PX for RO

Attended a seminar yesterday on Sea Water Desalination, jointly organized by Energy Recovery Inc (ERI) and Dow Water and Process Solutions. Seawater desalination is very energy intensive and to bring down the energy cost, the hydraulic energy of high pressure rejects has to be recovered. This is conventionally done through a turbine (Francis Turbine or Pelton Wheel) that is directly coupled to the feed pump. But the inefficiencies of turbine and pump limit the energy recovery to approximately 65%. ERI has introduced a revolutionary (literally!) device called the PX Exchanger with claims of 98% energy recovery. This consists of a spinning cartridge made from ceramic, where momentum of the high pressure reject fluid is transferred to the low pressure feed through actual contact. The contact time is however so small, of the order of 0.05 seconds, that the increase in salinity of the feed due to mixing with the concentrated rejects is only to the extent of 2.5%., which corresponds to an osmotic pressure increase of 1.3 bar. Since this increase in salinity is a function of the recovery in the RO module, ERI needs to work in close co-operation with the RO solution provider. The ERI website has a Power Model to carry out this preliminary calculation. PX devices are available in modular constructions with capacities ranging from 4.5 to 50 m3/hr. Adopting PX for energy recovery, the power required for seawater RO can be brought down to 3.5 KWH/m3.

What is Process Intensification?

Currently browsing through a book on Process Intensification authored by David Reay, Colin Ramshaw and Adam Harvey. It has a delightful definition of Process Intensification - It gives every molecule the same processing experience. And the URL of my blog is validated!

The Future Tense of Energy

On 5th June, World Environment Day, I gave a plenary lecture at CHEM09, the 2-day Kerala State Chemical Engineers Conference at Kochi. Organized by the Kochi Regional Centre of Indian Institute of Chemical Engineers. theme of the conference was Clean Air, Clean Water, Green Earth. I titled my talk: Energy Scenario – Turning Crisis into Opportunities. Here is a refined summary of a part of the lecture.

• Fossil fuels are a source of concentrated energy resulting from the photosynthesis efforts of thousands of millions of years and are irreplaceable in the true sense by any other conceivable alternative. In a mere couple of hundred years we have frittered away the energy accumulated over millions of years.
• Solar energy is the mother of all forms of energy and finally it is towards the mother we should look for our sustenance. Only a small fraction of sunlight hits our planet and even this is enough to meet our energy requirement few thousands of times over. The key is to improve the efficiency of photovoltaic cells (doubling from the present levels of 20%) so as to rapidly bring down the cost of solar energy.
• Among all the wonderful promises held out by nanotechnology, are advances in material science that can revolutionize photovoltaic cells through nanostructures and coatings of nano-particles.
• Bio-fuels can never fully replace fossil fuels and are not sustainable in the long run. The power density of biomass is less than 1 w/m2 compared to 100 w/m2 for fossil fuels. The land and water required to produce adequate biomass is enormous. The first charge on land and water should be for growing crops for food and not fuel.
• Coal meets 26% of the world’s primary energy needs and accounts for 41% of the world’s power generation, yet the perception of energy crisis is driven by oil price. The estimated reserves of coal at today’s consumption will last 130 years, more than 3 times longer than crude oil. Coal also has a better geographical spread unlike crude oil.
• As oil wells rapidly deplete, we will increasingly depend on coal and biomass during a short transition phase leading us to the age of solar energy in about 50-100 years.
• We are conditioned to think of coal as dirty. Coal emits more carbon dioxide than oil and gas and also leaves behind a residue that is problematic to dispose off. Integrated Gasification Combined Cycle (IGCC) and Underground Coal Gasification (UCG) are technologies which will allow us to use coal in a clean and efficient manner. IGCC is riddled with reliability issues due to short refractory life in Gasifiers, which operate at temperatures upto 1400 degrees C.
• Modern civilization is literally driven by liquid fuels, since more than 2/3 of it finds use in the transportation sector. This will put pressure to keep alive wasteful processes like coal to liquid, gas to liquid and production of liquid biofuels.
• Ethanol blends for gasoline, even at a modest 10% level cannot be sustained with corn or cane. Cellulose ethanol yields 80% more energy than that is required to grow and convert it, whereas Starch ethanol yields only 30% more. A big breakthrough in cellulosic ethanol production is only a few years away.
• In the long run hydrogen based fuel cells will drive our vehicles. But a hydrogen economy is possible only if the gas is derived by means of solar or nuclear energy or the miracle microbe that promises to split water.
• Algae is touted as the wonder fuel of the future because of its high photosynthesis efficiency and its ability to grow in saline water. This would ease the pressure on land and water for biomass cultivation. We need to mimic this nature’s model for efficient capture and storage of sunlight.