CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 193.are given an electrical charge and then subjected to a strong electrical field,leading to almost complete precipitation of the tar. Examination ofelectrically-precipitated tars showed that they are of lower specific gravitythan the ordinary tar recovered from the plant, and contain liquor of highammonia content together with hydrocarbon oils, large amounts of whichboil in the benzole range and can be recovered with suitable equipment(this system was mentioned in the previous article in this series - C.O.M.A.Bulletin, April 1992, p.l1.). The tars also contain resin-forming materialssuch as coumarone and indene, together with other unsaturated oils andpitch. If such materials were allowed to go forward in the gas stream,thickening of the creosote or petroleum oil used for benzole recoverywould rapidly take place.The author then considered the practical aspects of ensuring efficientrecovery of benzole from the gas, but noted that complete removal couldnever be achieved on the plant, owing to limitations in respect of contacttime between benzole vapour and scrubbing oil, mechanical efficiency ofthe scrubbers and the presence of traces of benzole in the debenzolised oil.In discussing topics such as scrubber design and the counter-currentprinciple of gas washing, oil circulation requirements and the maintenanceof low oil viscosity in relation to benzole recovery, the author stressed theimportance of adequate cooling of both gas and oil, but drew attention tothe need for the oil to be maintained at atemperature 1 to 2 0 C. above that of the gas, to avoid water deposition.Efficient recovery of benzole from the gas demands the circulation ofappropriate volumes of oil, and to illustrate the importance of temperaturein the process, the author presented a graph relating to practice at hisworks, which showed how the oil requirement increased from 55 gallonsper 10,000 cu. ft. at a washing temperature of 10 0 C. to 110 gallons at 25 0 C.in order to maintain efficiency, and posed the question “How manyoperators double the oil in circulation during the hot summer months?”. Agood wash oil should have a high capacity for benzole absorption, a lowviscosity to facilitate oil circulation and covering of the scrubber grids, lowspecific heat to assist in heating and cooling operations, low volatility toreduce distillation loss and a specific gravity which allows of readyseparation from any water present, thus preventing the formation ofemulsions. The oil should be resistant to sludge formation and also tochanges in its characteristics when subjected to repeated heating andcooling.
194. THE COKE OVEN MANAGERS’ YEAR-BOOKThe author then proceeded to describe the laboratory experiments hehad conducted into the causes of thickening of creosote oil, which occursafter repeated circuits round the benzole plant. The oil to be examined washeld at about 190 0 C. and various gases passed through the system. Withoxygen, the viscosity of the oil (Redwood No. 1, temperature unspecified)increased the 39 seconds to 60 seconds after 35 days, but with hydrogensulphide and viscosity had more than doubled (to 80 seconds) after only 14days. The effect of crude coke oven gas (containing 0.7 to 0.9% oxygenand 600 to 900 grains of hydrogen sulphide per 100 cu. ft.) was alsoexamined, and although, as expected, the results were less dramatic thanwith pure gases, the viscosity had nevertheless almost doubled in a month.Washing the oil with alkali and acid to remove basic and unsaturatedconstituents respectively, gave a product which was very resistant tothickening by oxygen and hydrogen sulphide, but the cost of manufactureof such an oil would prohibit its use in practice.In order to maintain the oil in good condition a Wilton pipe still with asingle fractionating column was installed at Clay Cross. In addition to thecreosote oil being regenerated, the feed to the pipe still included coolercondensates and black naphthalene (from steaming out coolers, etc.) as asource of make-up. The author described his experiences with this plant,which produced a light oil and naphthalene in addition to creosote oil, andoutlined the beneficial effects on benzole recovery which resulted from itsuse. To support the aim of increasing the make of benzole, there was a needfor regular laboratory tests on benzole in the gas before and after thebenzole scrubbers, together with information concerning temperatures andvolumes of oil and gas, provided by reliable recording instruments,properly maintained, so that any corrective action could be taken promptlyif necessary.Attention then turned to the debenzolising process, in which the rich oilis raised in temperature by heat exchange and closed steam prior to contactwith live steam in the stripping still, whereby the benzole is carried off asvapour. The author referred to the importance, for efficient stripping, ofadequate preheater surface area in relation to the oil flow, to ensure that asufficiently high temperature (usually about 120 0 C.) is achieved prior to thestill. Reference was made to the economies in steam consumption fordebenzolising which could be achieved where water temperatures alloweda smaller volume of colder scrubbing oil to be circulated, consistent with
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CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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