GB761079A - Improvements in or relating to the cooling of vaporous mixtures - Google Patents

Abstract

<PICT:0761079/III/1> Sensible heat is removed from a vaporous mixture which on cooling yields a solid as the first product of condensation, by flowing the vapour continuously through a metal cooling conduit the interior surfaces of which are maintained at a temperature below the snowpoint of the vapour, whilst maintaining the speed of vapour flow and the temperature and composition of the vapour substantially constant for a given cooler, thereby forming and maintaining throughout the cooling operation a protective layer of the condensed solid on the interior surface of the cooling conduit, the thickness of the protective layer formed and maintained being so controlled as to avoid progressive build up of condensed solid by correlating the following parameters: the inlet and outlet vapour temperature, its heat content, composition, flow rate and the snow and dew points of the components thereof, the thermal conductivities of the solid deposit and metal wall, and the cooling fluid inlet and outlet temperatures together with the cooling conduit diameter, which must be large enough to provide efficient free area after the protecting layer has been built up. When equilibrium conditions are attained the vapour leaves the conduit at a temperature above the snowpoint of the mixture without its composition being materially changed. The invention is applicable to removing sensible heat from vapour mixtures containing phthalic anhydride, beta napthol, salicylic acid, pyrogallol, anthracene or benzoic acid or inorganic compounds such as iodine, mercuric chloride or the halides of chromium, zirconium and aluminium. The invention is particularly adapted to the cooling of vapours of titanium and iron chlorides such as are obtained by the chlorination of ilmenite or rutile with chlorine in the presence of a solid or gaseous reducing agent. As shown in the Figure the cooler may consist of a metallic upright U-tube 2 having an inlet 3 and outlet 4 provided with a cleanout port 5 and cooled by annular spray ring 7. In the treatment of a titanium tetrachloride ferric chloride mixture, in the initial stages ferric chloride is condensed and deposits on the internal surfaces to build up a thin insulating layer 8, such build up continuing until the exit temperature of the gases leaving the unit becomes stablized, for example gases leaving an ilmenite chlorination furnace were passed to the cooling unit at 900 DEG C. whereupon a layer of up to 1/2 inch thickness at the inlet end and up to about 1 1/2 inches thickness at the exit end was built up until the exit temperature of the gas rose to about 300 DEG C. that is slightly above the snowpoint of the vapour. The temperature then became stabilized and no further ferric chloride was extracted from the vapour stream, sensible heat being thereafter removed without further build-up of ferric chloride or plugging of the cooler. Short period fluctuations in heat load caused by changes in vapour throughput rate or inlet vapour temperature are compensated automatically by the decrease in average thickness occasioned by inlet vapour temperature increase, or increase in thickness of the insulating layer caused by inlet vapour decrease. Gross variations in load may be compensated by increasing or decreasing the length of conduit or in the case where a drop in heat load would require shortening the conduit, the section of conduit in which cooling of the vapour below the snowpoint may occur can be contacted with a heat transfer fluid substantially at the snowpoint temperature, the temperature of such fluid serving to maintain a regulated thickness of deposit. The cooled exit gases may be treated in known manner for condensing the metal chlorides for example as in Specifications 550,750, 551,525 and 679,537.