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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.[1][2]
Direct spray distillation is a water treatment process applied in seawater desalination and industrial wastewater treatment, brine and concentrate treatment as well as zero liquid discharge systems. It is a physical water separation process driven by thermal energy. Direct spray distillation involves evaporation and condensation on water droplets that are sprayed into a chamber that is evacuated of non-condensable permanent gases like air and carbon dioxide. Compared to other vaporization systems, no phase change happens on solid surfaces such as shell and tube heat exchangers.
Applications
Currently, the only implementation of DSD technology is low-temperature distillation (LTD). The LTD process runs under partial pressure in the evaporator and condenser chambers, and with process temperatures of below 100°C. The first large-scale LTD systems for industrial water treatment are now in operation.[citation needed]
Process
Classification
Separation mechanism
Phase change through boiling
Physical process
boiling and condensation, multi stage
Energy supply
thermal energy driven
Scale
central plant, large scale
History
The DSD process was invented in the late 1990 by Mark Lehmann with the first successful demonstration of the process in a factory hall of the Obrecht AG, Doettingen, Switzerland. The results of the experiments were evaluated and double-checked by Prof. Dr. Kurt Heiniger (University of applied Sciences and Arts, Northwestern Switzerland)[3][4] and Dr. Franco Blanggetti (Alstom, Co-author of the VDI Wärmeatlas).[5] During the next years, the process has been further researched in the framework of many thesis[1] supervised by Heiniger and Lehmann.[2] The objective has been the examination of the influence of non-condensable gases in lowered pressure environments on the heat transfer during the condensation process on cooled droplets. It has been found that the droplet size and distribution as well as the geometry of the condensation reactor has the most significant influence on the heat transfer. Due to the absence of common tube bundle heat exchangers, the achievable efficiency gains result from the minimized heat resistance during the condensation process.
Technology description
Fig. 1: Two water columns with different temperatures
Low temperature distillation (LTD) is a thermal distillation process in several stages, powered by temperature differences between heat and cooling sources of at least 5 K per stage. Two separate volume flows, a hot evaporator flow and a cool condenser flow, with different temperatures and vapor pressures, are sprayed in a combined pressure chamber, where non-condensable gases are continuously removed.[3][4] As the vapor moves to a partial pressure equilibrium, part of the water from the hot stream evaporates.[5] Several serial arranged chambers in counter flow of the hot evaporator and cold condenser stream allow a high internal heat recovery by the application of multiple stages. The process excels in a high specific heat conversion rate caused by the reduction of heat transfer losses, which results in a high thermal efficiency and low heat transfer resistance. The LTD process is tolerant to high salinity, other impurities, and fluctuating feed water qualities. The precipitation of solids is technically intended to allow for zero-liquid-discharge operation (complete ZLD). It is possible to combine the low-temperature distillation process with existing desalination technologies serving as downstream process to increase the water output and reduce the brine generation.
Fig. 2: Different temperatures cause different vapor pressures
Physical principle
The following figures show and explain the thermodynamic principle on which the LTD technology is built. Considering Fig. 1, there are two cylinders given with open buttons and filled with water in two basins with two different temperatures (assumption: hot at 50°C and cold at 20°C). The temperature related vapor pressure of the water is 123 mbar for 50°C and 23 mbar for 20°C. It is assumed that the two cylinders are 10 meters long and allow to be pulled out the same distance.
Fig. 3: Boiling and condensing through thermodynamic imbalance
The pulled-out cylinders in Fig. 2 show now a different situation regarding the level of the water column. Due to the higher vapor pressure at 50°C, in the hot water column the Atmospheric pressure is capable to elevate the hot water column about 877 cm. In the remaining space, the water starts to evaporate at a pressure of 123 mbar. The cold water column at 20°C, the atmospheric pressure (1000 mbar) is 977 cm high in equilibrium with the according vapor pressure of 23 mbar. If no heat exchange takes place, this situation remains unchanged and is thermodynamically in equilibrium.
Now, the two tops of both columns are connected with a vapor channel in Fig. 3. If they are connected, the two vapor chambers (123 mbar and 23 mbar) spontaneously equalize their pressure to an average pressure. As a result, the two water columns tend to have the same level on both sides. However, this connection causes an energetical imbalance of the physical conditions of the water surface on top of the columns. On the 50°C hot column, the vapor pressure of the media is higher than the average pressure. On the 20°C cold side, the average pressure is higher than the vapor pressure of the water. This situation leads to a spontaneous boiling on the hot side and a vapor condensation on the cooler side on the water surface. This process continuous until the temperature on both sides has been balanced out in both columns. After the temperature adaption, both pressures and levels in the chambers are equal.
Fig. 4: External media circulations for heating and cooling
As a consequence of this, it can be assumed that as long as a temperature difference in both columns is maintained, a spontaneous evaporation and condensation of the surface water takes place in order to achieve equilibrium temperature and pressure. To make this technically possible, an additional external circulation in Fig. 4 can supply heat on the evaporation side and extract heat on the condenser side. As the reaction velocity is strongly depending on the available water surface, a specially designed spraying system creates millions of small droplets. This huge internal water surface results in very high internal heat transfer rates between evaporator and condenser.
This principle also works if the useless bottom of the open water column is cut off and replaced by a lid as shown in Fig. 5. Experiments on the demonstration plant have shown that a pressure differential of only a few millibar (1 mbar corresponds to 1 cm water column) is sufficient to run this distillation process. It corresponds with very small temperature differentials of a few Kelvin.
Fig. 5: Closing the evaporation and condensing reactors
If the temperature spread between the heat source and condenser is large enough, the condenser can act as a heater for the following stage. This has the advantage that the condensation heat is re-used multiple times at different temperature/pressures increasing the energetic efficiency with each additional stage. Depending on the available temperature difference, it can be multiplied several times resulting in an increase of the distillation capacity with the same amount of available heat. The result is the creation of the multi-cascaded direct spray distillation, visualized in Fig. 6.
Fig. 6: Cascaded direct spray distillation
Plant design
Fig. 7: General flow diagram of the LTD process
The low temperature distillation process needs reactors for evaporation and condensation equipped with the spaying system to generate the droplets, and three standard plate heat exchangers (heating, cooling, thermal recovery). The feedwater and distillate are pumped in two large circulation streams through the reactors. The thermal recovery is realized in a heat exchanger preheating the feedwater by the distillate after condensation. Saturated brine and distillate are removed from the process by valve locks. The process and media flows are visualized in Fig. 7 in a general process scheme.[6]
The thermal energy (1) is supplied at the main heat exchanger (HEX 1) by any available media heating up the intake water up to 95°C. In the evaporator cycle (green), the hot water is sprayed and evaporated in pressure reduced chambers (2) and flows by gravity to the subsequent chambers with lowered temperature and pressure environment. The generated vapor (3) flows from the evaporator to the condenser in every stage where it condenses on the cooled droplets of the sprayed distillate.[6]
The heat exchanger for cooling (HEX 3) reduces the temperature of the distillate (4) before it is pumped to the condenser cycle. In the condenser cycle (5), the cooled distillate is pumped and sprayed into the pressure chambers to allow for vapor condensation from the evaporators on cooled droplets. During this process, the temperature and pressure increases from stage to stage. After the last condenser, the increased heat of the distillate is recovered in the heat exchanger for thermal recovery (HEX 2) preheating the evaporator cycle. After the condensation in the first reactor, the distillate is hotter compared to the brine of the last evaporator. This condensation heat is recovered in HEX 2 and is used for heating the evaporator cycle (6). It is beneficial for the energetic efficiency to design this heat exchanger as large as possible.[6]
In order to run the process, a vacuum system (7) extracts non-condensable gases (like ), in the chambers. In the connection duct to the vacuum pump, an optional heat exchanger (HEX 5) cools down the vapor to condense as much water as possible (8). The gained distillate is transferred after an optional heat recovery (9) out of the process. A post-treatment system can treat the distillate according to the desired requirements (remineralization). The brine is extracted at the evaporator cycle after the last evaporator stage (10). The over-saturation and precipitation of salts for zero-liquid discharge (ZLD) application requires an additional evaporator acting as crystallizer which is not shown in Fig. 7.[6]
Plant layout
The main components to fow temperature distillation plants are the pressure vessels and the spraying facilities. Further important components are an adapted instrumentation and controlling system as well as a vacuum system. A low temperature distillation plant has no membranes and no tube bundles, and consists of the following main elements:
Pressure vessels with unique pressure control system
Evaporator and condenser vessels are constructed for vacuum pressure conditions up to 20 and include the spraying installations for the evaporation/condensation reactors.[6]
For the energy supply of the process itself, only standard plate heat exchangers are installed. A low temperature distillation plant consists of one heat exchanger for the heat transfer from heat source into water and one for the heat transfer from distillate to the re-cooling media. A plant with several cascades has one additional heat exchanger for internal heat recovery (HEX 2) increasing the thermal efficiency of the plant.[7] Due to the flexibility of the low temperature distillation process, various arrangements are possible to adapt each plant to the given application. If only a small overall temperature spread or a limited heat source is available, internal flows can be adjusted for maximised internal heat recovery. Additional low-temperature heat sources such as solar collector systems can also be integrated.[7]
The media supply is mostly realized with standard centrifugal pumps. The process conditions favor a low NPSH construction in order to facilitate hot media leaving the system from vacuum conditions. Due to the lowered volume flows in small scale plants, the application of displacer pumps is recommended.
Comparison with other thermal desalination technologies
Low temperature distillation operates at low temperature and low pressure, similar to Multi-effect distillation (MED) and Multi-stage flash distillation (MSF).[8] While the process flow is similar to a MSF plant, the temperature and pressure dynamics are more comparable to a MED system.[7] It is designed use low grade or waste heat from other industrial processes or renewable sources, like solar thermal collectors.[9] The most significant difference compared to MED and MSF technologies is that there are no tube bundles heat exchangers within the pressure chambers. This permits enhancements [6] of the thermal distillation process:
Treatment of high saline or heavily polluted waste water, even with precipitation of solids
Optimized heat transfer and overall high thermal efficiency of the plant
No phase changes on solid surfaces, danger of scaling, fouling or clogging is prohibited
Reduced internal installations (mainly in the evaporator), resulting in lower material consumption
Tolerant to part load operation or fluctuating process conditions (power supply with renewable energy possible)
Fig. 8: Relative energy and costs depending on feed water salinity
Due to the relative high energy demand of thermal distillation processes for water treatment, low temperature distillation is most economically applicable for high saline feed waters. Fig. 8 compares the relative energy and plant costs in comparison with membrane-based desalination processes like reverse osmosis (RO) from sea water desalination. The possible feed waters may contain a wide range of impurities like brines from desalination plants, radioactive ground water, produced water from oil production, hydrocarbon polluted water, and high salinities up to 33% NaCl. The plant operates even under high concentrations up to the precipitation of anorganic compounds. Also, the effluent of existing sea water desalination plants can be treated further in a low temperature distillation to maximise the dewatering capacity of a desalination system.
Low temperature distillation can accommodate variations in the plant load, running efficiently from 50 – 100% of plant design capacity depending on the available heat supply. The spraying process is self-adjusting, and the amount of water produced is proportional to the amount of heat provided.
High saline feed water
The LTD process is most suitable for high saline feedwaters starting from typical concentrations of sea water to concentrated wastewater solutions from various industrial processes.[10] One possible application is the capacity duplication of RO based desalination systems by further treatment of the evolving effluents to the precipitation of salts. Brackish water desalination is principally also possible, but other desalination processes tend to be more economical due to low osmotic pressure and resulting low specific energy consumption.[6]
Scaling and fouling
Low temperature distillation plants are not prone to scaling or clogging even with very high TDS in the feed water. There are no installations within the pressure vessels that could scale. Phase changes (evaporation and condensation) only take place on the surface of the water droplets, never on solid surfaces. The following design features ensure the minimal risk of scaling within the plant:
The LTD pressure vessels (evaporators and condensers) droplets are in free fall. The evaporation and condensation happen directly on the surface of the droplets during the residence time inside the reactor (less than one second).
The plant controls avoid that the concentration of dissolved solids in the evaporator cycle never reaches the point of precipitation. In a specially designed high saline loop (crystallizer), the brine from the evaporator cycle is further concentrated until solids are precipitating. All sedimentations of solids are continuously extracted from this loop.
There is no phase change in the standard plate heat exchangers. Within the main heat exchanger all thermal energy is transferred to the internal water streams. Therefore, there never is an operational risk on the heat supply side.
Low temperature distillation plants are able to treat feed waters such as:
High saline brine from any other desalination plant, e.g. from inland desalination via BWRO plants, that would otherwise not receive a permission of operation because of the lack of a brine disposal solution
Produced/fracking water from oil or gas fields with salinities of up to 300.000
Industrial waste waters from textile production and drying
Desalinated water and brine
The desalinated water quality from the low temperature distillation process is almost demineralized water with a remaining salinity of 10 ppm. Residual contaminants result from demister losses and depend on the treated feedwater as well as vapor velocities between evaporator and condenser. The brine concentration in the LTD process can be adjusted to the site conditions and disposal options. Current research focuses on selective crystallization to recover various salt species beyond NaCl.
Specific data and information
Specific energy consumption (energy/m³ output), electricity: 0.8 to 2.5 for internal pumping and continuous non-condensable gas removal [8][9][10]
Specific energy consumption (energy/m³ output), thermal: 80 to 200 , depending on the number of stages and the temperature of the heat source. The use of low grade heat at 45 – 95°C is possible.[7][8][9][10]
Recovery rate, concentration factor: LTD converts up and into precipitation for liquid brine discharge or into precipitation for ZLD (wet), where water and solids are completely separated. For seawater (4% salinity), this results in a water extraction from feed (96% water and 4% NaCl) of 98.95% for ZLD (wet), and for liquid brine disposal a water extraction of 91.6%.
Chemical additive consumption: No chemical additives are required for common seawater, as well as brine from desalination processes like RO/MSF/MED.
Personnel intensity: LTD is automated. Periodic checks and maintenance are according to industrial practice.
Replacements (e.g. membrane replacements): There are no membranes or filters to replace. Low pressure pumps, valves, and gaskets need to be maintained according to industry practice.
Preferred use
The application of the LTD process becomes economically feasible starting with salinities more than 4%. LTD can be useful for normal seawater desalination if high recovery rates or further treatment of the RO brine are required. High saline effluents from industrial processes such as the oil and gas industry, the textile industry, and the chemical industry are more advantageous. In general, pretreatment for zero-liquid discharge systems with LTD is the most economical option. The treatment of brackish water is possible in principle, but the energy consumption required for evaporation is higher compared to conventional reverse osmosis.
Environmental impact
Due to the reduction of the brine volume, the environmental impacts are significantly lowered compared to standard seawater RO units. The recovery of NaCl in high purity is possible and can be used e.g. as regenerative salt for ion exchangers or water softeners.
The LTD process has a stable part-load behaviour which facilitates the use of renewable energy sources. Thermal energy can be supplied by solar collectors like flat plate or evacuated tube, solar ponds, concentrating solar collectors, or in co-generation with solar power plants.[7][8][9][10]
Further developments
Opportunities for improvement focus mainly on integration in an appropriate operating environment with heat management. The combination of LTD plants with thermal power plants as heat sources seems advantageous. Combinations with other desalination processes, like thermal or mechanical vapor compression (MVC) are also possible. Under certain process conditions, such systems can compensate for fluctuating heat supply by substituting electric power in an integrated MVC unit.
Current research focuses on the reduction of the heat and electricity consumption of auxiliary systems. The selective crystallization of the brine and recovery of salts are also being researched (in cooperation with TU Berlin, Germany). Further development potential lies in the integration of adsorption and absorption technologies for integrated cooling and desalination.
Related Research Articles
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heating of solid materials to produce gaseous products ; this may involve chemical changes such as destructive distillation or cracking. Distillation may result in essentially complete separation, or it may be a partial separation that increases the concentration of selected components; in either case, the process exploits differences in the relative volatility of the mixture's components. In industrial applications, distillation is a unit operation of practically universal importance, but is a physical separation process, not a chemical reaction. An installation used for distillation, especially of distilled beverages, is a distillery. Distillation includes the following applications:
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to liquid water when in contact with a liquid or solid surface or cloud condensation nuclei within the atmosphere. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition.
Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltwater is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water resources.
Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Current MSF facilities may have as many as 30 stages.
Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic).
An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. 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.
The solar humidification–dehumidification method (HDH) is a thermal water desalination method. It is based on evaporation of sea water or brackish water and subsequent condensation of the generated humid air, mostly at ambient pressure. This process mimics the natural water cycle, but over a much shorter time frame.
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.
Dewvaporation is a novel desalination technology developed at Arizona State University (Tempe) as an energy efficient tool for freshwater procurement and saline waste stream management. The system has relatively low installation costs and low operation and maintenance requirements.
An evaporator is a device used to turn a liquid into a gas.
A solar-powered desalination unit produces potable water from saline water through direct or indirect methods of desalination powered by sunlight. Solar energy is the most promising renewable energy source due to its ability to drive the more popular thermal desalination systems directly through solar collectors and to drive physical and chemical desalination systems indirectly through photovoltaic cells.
In chemistry, a condenser is laboratory apparatus used to condense vapors – that is, turn them into liquids – by cooling them down.
In systems involving heat transfer, a condenser is a heat exchanger used to condense a gaseous substance into a liquid state through cooling. In so doing, the latent heat is released by the substance and transferred to the surrounding environment. Condensers are used for efficient heat rejection in many industrial systems. Condensers can be made according to numerous designs, and come in many sizes ranging from rather small (hand-held) to very large. For example, a refrigerator uses a condenser to get rid of heat extracted from the interior of the unit to the outside air.
A seawater greenhouse is a greenhouse structure that enables the growth of crops and the production of fresh water in arid regions which constitute about one third of the earth's land area. This in response to the global water scarcity and peak water and the salt-infecting soil. The system uses seawater and solar energy. It uses a similar structure to the pad-and-fan greenhouse, but with additional evaporators and condensers. The seawater is pumped into the greenhouse to create a cool and humid environment, the optimal conditions for the cultivation of temperate crops. The freshwater is produced in a condensed state created by the solar desalination principle, which removes salt and impurities. Finally, the remaining humidified air is expelled from the greenhouse and used to improve growing conditions for outdoor plants.
Multiple-effect distillation or multi-effect distillation (MED) is a distillation process often used for sea water desalination. It consists of multiple stages or "effects". In each stage the feed water is heated by steam in tubes, usually by spraying saline water onto them. Some of the water evaporates, and this steam flows into the tubes of the next stage (effect), heating and evaporating more water. Each stage essentially reuses the energy from the previous stage, with successively lower temperatures and pressures after each one. There are different configurations, such as forward-feed, backward-feed, etc. Additionally, between stages this steam uses some heat to preheat incoming saline water.
Vapor-compression desalination (VC) refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapor.
Pumpable icetechnology (PIT) uses thin liquids, with the cooling capacity of ice. Pumpable ice is typically a slurry of ice crystals or particles ranging from 5 micrometers to 1 cm in diameter and transported in brine, seawater, food liquid, or gas bubbles of air, ozone, or carbon dioxide.
Membrane distillation (MD) is a thermally driven separation process in which separation is driven by phase change. A hydrophobic membrane presents a barrier for the liquid phase, allowing the vapour phase to pass through the membrane's pores. The driving force of the process is a partial vapour pressure difference commonly triggered by a temperature difference.
Zero Liquid Discharge(ZLD) is a water treatment process designed to remove liquid waste from a system. The focus of ZLD is to reduce wastewater economically and produce clean water that is suitable for reuse (e.g. irrigation), thereby saving money and benefitting the environment. ZLD systems employ advanced wastewater/desalination treatment technologies to purify and recycle virtually all of the wastewater produced.
A concentrated solar still is a system that uses the same quantity of solar heat input as a simple solar still but can produce a volume of freshwater that is many times greater. While a simple solar still is a way of distilling water by using the heat of the sun to drive evaporation from a water source and ambient air to cool a condenser film, a concentrated solar still uses a concentrated solar thermal collector to concentrate solar heat and deliver it to a multi-effect evaporation process for distillation, thus increasing the natural rate of evaporation. The concentrated solar still is capable of large-scale water production in areas with plentiful solar energy.
References
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1 2 F. Hug. Niederdruckdestillation - Untersuchung der Einflussfaktoren. Diplomathesis, University of Applied Sciences, Northwestern Switzerland, 2005.
1 2 S. Meier and D. Altdorfer. Niedertemperaturdestillation im fremdgasbefreiten Raum. Diplomathesis, University of Applied Sciences, Northwestern Switzerland, 2000.
1 2 S. Martin and L. Treier. Messungen und Simulationen zur Niedertemperaturdestillation und Aerosoltransport. Diplomathesis, University of Applied Sciences, Northwestern Switzerland, 2002.
1 2 G. Kotrle. Niedertemperaturdestillation - Messungen und Simulationen. Diplomathesis, University of Applied Sciences, Northwestern Switzerland, 2003.
1 2 3 4 5 6 7 Low Temperature Distillation, General plant and process description, Watersolutions AG, Mark Lehmann, Buchs, Switzerland, 2012
1 2 3 4 5 J. Wellmann, Conceptual design of a concentrating solar power plant for a combined electricity and water supply of the city El Gouna, PhD thesis, Technische Universität Berlin, Institut für Energietechnik, Berlin, 2015, ISBN978-3-945682-21-0
1 2 3 4 J. Wellmann, K. Neuhäuser, M. Lehmann, F. Behrendt, Modeling the cogeneration of power and water with CSP and low temperature desalination, Desertec Best Paper Award, Berlin, 2012
1 2 3 4 J. Wellmann, K. Neuhäuser, M. Lehmann, F. Behrendt, Modeling an innovative low-temperature desalination system with integrated cogeneration in a concentrating solar power plant, Desalination and Water Treatment 1-9, 2014
1 2 3 4 J. Wellmann, B. Meyer-Kahlen, T. Morosuk, Exergoeconomic evaluation of a CSP plant in combination with a desalination unit, Renewable Energy 1-17, 2017
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