Sloley2

Don’t Hobble Your Heat Transfer Fluid

Nov. 4, 2020
Pay careful attention to temperature limits and fluid condition

A plant’s problems with wiped-film evaporators (WFEs) used for recovery of heat-sensitive distillates spurred a heated discussion when I, the consultant called in to help address the issues, got the process engineering and operations staffs together in a room. The two groups dramatically differed on which problem merited attention first. Process engineering wanted to investigate the WFE speed, recycle system and operating pressure. Operations instead insisted the heat transfer fluid (HTF) for the WFEs deserved priority.

[pullquote]

To back up its case, someone from the operations staff stressed that the HTF solidified if its temperature dropped too low. That person then went out and returned with a solid chunk of the HTF. My initial thoughts on looking at the chunk were, first, that it shouldn’t be a solid and, second, it shouldn’t be dark grey-black.

The first point reveals my lack of experience with HTF systems. Some high-temperature HTFs actually are solid at ambient temperature. This material formed a slurry starting at around 150°F. The high slurry temperature created problems for startup and shutdown but wasn’t a defect in the fluid.

However, the grey-black color raised serious concerns. HTFs require filtering, cleaning and even scheduled replacement to ensure they remain effective. This HTF no longer was filtered because all the filters plugged more quickly than they could be changed and so were removed, noted someone from operations.

Further investigation showed the HTF had a manufacturer’s recommended temperature limit for extended operations of 752°F and a maximum film temperature of 770°F. The WFEs operated at a process temperature of 805°F, necessitating a hot oil temperature of 810°F. Film temperatures in the hot oil heater weren’t evaluated but were even higher.

As a rule-of-thumb for organics, thermal cracking rates roughly double for every 18°F (10°C) rise in temperature. The fluid temperature exceeded the recommended temperature by 58°F (810°-752°). This made thermal cracking rates more than nine times higher than the normal maximum rate. Catalytic reactions from degradation products increase even faster with temperature. The oil sample was black due to extreme HTF degradation. Reduced heat transfer due to the fouling on the HTF side of the WFEs limited their performance.

The current dilemma stemmed from a combination of poor plant design and changes in operating conditions. The original design point exceeded the HTF capabilities. Also, over time, the plant had raised WFE operating pressure to boost capacity. The increased process pressure required higher HTF temperatures to recover the product.

The plant faced a difficult situation. The WFEs relied on conventional HTF systems designed for liquid-phase operation. The HTF had to have a low enough vapor pressure to remain liquid at high temperatures. Investigation revealed that no commercial HTF fluids available met the constraints imposed by the process operating conditions, HTF system capability, and equipment materials of construction and mechanical limits. In fact, the current fluid was the best of a set of unsuitable choices. So, continuing to use that HTF required taking some steps to better handle the severe conditions.

First, HTF filtration resumed. Removing contaminants is critical because many degradation products are catalysts that promote further degradation. However, a reasonable frequency for filter changes is essential to avoid the task getting abandoned as being too much trouble. Achieving a viable frequency necessitated installing larger filters.

Second, the plant reinstituted fluid quality monitoring. It had abandoned the practice because the abysmal quality found had made the exercise seem almost pointless. Of course, monitoring only matters if the plant actually acts based on the results.

Third, the plant relied on the results of the quality monitoring to define fluid replacement periods; these, as expected, were much more frequent. Shorter operating periods between fluid changes increased plant capacity by improving WFE heat transfer.

Fourth, replacement fluids were run through a nitrogen stripping step to remove trace oxygen before loading into the system. This helped reduce oxidative decomposition — extending fluid life significantly at these high temperatures.

Fifth, work on a planned plant expansion focused on using different heating technologies for the WFEs being added. The choices investigated were a molten salt system or an electrical heater integrated with the WFE — but that’s a discussion for another column.

ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at [email protected]