Perspectives: Process Puzzler

Process Puzzler: Hold Off on a Hydrogen Hike

Consider other options than increasing the feed of fresh hydrogen

*THIS MONTH’S PUZZLER* We’re planning to increase the capacity of a diesel hydrotreater unit by replacing part of the recycle hydrogen currently used with fresh hydrogen from an existing 1,200-psig system. The header begins as an 18-in., schedule-80 pipe. The control valves are each fed from a line right off the header.* Figure 1* (http://goo.gl/S801qN) shows what the header system looks like: the 1,600-ft line now handles a flow of 868 million standard cubic feet per day (MMSCFD) of hydrogen; the expansion will raise the total flow to 918 MMSCFD. One branch delivers 5 MMSCFD to isomerization. Another branch services a variety of hydrotreaters. It runs 2,500 ft and furnishes 200 MMSCFD to a hydrocracker, and then goes 3,100 ft where a common branch feeds two diesel hydrotreaters, which get 75 MMSCFD and 63 MMSCFD, respectively, and an ultra-low-sulfur diesel (ULSD) hydrotreater, which is fed 75 MMSCFD. After the diesel header, the line decreases in size to 12 in. and goes 2,700 ft to a kerosene hydrotreater fed 85 MMSCFD. It then continues 2,600 ft to a naphtha hydrotreater that gets 75 MMSCFD and to another header that delivers 140 MMSCFD to a gasoline hydrotreater and 150 MMSCFD to a fluid catalytic cracker (FCC) hydrotreater. The expansion will increase the hydrogen flow to the diesel hydrotreater 2 to 113 MMSCFD from 63; the % open for the control valve only will increase to 76% from 61%, so the project has been approved. The flows are all nominal-high values. A production engineer now is questioning the idea of increasing the fresh H2 output; he’s afraid it will put a strain on the system. What do you think?   *LOOK AT THE BIG PICTURE* Anytime you increase the system demand by 50 MMSCFD, i.e., 6%, with a pressure control valve (PCV) already at 72%, there will be trouble. Obviously, the compressor supplying the H2 and the control valve are sized together. With higher flow, head will decrease eventually. The rise to 79% open for the PCV probably is caused by a drop in the compressor head.

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