1. Application Review

Hydroprocessing consists of hydrotreating and hydrocracking processes.

Hydrotreating is used to remove undesirable contents in the feedstock such as sulfur, nitrogen and other metal contaminants. Meanwhile hydrocracking cracks long chain hydrocarbons feedstock into lighter components.

Both processes require hydrogen at high temperature and pressure with the presence of catalysts for a reaction to take place. The effluent at the reactor outlet contains a combination of trapped, dissolved gases, like unreacted hydrogen, light hydrocarbons and Hydrogen Sulfide. The effluent will later let down pressure to a stripper or distillation column. When the high pressure is being let down in this application, the fluid will undergo outgassing, flashing and cavitation with entrained catalyst, therefore special attention is necessary for selecting the right valve to address erosion, corrosion and vibration. The most common service names found in both the hydrotreating and cracking units are hot high and cold high pressure separators.

2. Challenges

Each HHPS and CHPS has two letdown valves that maintain the separator level to ensure proper separation of liquid and gas products. The valves are exposed to various severe challenges because of the multiple constituents involved along with the entrained gas and abrasive catalysts:
• Vibration due to outgassing of entrained gasses
• Erosion due to outgassing, cavitations, flashing and entrained catalysts 
• Special trim selection and sizing due to flashing and outgassing 
• Corrosion due to entrained corrosive gas (H2S)

3. Recommended Control Valve Solution

Cavitations and Catalyst
The presence of fine catalysts from the reactor causes more challenges to the conventional anti-cavitation cage design. This fine catalysts tend to clog the trim flow passage. Hence special requirements like anti-clog, anti-cavitating with multi-stage pressure design dirty service trim are required to ensure smooth operation and long service life of the control valve.

Flashing/Outgassing Fluid and Vibration
ISA flashing calculation based on a single component is not suitable as the process contains multiple components and will therefore require two phase sizing to determine the control valve capacity.

If the outgassing fluid has more than 20% vapor at the outlet, the standard dirty service trim might not be able to handle the large amount of outgassing fluid. Special attention is required on the valve outlet area and trim design to provide adequate flow area and reduced vibration due to the turbulent fluid.