Flow control in a novel heavy water generation process
What is heavy water?
Isotopologues are molecules which have the same chemical formula and whose atoms bond in similar arrangements, but have one or more atoms with a different number of neutrons than the parent molecule. Deuterium oxide, chemical formula D2O, more commonly referred to as heavy water, is a valuable water isotopologue in nuclear reactors, medicine, and spectroscopy.
In nuclear reactors, heavy water is used to moderate neutrons, slowing them down to speeds at which fission can occur. Nuclear reactor grade deuterium oxide typically exceeds 99.98% isotopic purity. In medicine, heavy water is used as a radiotracer to visualize and measure changes in physiological activities including blood flow, chemical adsorption, metabolism, and more. In spectroscopy, deuterium oxide is used as the solvent instead of regular water if the nuclide of interest is hydrogen as regular water interferes with the signal from hydrogen solute molecules of interest.
Compared to regular water, heavy water has a density which is 10.6 times larger as well as a higher viscosity. Additionally, as an isotopic form of water which is always stable and non-radioactive, deuterium oxide is non-toxic and safe for human consumption in small amounts.
Due to these unique properties and uses, heavy water’s demand is high. Since ordinary water only contains one part of heavy water per 6000 parts ordinary water by weight and is difficult to separate, traditional heavy water production is energy intensive and costly. As a result, the price of heavy water can be as high as $0.5-$1 per Kg based on desired purity.
A novel heavy water production method
Traditionally heavy water is produced from electrolysis, chemical exchanges, or fractional distillation of regular water. The most cost-effective traditional process for producing heavy water is the dual temperature exchange sulfide process referred to as the Girdler sulfide process requiring a supply of hydrogen sulfide and operating temperatures (in one column) of 130 C.
However, recently a group of researchers at Kyoto University’s Institute for Cell-Material Sciences (iCeMS) discovered a much less energy intensive method to produce deuterium oxide in room temperature operating conditions.
In their process, two porous coordination polymers (PCPs), or porous crystalline materials made with metal nodes and organic links provide a diffusion-regulatory functionality. Undergoing moderate temperature changes, H2O vapor is adsorbed into the PCPs at a faster rate than D2O, allowing for D2O to be easily separated from it.
Steps to heavy water production
- In the research configuration, helium gas is bubbled into the starting mixture of D2O and H2O at 298K at a rate of 10 SCCM at ambient pressure, allowing for water vapor to be analyzed with a mass detector to determine the beginning ratio of heavy to normal water.
- After this step, the PCPs are activated in a vacuum chamber at 393 K for 10 hours, removing any residual water in the PCPs.
- In the next step, the temperature is decreased to 298 K and He flow sparges the water into vapor.
- After this, the bubbling stops and He flows to remove water vapor to carry to the mass detector to confirm the separation of heavy water from the normal water.
Flow regulation for novel heavy water production method
High levels of accuracy and repeatability are required in the above process in order to achieve the 10 SCCM flow rate used in order to correctly confirm ratios of normal to heavy water before, during, and after the process. Additionally, due to the prevalence of water vapor, an anti-corrosive mass flow controller is ideal. As a result, a high accuracy and repeatability, low flow, mass flow controller is preferred. Alicat’s MCS-Series and CODA KC-Series can each provide optimal customizable solutions, including features and specs such as:
- 0.5 SCCM full scale to 5,000 SLPM full scale with a turndown of 1% – 100% of full scale
- NIST-traceable accuracy up to ±0.4% of reading or ±0.2% of full scale
- Repeatability ±0.2% of full scale
- Control response times as fast as 30 ms
- 40 g/h full scale to 100 kg/h full scale with a turndown of 2% – 100% of full scale
- NIST-traceable gas accuracy up to ±0.5% of reading or ±0.05% of full scale, whichever is greater
- Repeatability ±0.05% of reading + 0.025% of full scale
- Control response times as fast as 500 ms