Alkaline water electrolyser system
Alkaline water electrolyser system
(OP)
Hi, this is my first time posting. I hope you can help.
I am struggling with the separation from hydrogen and water in my alkaline water electrolyser system, which was simulated on Aspen plus. In the electrolyser, energy has to be supplied to separate water into hydrogen and oxygen gas, which exit at the cathode and anode respectively. The electrolyser is assumed to have an efficiency of 80%, so some electrolyte will be leaving the electrolyser. The major problem is separating the liquid water and steam (steam is produced due to the energy from the source, in this case, the power is coming from the wind turbine) from the hydrogen gas. The most effective method I have is using a flash separator. However, I would like a separation close to 100% because the hydrogen product stream will be passed into a compressor, and I don't want any liquid water. Moreover, I have read that water in the hydrogen will reduce the product quality. I am producing 1002.06 kg/day of H2 and 6.17 kg/day of H2O. In order to arrive at this product flow rate, I had to lower the temperature from 70 deg C to 2 deg C using multiple heat exchangers. This cooling allowed some water to condense in the liquid phase. I also used a throttling valve to reduce the pressure from 7 bar to 3 bar because that is how a flash separator works, i.e. a reduction in pressure causes a richness in the vapour phase. But this is not necessary because my hydrogen gas is 100% in the vapour phase. Instead, I would require high pressures to condense water in the liquid phase. I have looked in literature and research papers and found that most people use a horizontal gas separator with a demister pad, which is very useful because the KOH and H2O are much denser than hydrogen and will be collected at the bottom due to gravity. A demister pad provides further separation by coalescing tiny liquid droplets into larger ones, allowing them to gather at the bottom due to gravity. However, Aspen does not have a block that mimics a gas separator, and assuming a 100% separation without simulating it on Aspen is not sensible. I have tried using a gas scrubber with acetic acid as my solvent. This method helped remove the water, but it creates an additional problem in separating the acetic acid from hydrogen gas, which results in a worse purity of hydrogen gas. Is there another way to separate a vapour-vapour-liquid mixture? Also, I have a question on Aspen that is why does the software allow water to be a liquid at negative temperatures and atmospheric pressures, and because of this I doubt the accuracy of the simulation. Thank you for taking your time in reading this lengthy post. I have also attached my Aspen file for you to look at.
I am struggling with the separation from hydrogen and water in my alkaline water electrolyser system, which was simulated on Aspen plus. In the electrolyser, energy has to be supplied to separate water into hydrogen and oxygen gas, which exit at the cathode and anode respectively. The electrolyser is assumed to have an efficiency of 80%, so some electrolyte will be leaving the electrolyser. The major problem is separating the liquid water and steam (steam is produced due to the energy from the source, in this case, the power is coming from the wind turbine) from the hydrogen gas. The most effective method I have is using a flash separator. However, I would like a separation close to 100% because the hydrogen product stream will be passed into a compressor, and I don't want any liquid water. Moreover, I have read that water in the hydrogen will reduce the product quality. I am producing 1002.06 kg/day of H2 and 6.17 kg/day of H2O. In order to arrive at this product flow rate, I had to lower the temperature from 70 deg C to 2 deg C using multiple heat exchangers. This cooling allowed some water to condense in the liquid phase. I also used a throttling valve to reduce the pressure from 7 bar to 3 bar because that is how a flash separator works, i.e. a reduction in pressure causes a richness in the vapour phase. But this is not necessary because my hydrogen gas is 100% in the vapour phase. Instead, I would require high pressures to condense water in the liquid phase. I have looked in literature and research papers and found that most people use a horizontal gas separator with a demister pad, which is very useful because the KOH and H2O are much denser than hydrogen and will be collected at the bottom due to gravity. A demister pad provides further separation by coalescing tiny liquid droplets into larger ones, allowing them to gather at the bottom due to gravity. However, Aspen does not have a block that mimics a gas separator, and assuming a 100% separation without simulating it on Aspen is not sensible. I have tried using a gas scrubber with acetic acid as my solvent. This method helped remove the water, but it creates an additional problem in separating the acetic acid from hydrogen gas, which results in a worse purity of hydrogen gas. Is there another way to separate a vapour-vapour-liquid mixture? Also, I have a question on Aspen that is why does the software allow water to be a liquid at negative temperatures and atmospheric pressures, and because of this I doubt the accuracy of the simulation. Thank you for taking your time in reading this lengthy post. I have also attached my Aspen file for you to look at.
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