Xzero Technology
Process
The Xzero process consists of four basic steps: degassing, membrane distillation, polishing and concentration. First, volatiles are removed from the feed by degassing. Then, non-volatile contaminants are removed from the feed through membrane distillation (MD). The permeate is then polished before use as UPW and the brine is concentrated to solid waste.
After simple pre-treatment, the water goes to the degasser. In the degassing step, the exposure area of the pre-heated feed water is optimized which allows volatiles to escape through evaporation. The heated water, which has to be at a temperature lower than boiling point, is then guided along a hydrophobic microporous membrane in a cassette. On the other side of the membrane, within the cassette, there is a cooling surface. The temperature gradient between the hot water and the cooling surface creates a vapor pressure differential that moves the vapor within a milieu of ambient pressure from the warm side to the cool side where it condenses. Thus the primary loop is completed. The condensed water is fed to a secondary loop (a polishing loop) in order to be kept absolutely pure. To keep feed water concentration at allowable levels, part of the brine is continuously tapped to be concentrated so that solid residue can be disposed of. The water from the concentrator is fed back into the primary loop.
The membrane
The membranes have more than one million pores per square centimeter. The pores are 20,000 times smaller than a droplet of water, but 700 times larger than a molecule of water. The membrane should be made of hydrophobic, i.e. water-repellent material. Vapor, however, will leave the surface of the water, pass through the membrane and be condensed on the other side. At the same time, the surface tension of the water keeps all types of organic and non-organic non-volatile components in the non-vaporized part of the water, which is then recirculated.
Requirements for the membranes and the process through the membranes is that no capillary condensation should take place inside the pores of the membrane, that only vapor should be transported through the pores, that the membrane must not alter the vapor-liquid equilibrium of the different components in the process liquids, that at least one side of the membrane should be in microscopic proximity with process water and that, for each component, the driving force is a partial pressure gradient in the vapor phase.
Because of the low temperature and ambient pressure, the vaporization does not create bubbles and no impurities or droplets will accompany the vapor (there is no entrainment) through the membrane. The permeate will thus be virtually free from organic and non-organic particles and also from dissolved ions. The permeate is thus not only purer than what can be achieved with the best of other filter technologies but also several times purer than with conventional distillation.
Cassettes, modules and systems
A large exposure surface is required in a confined space. This is achieved by means of finely engineered cassettes that are made up of membranes, cooling surfaces, and frames with capillary systems for separated movement of feed, coolant and purified water allowing a modular structure wherein the capacity can be increased through use of numbers of cassettes in a module. The modules are then assembled into a system.
A full system will contain: degassers, modules, polishing loop, brine concentrator, energy recovery and an assortment of ancillary components such as piping, pumps, tanks, valves and instrumentation.