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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You are a chemical engineer with the modest assignment to debottleneck your plant to boost production. The direct approach seems obvious: assess current capacity, determine output limitations, and then vigorously employ standard engineering techniques to upgrade or replace equipment. Perhaps you find 20% reserve capacity, eliminating the need for major capital expenditure. Even if you achieve good results, however, you can't necessarily be sure that your straightforward analysis gives the optimum solution.

 

Now, consider a different assignment: expand the plant capacity by 75%, reduce the specific energy consumption by 20%, increase the product yield by 5% -- and do it on a shoestring budget. This presents more of a challenge and definitely requires a different, more creative approach to the problem.

Not every plant can be revamped to gain such improvements. However, many plants can operate much more efficiently or at a substantially higher capacity than a routine analysis might suggest. This article presents some ideas and case studies to help you critically evaluate your process to see whether it offers exceptional revamp opportunities.

Beyond troubleshooting
Both revamp work and troubleshooting can be part of a plant debottlenecking effort. They share many tactics, but differ in their intent.

The goal of a revamp is to improve some basic parameters such as capacity or processing efficiency. In contrast, troubleshooting merely aims to solve a problem that hampers current operation. Often, the problem exposes an opportunity for much greater gain in performance.

Both revamp and troubleshooting require open-minded thinking and proficiency with engineering tools. Both often rely on test runs to diagnose problems and uncover design errors or inaccuracies in equipment ratings. However, for a major improvement in the fundamental performance of a unit, you must go beyond this first level of investigation and intentionally look for the greater gain.

The exceptional plant revamp will require, in one way or another, each of the following five elements:

1. Thorough knowledge of process fundamentals;

2. A critique of the original design;

3. Proper approach to the problem;

4. Creative and talented individuals; and

5. A focus on developing the simplest solution.

Of course, you must also give due consideration to the specific problem and common sense.

Understand the process
A prerequisite for making a reasonable troubleshooting effort or having any hope of a successful revamp is to have a thorough knowledge of the process. This must go beyond the mechanics of operation and control points and include "what and why."

Let's review, for example, a traditional aromatics extraction plant. Most operators of Sulfolane or UDEX plants do not fully understand the operation of the extractor/stripper and the various recycle loops. What factors most affect the phase separation of the liquid-liquid extraction (LLE)? What about water or other co-solvents? What is the loading profile in the towers? What is the stage efficiency? What is the effect of process temperature? Do C5s in the feed help or hurt the operation? How do you deal with three-phase distillation? There are a host of other questions to answer before optimizing the process, let alone before proposing a revamp.

 

Figure 1: An LLE unit for aromatics recovery started to lose capacity because of flooding. Initially it was assumed that the usual flooding mechanism (A) was occuring, but deeper analysis pointed to the mechanism that was really responsible (B).

The following case study shows how an inadequate understanding of the process led to a misdiagnosis and delay in resolving an operating constraint. Throughput of an LLE unit used for aromatics recovery started to decrease. The tower design had the heavy-phase dispersed with the interphase at the bottom. All parameters seemed normal, but the extractor would flood at high feed rates. Typically, such flooding is due to restriction in the tower internals, as shown in Figure 1. However, in this case, it occurred for a very a subtle and unexpected reason.

It was originally thought that the sieve trays were plugged because the tower would hold up solvent as it became loaded. However, water-washing the debris from the tray decks did not restore the capacity.

Pressure drop only measured the head of the continuous phase. To further debunk the theory of holes being blocked, the tower could operate at virtually any flow rate of solvent. It only was sensitive to higher flow rates of the continuous phase, or raffinate. Furthermore, an inspection indicated that the tower was clean and mechanically intact, with no apparent restriction to the solvent flow.

The troubleshooting effort then turned to a trial-and-error manipulation of various, unrelated operating variables and external factors. Such efforts are costly and time consuming and, in this case, were not based on fundamental process knowledge or all the phenomena occurring.

Hydraulic ratings showed that the sieve trays were operating in the normal range with no entrainment of heavy phase into the upcomers. The overall aromatics recovery was as high as it had always been, regardless of the feedstock or operating conditions, suggesting that back mixing of the phases did not materially affect the efficiency.

At this point, experience and operation knowledge finally came in to play.

The trays were designed with 5/32-in. (4 mm.) nominal diameter sieve holes. Drill bits were used to verify the hole size. Although the tray decks appeared clean, the 5/32-in. bits did not pass through the perforations. There was a slight polymer buildup around the circumference of the holes. This buildup was sufficient to shear the heavy phase into smaller droplets that became entrained in the raffinate phase at higher continuous-phase rates. A simple chemical cleaning of the tower rrestored the capacity to its prior level.

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