Heavy water production with geothermal steam

Abstract

As the separation of heavy water from ordinary water is a process which is highly energy consuming it is in some respects the ideal chemical process for utilizing geothermal steam as process heat and, conversely, cheap geothermal steam is an ideal energy source for the process. A calculation has been made of the manufacturing cost of heavy water by the H2S/H2O ion exchange method with geothermal steam supplying the process heat. The results of this calculation show that heavy water produced in such a manner would at present be 10–15% less costly than heavy water produced with heat derived from natural gas or from steam economically available as exhaust steam from turbines, which is the cheapest way of producing heavy water. The characteristics of geothermal steam cause modifications in the optimization of the heat recovery system of a heavy water plant, namely that low cost leads to a lesser optimal degree of heat recovery and the price vs temperature characteristics of the steam make it economical to split the heaters into low and high temperature sections. The relatively low pressures of geothermal steam make it necessary to strip the H2S from the waste water at a lower pressure than the main process pressure. This in turn leads to a modification of the process and the introduction of an absorber in the feed water stream where the H2S stripped from the waste water is returned to the system so as to minimize compression work. The cost of heat delivered to the process will be further reduced by a factor of two if the water accompanying the steam from the borehole can also be used, but this is as yet uncertain because silica and other solids in the hot water may lead to scaling in the heat exchangers.