubbing towers installed in series are common in HCl electrolysis plants. 

The correlation for distillation is based on limited data for systems of hydrocarbons and chlorinated hydrocarbons, that for absorption on the performance of hydrocarbon absorbers and the scrubbing of ammonia and carbon dioxide with water, p/ is in units of centipoises, H in imits of mole-fraction ratios, and p L is the molar density of the liquid solvent in lb mol/ft. Note that gas absorbers generally have much lower plate efficiencies than distillation coliunns due primarily to their lower operating temperatures. 

---