Startup OptionsFigure 2. To get inter-reboiler E33 operating requires temporary use of either inert lift gas or some distillate.
As far as the second question, let’s analyze E33’s impact on duty. The first thing that inter-reboiler does is shift duty from the existing feed-preheat exchangers (E30AB) to a new place. Net energy savings depend upon two factors: how well the existing feed-preheat exchangers work in the first place, and how effective heat in the feed is versus heat in the side reboiler.
Existing feed rate was 225,000 lb/hr at 165°F and bottoms rate was 38,000 lb/hr at 409°F. Before E33 was placed into service, bottoms product temperature after heat integration was 164°F. Temperature pinch limited the existing E30AB exchangers but, from a heat-recovery perspective, they worked very well indeed. Any duty in the new E33 exchanger would be matched 100% by a duty loss in the existing feed-preheat exchangers.
Before E33 was operating, the feed-bottoms exchangers slightly vaporized the feed (~3.4% by weight). In general, shifting duty from feed to reboiler makes fractionation more efficient — as vapor and liquid rates in the tower are higher over more trays. The question is how much more efficient. T07 splits the feed so 187,000 lb/hr go overhead and 38,000 lb/hr exit as bottoms. Simply vaporizing the required amount of feed in the reboiler provides high internal flow rates. Efficiency gain on moving duty from preheat to bottoms is only about 10% of shifted duty. Energy savings totaled only 200,000 Btu/hr or 0.6% of reboiler duty.
The addition of the side reboiler failed on three counts. First, designers didn’t consider the effect of static head, particularly its implications for startup. Second, the inter-reboiler stole duty from the feed preheat instead of adding duty to the system. Third, the process gained only minor efficiency benefits for shifting duty.
Heat integration can save money. However, make sure you understand the entire system before making changes. Also, don’t forget startup requirements. More complex heat integration often means more complex startup procedures or adding special startup lines.
Andrew Sloley is a Chemical Processing contributing editor. You can e-mail him at ASloley@putman.net.