By Alan Rossiter, PhD, PE, Principal, Rossiter & Associates. Article first published on Rossiter & Associates on January 4, 2016.
Operating practices tend to become entrenched over time. Operators may dutifully follow procedures that were developed when their plant was commissioned, even though throughput, feed stocks, product slates, and a host of other factors have changed. Moreover, many established operating practices are unnecessarily conservative. Correcting these issues can yield large savings.
One of my favorite examples comes from a study I carried out a number of years ago at a petrochemicals facility. This was first published in Hydrocarbon Engineering .
In this example, the design of a distillation column includes an airfin cooler on the feed (see Figure 1). The cooler was installed to prevent overloading of the overhead condenser under certain abnormal operating conditions. However, it also ran continuously during normal operations, too. This removed heat from the feed, and consequently the heat load in the reboiler increased.
As a result of the study the procedures were changed and the air cooler fan was shut off during normal operations. This reduced the reboiler duty by more than 30%, a saving worth more than $1,000,000/year, at no cost to the facility.
Even with the fan off there was still a significant loss of heat due to convection in the airfin. This loss can be eliminated by installing a bypass around the cooler (Figure 2), a small project that saves an additional $200,000/year with a very good payback.
You might think this is an isolated example of inefficient operation. However, I have found many similar situations over the years. Indeed, the unnecessary cooling of streams that should be kept hot is one of the most common forms of inefficiency in process plants. There are additional examples in my book, Energy Management and Efficiency for the Process Industries – see, especially, Chapter 8.
1.Rossiter, A. P., Back to the Basics, Hydrocarbon Engineering, Vol. 12, No. 9, pp. 69-73, September 2007