Hot High Pressure Separator

Hot High Pressure Separator Letdown Application Discussion AD125 June 15, 2003

As environmental regulations tighten relating to sulfur, heavy metals and aromatic hydrocarbons, hydroprocessing has become a much more utilized process in the refining industry. Hydroprocessing Hydroprocessi ng enables the processing of many different feedstocks to produce products that are economically favorable at a given time while removing entrained contaminants. Hydroprocessing Hydroprocessi ng includes processes such as hydrotreating and hydrocracking. hydrocracki ng. Hydrotreating Hydrotreati ng processes are used to remove undesirable materials from a feedstock by selective reactions with hydrogen in a heated catalyst bed. This removes sulfur, nitrogen and certain metal contaminants. This process is often used to remove catalyst poisons from a feedstock before downstream processing. In this process, olefins and aromatics are converted to saturated hydrocarbons. The hydrocracking process converts (cracks) heavy feedstocks into lighter components by selective reactions with hydrogen in multiple heater catalyst beds. This process is most commonly used to create gasoline or diesel product streams. Most hydroprocessing today utilizes a single stage fixed-bed catalytic process. The fresh feed is mixed with makeup hydrogen and recycle gas (rich in hydrogen content) and sent through a heater to the first reactor. If the feed has not been hydrotreated, there is a guard reactor before the hydrocracking reactor. The catalyst in the guard reactor converts organic sulfur and nitrogen compounds to hydrogen sulfide (H2S), ammonia (NH3) and additional hydrocarbons to protect the precious metal catalysts in the other reactors. The hydrocracking reactor is operated operated at a sufficiently high temperature to convert 40 – 50 percent (volume) of the reactor effluent to material boiling below 400 degrees. The reactor effluent goes through heat exchangers to the hot high pressure separator (HHPS) where the hydrogen-rich hydrogen-ric h gases flashed off overhead. The hydrogen rich gases are then sent to the cold high pressure separator (CHPS) for additional separation from which the hydrogen rich gases are recycled to the first stage of the process for  mixing with additional hydrogen and the fresh feed. The liquid effluent from both the HHPS and CHPS is sent to a fractionalization column where the butane and lighter gases are taken off  overhead and the light and heavy naphtha, jet fuel and diesel fuel are removed as liquid side streams. Figure 1 shows the process flow diagram of a hydrocracking operation.

Figure 1: Generic Hydrocracking Process Flow Diagram

The separator letdown valves control the liquid level in the HHPS and CHPS. In some processes, these valves may dump the liquid effluent from the HHPS and CHPS to a low pressure separator before flowing to the fractionalization tower. Utilizing a low pressure separator allows additional removal of hydrogen and light hydrocarbons. In order to control the level in the HHPS, two valves (typically angle style) are normally used to control flow to the fractionalization column or low pressure separator. With pressures in the HHPS ranging between 1800 and 3500 psig and pressures in the fractionalization tower ranging from 100 to 300 psig, concerns arise due to flashing damage and vibration. High temperature (400 – 850 deg F) is another issue that must be dealt with. Not only will the flashing fluid be erosive it can also be corrosive. This is because the entrained H2S and NH3 can attack the trim and body materials. Because there may be some entrained catalyst from the reactor, the valve also must be able to pass the flowing particulate without plugging the flow passages and without causing erosion damage. The most common material used for the trim components is 316 SST with an Alloy 6 overlay. Valve bodies can be either  WCC (heat treated for NACE) or 316 SST. For this application, Fisher recommends the use of the Dirty Service Trim (DST) or the Type 461 Sweep-Flo control valve. The 461 Sweep-Flo is a rugged valve designed for high pressure drops where erosion, flowing particulate or flashing can cause severe valve damage. The angle valve design protects the valve body from erosion concerns due to flashing and flowing particulate. DST utilizes a staged pressure reduction to eliminate the formation of damaging cavitation and also compensates for volume expansion of flashing fluids via expanded area staging. DST is also designed to pass particulate up to ¾” in diameter ensuring the plugging due to catalyst fines will not occur.