Jim Norton and Chris Steves (Norton Engineering)
Diagnosing poor vacuum jet ejector performance can be tricky, but some things to look at while troubleshooting include:
•Look for leaks – even small pinhole leaks anywhere in the ejector, condenser, or diplegs can lead to severe vacuum problems. Consider helium testing to check for leaks, but also get as many eyes on the equipment as possible.
•Check condenser performance for problems in water flow.
•Check for pluggage in the diplegs that could be backing up liquid into the condensers.
•Check to ensure that the ejectors are not overwhelmed by cracked material from overfiring the vacuum heater.
Andrew W. Sloley (CH2M HILL)
At vacuum tower top temperatures over 135-150°F some material heavy enough to condense after the pre-condenser goes overhead. In units with extremely paraffinic feeds the hydrocarbon can form wax in the pre-condenser, leading to reduced heat transfer and higher vacuum tower pressures. Other units get liquid carryover from the upper pumparound. This liquid shows up in the precondenser liquid.
Andrew W. Sloley (CH2M HILL)
Thermal cracking of liquid in the vacuum tower bottoms depends upon time and temperature. The higher the temperature, the higher the cracking rate. The longer the residence time, the more cracking. The key to reduced cracking in the vacuum tower bottoms is to keep the bottoms temperature down and the residence time low.
Temperature limits to avoid cracking depend upon the crude. The most common method to reduce thermal cracking uses a quench recycle to cool the boot. Most plants target a bottoms temperature of ~650°F to avoid cracking.
If you cannot reduce the bottoms temperature, the other strategy is to run with minimum liquid inventory in the bottoms.
One of the most sensitive measures of thermal cracking is non-condensable gas make from the vacuum system. Monitor the flow rate of the vacuum system off-gas from the hot well. Higher cracking rates give higher off-gas production.
Andrew W. Sloley (CH2M HILL)
Wash oil loss can result in coking in either the wash oil delivery system or in the wash bed, or both. The best method is to prevent coking is to keep the wash oil in service. Auto-start of standby wash oil pumps, alternate wash oil supply sources (AGO), and putting pumps on critical service power supply are used.
Steam purge of wash oil spray distributors has been used to prevent nozzle coking.
Coking rates depend upon oil composition, temperature, and residence time. If wash oil is not available, the vacuum heater outlet temperature should be dropped to reduce operating temperature. This will reduce unit yields and put more light material into the vacuum tower bottoms. The lower operating temperature reduces coking directly by cooler operation, and indirectly by reducing the amount of entrainment into the wash bed as a result of lower vapor velocities in the vacuum tower.