Alloys for hydrocracking process in refineries

Hydrocracking is a combined desulfurization and cracking operation that can convert a complete range of hydrocarbon feedstocks into valuable products. The transformations occur in the presence of high pressure hydrogen, so hydrocracking is hydrogenation or inclusion of hydrogen to molecules. Although differences occur from plant to plant, specifically in the various reactors and arrangement of heat exchangers. Many systems have more than one reactors with large plans having a desulfurized reactor first, after a hydrocracking reactor.

The reactions occur at temperatures of 650oF to 8500oF at pressures about 1200 to 3000 psi and in the availability of a hydrogen rich recycle gas stream. After heat exchange with the hot effluent, the charge oil and recycle gas streams are combined and heated in a feed heater to the required inlet temperature. To control the temperature of the highly exothermic reactions, cool recycled quench gas is injected between the beds in the reactor. The reactor effluent is quenched, flashed and washed in the high pressure separator, flashed at low pressure and fractionated.

The corrosion issues and factors and compensatory remedies are much similar to those in hydrotreating. The stainless steels used in hydrocracking desulurizing reaction section are stainless steels 304, 321 and 410. The austenitics are recommended because of their high temperature strength and due to the problems of 885oF embrittlement with ferritic stainless steels. Although ferritic stainless steels the grades are used as tube material for feed- effluent exchangers operating below 700oF. 

The stabilized grades of Inconel bar are used to prevent sensitization particularly in the welds and the feasibility of polythionic stress corrosion cracking.

With feed and recycle gas flowing through heat exchangers, it is safe to consider that the steam temperature will be above 550oF or 288oC and the application of stainless steel is justified. The reactor effluent on the opposite side is specifically above 650oF or 343oC, so irrespective of the material types used on the feedside, the effluent needs stainless steel materials.

The tube sheets for feed effluent exchangers are made from stainless steel 304 or 321. The channel section is made of the same grades. The shell is often clad with stainless steel to prevent sulfidation from the hot effluent stream.

In the feed pipes as soon as the temperature reaches 550oF, it depends on the plant size whether it is made from wrought austenitic stainless steel or centrifugally cast HF modified. In the modification, there is often lower carbon and controlled ferrite, it offers more resistance to polythionic cracking. The wrought alloys are popularly chosen for pipes are grades 347 and 321. In the feed heater, grades 304 or 321 furnace tubes are used, even they are aluminized to reduce scaling.

Some service companies aluminize stainless steels for added security against polythionic corrosion feasibilities and to decrease high temperature sulfide scale development. This method is used by these firms in hydrotreaters and for the desulfurization stage in hydrocracking. An alloy is aluminized that is corrosion resistant, the feed effluent exchanger is used as a stabilizer feed heater.