Nevertheless, the permeate vacuum is not the higher the better. The higher permeate vacuum means greater cooling water flux (the cooling water flux of 2.5 L/min corresponds to the permeate vacuum of around −50 kPa in experiments), which also means more bubbles of non-condensable gas permeate through the pore into the cooling water side. This demonstrates that the water can be recovered due to the permeation of water liquid when the feed gas is saturated, while the higher pressure difference makes the liquid permeate too quick to block the membrane pore, resulting in the passage of the non-condensable gas.
The advantage of choosing a low permeate vacuum (i.e. low cooling water flux) appropriately is that barely no bubbles exist in the water side, and higher selectivity and lower energy consumption can be achieved. Ref. [10] argues that the amount of recovered water mainly depends on the mass flow rate ratio of cooling water and flue gas. Compared to that, the relationship between the permeate vacuum caused by cooling water and the gathering velocity of the condensed water is of greater importance.
Compared with the experimental system referred in literature [19], a gas compressor is used to maintain the pressure difference, this study sets a pump in the water outlet of module to produce the stable vacuum in the cooling water side, in which the energy consumption of gas compressor can be saved. What’s more, the experimental system in this study is much closer to the reality situation.
The amount of absorbed heat of the cooling water increases with the rising of water flux, as shown in Fig. 15 and Fig. 16. Due to the heat of the membrane surface being carried away by adequate cooling water timely, the heat of the feed gas can be recovered efficiently. Therefore, the optimum flux of the cooling water (i.e. the optimum vacuum) for the certain flue gas condition exists and its effect on the selectivity and heat recovery should be concerned.