Succeed at plant debottlenecking

Critical evaluation can enable you to turn up capacity and improve performance. This article presents some ideas to help you evaluate your process to see whether it offers exceptional revamp opportunities.

By Joseph C. Gentry, P.E.

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Sometimes, the evolution of knowledge and experience can lead to improvements. You should question long-standing beliefs that govern important or peculiar operating practices.

At an olefins plant, troubleshooters were trying to discover the source of methanol found in polymer-grade propylene. The cracker feedstock was immediately suspected, but the idea was dismissed as unreasonable. A former staff member had read in a technical article that methanol would not survive without decomposing in the cracking furnaces. An investigation found that the article, written in 1929, described methanol decomposition after a few seconds at cracking furnace temperatures. The cracking furnaces in this plant, however, provided only a fraction of a second residence time and, therefore, could not decompose the methanol. The feed source was addressed and the problem was solved. More reliable data in the beginning and closer scrutiny of what was fact versus myth would have led to a quicker solution.

 

Figure 2: A xylene tower operated at the far end of the R/D versus NTS curve, near the point of minimum NTS. Revamping, by replacing each tray with two at closer spacing, moved the operating point and led to increased production and lower energy consumption.

Likewise, don't assume that popular technology represents the current state of the art. Process engineers tend to be a conservative lot. For the most part, this is prudent because the business risks of failure outweigh potential gains from minor improvements. But, consider the example of aromatics recovery technology where the LLE process had remained unchallenged for 40 years. Now, extractive distillation (ED) with modern solvents and design techniques is accepted as the superior process and current state of the art.

 

Figure 3: Structured packing gave substantially higher HETP than expected, but this went unnoticed because the curve was steep in that region. Revamping moved the operating point to a more favorable location.

Approach the problem right
Before you can embark on solving a problem, you have to recognize that you have a problem and, perhaps, a significant opportunity. The xylene-splitting column in the example above had been in operation for many years before anyone saw the revamp opportunity. Countless other cases throughout the industry await discovery and process engineers need to take the initiative to find and make improvements to their units.

Ensure that the problem to be resolved is not defined too narrowly. Before you try to increase the capacity of a unit, make sure that you are effectively using its current capacity. For example, one plant wanted to revamp its toluene product column to gain more capacity. While that could have been done, a broader investigation revealed five distillation columns within the complex that were already separating C7 from C8 components. The streams were re-mixed after an intermediate processing step, only to be separated again. A proper redesign of the process configuration would bring this down to two separations and free up the desired capacity.

Remember: solving the problem in the traditional manner will get you whatever spare capacity the original designer left in the unit, at about the same yield and specific energy consumption. Tackling the same problem with a creative approach could gain much more capacity and reduce operating cost at the same time.

Perhaps you have heard, "I've already revamped my unit," from someone who is proud of increasing capacity by, say, 80% more than the original design. The implication is that the maximum capacity possible has been reached. Don't blithely accept that. My company re-engineered a Sulfolane unit to boost its capacity by another 80% (to 320% of the original design) by adding only one new distillation column (Figure 4). The solution was to convert the LLE unit equipment to ED service. The revamp cost only one-third as much as that of building a completely new unit, which is the conventional approach. Lesson learned: make sure that you do not kill the project before reviewing it from more than one viewpoint.

Find creative people
If you need technology experts or process licensors to help with the revamp design, ask some basic questions. What are their skills with this type of revamp? How do they approach the problem? Are they proficient with the process engineering? Certainly, major firms have qualified engineers -- but that doesn't mean these particular engineers will work on your project. Exercise your right to select the team.

 

Figure 4: Capacity of an aromatics extraction unit was boosted by an additional 80% (to 320% of original design) by converting the LLE unit equipment to ED service at one-third the cost of building a completely new unit.

When assessing licensors, remember that a long list of licensed units does not necessarily mean the technology is state of the art or the units were properly designed. Neither of these may be true.

Many engineers don't take the trouble to understand the distinctions among available processes and instead believe the safe selection is the most widely known licensor. Maybe this is the best choice, but it is essential to carry out a thorough investigation. Otherwise, you will not know what has been missed and may find yourself in a competitively difficult situation against others who have chosen a more creative approach.

Strive to put together a team with a variety of skill sets. Engineers who have worked in regions where energy costs are high will likely keep energy conservation and process integration clearly in mind. Engineers from relatively "technology-poor" regions usually have well-developed basic skills and practical insight. A variety of backgrounds often foster fresh ideas.

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