Eventually, the staff might have decided to chemically clean the tower and gained the improvement in capacity. However, without understanding the cause of the problem, the plant would not have been in a position for further improvements.
A key relationship
It is surprising how often the simple relationship of reflux-to-distillate (R/D) ratio versus the number of theoretical stages (NTS) in a distillation application is overlooked. Yet, it can lead to a great return on investment or a simple resolution to a problem.
In the post-fractionation section of one particular aromatics plant, three sets of xylene splitters operated in parallel to produce ortho-xylene. The para- plus meta-xylene separation from ortho-xylene typically requires about 120 trays at a moderate reflux-to-feed ratio. One of the xylene splitters had only 51 trays. The tower was brought into xylene-splitting service several years earlier, but had never produced much on-specification ortho-xylene. Higher reflux rates did not improve the operation, as they did with the other towers. This was a classic case of operating at the far end of the R/D versus NTS curve, near the point of minimum NTS (Figure 2). The tower was revamped by replacing two trays for every original one. The result: a threefold increase in ortho-xylene production at a lower energy consumption.
A different petrochemical application, this time involving structured packing, illustrates the same oversight, but at the other end of the spectrum. Structured packing is commonly considered for low-pressure or vacuum applications, but not those with high liquid loadings resulting from high vapor density. Yet, the packing has been used in a successful revamp of a high-pressure depropanizer application.
The next application of structured packing in the same service, however, did not perform nearly so well. A closer examination of the first case showed the reason why. The first column was operating at the edge of the R/D versus NTS curve, near the point of minimum reflux, as shown in Figure 3. The packing gave substantially higher height equivalent of a theoretical plate (HETP) than claimed, but this went unnoticed because the curve was steep in that region and the small loss in efficiency was dismissed as inconsequential. The second case operated at a more normal position on the curve. There, the loss of NTS was more pronounced, which exposed the true performance of the packing.
The first revamp technically was not a failure because the depropanizer realized the claimed capacity. The mistake, as shown in the second case, was in wrongly assuming that the revamp would work in the same application without understanding the limitations of the technology.
Know where your towers operate on the R/D versus NTS curve. And understand how fluctuations in operation over time will affect performance.
Critique the design
It is counterintuitive to question the integrity of a design that may have given years of adequate service. However, the original design should be critically reviewed to address problems and to understand the basis for significant improvement. The intent is not to denigrate the designers, but to see where new methods and recently developed technologies may apply.
In some cases, the original designers used rules of thumb and practical guesswork. Therefore, designs may appear to work well from the outset but may be on the verge of imminent failure. If possible, contact the original designers to evaluate their methods. Do not automatically assume that the design was done correctly.
Within a reasonable range, it is possible to exchange efficiency for capacity by adjusting the tray design and process conditions. Like the high-pressure depropanizer application, the aromatics LLE tower had far more efficiency than needed and great potential for capacity increase. Ultimately, the column was debottlenecked with only minor modifications to approximately double the original capacity at inconsequential loss of efficiency.
Some older plants may have been designed with rudimentary computer equipment, or perhaps even slide rules. So, the designers may have missed opportunities for optimization due to the difficulty of reviewing multiple design cases. In addition, all basic unit operations have benefited from great improvements during the last several years. If you have a vintage plant that has not been reviewed recently, chances are that such a review can uncover cost-effective revamp opportunities.
Pay particular attention to any unit with an unusually large surplus capacity in one part. This often means that the designer did not know what to do in that section and took the conservative approach of oversizing the equipment. A mismatch in capacity within a unit presents a good opportunity to find creative revamp solutions.
You may want to correct the imbalance first. Consider the example of some aromatics extraction plants, where the capacity of the solvent stripping part of the unit substantially exceeds that of the extraction part. An all-too-typical discussion during a plant walk-through goes like this:
"What is that tower?" a troubleshooter asks.
"It's our new extractor," the plant engineer replies.
"Why did you need a new extractor?"
"The first one couldn't handle the capacity we wanted."
"What other equipment did you have to replace to gain the higher processing rate?"
"Nothing else, only the extractor."
Adding a new extractor is a high price to pay for an improper design. If the original designer could not balance the major pieces of equipment in the unit, you can be sure that the other parts are not optimally designed either. Look for opportunities here.