What is the effect of reducing the system pressure in a gas oil/naphtha hydrotreater? Will this be able to reduce the hydrogen consumption given that there is still allowance on catalyst deactivation? What parameters do we need to consider before reducing the system pressure?
Answers
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15/08/2017
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Ganesh Maturu, Self, maturu.ganesh@gmail.com
I have slightly different opinion.
Gas Oil Hydrotreater: I don't understand why you wanted to reduce system pressure for Gas Oil Hydrotreater where our objective is to increase Cetane number/density reduction/aromatic saturation is the main objective to get high quality diesel product.
Naphtha Hydrotreaters: We can optimize system pressure in this unit to minimize hydrogen consumption. Typically HDS is the main objective in NHT units to provide feed to downstream platformers. We may not require high system pressure to remove sulfur species from naphtha as it contains easy sulfur. However, naphtha removal also one of the requirement to take care of NH4Cl issues in platformer and hence need to maintain sufficient pressure to remove nitrogen. Reducing system pressure reduces aromatic saturation and is advantageous some times which reduces H2 consumption. Careful review of the unit with respect to deactivation/increase in WABT to be done before taking final decision. Also reducing system pressure reduces recycle gas circulation to the unit for the given compressor capacity.
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20/07/2017
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
First, we assume the unit exists and is not being designed. For the same conversion, reducing pressure will require increasing temperature and/or H2/H.C. ratio. The other variable is LHSV, which cannot be changed in an existing unit at a fixed charge rate. Increasing temperature will shorten run length. If you have correlation curves used by the licensor, you can determine the optimization.
If you perform such an optimization study, use a long time period, say, 5 or 10 years, because the losses during additional downtimes can be a significant factor in the calculations. If your unit is oversized and therefore can play “catchup” by gradually processing any held back feedstock, that is an important factor as well.
Finally, if the hydrogen consumption is reduced, the conversion is reduced, and that is not one of your objectives. Yes, at different pressures, the dissolved hydrogen to the fractionation section is different. That is insignificant unless you are extremely short of available makeup hydrogen.
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