Compressibility, liquid chlorine

With modern EDC unit design, it is no longer necessary to provide a buffer of liquid chlorine between the chlor-alkali plant and the EDC or EDC/VCM plant. The EDC unit can easily and safely accept the gaseous feed of chlorine (after drying and compression) directly from the chlor-alkali plant. 

Process pumps convey a variety of liquids required in processes involving acids, caustics, liquid chlorine, liquid vinyl chloride, and other hazardous fluids. Compressors at this location convey gaseous chlorine, anhydrous hydrogen chloride, refrigerants, and compressed air. 

The critical temperature of chlorine is 144°C and its critical pressure is 76.1 atm. Boiling point of liquid chlorine is — 34.6 °C. Thus chlorine is normally below its critical temperature and can be liquefied either by compression or by refrigeration. Usually a combined method is used i. e. compression to a medium pressure and suitable cooling. 

If pure chlorine is to be liquefied at a constant temperature, it must be compressed to a pressure equalling the equilibrium vapour pressure of the liquid chlorine at the same temperature. The evolved condensation heat must be then lead away. If the chlorine is not entirely pure, compression must be increased by partial pressure of all inert gases. If chlorine containing other gases which cannot be liquefied readily is compressed it will be liquefied alone and its concentration in the gas phase will be decreased. Should further portions of chlorine be liquefied it would be necessary to increase the pressure in proportion to the increasing concentration of the inert gases in the gas phase. 

First of all liquid chlorine is measured in a pulsometer, then collected in a liquid chlorine receiver equipped with a safety valve as a safeguard against increased pressure. Outside, it is insulated with cork to keep the temperature as low as possible. The volume of the receiver usually amounts to 20 cu. m. The liquid chlorine is pumped by means of air, compressed to 12 atm. and dried with concentrated sulphuric acid or silicagel. 

Supercritical fluids (scf) are highly compressed liquids or gases. The latter already have an established role in "clean extraction (substitution of chlorinated/organic solvents) on an industrial scale (e. g. decaffeination of coffee and tea, extraction of hops, spices, etc.). The specific physical and chemical properties of scf make them particularly suitable for a variety of other applications, e. g. reactions, powder technology and impregnation. 

Gaskets in both dry gas and liquid chlorine systems are made of rubberized compressed asbestos. For wet chlorine gas, rubber or synthetic elastomers are acceptable. PTFE is resistant to both wet and dry chlorine gas and to liquid chlorine up to 200 °C. Tantalum, Hastelloy C, PTFE, PVDF, Monel, and nickel are recommended for membranes, rupture disks, and bellows. 

Liquid-liner compressors produce an oil-free discharge of up to 125 psig. The efficiency is relatively low, 50% or so, but Wgh enough to make them superior to steam jet ejectors for vacuum service. The liquid absorbs the considerable heat of compression and must be circulated and cooled a 200 HP compressor requires 100 gpm of cooling water with a 10°F rise. When water vapor is objectionable in the compressed gas, other sealing liquids are used for example, sulfuric acid for the compression of chlorine. Figure 7.19(e) shows the principle and Table 7.10 gives specifications of some commercial units. 

At room temperature and atmospheric pressure, CI2 is a pale-green gas. It can be compressed at room temperatures to a yellow-green liquid. Both the gaseous and liquid chlorine react with water to become hydrated. Liquid chlorine forms the compound C12-8H20, "chlorine ice," below 9.4 C. 

The liquefaction of chlorine and sulphur dioxide by compression by North-more, published in an easily accessible English journal, should have been known to Davy (Northmore says he was in touch with the chemical operator at the Royal Institution). Faraday says there was no doubt that Northmore obtained liquid chlorine and sulphur dioxide, but not liquid hydrogen chloride, as he claimed. The supposed liquefaction of air by compression, claimed by Perkins, an American who settled in London, was, said Faraday, very doubtful a specimen of liquid air sent to him, as far as I could by inquiry make out its nature , was water. 

An important discovery during this period was the fact that steel is immune to attack by dry chlorine [7]. This permitted the first commercial production and distribution of dry liquid chlorine by Badische Anilin-und-Soda Fabrik (BASF) of Germany in 1888 [8,9]. This technology, using H2SO4 for drying followed by compression of the gas and condensation by cooling, is much the same as is currently practiced. 

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

C. Formation of Chlorine Hydrate. Because of the presence of traces of water in compressed chlorine, the chlorine hydrate discussed in Section 9.1.3.5 again becomes a problem. As chlorine condenses, some of the water accompanies it. Depending on the temperature, a certain amount of water is soluble in the chlorine. So long as this solubility is not exceeded, the condensate remains homogeneous and solid hydrate does not form. Below we develop an estimate of the solubility of water in liquid chlorine and show that, because of its very low solubility in chlorine and therefore its very high activity coefficient in solution, it behaves as a volatile component. The practical effect of this is that water tends to concentrate in the gas phase in most first-stage liquefiers. 

Direct feed of compressed chlorine to another process is possible only when the quality of the chlorine meets the user s needs. The receiving process determines and controls how much chlorine is taken from the header, but another control valve is necessary at the compressor discharge header in case the user attempts to take more chlorine than is available. The liquefiers normally handle the chlorine not taken by the direct user. It has been common practice to design the liquefaction plant for full cell output, so that the cells can operate at full rate during short upsets in the user s process. This approach may be modified to suit restrictions on maximum chlorine inventory. In any case, the liquefiers should always have some chlorine gas fed to them to keep them operational and ready to handle full chlorine production should the direct user suddenly stop taking gas. In addition, a supply of liquid chlorine may be needed for a suction chiller. A bypass line around the control with a restricting orifice sized for about 10% of full capacity at the control valve drop can meet this requirement. Any chlorine not taken... 

Many liquid chlorine lines are fitted with expansion chambers to provide some volume for thermal expansion. The intent is to prevent dangerous increases in pressure as liquid chlorine in a static line becomes warmer. These chambers, their function, and their sizing are the subject of Section 9.1.10.4. When called for, they should be installed above the lines in question and subjected to the routines called for in the precommissioning procedures. Each chamber should be filled with dry air or an inert gas. Compression of this gas provides the buffer against expansion. Part of the commissioning documentation should be a schedule giving the appropriate pressures for all chambers. 

Usually, chlorine tank cars are filled at low temperature and pressure. The inherent pressure of the vapor in the tank car is normally sufficient to accomplish withdrawal of the liquid chlorine to the process, but sometimes, especially during the winter, the car is air padded to accomplish liquid withdrawal. When air padding is required, introduce only clean, oil-free, cooled, dry compressed air into the tank car through its vapor valve. 

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