In chloralkali plants, the chlorine gas is produced at elevated temperature from aqueous salt solutions. This chlorine gas is saturated with water vapour at the electrolysis cell operating temperature and further contains some brine mist. For liquefaction and many direct applications for chlorine gas, the water vapour and the brine mist must be removed completely. Significant amounts of moisture result in severe corrosion in the carbon steel equipment and piping used for dry chlorine gas and liquid chlorine. Most of the water vapour and all brine mist are usually removed by cooling and filtering the hot wet chlorine gas. Depending on the cooling end temperature, the cooled and filtered chlorine gas still contains a substantial amount of water vapour. For the removal of this residual water vapour the chlorine gas is further dried.
Chlorine gas from sources other than chloralkali electrolysis frequently contains other impurities which are removed by washing with water, so that such chlorine gas usually also must be dried.
Efficient and reliable chlorine
gas drying is essential for the safety and long life of the downstream equipment
The most common process for drying chlorine gas is the absorption of the residual water vapour in concentrated sulfuric acid with minimum 98 % H2SO4. Such acid is readily available in most places and has a very low water vapour pressure. By the absorption of water vapour, the acid is diluted to about 75 to 80 % H2SO4. At this concentration, the spent sulfuric acid after removal of the small amount of dissolved chlorine can still be used for many other applications or is reconcentrated in a sulfuric acid plant.
The absorption of the residual water vapour is connected with the generation of a substantial amount of heat which corresponds about to the heat of condensation of the water vapour plus the heat of dilution of the sulfuric acid. For maintaining a low operating temperature, this heat must be removed within the drying system.
Any sulfuric acid mist entrained with the dried chlorine gas is usually separated in an other gas filter, unless the dried chlorine gas is fed to sulfuric acid ring compressors where it anyway is again in contact with concentrated sulfuric acid.
The required very high absorption efficiency results from the installation of special baffles above the perforations of a kind of sieve trays. Size, shape and location of the baffles are carefully optimised for very intensive mixing of the acid and the gas and thus for unique absorption efficiency. Because only a small part of the total water vapours are absorbed in the tray column and no pump power is added to the column, there is no need for cooling the tray column. The gas temperature increases only marginally. Since the tray column has no acid recirculation, chlorine gas and acid are flowing strictly countercurrent. The feed acid is not diluted with recirculated acid. This further increases the absorption efficiency in comparison with a packed column with acid recirculation.
The overall gas side pressure drop of Krebs Swiss drying systems is comparable with that of conventional systems and very high turndown ratios are easily possible by integrating the system with the downstream chlorine gas compression.
Such systems comprising
only one packed column and a column with special impingement baffle trays
are operated since more than twenty years in the chloralkali plants designed
and supplied by Krebs Swiss. In comparison with conventional systems, the
Krebs Swiss system has several substantial advantages:
- Substantially lower cost for equipment, piping, civil constructions and erection work.
- Substantially reduced cost for electric power.
- Substantiaily lower maintenance cost.
The services rendered include all services required for the execution of turn key projects. They are made available to our clients as required by the specific project.
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The moisture content of the dried chlorine gas from Krebs Swiss drying systems is reliably reduced to below 10 ppm w/w, which is fully adequate for liquefaction and most uses for direct chlorine gas. Krebs Swiss is very familiar with such downstream installations and, if required, assists also in these areas.
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The operational requirements for drying 1000 kg Cl2 in a Krebs Swiss drying system are typically as follows:
|Sulfuric acid (98 % H2SO4
dilution to 75 to 80 % H2SO4)
|25 to 40 kg
|Cooling water or chilled water
The drying system is usually operated together with all other upstream and downstream chlorine processing installations by one shift operator.
In the last two years KREBS SWISS took orders for five chlorine drying systems with capacities ranging from 110 to 600 tpd.
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