Questions about the purified water produced by electrodesalination technology in atmospheric and vacuum distillation units?
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In atmospheric and vacuum distillation units, two-stage electrodesalination is used. I would like to ask: is the purified water and fresh water from various sources mixed with crude oil before the first-stage electrodesalination tank, or before the second-stage electrodesalination tank? I have seen that in some cases, the purified water is mixed before both the first and second-stage electrodesalination tanks; in other cases, it is only mixed before the second-stage tank, with the water used before the first stage coming from the bottom water of the second-stage tank. What impact does this have on the desalination efficiency? What are the implications in terms of process design and logic? I appreciate your guidance. Thank you!
This post was last edited by chengkang on 2009-8-19 20:50. It should be mixed in front of the primary and secondary electrodialysis tanks. Since the salt content in the bottom water from the secondary electrodesalination tank is much lower than that of the crude oil in the primary tank, in order to save water, this bottom water is fed into the primary electrodesalination tank as injection water for the primary tank! Mixing only before the secondary electrodeionization tank may be specific to the situation where the crude oil contains low levels of salts.
The situation you mentioned is the result of energy-saving upgrades. The original design for electric desalination involved the use of brine for injection; with these energy-saving upgrades, secondary electric desalination is used for dehydration, and the resulting fluid can be reused in primary electric desalination processes, which is more than sufficient to meet the requirements for desalination and dehydration. Even better are heat exchangers, which use demineralized water to heat the injection water, and they yield excellent results.
Injection is done before both stage 1 and stage 2; stage 2 receives purified water or fresh water. Stage 1 water is removed using stage 2 desalination; this saves water, increases the injection water temperature, and helps with desalination. It’s also good to use primary water dragging down combined with secondary water injection for heat exchange.
Before stage two, it must be fresh water; before stage one, the water from the bottom of the second-stage tank can be used. This is done to save energy – as long as the water from the bottom of the second-stage tank is suitable and does not affect the desalination process at stage one. After all, the salt content in the water from stage two is much lower than that in the water from stage one. Then, the water from stage one and stage two are exchanged regarding heat: the water from stage one is cooled and discarded, while the water from stage two is reheated. It’s great!