The effects and limitations issued by non-condensible gases in sea water distillers

Abstract

The paper tries to give a review of our knowledge of the effects of non-condensible gases on heat transfer, on energy demand, and on operation of desalination distillers. This and consequences of these effects on design are presented and discussed for the group of multi-stage-flash (MSF) distillers and for the alternatives in multiple-effect (ME) distillation. Each topic is treated, if necessary, separately for MSF and for ME evaporators. The total releases of the different non-condensible (NC) gases are summarized and the problem of predicting the bicarbonate decomposition is discussed. A closer view is taken into the influence of the NC gases in the individual stages of an evaporator: the change of heat transfer and saturation temperature along the flow path of vapour in the condenser, the effect on total performance and the significance of the cooling section on venting flows and energy consumption. The problem of how the carbon dioxide releases are distributed between the evaporator stages is discussed with the results of an analytical model for this complex process. Finally the practical design concepts for venting are briefly summarized and compared. Efforts to develop low energy evaporators also require an optimization of venting system.