Open Discussion Forum Question 6

6.    Asian Ethylene Producer (December 2004)

We have an Ethane Cracker, where we face fouling in the Cracked Gas Compressor Intercoolers. The compressors are treated with a major anti-foulant program and are injecting with wash water. Wash oil is also injected intermittently. Each stage outlet temp is between 95-105 degrees C. The compressors themselves appear clean, since their polytrophic efficiency is steady. 

A black polymer deposit is however noticed on inlet tube sheet of the inter-stage coolers (process gas is on tube side). As a result pressure drop across the system increases causing plant shutdowns. We also notice that the inter-stage knock drums are inadequate causing liquid entrainment to compressors; can this result into fouling?

Compressor Industrial Practices (December 2004)

Current industry practice is to utilize a continuous wash water rate of about 1-2% of process flow. If the feedstock is a heavy naphtha or gas oil, the foulant material may contain some inorganic and organic acid materials. They may be corrosive and can often drive the water pH in the knock out pot down below 4. Then a neutralizer to raise the pH is recommended.

Wash oil of about 2-3% of process flow can be injected intermediately - maybe on a once per week basis. Antifouling and dispersant are recommended with the wash oil flush.  Insure that your wash oil does not contain any styrene, as it will have the exact opposite effect than you are trying to achieve.  A check of the composition of the wash oil being used, to ensure compatibility, should be completed.

The compressor outlet temperature may be higher than recommended. Typical recommendations to reduce butadiene fouling are try and maintain below 90 degrees C. The material passing through a compressor is at it highest temperature as it leaves the compressor -- thus polymeric fouling is more likely in the compressor itself than in the intercooler. An ethane cracker in Asia saw heavy polymeric fouling in the compressor (at the end of a 4 year run despite weekly flushing), and intercooler performance limited by inter-stage cooling water side fouling only.  If the compressor is not fouling maybe the problem is related to other factors such as composition or separation.

The black polymer on the tube sheet may be from the polymer in the piping.  Over time the piping will build a layer of polymer which will eventually go to the tube sheet. You may want to consider cleaning the interconnecting piping.

The inter-stage knock out drums can be upgraded with improved liquid removal systems to reduce the liquid carry over to the compressor. Since the hydrocarbon will float on top of the water layer, and what is carried over first, is the light hydrocarbons, which can lead to some fouling if they are polymer producers.