Don’t Let Cold Box Fouling Bring Your Plant Down

In many cryogenic processes [RSV gas plants, nitrogen recovery units (NRUs), LNG facilities] the cryogenic exchangers and the cold box can be a potential bottleneck to production.?The exchangers have very fine channels which can easily trap dust and solids, particularly if they mix with high viscosity fluids or entrain sticky solids. Fouling of these exchangers can render a plant inoperable and the opportunity cost due to lost production or impaired product recovery can quickly exceed millions of dollars. While the opportunity cost can be tremendous, the exchanger cleaning, repair, or replacement can also be extremely expensive.

Cryogenic exchanger fouling usually has several sources: [1] Carryover of lube oil from upstream compression, [2] Carryover of molecular sieve fines from dehydration beds, [3] Liquid aerosol carryover of amines or hydrocarbons into the molecular sieve beds which crack to sticky tars upon bed regeneration.

Carryover of lube oil from residue gas compression into RSV reflux exchangers or into NRU systems is common.?At sub-zero to cryogenic temperatures, the lube oil viscosity increases to the extent that it deposits in the exchangers as a solid or semi-solid, quickly increasing differential pressure through the exchangers and leading to shut down.?The culprit here is generally ineffective residue gas coalescing.

Carryover of molecular sieve fines from upstream dehydration can be an equally damaging and all too frequent occurrence.?The culprit in this case is most often ineffective dust filters or ineffective sealing of the filters which allows bypass of solids.?While some exchanger capacity can be recovered with exchanger cleaning or a “blow back”, these require system shutdown and significant maintenance expense.

Liquid carryover of amines or hydrocarbons into the dehydration beds can degrade the molecular sieve leading to ineffective dew point suppression or shortened bed cycle life. As we have seen in a number of facilities, the regeneration of the bed at 500 F can lead to cracking and coking of the contaminants to generate sticky tars which move downstream after the bed is placed back in service.?The culprit here is generally ineffective inlet coalescing prior to the dehydration beds.

Best practices to mitigate cryogenic exchanger fouling are:

[1] Measure: Know how your filters and coalescers are performing through on-line measurement.?Do not rely simply on the filter or coalescer datasheet to determine performance. Sophisticated system surveys and gravimetric analysis are available as part of Transcend Solutions offering to validate filter or coalescer performance efficiency or to assist in troubleshooting process challenges by quantifying and characterizing contaminants in the system.

[2] Inspect: Inspect the filter and coalescer systems to make sure they are mechanically sound and do not have avenues for bypass from poor equipment design, inappropriate separator elements, or mechanical or corrosion induced failure. Transcend Solutions’ field service team regularly inspects separator vessels and can often make simple mechanical upgrades that improve capacity, performance, or seal integrity.

[3] Maintain: Create a regular maintenance program that includes gravimetric sampling of your system to monitor separator performance and to evaluate changes in feed quality or contaminant levels. Transcend Solutions can regularly perform contaminant audits to establish baseline contaminant levels as well as monitor contaminant changes under dynamic operating conditions.?Transcend has extensive experience troubleshooting cold box issues and cryogenic exchanger fouling and delivering solutions to prevent contamination induced bottlenecks.?

Effective separations solutions and regular contamination monitoring are hallmarks of well-run process facilities. Don’t let preventable contamination control issues bring your plant down.??