base metal with a lining of stabilized austenitic stainless steel for added corrosion
protection. This choice of alloys gives the high strength of the base metal and the
excellent corrosion resistance of the inner lining. There are several items
concerning the selection of materials that must be taken into consideration during
the operation of the unit. Concerning the use of austenitic stainless steels in
hydrotreating units, the possibility exists for corrosion cracking to occur if proper
procedures are not followed. Corrosion cracking in a hydrotreating unit can occur
through chloride attack or polythionic acid attack. Chloride attack can be prevented
by minimizing the amount of chloride in the process material that will come in
contact with the austenitic stainless steel during normal operations. In addition,
during start-up and shutdown operations, precautions should be taken to limit the
chloride content in any flushing, purging, or neutralizing agents used in the system.
Polythionic acids occur as the result of the action of water and oxygen on the iron
sulfide scale that forms on all items made of austenitic stainless steel. Once formed
these acids can attack the austenitic steel and cause intergranular corrosion and
cracking. To prevent polythionic acid attack, it is necessary to maintain the temperature above the dew point of water in those areas containing stainless steel.
Under normal operating conditions, the system is essentially free of oxygen.
However, when the system is depressurized and the equipment is opened to air, it
becomes necessary to maintain a nitrogen purge to prevent air from entering.
In cases where adequate temperatures or purges cannot be maintained, a protective
neutralizing environment should be established. Generally, a 5 % soda solution is
used to neutralize the austenitic stainless steel.
Figure 26 shows a hydrotreating reactor with two beds of catalyst and one
interbed quench zone pictured, but the number of beds can vary for different
designs. As already indicated, most naphtha hydrotreaters only have one catalyst
bed. Many reactors processing cracked feedstocks will have several beds to facilitate temperature control by cooling with hydrogen quench between the catalyst
beds. For example, a reactor design could require three catalyst beds and two
interbed quench zones.
The reactor vessel is designed to allow maximum utilization of catalyst. Creating
equal flow distribution, providing maximum liquid/vapor mixing, and providing
multiple beds with quench zones for efficient catalyst usage achieve this. The
internals of the reactor found in a reactor are the following (though not all reactors
necessarily have all of them):
• Inlet diffuser
• Top vapor/liquid distribution tray
• Quench section (present only when there are multiple catalyst beds)
• Catalyst support grid (present only when there are multiple catalyst beds)
• Outlet collector
The size of hydrotreating reactors varies widely depending on the design
conditions and is dependent on the desired mass velocity and acceptable
Hydrotreating in Petroleum Processing
425
protection. This choice of alloys gives the high strength of the base metal and the
excellent corrosion resistance of the inner lining. There are several items
concerning the selection of materials that must be taken into consideration during
the operation of the unit. Concerning the use of austenitic stainless steels in
hydrotreating units, the possibility exists for corrosion cracking to occur if proper
procedures are not followed. Corrosion cracking in a hydrotreating unit can occur
through chloride attack or polythionic acid attack. Chloride attack can be prevented
by minimizing the amount of chloride in the process material that will come in
contact with the austenitic stainless steel during normal operations. In addition,
during start-up and shutdown operations, precautions should be taken to limit the
chloride content in any flushing, purging, or neutralizing agents used in the system.
Polythionic acids occur as the result of the action of water and oxygen on the iron
sulfide scale that forms on all items made of austenitic stainless steel. Once formed
these acids can attack the austenitic steel and cause intergranular corrosion and
cracking. To prevent polythionic acid attack, it is necessary to maintain the temperature above the dew point of water in those areas containing stainless steel.
Under normal operating conditions, the system is essentially free of oxygen.
However, when the system is depressurized and the equipment is opened to air, it
becomes necessary to maintain a nitrogen purge to prevent air from entering.
In cases where adequate temperatures or purges cannot be maintained, a protective
neutralizing environment should be established. Generally, a 5 % soda solution is
used to neutralize the austenitic stainless steel.
Figure 26 shows a hydrotreating reactor with two beds of catalyst and one
interbed quench zone pictured, but the number of beds can vary for different
designs. As already indicated, most naphtha hydrotreaters only have one catalyst
bed. Many reactors processing cracked feedstocks will have several beds to facilitate temperature control by cooling with hydrogen quench between the catalyst
beds. For example, a reactor design could require three catalyst beds and two
interbed quench zones.
The reactor vessel is designed to allow maximum utilization of catalyst. Creating
equal flow distribution, providing maximum liquid/vapor mixing, and providing
multiple beds with quench zones for efficient catalyst usage achieve this. The
internals of the reactor found in a reactor are the following (though not all reactors
necessarily have all of them):
• Inlet diffuser
• Top vapor/liquid distribution tray
• Quench section (present only when there are multiple catalyst beds)
• Catalyst support grid (present only when there are multiple catalyst beds)
• Outlet collector
The size of hydrotreating reactors varies widely depending on the design
conditions and is dependent on the desired mass velocity and acceptable
Hydrotreating in Petroleum Processing
425