A critical review of Adsorption Desalination

Abstract

Water is the most dominant substance on the planet and has always been a synonym for life. On this planet, water has always been plentiful. Therefore, the conclusion of the United Nations in 2015 was even more shocking. The United Nations estimated that in 2030 a water gap would exist of 40%. The withdrawal of fresh water would be 40% larger than the amount that is replenished every year. We are living on borrowed time, and we need to close this gap. Since most water on the earth is saline, desalination could be a possible solution. Unfortunately, desalination is an energy-intensive process, and it will always be. There are different methods. However, energy consumption would always be rather large. Much of our energy is still created by fossil fuels in most areas in the world. Our increasing demand for fresh water can lead to a further large fossil footprint and therefore more climate change. Climate change negatively impacts our water supplies. A negative spiral could be the result.
The world is consuming a large amount of energy. Unfortunately, this is not done efficiently. It is estimated that 72% of worldwide energy consumption is lost after conversion. Waste heat is an unwanted product as a result of this inefficient conversion. It is estimated that 63% of this waste heat has a temperature of 100 degrees Celcius or lower. A temperature that is not even sufficient to boil an egg, but it is energy nonetheless. If this energy could be harvested, the water gap could be closed by desalination without increasing the carbon footprint. However, how to do this? A solution could be found in Japan and your dry cleaner. In the seventies, when the oil crisis was at its peak, Japan had a problem. It has a high need for air conditioning which has a high need for energy. A device was created that could use waste heat to create cooling with silica gel. The general public knows silica gel as moist eaters that are included in your dry cleaning. By placing silica gel in a closed environment, liquid water could be evaporated. Evaporation needs energy, and this could create cooling. In early 2000, some folks in Singapore realised that this technology could also be used to desalinate. Adsorption Desalination was born. Adsorption desalination (also known as AD) is a boiling point elevation and vapour pressure lowering desalination technique. Silica gel has a high affinity to adsorb water vapour. Since this is done in a closed environment, the pressure is lowered till boiling. When an energy source is placed inside the liquid, an equilibrium can be found without lowering the pressure further. Once the silica gel is saturated, it is heated till a certain temperature that it starts to desorb the vapour again. This vapour is collected and condensed into a clean product. The mass transport takes place by a pressure difference. Since the evaporation takes place in vacuum conditions, it is around room temperature. Therefore it can have a low sensitivity for erosion. Since it is an evaporation technique, it also has a low sensitivity for fouling. On paper, it can be a desalination technology with low operational costs compared to others. In reality, a couple of barriers have to be conquered. One of the barriers is the characteristics of the core element of this technology, the silica gel. The reason that silica gel can adsorb water vapour so well is due to two main elements. First, it is an incompletely dehydrated polymeric structure of colloidal silica acids. Therefore it can create hydrogen bonds, polar bonds and weak electron bonds with other molecules. Water is a bipolar molecule, and therefore silica gel can adsorb water exceptionally well. It can create hydron bonds efficiently, and due to the bipolar nature of water, multiplayer on top of each other can be formed. The second element is the high specific surface of silica gel. It has many pores, and therefore a specific surface equal to human lungs can be formed. In the end, the best qualities can adsorb until 40% of their body weight.
These two elements determine the maximum amount of vapour silica gel can adsorb. However, two parameters determine if silica gel can adsorb. These two parameters are the temperature and vapour pressure. If the temperature is high, the vapour pressure has also to be high to ensure adsorption and the other way around. The same account for the opposite effect. Therefore energy transfer is vital for smooth operation. Unfortunately, the thermal conductivity of silica gel is extremely low. Besides energy transfer to change the temperature of silica gel to go from adsorping to desorbing and reverse, much energy has to be transported during the adsorption and desorption phase. During adsorption, water vapour transforms from a gas phase to a semi-solid phase. In this phase, the energy level is lower. Since energy cannot be destroyed, this energy has to be removed. The same applies to the opposite. If the heat exchanger is not well designed, the adsorption and desorption phase are prolonged, and the daily water production diminished. Even if the whole set-up is well designed, there is still one element of silica gel that can also diminish the water production of the set-up: the degradation of silica gel. Multiple mechanisms can degrade silica gel, but the two most important are fragmentation and pore pollutions by metal ions. Fragmentation (the breaking up into smaller parts) occurs by the high thermal stress the silica gel endures and the pollutions by the feed. A part of the sorption ability is destroyed or temporarily inhibited. The use of acid water can remove the pollution, so maintenance on the silica gel is necessary. Replacing the silica gel by other sorbents like zeolite is possible. Unfortunately, it has a lower adsorption capacity which leads to a less compact device and is, therefore, more expensive. Another unfortunate fact is that zeolite desorbs at a higher temperature. When using zeolite, adsorption desalination can no longer use waste heat of low quality to desalinate. The last problem with using silica gel, but also other sorbents, are the instabilities with the sorption characteristics in vacuum conditions. The sorption characters depend as said before on the temperature and the vapour pressure. Inside vacuum conditions, a disturbance of the vapour pressure can negatively influence the sorption characteristics and can lead to desorption at the wrong moment. Since it is done in a closed environment, it can lead to condensation which lowers the vapour pressure further. A complete breakdown of performance can be the result. Disturbances can happen through air leakage, which is are a common problem in industries. There are some solutions, but in the end, the permanent one is a high-quality vacuum system and intensive maintenance. Even with these barriers, adsorption desalination is still an attractive technology due to its relationship with its own energy consumption pattern. The energy consumption is high but stable and barely sensitive to change in conditions like feed salinity and recovery. Its energy consumption stays around the 40 $kWh/m^3$ in most cases. Only a small part, around 1.38 $kWh/m^3$ or even less has to be mechanical energy. All others can come from waste heat. If the other operating costs are kept low, it is an exciting technology to use, especially for brine treatment since waste heat is an inexpensive energy source. Some researcher claim it is a free energy source, but that statement cannot be valid. To use waste heat, infrastructure is necessary to collect it and transport it. It is not the same as putting a plug in the wall. There is probably enough waste heat in the world to close the water gap. As an example, the Netherlands is the second biggest producers of waste heat in Europe and creates enough to produce probably more 850 $km^3$ potable water. Unfortunately, there are some uncertainties. Waste heat is only classified in three rough categories, low, medium or high. Low is everything below 100 degrees Celcius. It could be that most energy is trapped in waste heat with a temperature of 30 degrees. More validation is necessary since the future of adsorption desalination is intertwined with waste heat. Without waste heat, it cannot be an inexpensive desalination technique. Since it is still an experimental technology, not a lot of data is available to create a solid picture of the costs involved with adsorption desalination. Only two studies exist today, and both estimate that the cost for seawater desalination using adsorption desalination is between 50% and 70 % of the cost while using reverse osmosis. Only one study gives insight into the different sectors of cost. What is notable is that the other costs, replacement cost for parts and chemical cost for adsorption desalination are kept at zero. A strange conclusion since silica gel can degrade and replacement can be necessary, chemicals are necessary for the heating and cooling water AD uses. No conclusion can be made about the final costs of AD, but it the conclusion of these studies seems unlikely. For seawater desalination, the costs are probably in the same range as reverse osmosis. Furthermore, there is a strong case that AD can remove biological pollutants since it is an evaporation technique where temperatures are reached until 80 degrees Celcius. There are a few remarks and only solid empirical data can prove these claims. The original intent of this research was to research these claims. Unfortunately, the set-up was not functioning properly due to practical problems like air leakages. Even with this result, much knowledge was gained, and a few remarks and conclusions can be made. To conclude, adsorption desalination is a newly emerging technology with many interesting aspects. It can provide an alternative for standard desalination techniques while being sustainable. There are a few barriers to overcome, but it deserves attention. Further developments should be stimulated since the water should be close in an environmentally friendly matter. Adsorption desalination is a sustainable solution that should always be considered.