Patent GB-720288-A (GB)
Title: Improvements in or relating to method of and apparatus for treating liquors containing tar acid
IPC: C10C1/04
Filed: 1951-10-19
Abstract: <PICT:0720288/III/1> <PICT:0720288/III/2> In a process for the removal of tar acids from liquors such as ammoniacal gas liquors, wherein the liquor, after being freed from acidic gases CO2, H2S, H C N), is stripped with an inert gas such as steam whereby the tar acids are volatilized and carried away in the inert gas, the tar acids are removed from the inert gas by scrubbing it in a bubble cap column with an aqueous solution of an alkali; the effluent gas from the column is then used to strip tar acids from another quantity of liquor.  The use of a bubble cap column in the absorption stage enables efficient contact to be made between large volumes of inert gas with relatively small volumes of alkaline solution, and consequently stronger solutions of the alkali (viz., 20-50 per cent NaOH) may be employed than in those processes which make use of packed towers.  Ammonia liquor from tank 1 (Fig. 1) is pumped through conduit 2 and filter 3 into still 5, which is supplied with steam through connection 6 and duct 7 so that the free ammonia and the acidic gases are carried off in the steam, but the tar acids remain in the liquor.  The vapours withdrawn from still 5 are passed, after dephlegmation, to the main coke oven by-product gas line before the primary coolers.  The hot liquor which collects at the base 63 of still 5 is passed through conduits 12, 13 to sprays 14, through which it is sprayed into stripping section 15.  This section contains suitable contact material such as wooden hurdles or spiral tile packing, and is supplied at the base with steam through duct 40.  The dephenolized liquor collects in well 19, and is passed through conduit 20 into tower 21, wherein it is mixed with lime, introduced through inlet 22, by means of steam supplied through inlet 23.  The steam leaving tower 21 is passed into still 5 through connection 6. The liquor overflows from tower 21 through conduit 24 into fixed ammonia still 25, where it is distilled with steam supplied through inlet 26.  The vapours from the head of still 25 are passed through duct 7 into still 5.  The liquor withdrawn from still 25 through outlet 27 is discharged to the sewer.  The steam charged with tar acids flows from the top of stripping section 15 through downcomer 18, and is pumped into the bottom of bubble cap column 28.  Hot caustic soda solution, prepared in storage tank 40 and dilution tank 41, is pumped through flowmeter 43 onto the top tray of the column. The soda solution forms a thin layer on each tray of the column.  The vapours which reach the top of the column are free from tar acids; these vapours are passed through a bank of steel spirals 39 (to remove entrained solution), and introduced into stripping section 15 through duct 40.  The solution reaching the bottom of the column is passed through pipe 47 to tubular evaporator 48, where part of the water is evaporated from the solution.  The concentrated solution is collected in tank 50; and the generated steam is passed into the bottom of the column through line 51, where it maintains the solution therein at the boiling point and serves as the source of the required inert gas.  The temperature of the evaporator 48 may be controlled thermostatically by a device associated with live steam valve 65. Excess steam delivered by evaporator 48 and inert gases not p removed in still 5 are vented from the column through line 60, the pressure in the column being automatically controlled by valve 62. The overflow portion (i.e. solution outlet) of each tray in the column is formed as a cut out segment bounded inwardly by two weirs, which extend across the entire chord of the segment.  Each tray (Fig. 3) has a fixed weir 44 to which is attached a threaded stud.  Weir 45 has an elongated slot, and is adjustably attached to the fixed weir 44 by the threaded stud and a tightening nut. The height of weir 45 is adjusted to maintain the desired depth of solution (e.g. 1/2 inch) on the tray.  A hood 53 is provided to prevent solution from the tray being thrown into the downspout by the upflowing gas.  The effective hood length is adjustable. The bubble caps adjacent the tray outlet are provided with straps 56 which prevent vapour bubbling up from under the half of the cap facing the tray outlet.  The optimum rate of feed of liquor (containing 0.75 to 3.5 grams per litre of tar acids) to the stripping section is 0.9 gallons per minute for each square foot of cross-sectional area of the stripping section.  If 10 per cent caustic soda solution is used in the bubble cap column, then the optimum ratio of the rate of flow of the caustic solution in the column to the liquor rate in the stripping section is 0.01, and the absolute rate of flow of the caustic solution is 0.009 gallons per minute per square foot of cross-sectional area of column.  The time of residence of the solution on each tray may be over 3 hours.  Air, nitrogen or coke oven gas may be used instead of steam as inert gas in the process, but steam is preferred.  The stripping section and the column are thermally insulated. Specifications 299,837, [Class 32], and 463,217 are referred to.