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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.
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. [1] 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. [2] 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 [3] 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. [1]
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. [2] [3] 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, [2] [3] 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.
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.
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. [1] 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. [4]
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. [4] 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. [5] To avoid fouling a high circulation rate is used, typically between 1.5 – 4 m/s [6] 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. [1]
Heating of the liquid across the heat exchanger is kept minimal with a standard temperature difference of 2 - 3 K. [1] 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. [1] It is important to note that 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.[ citation needed ]
When designing a forced circulation evaporator there are 3 considerations to address; the heat transferred, the liquid vapor separation and the energy consumption efficiency. [4] 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 [7] and as such optimum tube velocities can be reached resulting in an efficiently designed system that addresses the design considerations. [8]
Forced Circulation Evaporators have high liquid velocity and therefore a high turbulence [6] 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. [8]
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. [9]
Multiple heating effects can be used to increase thermal efficiency. [10] [4] 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 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. [11]
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) [1] 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. [1]
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, [11] 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.
Currently, a wide range of forced circulation evaporators are available that are specifically tailored to carry out distinct applications.
Plate Forced Circulation Evaporators utilize a centrifugal pump which forces liquid to circulate through the plate structures and heat exchanger. [12] The flexibility of this design is a major advantage, as the rate of evaporation can be manipulated by either adding or removing extra plates, allowing it to perform a wide range of duties that require greater heat transfer co-efficient. More specifically, products with higher viscosity have been better suited to this design, with the plate forced circulation evaporator demonstrating higher performance and improved evaporation with comparison to the tubular forced circulation system. The liquid must undergo superheated temperature, which exceeds the original boiling point of the liquid by a large degree, forcing rapid evaporation. In addition, to flexibility, this system is compact, only needing small space and is easy to clean [13] and maintain as plates are readily accessible. With regards to suitability, this design is currently being used in processes that involve liquids with low to medium evaporation rates and consist of minute portions of undissolved solutes with close to no capacity to induce fouling.
Tubular forced circulation evaporators employs an axial circulation pump which navigates the flow of liquid in a circular motion through the system's heat exchanger in which it is superheated. Thereafter, when the liquid reaches the separator the liquid pressure decreases dramatically forcing a portion of the liquid to be rapidly boiled off. This design is specifically for products and/ or particulates with a diameter of over 2mm. [14] As the evaporation action occurs only in the separator and not in the heat exchanger, fouling is reduced despite higher levels of turbulence in the design. Alternatively, another design parameter is the optimisation of liquid velocity in the tube side flow which is regulated by the circulation pump. [1]
Forced circulation evaporators in the food industry use modified designs that mimic the original system however involve extra secondary steam units to enhance forced circulation flow. [13] Whilst the single effect design employs a condenser unit to stimulate a condensation action subsequent to vapour inflow from the heat exchanger, the double effect design does a similar duty however the extra component acts to reduce the overall pressure in the system. In comparison, the triple effect system is used when high levels of effective evaporation are needed with minimum labour. [1] In this design, the liquid enters the third effect at a low temperature and moves to the second stream in which concentration is increased due to the previous evaporation effect. Finally, the optimum product concentration is achieved in the first effect. [1]
With regards to the design components within forced circulation evaporation systems, the heat exchangers can vary. Shell and tube exchangers are the most widely apparent as a result of the flexible design that can accommodate various pressure and temperature values. [1] Forced circulation exchangers can employ either horizontal or vertical shell and tube heat exchangers, allowing the exchange of heat between fluids within and outside the tubes (that exist inside the heat exchanger). Liquids with high levels of solute usually require vertical heat exchangers which are more commonly used. [1]
Evaporation generally deals with evaporation of water from a mixture or solution, containing another liquid or fine solids. This concentrated stream is in most cases the product and as such the only waste stream is pure water, which poses no risk to the environment and may be disposed into the stormwater/ sewage system. For the case where the concentrate is the waste stream, such as in evaporation of saltwater to produce potable water, the salt concentrate should be dispersed back into the oceans, or further dried and sent off for disposal/ use in other operations. For most cases, there are no hazardous waste streams associated with natural and forced circulation evaporators.
Natural/forced circulation evaporators have many advantages, making them the more popular choice of evaporator in industry. [15]
The liquid entering the circulation evaporator will boil in the separator, not on a heating surface, hence minimising fouling, whereas with plate evaporators, boiling will occur on a heating surface. [15] It is for this reason that circulation evaporators are preferred for liquids with a higher tendency to foul. Minimal fouling also means that the cleaning cycles are not as frequent as with other evaporators such as plate evaporators. [4]
Circulation evaporators are fairly compact and are easy to clean and operate. [13] They can also be easily adapted according to the product that needs to be obtained. [15] They have a high heat transfer coefficient as well as a high circulation flow, [4] [13] which both work to increase the efficiency of the evaporator.
One of the main limitations of the forced/natural circulation evaporators is the cost. Circulation evaporators have particularly high construction costs, whereas falling film evaporators have a low investment cost. Falling film evaporators has no rotating internal part, and hence experience no mechanical deterioration, whilst circulation evaporators have high maintenance costs. [13] [15]
Although previously described as an advantage, there is also a down side to the high circulation flow. The increased velocity can cause the equipment to corrode at a faster rate, which will increase the overall cost of running the evaporator considering how expensive it is to maintain compared to other evaporators.
Natural/forced circulation evaporators have a major role in the food and beverage industry. Specifically, they can be used for processes that produce tomato juice concentrate, (tropical and berry) fruit concentrate, and when water needs to be removed from certain raw materials in such a way as to maintain the raw material properties. [4]
In general, forced circulation evaporators are required when the fouling characteristics of a liquid will cause problems if the liquid boils on a heating surface. [16] [8] These evaporators are also used for liquids with a high solids content and a high viscosity. [4] [9]
There are several other processes that require the use of forced circulation evaporators, which work particularly well as crystallising evaporators. [15] These include processes that produce salt, corn steep water and calcium carbonate. [9]
Natural circulation evaporators are used in other processes such as those that produce anhydrous sodium hydroxide (caustic), sugar beet, liquors that are particularly foamy, or those that have a low to moderate viscosity, and precipitating liquids. [15]
Natural/forced circulation evaporators are also necessary in effluent treatment plants, and in both the chemical and pharmaceutical industry. [8]
Improvements in the design of the forced/natural circulation evaporators have had significant implications for industrial products and processes. The advent of self-cleaning exchangers installations containing an external circulating motion for particles has drastically reduced levels of fouling. [17] Moreover, the use of forced circulation evaporators in multi-effect evaporation plants, [5] as described earlier in the designs available section, have significantly broadened the applications for liquids that have high viscosities, can be easily deposited and require higher concentrations. Further evidence can be extracted from the case study regarding the North Italy landfill, in which biogas in a single effect evaporator could not completely evaporate the leachate. As a result, a triple effect forced circulation evaporator was utilized. [18]
A heat pump is a device that consumes work to transfer heat from a cold heat sink to a hot heat sink. Specifically, the heat pump transfers thermal energy using a refrigeration cycle, cooling the cool space and warming the warm space. In cold weather, a heat pump can move heat from the cool outdoors to warm a house ; the pump may also be designed to move heat from the house to the warmer outdoors in warm weather. As they transfer heat rather than generating heat, they are more energy-efficient than other ways of heating or cooling a home.
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream. As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function.
A falling film evaporator is an industrial device to concentrate solutions, especially with heat sensitive components. The evaporator is a special type of heat exchanger.
Thermosiphon is a method of passive heat exchange, based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heating and cooling applications such as heat pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs across air temperature gradients such as those utilized in a wood fire chimney or solar chimney.
An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of convenient heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. Absorption refrigerators are commonly used in recreational vehicles (RVs), campers, and caravans because the heat required to power them can be provided by a propane fuel burner, by a low-voltage DC electric heater or by a mains-powered electric heater. Absorption refrigerators can also be used to air-condition buildings using the waste heat from a gas turbine or water heater in the building. Using waste heat from a gas turbine makes the turbine very efficient because it first produces electricity, then hot water, and finally, air-conditioning—trigeneration.
Vacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Although the process can be applied to any type of liquid at any vapor pressure, it is generally used to describe the boiling of water by lowering the container's internal pressure below standard atmospheric pressure and causing the water to boil at room temperature.
Economizers, or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.
Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using two compressors.
Vapor-compression evaporation is the evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the pressure of the vapor produced. Since the pressure increase of the vapor also generates an increase in the condensation temperature, the same vapor can serve as the heating medium for its "mother" liquid or solution being concentrated, from which the vapor was generated to begin with. If no compression was provided, the vapor would be at the same temperature as the boiling liquid/solution, and no heat transfer could take place.
A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial-flow turbine, through which a high-pressure gas is expanded to produce work that is often used to drive a compressor or generator.
Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation. The heat supplied to the column by the reboiler at the bottom of the column is removed by the condenser at the top of the column.
An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer, which provides the necessary thermal energy for phase transition from liquid to vapour. Within evaporators, a circulating liquid is exposed to an atmospheric or reduced pressure environment causing it to boil at a lower temperature compared to normal atmospheric boiling.
Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that transmits heat from one location at a certain temperature to another location at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink, or a "refrigerator" or “cooler” if the objective is to cool the heat source. The operating principles in both cases are the same; energy is used to move heat from a colder place to a warmer place.
HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.
In refrigeration, flash-gas is refrigerant in gas form produced spontaneously when the condensed liquid is subjected to boiling. The presence of flash-gas in the liquid lines reduces the efficiency of the refrigeration cycle. It can also lead several expansion systems to work improperly, and increase superheating at the evaporator. This is normally perceived as an unwanted condition caused by dissociation between the volume of the system, and the pressures and temperatures that allow the refrigerant to be liquid. Flash-gas must not be confused with lack of condensation, but special gear such as receivers, internal heat exchangers, insulation, and refrigeration cycle optimizers may improve condensation and avoid gas in the liquid lines.
A climbing/falling film plate evaporator is a specialized type of evaporator in which a thin film of liquid is passed over a rising and falling plate to allow the evaporation process to occur. It is an extension of the falling film evaporator, and has application in any field where the liquid to be evaporated cannot withstand extended exposure to high temperatures, such as the concentration of fruit juices.
A rising film or vertical long tube evaporator is a type of evaporator that is essentially a vertical shell and tube heat exchanger. The liquid being evaporated is fed from the bottom into long tubes and heated with steam condensing on the outside of the tube from the shell side. This is to produce steam and vapour within the tube bringing the liquid inside to a boil. The vapour produced then presses the liquid against the walls of the tubes and causes the ascending force of this liquid. As more vapour is formed, the centre of the tube will have a higher velocity which forces the remaining liquid against the tube wall forming a thin film which moves upwards. This phenomenon of the rising film gives the evaporator its name.
The low-temperature distillation (LTD) technology is the first implementation of the direct spray distillation (DSD) process. The first large-scale units are now in operation for desalination. The process was first developed by scientists at the University of Applied Sciences in Switzerland, focusing on low-temperature distillation in vacuum conditions, from 2000 to 2005.