(187f) Overcoming Seasonal Cooling Problems in Distillation Column Overheads - Case Studies

(1) Inadequate pressure control. Some columns with total condensers allow the pressure to float with ambient air or cooling water temperature, as they have no effective pressure control strategy at all. Others have improper regulatory pressure control schemes that make it difficult or impossible to maintain the desired set-points adequately. Such inadequacies cause periodic fluctuations in product quality, especially when relative volatilities are influenced strongly by tower pressure.

(2) Throughput loss caused by Inadequate overheads cooling. During the summer, ambient air temperature can be 40-50°F higher than in the winter, and cooling water temperature is often is higher by 30°F or more. This reduces the temperature driving force in the overhead condensers and requires column pressure set-points to be raised considerably. The resulting reduction in relative volatility for the separation can require higher reflux ratios (and reboiler duties) for maintaining the same product quality. In cases where the column is close to its hydraulic vapor-handling limit (flooding), the operator has no choice but to cut unit throughput.

(3) Product quality violations. Fluctuations in tower pressures, feed rates, or reflux rates invariably set off composition transients that require significant countervailing moves for maintaining product quality at both ends of a tower. Often, operators intervene in ways that, eventually, lead to potential violations of product quality specifications, or throughput reductions, or both. One tactic, highly inadvisable but often practiced routinely, is to make overheads product of a far better quality than specification so as to be able to absorb occasional violations, especially for high-purity specifications. Such giveaways lead (a) to lost production when column capacity is near its limit, or (b) excessive utility consumption, which can be quite costly especially in columns using refrigerated overheads cooling.

These points have been noted in the distillation literature repeatedly. However, the traditional approach is to focus on improved regulatory and advanced control schemes to minimize production losses and product quality excursions.

In this paper, we will present several innovative and highly profitable ways to provide direct cooling of column overheads (during the summer months) that have been field-proven over many years. The goal of these approaches is to move column operation back to winter-like conditions and restore production and product quality reliably to desired levels. These solutions require application of sound engineering principles, using the proper process simulation, phase equilibrium thermodynamics, and equipment rating techniques.

Capital appropriations are hard to justify for seasonal cooling solutions. There is a paramount need for reliable solutions that can be engineered, installed, and commissioned in days or weeks to capitalize on short-term product margins. This situation is best handled using industrial-grade rental equipment such as heat exchangers, refrigerated chillers, cooling towers, etc. The key is to be able to insert such equipment into a running column in a way that does not require a shutdown and causes no disruption or disturbance in column operations. Such seasonal projects have been proven in numerous successful industrial applications.

This paper will present examples of such engineered solutions pertinent to the refining and petrochemical industries that have yielded benefit/cost ratios of ranging from 4:1 to 10:1. Best of all, the operating problems mentioned above are eliminated, or at least ameliorated significantly, using these approaches.