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r 16v*e 12.17.1 Lnn Oil Still with Ptod - Bottom Interchangor

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INSTALLATION. An olefins plant debutanizer, which separates ! butadiene and butenes as the top product from pyrolysis gasoline tha-_ i leaves in the column bottoms. The bottom product flowed to a j hydrogenation reactor after being preheated by reactor effluent in ' the reactor feed-effluent exchanger (Figure 12.18.1). The column | received two feeds. The smaller feed stream, which entered at a | higher point up the column, contained most of the C.s. The lower feed contained less C4s, but was larger in quantity than the top feed.

PROBLEM Toward the end of a run, and about six months prior to a scheduled shutdown, fouling at the bottom of the column caused it to reach a capacity limitation. This resulted in excessive heavies in the C product, although the column bottom stream remained on-spec | for C.s. The off-spec C. make could not be sold, and had to be ; flared. The loss of C, make was costly, but was a preferred j short-term solution to shutting the plant down or cutting plant | rates, which would have been even more costly because of lost production. Flaring the C. make, however, was only acceptable as a short-term solution because of environmental considerations. The plant was facing a shutdown unless the problem could be quickly resolved.

OPTIONS Analysis of the column feed streams revealed that the lower feed contained only about 8 percent C.s, half of which was butadiene. A physical inspection of the piping revealed that some of the start-up lines could be utilized to provide a route through which th ! lower feed could be diverted to the bottom of the column. The possibility of bypassing this lower feed around the column was then considered. The main fear was that the butadiene contained in that j stream would disable the hydrogenation reactor to an extent that it would beccr.e inoperable. Nevertheless, it was decided to go ahead with the operation, realizing there was little to lose. The cost of a new charge of reactor catalyst was negligible compared to the cost of either the flared C4 make or a plant shutdown.

A shutdown would be required whether the operation failed or was not carried out at all, while success would have avoided the need to shut down or flare.

CURE Prior to bypassing the lower feed, the upstream plant was trimmed in order to minimize the butadiene in that stream. This reduced the concentration of butadiene to about 2-3 percent, giving about 1-2 percent butadiene in the hydrogenation reactor feed.

Bypassing the lower feed unloaded the column, and the C. make achieved its purity specs. The greater quantity of butadiene in the feed did not disable the catalyst, although it shortened its life to an extent that one additional catalyst charge was required. This was considered a relatively minor expense.

One surprising side-effect of this operation was a great reduction i overall steam consumption, which resulted from unloading the debutanizer. The steam savings achieved in the following six-month period (to the next scheduled shutdown) was alone more than sufficient to pay for the new catalyst charge.

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