SINGLE-EFFECT EVAPORATORS

A single-effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50 per cent solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. If the overall coefficient of heat transfer is 3 kW/m2 deg K, estimate the heating surface required and the amount of steam used if the feed to the evaporator is at 294 K and the condensate leaves the heating space at 352.7 K. The specific heats of 10 and 50 per cent solutions are 3.76 and 3.14 kJ/kg deg K respectively. 

Assuming that the steam is dry and saturated at 205 kN/m2, then from the Steam Tables in the Appendix, the steam temperature = 394 K at which the total enthalpy = 2530 kJ/kg. 

5 kN/m2, water boils at 325 K and, in the absence of data on the boiling point elevation, this will be taken as the temperature of evaporation, assuming an aqueous solution. The total enthalpy of steam at 325 K is 2594 kJ/kg. 

Thus the feed, containing 10 per cent solids, has to be heated from 294 to 325 K at which temperature the evaporation takes place. 

As the preheating of the solution and the sub-cooling of the condensate represent but a small proportion of the heat load, the temperature driving force may be taken as the difference between the temperatures of the condensing steam and the evaporating water, or  

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Wlien solid soda ash is used to supply all the carbonate v alues in the precipitation step (eq. 9), a ca 10% Na2S solution results from the primary filtration step wliich can be concentrated to 40% Na2S in a three-effect evaporator train. Final concentration to 60%Na2S occurs in a high v acuurn single-effect evaporator. Hiis concentrated solution can then be fed to a Baker to produce a 60% sodium sulfide flake wliich is sold as co-product. 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

A solution containing 10 per cent of caustic soda is to be concentrated to a 35 per cent solution at the rate of 180,000 kg/day during a year of 300 working days. A suitable single-effect evaporator for this purpose, neglecting the condensing plant, costs 1600 and for a multiple-effect evaporator the cost may be taken as 1600TV, where N is the number of effects. 

A single effect evaporator operates at 13 kN/m2. What will be the heating surface necessary to concentrate 1.25 kg/s of 10 per cent caustic soda to 41 per cent, assuming a value of U of 1.25 kW/m2K, using steam at 390 K The heating surface is 1.2 m below the liquid level. 

Distilled water is produced from sea water by evaporation in a single-effect evaporator working on the vapour compression system. The vapour produced is compressed by a mechanical compressor of 50 per cent efficiency, and then returned to the calandria of the evaporator. Extra steam, dry and saturated at 650 kN/m2, is bled into the steam space through a throttling valve. The distilled water is withdrawn as condensate from the steam space. 50 per cent of the sea water is evaporated in the plant. The energy supplied in addition to that necessary to compress the vapour may be assumed to appear as superheat in the vapour. Calculate the quantity of extra steam required in kg/s. The production rate of distillate is 0.125 kg/s, the pressure in the vapour space is 101.3 kN/m2, the temperature difference from steam to liquor is 8 deg K, the boiling-point rise of sea water is 1.1 deg K and the specific heat capacity of sea water is 4.18 kJ/kgK. 

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. 

A single-effect evaporator with a heating surface area of 10 m2 is used to concentrate a NaOH solution flowing at 0.38 kg/s from 10 per cent to 33.3 per cent. The feed enters at 338 K and its specific heat capacity is 3.2 kJ/kg K. The pressure in the vapour space is 13.5 kN/m2 and 0.3 kg/s of steam is used from a supply at 375 K. Calculate ... 

The single effect evaporator uses rather more than 1 kg of steam to evaporate 1 kg of water. Three methods have been introduced which enable the performance to be improved, either by direct reduction in the steam consumption, or by improved energy efficiency of the whole unit. These are ... 

Example 13.6 A single-effect evaporator concentrates 10,000(lbm)(hr) l of a 10% (by weight) aqueous solution of NaOH to 50%. The feed enters at 70(°F). The evaporator operates at an absolute pressure of 3(inHg), and under these conditions the boiling point of a 50% solution of NaOH is 190(°F). What is the heat-transfer rate in the evaporator ... 

JO A single-effect evaporator concentrates a lS-wt-% aqueous solution of H2S04 to 60%. feed rate is 20(lbm)(s)"and the feed temperature is 100(°F). The evaporator is maintained at absolute pressure of l.5(psia), at which pressure the boiling point of 60% H2S04 is 176( F). is the heat-transfer rate in the evaporator ... 

Example 13.5 A single-effect evaporator operating at atmospheric pressure ooncen-trates a 1S% (by weight) LiCl solution to 40%. The feed enters the evaporator at the rate of 2kgs-1 at 25°C. The normal boiling point of a 40% LiCl solution is about 132°C, and its specific heat is estimated as 2.72 kJ kg-1 °C 1. What is the heat-transfer rate in the evaporator ... 

The dark-colored clarified liquor is presure-filtered and concentrated to 60 percent solids in multiple-effect evaporators. The concentrated liquor is decolorized with granular carbon in columns 12 ft in diameter and 30 ft high in a countercurrent manner that is, liquor flows upward in the columns, while a portion of the carbon is removed from the bottom periodically. Carbon is used at a rate of 2.5 percent of dry solid processed, and approximately 5 percent of carbon is lost during revivification.91 In some cases, granular carbon has been replaced by synthetic polymeric adsorbents to decolorize the syrup. Low ash syrups usually are deionized with ion-exchange resins. The processed liquor is evaporated to a final solids content of 75-85 percent in a single-effect evaporator. 

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