1. Field of the Invention
The present invention is directed to distillation. It has particular, but not exclusive, application to using rotary heat exchangers to purify water by distillation.
2. Background Information
One of the most effective techniques for purifying water is to distill it. In distillation, the water to be purified is heated to the point at which it evaporates, and the resultant vapor is then condensed. Since the vapor leaves almost all impurities behind in the input, feed water, the condensate that results is typically of a purity much higher in most respects than the output of most competing purification technologies.
One of the distillation approaches to which the invention to be described below may be applied employs a rotary heat exchanger. Water to be purified is introduced to one, evaporation set of heat-exchange surfaces, from which the liquid absorbs heat and evaporates. The resultant water vapor is then typically compressed and brought into contact with another, condensation set of heat-exchange surfaces that are in thermal communication with the set of evaporation heat-exchange surfaces. Since the water vapor on the condensation side is under greater vapor pressure than the water on the evaporation side, vapor that condenses on the condensation side will be hotter than the evaporating liquid on the evaporation side, and its heat of evaporization will therefore flow to the evaporation side: the system reclaims the heat of evaporization used to remove the relatively pure vapor from the contaminated liquid. To minimize the insulating effects to which a condensation film on the condensation surfaces would tend to contribute, a rotary heat exchanger""s heat-exchange surfaces rotate rapidly, so the condensate experiences high centrifugal force and is therefore removed rapidly from the condensation surfaces.
This removal of liquid from the condensation-side heat-exchange surfaces is important, because a significant drawback of using distillation for water purification is the energy cost that it exacts. That cost tends to be greater when the temperature difference between the rotary heat exchanger""s evaporation and condensation sides is relatively great. On the other end, a low temperature difference tends to result in a lower rate of heat exchange, and this then necessitates a greater heat-exchange area for a given volume rate of distillation. Such an additional heat-exchange-surface area exacts its own cost penalties not only in initial equipment cost but also in the power needed to operate the unit. The reason why rapid condensate removal tends to ameliorate the energy-cost problem is that reduction of the condensate film""s insulating effects tends to increase the heat-exchange rate for a given temperature difference.
The rotary heat exchanger""s centrifugal force also tends to reduce the water-film thickness on the evaporation side and thereby further benefit heat-exchange efficiency. Of course, introducing liquid to the evaporation side at too great a rate will compromise the centrifugal force""s beneficial effect on heat transfer, so evaporator efficiency is best served by keeping the rate of feed-water introduction relatively low. Unfortunately, too low a rate of feed-water introduction is counterproductive; it allows surface tension to defeat proper surface wetting and thus heat transfer to the liquid.
But I have recognized that heat-exchanger efficiency can be improved by employing a technique that keeps the evaporator surfaces substantially wetted but uses an average rate of liquid feed substantially lower than the steady-state rate required to maintain proper wetting. In accordance with my invention, the rate at which the evaporator-side heat-exchange surfaces are irrigated so varies as repeatedly to reach a peak irrigation rate that is at least twice its average rate. Preferably, that average rate is less than half the steady-state rate required to maintain proper wetting, while the peak rate preferably exceeds that steady-state rate. Even though the average rate is low, the repeated increases to such a peak rate can prevent those surfaces from dewetting. The result is a significantly greater heat-exchange rate, and less power consumption, than in a similar system employing the minimum steady-state rate required to maintain wetting.