Liquor circulation effect

Amongst the first quantities that may have to be specified, or calculated, in a design procedure are the crystal production rate, a clearly defined mean product crystal size, the crystallizer working volume, liquor circulation rate, magma density, mass of crystals in the suspension and the residence times of both crystals and liquor. Several of these quantities are interrelated and will not need separate calculation. Some are directly linked to the effective overall crystal growth rate, a sensitive system-specific quantity. 

The rate of formation of thiosulfate is not only dependent upon the degree of conversion of the hydrosulfide to sulfur prior to contacting with oxygen, but also on the pH of the solution, the operating temperature, and the concentration of dissolved solids in the circulating Stretford liquor. The effects of temperature and pH are shown in Figures 9-19 and 9-20... 

Various authors have studied the effect of the rate of liquor circulation in dyeing. Vickerstaff pointed ouT that, because of a lower concentration of the dye in the hquor near the surface of the fibre or trapped in the spaces of the yam compared with the external dyebath, the rate of dyeing is dependent upon the efficiency with which the dye is transported to the material being dyed, either by liquor circulation throngh the material or by the material s movement with respect to the dyebath. 

A single-effect evaporator is used to concentrate 0.075 kg/s of a 10 per cent caustic soda liquor to 30 per cent. The unit employs forced circulation in which the liquor is pumped through the vertical tubes of the calandria which are 32 mmo.d. by 28 mmi.d. and 1.2 m long. Steam is supplied at 394 K, dry and saturated, and the boiling-point rise of the 30 per cent solution is 15 degK. If the overall heat transfer coefficient is 1.75 kW/m2 K, how many tubes should be used, and what material of construction would be specified for the evaporator The latent heat of vaporisation under these conditions is 2270 kJ/kg. 

Cascara, Coffee, Licorice, Nicotine, Tea.—The original density of these extracts depends entirely on the manufacturing process, and no average can be given. The liquors are usually concentrated to from 40 to 50 per cent solids, and the capacity is about 2 gal. per square foot with a steam pressure of 5 lb., and a vacuum of from 27 to 28 in. Single effects of the horizontal-tube or rapid-circulation type are standard equipment sometimes a plain coil pan with or without steam jacket is used. Heating surface must be of copper and the shells can be of either copper or enameled steel. 

Lactic Acid.—The filtered solution is usually concentrated from 8 to 50 per cent in a single-effect evaporator of the vertical-tube or rapid-circulation type. All parts of the evaporator coming in contact with the liquor or vapor must be of copper. The solution contains calcium sulphate, and the deposit of hard scale must be removed very frequently from the tubes. 

Grape Juice and Cider.—The initial density of these liquors will vary from 6 to 8 Bd., and the final concentration is usually carried to from 30 to 33°Bd. The capacity is from 1J2 to 2 gal. per square foot with a steam pressure of 5 lb. and a vacuum of from 26 to 28 in. Only single effects can be used in order to avoid high temperature and a cooked taste in the liquor. The evaporators must be built of copper, which is sometimes tinned. Machines of the vertical-tube or rapid-circulation type are commonly used, but a number of factories are also using the ordinary coil pan with shells made of copper or enamel steel. 

Sulphite Waste Liquor.—One ton of pulp will produce from 2,500 to 5,000 gal. of waste liquors, the density of which will vary from 6 to 10 per cent. It is usually concentrated after neutrahzation to about 30°B6. (50 per cent solids) in multiple-effect evaporators of the vertical or rapid-circulation type, and the capacity is from 2 4 to 3J gal. per square foot, with a steam pressure of 5 lb. and a vacuum of from 27 to 28 in. The tubes will foul very quickly, on account of the large amount of calcium sulphate contained in the weak solution, and therefore evaporators are to be constructed so that tubes can readily be cleaned mechanically. 

Mercerizing Liquors.—Quantities of liquor will vary considerably with the process of washing used in each plant, and a reasonable average will give 2 gal. of 6°Twaddell waste liquor for each pound of cotton mercerized. These liquors are usually concentrated to from 70 to 80 Twaddell in a triple- or quadruple-effect evaporator of the horizontal-tube or rapid-circulation type. The capacity is from 2H to 3H gal. per square foot, with a steam pressure of from 10 to 15 lb. Special construction and entrainment separators are necessary to prevent losses by foaming. The last effect will frequently show incrustations of sodium carbonate which can be dissolved in hot water or weak hquor. Evaporators are usually built of steel with steel or charcoal-iron tubes. 

Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Circulatory systems are therefore not well suited for evaporating heat sensitive materials. Circulatory evaporators, however, can operate over a wide range of concentrations and are well adapted to single-effect evaporation. 

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