4) Fluid Movers

(1) Chlorine Movers

The selection of the proper mover for chlorine is completely dependent on the exact specifications of the situation. In general, the proper material of construction depends on whether or not chlorine is either "wet" or "dry". For dry service carbon steel is suitable. For wet chlorine service the choice materials of construction are selected FRP and CPVC. Titanium is very useful where water concentration will always be above 2000 ppm. Under no conditions should titanium be used for dry chlorine service as titanium "fires" can result.

For emergency chlorine spill control the conservative design system would assume the chlorine to be wet.

There are a number of methods to move chlorine from one point in a system to the next. The following four sections identify the most commonly used chlorine movers. The main variables to be considered are required volumes to be moved and the required pressures. Each method listed has limitations for either volume or pressure.

(2) Process Pressure

Chlorine can be transferred from one vessel to another by the differential pressure between the two vessels. In warm weather chlorine may be transferred by its own vapor pressure. Because vapor pressure is a function of temperature, cold weather usually reduces transfer rates unless the pressure is increased by padding with "dry" air or inert gas. Reference Chlorine Institute Pamphlet #66, see 6.1.4.

(3) Ejectors

An ejector is a simple mechanical device with no moving parts. All types of ejectors operate on the principle of one fluid entraining a second fluid. The ejector is constructed in three sections: motive fluid inlet, suction chamber and discharge. An ejector consists of a relatively high pressure motive fluid nozzle discharging a jet across a suction chamber into a ventrui shaped diffuser. The jet stream across the suction chamber entrains the secondary fluid into the motive fluid. The mixture enters the discharge section where the velocity energy is converted to pressure by the diffuser shape geometry.

For chlorine service an ejector must be constructed of materials suitable for wet chlorine service. True performance of an ejector is difficult to predict and usually requires close testing.

(4) Compressors

Due to the economics of the required materials of construction, conventional compressor designs (reciprocating and rotary) are not recommended for wet chlorine service. Typically, the liquid ring compressor is used in dry chlorine service but in limited situations this design can be used in wet chlorine service. The liquid ring compressor is a positive displacement machine is which a barrier fluid is used as the piston to compress and displace the gas stream. For dry chlorine service the barrier fluid will be high strength (98%) sulfuric acid. The acid will tend to dry the chlorine gas during compression. The danger in wet service is the dilution of the acid and destruction of the equipment. Where water content of the chlorine is always above 2000 ppm, titanium liquid ring compressors, using water as a seal fluid, have been used. Special design expertise is required.

(5) Blowers

The term "blowers" is a common industrial term to refer to a single stage centrifugal compressor. Typically a blower will deliver high volumes at low discharge pressures. FRP or titanium blowers are used in wet chlorine service. Steel or Alloy 20 blowers with Hastelloy C shafts are used in dry chlorine service.

(6) Scrubbing Solution Movers

Scrubbing solutions are generally transferred using a centrifugal pump, but gravity or feed from pressurized tanks can not be excluded. Specifics of pump, vessel design and pump seal needs must be engineered for each site. Following are comments on pumping caustic and hypochlorites.

• Centrifugal Pumps: For pumping caustic liquor to scrubbing systems, all-iron construction is generally suitable, although nickel and nickel-cast-iron pumps give longer service life. Above 140^oF (60^oC), a nickel or nickel alloy pump must be used. Brass fittings and bearings should be avoided on wetted parts, especially at high temperatures or concentrations.
  
  For pumping various hypochlorite solutions the materials of construction of the pump are dependent on the temperature and concentration of the solution. At lower temperatures, [below 70^oF (21^oC)] high silicon iron and other non-metallic corrosion resistant materials work well. Titanium may be used at any temperature but as the temperature rises above 120^oF (49^oC), it is the only economical material. It is important when using titanium that complete wetting is maintained.
  
• Differential Pressure: Caustic can be fed to scrubbers from head tanks or pressurized feed tanks. These systems may be particularly useful to assure feed during transient conditions such as during the time required to put a backup power source into service.

5) Heat Removal

It may be desirable to remove the heat of reaction and the heat of solution of the caustic soda or process reasons, (i.e., reduced chlorate formation, equipment temperature design limits, etc.).

Regardless of the scrubber design, heat removal is accomplished by use of circulating pumps and heat exchangers. The process fluid is circulated in a closed loop and the cooling liquid or gas does not come in direct contact with the scrubbing liquid. The typical and most commonly used material for construction is titanium.

Sizing of the system is accomplished by review of the flow rate of the chlorine gas or liquid and using 526 BTU/lb (292 cal/g) for liquid and 626 BTU/lb (348 cal/g) for vapor, plus the calculated heats of solution for the caustic if applicable. The heat of the NaOCl decomposition reaction must be added if conditions are appropriate.

Heat exchangers, pumps, and process flow rates can only be determined by individual scrubber design and is left to the engineer. Plate and frame heat exchangers can more easily "salt out" and pluggage may occur.

6) Storage and Decomposition

Tanks for storage of scrubbing solution and/or decomposition of spent scrubbing solutions may vary according to the end use; i.e. sale of the hypochlorite by-product or decomposition for environmental reasons. If the solution is to be sold, the fluid is usually cooled during scrubbing and may be stored as a cool, hypochlorite solution in lined steel tanks or polyester and vinyl ester fiber-glass-reinforced plastic tanks. Linings that may be economically used are chlorbutyl and ethylene propylene rubbers, polyvinyl chloride, and polypropylene.

Decomposition tanks usually handle solutions at elevated temperatures, and therefore are in extremely aggressive service. At the higher temperatures, titanium, brick lined steel and in some cases rubber lined steel are the materials of choice. It should be noted that titanium is attacked by dry chlorine, therefore care should be taken to keep all parts of the decomposition system wetted.

7) Controls

The use of any necessary controls with the attendant instrumentation is dictated by the type of scrubbing system chosen, the conditions of the installation and the degree of automation desired. Controls are necessary to start up or shut down systems, alert operating personnel to any problems - real or potential, and maintain operation within design parameters.

In general, the keys to successful control are the availability of operating condition information. the designer should evaluate the need for instrumentation in the following areas:

• Chlorine Gas Detector - scrubber gas vents; chlorine gas inside the isolated area and external around the area and some process relief headers
• Pressures - Pumping solutions
• Temperature - Scrubbing Liquids
• Scrubbing Liquid End Point Indicator - oxidation reduction potential or pH
• Level Control - Storage and Reaction Tanks
• Pressure - Chlorine process
• Flow of scrubbing liquids

Caution: Although pH and oxidation reduction potential are useful indicators of reaction end points, they should not be relied upon for insurance against chlorine release mitigation. Periodic analysis of the scrubbing solutions must be performed to ensure that adequate excess caustic is maintained.

Alarms should be considered to alert personnel to potential or actual abnormal performance. Stand-by electrical power supply should be considered.

The process and the desires at the installation should dictate whether a sensor is indicating only or transmitting a signal for indication or control.

Start-up or shut down can be automatic or manual, local or remote, as the needs and design of the situation dictate.

Housing for instrumentation should conform to NEMA 4X standards as a minimum to ensure proper corrosion protection. Review all local regulations and follow good instrumentation practice and principles in the design and choice of components.

8) Analysis

From time to time, the analysis of the scrubbing solution is recommended to ensure that the solution is at sufficient strength and can handle the amount of chlorine release for which the system is designed.

Analysis of solution strengths are determined normally by titration. Care should be taken to differentiate carbonate alkalinity from caustic alkalinity.