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Direct Spray Distillation (DSD)
The Direct Spray Distillation is a water treatment process applied in seawater desalination, brine and concentrate treatment as well as zero-liquid discharge systems. It is a physical water separation process driven by thermal energy. This article explains the physical principles, plant layout, design and possible feedwater characteristics.

Technology Description
The DSD (or also called Low Temperature Distillation (LTDis)) is a thermal distillation process over several stages and is powered by a temperature difference between heat and cooling source of at least 5 Kelvin for one stage. Two separate volume flows, a hot evaporator flow and a cool condenser flow, with different temperatures and vapour pressures, are sprayed in a combined pressure chamber, where non-condensable gases are removed. As the vapour strive for a pressure equilibrium, part of the water from the hot stream evaporates. The vapour is transferred to the cooler stream where it condenses on the surface of the sprayed cooled distillate droplets and not on tube bundle heat exchangers. 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 DSD process is tolerant to high salinities, other impurities and fluctuating feed water qualities. The precipitation of solids is technically intended in order to allow for zero-liquid-discharge operation (complete ZLD). It is possible to combine the DSD process with existing desalination technologies serving as downstream process to increase the water output and reduce the brine generation.

The development of the DSD process reaches back to several scientific works focusing on low temperature distillation in vacuum conditions and the condensation on cooled droplets which have been created in the University of Applied Sciences in Northwestern Switzerland in 2000 to 2005. 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.

Physikal principle
The following figures show and explain the thermodynamic principle after which the LTDis 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 vapour pressure of the water is 123 mbar for 50°C and 23 mbar for 20°C. We suppose that the two cylinders are 10 meters long and allow to be pulled out the same distance.



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, the 1 bar 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 the equilibrium.



Now, we connect the two tops of both columns with a vapor duct 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.



Now we can conclude, 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 wise. To make this technically possible, an additional external circulation in Fig. 4 can supply heat (EIN) on the evaporation side and extract heat (EOUT) on the condenser side. As the reaction velocity is strongly depending on the available water surface, a special designed spraying system creates millions of small droplets allowing for very high internal heat transfer rates.



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.



If the temperature spread between the heat source and condenser is big enough, the condenser can act as a heater for the following stage. This has the advantage, that the condensation heat is used multiple times at different temperature/pressures increasing the energetic efficiency. 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 low temperature distillation.



Comparison with other thermal desalination technologies
The DSD process is a low temperature and low-pressure distillation system similar to MED (Multi-effect distillation) and MSF (Multi-stage flash distillation). While the process flow is similar to a MSF plant, the temperature and pressure dynamics is more comparable to a MED system. It is designed as a highly efficient desalination solution using low grade or waste heat from other industrial processes or renewable sources, like solar-thermal collectors, to produce fresh water. The most significant difference compared to MED and MSF technologies is that there are no tube bundles or heat exchangers within the pressure chambers. This unique advantage ensures some major enhancements of the thermal distillation process:


 * LTDis can treat high saline or polluted water even under precipitation of solids
 * Very efficient heat transfer and thus overall high thermal efficiency of the plant
 * No phase change on solid surfaces, the plant is not prone to scaling or clogging
 * Very reduced inner parts of the plant (internal installations mainly in the evaporator), lower material consumption resulting in lower plant price
 * The plant is very tolerant to part load operation or fluctuating process conditions

The classification of the most important mechanisms of the LTDis process is visualized in Tab. 1. It is able to treat feed waters with a wide range of impurities (e.g. radioactive ground water, produced water, hydrocarbon polluted water) and high salinities up to 330’000 ppm TDS (e.g. sea water, RO brine). The plant operates even under high concentrations up to the precipitation of certain solids. Also, the effluent of existing sea water desalination plants can be treated further in a LTDis plant to maximise the dewatering capacity of a desalination system.

LTDis can also accommodate variations in the plant load, running efficiently from 20 – 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.

Plant design
The DSD/LTDis 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.



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.

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.

In order to run the process, a vacuum system (7) extracts non-condensable gases (like CO_2, N_2, O_2) 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.

Layout of the DSD plant
The simple and modular plant design of LTDis, using non-corroding materials and durable standard components, results in a speedy installation and moderate investment costs. Main components of the LTDis 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. An LTDis plant has no membranes and no tube bundles and consists of the following main elements:


 * Pressure vessels with unique pressure control system
 * Piping in PP-plastics or fibre reinforced plastics (FRP)
 * External heat exchanger (standard component)
 * Water circulation pumps (standard component)
 * Process control system (control panel)



Plant components
Evaporator and condenser vessels are constructed for vacuum pressure conditions up to 20 mbar_{abs} and include the spraying installations for the evaporation/condensation reactors. For monitoring and regulating the plant, several pressure, temperature, level and conductivity sensors need to be installed in the reactor vessels. The plant itself can be pre-assembled in standard containers for shipping. An exemplary plant layout for a four-stage unit is visualized in Fig. 8. Here, the evaporators and condensers as well as the three heat exchangers can be seen. The latest research and development has shown that a vertical alignment of the reactors can save valuable energy for pumping and improves the dynamics of the plant.

For the energy supply of the process itself, only standard plate heat exchangers are installed. Thus, maintenance and cleaning are very simple. Every DSD 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. Due to the flexibility of the DSD process, various process 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.

The media supply is mostly realized with standard centrifugal pumps. The process conditions favor a low NPSH contruction 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.

Feedwater
The DSD process is most suitable for high saline feedwaters starting from typical concentrations of sea water to concentrated wastewater solutions from various industrial processes. One possible application is the capacity duplication of RO based desalination systems by further treatment of the evolving brines to the precipitation of salts. Brackish water desalination is principally also possible, but other desalination processes tend to be more economically due to low osmotic pressure and resulting low specific energy consumption.