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 with 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 greatly affects evaporator design, as it represents 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 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 condense 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 on the specifications determined in the calculations above.
Main characteristics The main process characteristics are those based around evaporation specifically through heat exchange and pressure manipulation. It is a flash separation procedure that includes the heating of a base liquid mixture and forced circulation through the system via pumping.
Physical characteristics Forced/Natural Circulation Evaporation is used when boiling of base liquids is undesired. It was developed specifically for processing and separation of liquids in which crystallising and scaling occurs. The evaporator uses separate parts to create the overall system; a heat exchanger, separation tank and for the forced circulation system (as opposed to the natural circulation system) a circulation pump are standard although can be subject to change depending on the liquids properties of the mixtures being separated and specific design. The units in the heat exchanger (where thermal transfer takes place) are called the heating units or calandria (for single tube heat exchangers). The liquid-vapor separation tank is called a flash separator, flash chamber or flash vessel. The basic module of an evaporator is known as the “body” of the evaporator and refers to the calandria and the flash chamber. The term “effect” is used to describe the body where vapor is extracted from the raw material and is operating at the same boiling point.Forced circulation Evaporators. Gemina, Machine Point Food technologies, 2012 Date reviewed: 9/10/2013
System characteristics Evaporation is the elimination of the solvent in form of vapor from a solution. For most evaporation systems, the solvent is water and the heat is provided by steam condensation. In a forced circulation evaporation liquid is constantly circulated through the system. The mixture moves through the heat exchanger where it is superheated under pressure."2010-10 Functional Principle of a Forced Circulation Evaporator." Functional Principle of a Forced Circulation Evaporator by GEA Wiegand GmbH. Web. 27 Sept. 2013. To avoid fouling a high circulation rate is used, typically between 1.5 – 4 m/s “Forced Circulation Evaporator.”, Rising Film Evaporator, Agitated Thin Film Evaporator. Web. 27 Sept. 2013. <http://www.sspindia.com/forced-circulation-evaporator.html>. although this ultimately depends on the component properties and is easily manipulated by the circulation pump. The liquid is pressurised through the heat exchanger externally by pressure stabilisers such as valves or orifices or hydrostatically within the system.
Heating of the liquid across the heat exchanger is kept minimal with a standard temperature difference of 2 - 3 K. As the liquid enters the flash vessel the pressure is reduced to slightly below that of the heat exchanger and flash evaporation occurs. The vapor stream is separated out of the liquid stream. This vapor is usually not the desired product from the evaporation unit. As such the vapor can be either collected or disposed of depending on the system. The enriched liquid solution is then either collected in the same way as the vapor or recirculated through the system again.
This results in a high recirculation ratio within the range of 100–150 kg of liquid (solvent) recirculated per Kg of vapor removed. These high recirculation rates result in high liquor velocities through the tube and in turn minimize the buildup of crystals, other deposits and in turn minimize fouling. In crystallisation applications, crystallisation still occurs in the flash separator and in some specific systems a further separation of solid particles from the recirculated slurry is needed.
Assessment of characteristics When designing a forced circulation evaporator there are 3 considerations to address; the heat transferred, the liquid vapor separation and the energy consumption efficiency. All of these considerations need to be maximized in order to create an efficient system. As circulation and heating are maintained for the system, liquid temperatures and flow rates can be controlled specifically to suit the product requirements “Forced Circulation Evaporators.” Forced Circulation Evaporators. Web. 27 Sept. 2013. <http://www.gea-wiegand.com/geawiegand/cmsdoc.nsf/WebDoc/webb89bagw>. and as such optimum tube velocities can be reached resulting in an efficiently designed system that addresses the design considerations.“Forced Circulation Evaporators.” Industrial Evaporators. Comp Engineering and Exports, 2011. Web. 8 Oct. 2013.
Forced Circulation Evaporators have high liquid velocity and therefore a high turbulence this ergo equates to high heat transfer coefficients. The system contains positive circulation, freedom from high fouling, scaling or salting and is suitable for corrosive and viscous solutions.
The operating characteristics are specifically manipulated to fit the application criteria. Forced Circulation Evaporators are however versatile in their application and can be used in a wide variety of applications (see applications). For instance they are ideal for crystallising operations. Concentration values of forced circulation evaporators can handle more than the limits of conventional tubular evaporators when handling feed with dissolved salts and is often used as a finishing evaporator for concentration of liquids to high solid content following low solids multi-stage, TVR or MVR evaporation.Anhydro Evaporators - Forced Circulation Evaporators.” Global Industrial Equipment & Global Manufacturing. SPX Andryo, 2012. Web. 12 Oct. 2013
Multiple heating effects can be used to increase thermal efficiency."Evaporators." - Agitated Thin Film, Forced Circulation, Falling Film. Web. 27 Sept. 2013. <http://www.rajprocess.com/evaporators.html >. In this system design extracted vapor is used as a heating medium for the 2nd heating effect at a lower pressure than the first effect. This can be repeated for multiple effects.
Natural Circulation evaporator characteristics Natural Circulation evaporation is essentially based upon natural convection currents manipulated through the system piping to create circulation. Circulation through convection is achieved through bubble formation. Bubble are of lower density and rise through the liquid to promote upward lift into the evaporating vessel.“Natural Circulation Evaporator.” GIG Karasek Verfahrenstechnik Verdampfer Prozesslsungen TurnKey Anlagen. Web. 14 Oct. 2013.
Physically Natural circulation evaporators use a short tube bundle within the batch pan or by having an external shell and tube heat exchanger outside of the main vessel (as shown in the diagram) External heating through heat exchangers is normally used as it has the advantage that it is not dependent on the calandria size or shape. As such larger capacities for the flash separation tank can be obtained.
Removing of the circulation pump reduces the operating costs, however due to characteristics of the system as mentioned above the evaporator has a long residence time and low flow rates, making its uses severely more limited than a forced circulation evaporator. The most common application of Natural Circulation evaporation is as a reboiler for distillation columns.
System designs available