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Circulation evaporators are a type of evaporating unit designed to separate mixtures unable to be evaporated by a conventional evaporating unit. Circulation evaporation incorporates the use of both heat exchangers and flash separation units in conjunction with circulation of the solvent in order to remove liquid mixtures without conventional boiling. There are two types of Circulation Evaporation; Natural Circulation Evaporators and Forced Circulation Evaporators, both of which are still currently used in industry today, although forced Circulation systems, which have a circulation pump as opposed to natural systems with no driving force, have a much wider range of appropriate uses. ==Design of natural/forced circulation evaporators== Evaporators are designed based on criteria that are the same regardless of the industry in which they are employed, and these answer the two key objectives: Is the equipment to be selected best suited for the duty, and is the arrangement the most efficient and economical.〔Evaporator Handbook. Chicago, IL: APV Crepaco, [198. Print.〕 Heat transfer is the single most important factor in evaporator design, representing the greatest cost in its operation. The most suitable evaporator will have the highest heat transfer coefficient per dollar of equipment cost.〔Coulson and Richardson. Chemical Engineering Volume 2, 5th Ed. p771-822. Butterworth-Heinemann, 2002.〕 In optimising the design of an evaporator, another important consideration is the steam economy (kg of solvent evaporated per kilogram of steam used). The best way to achieve high economies (which can be well over 100%) is to use a multiple effect evaporator, whereby the vapour from one evaporator – or effect – is used to heat the feed in the next effect, where boiling occurs at lower pressure and temperature 〔Perry, R and Green, D. Perry’s Chemical Engineering Handbook. sect. 11-107 p 1141〕 Thermo-compression of the vapour, whereby the vapour will condensate at a temperature high enough to be reused for the next effect through compression, will also increase efficiency. However, increased energy efficiency can only be achieved through higher capital costs and a general rule is the larger the system, the more it will pay back to increase the thermal efficiency of the evaporator.〔 Heat transfer is not the sole design criteria however, as the most appropriate evaporator also depends on properties of the feed and products. Crystallisation, salting and scaling, product quality and its heat sensitivity, foaming potential of the solution, viscosity of feed (which increases with evaporation) and its nature (slurry or concentrate) all need to be considered.〔〔 For Single Effect Evaporators that are used in small scale processes with low throughput of material, material and energy balances can be used to design and optimise the process. In designing multiple effect evaporators, trial and error methods with many iterations are usually the fastest and most efficient. The general steps in design are as follows,〔〔 and would be carried out in excel for ease of calculation. Other design software such as Aspen Plus could also be used with built in functions for process equipment. 1) Estimate temperature distribution in the evaporator, taking into account boiling-point elevations. If all heating surfaces are to be equal, temperature drop across each effect will be approximately inversely proportional to the heat transfer coefficient in that effect. 2) Determine total evaporation required, and estimate steam consumption for the number of effects chosen. 3) Calculate evaporation in the first effect from assumed feed temperature or flowrate. Repeat for following effects, and check that initial and intermediate assumptions are still valid. Also, determine whether product quality has met required specifications at the last effect. 4) . Check to see if the heat requirements have been met and product meets desired specifications. If not, repeat previous steps with different assumption of steam flow into the first effect. 5) Now that concentrations in each effect are known, recalculate boiling point rises to determine the heat loads. Using this information revise assumed temperature differences heat transfer coefficients, then determine heating surface requirements. 6) Given enough data, based on the above conditions, heat transfer coefficients can then be calculated more rigorously, and surface heating requirements adjusted accordingly to give a more reliable design representative of the physical system itself. Once the evaporator components themselves have been designed, ancillary equipment such as pumps (particularly for forced circulation evaporators) and heaters would need to be designed and/or specified for the system to give a reliable performance and cost estimate of the system as a whole. These would be based off the specifications determined in the calculations above. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Circulation evaporators」の詳細全文を読む スポンサード リンク
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