According to the Hydrogen selectivity definition (H2 selectivity (%) = Moles of H2/Sum of the moles of CO, CO2, CH4 and C2H6), it should be a number less than 100%, but in my experimental work, some of the experiments have hydrogen selectivity more than 100%, which is due to the molar fraction of produced hydrogen more than 50%, after all i want to know is it possible to have hydrogen selectivity more than 100%?
I think you have a very odd definition of selectivity. The normal definition of selectiviy the denominatoor would include all of the H that could become H2 and 100% would be the max. Your definition of selectivity allows > 100% .
Here are two hits to help you understand and, hence, be able explain the observation that you indicated:
1) In chemical reactions, you conserve mass and not number of moles.
2) Please review Le Chatelier's principle which is also called the 'Equilibrium Law.'
Please describe the experiment you are performing. Are you trying to produce supercritical H2 by burning ethane at supercritical conditions? If you share your data we might be able explain the phenomenon you believe you are observing.
I agree with others that the definition of selectivity mentioned in your question is odd. Generally speaking for SCWG you could possibly reach hydrogen gasification efficiency (H2_product/ H2_organic feed) of over 100 % due to the participation of reactive water (from the solvent) in the gas shift reaction which increases H2 gas yield.
Rick Manner
Dear Mr. Manner thank you so much for your response, the experiments that i had been performing was about super-critical water gasification of biomass, and in order to find the optimum condition of hydrogen rich syn-gas production, i had varied temperature, feed concentration and residence time of experiments in the rang of 375-775°C, 1- 9 wt% and 10 to 50 minutes respectively. as it is known higher temperatures and lower feed concentrations are very suitable for both the total gas yield , and hydrogen production( because of Water gas shift reaction promotion), While the effect of the residence time is not very noticeable. after all in the optimum condition(775°C, 1 wt% and 27 min) ,the gas composition is as follows: 17.38 mmol CH4/each gram of feedstock, 12.63 mmol CO2/each gram of feedstock, 44.86mmol H2/each gram of feedstock and 0.74mmol CO/ each gram of feedstock. According to the definition of Hydrogen Selectivity which is : Hydrogen selectivity(%)= (Yield of H2/ Yield of other gaseous products)*100, in the optimum condition hydrogen selectivity =(44.86)/(17.38+12.63+0.74)*100 = 145.88%
Yehia F. Khalil
Dear professor Yehia F. Khalili , thank you so much for your guidance
Babak Arjmand
I would call that 59.3% selectivity to H2 because you need to count the 44.86 H2 as part of the total product in the denominator. You are free to define selectivity however you like - as long as you define it properly when you discuss it.
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