Overview on C2 and C3 Selective Hydrogenation in Ethylene Plants
Edgar L. Mohundro
Prepared for Presentation at the
American Institute of Chemical Engineers
15th Ethylene Produces Conference
2003 Spring National Meeting
New Orleans, LA
March-April 3, 2003
Session: Ethylene Plant Technology
C2 and C3 Hydrogenation Technology Review
This paper will provide an overview on C2 and C3 selective hydrogenation operations in ethylene plants. The primary focus will be on selective hydrogenation of acetylene. The reason for this is that most C3 selective hydrogenation units installed since the mid-late
1970's are liquid phase units. For the most part, the details about the design and operation of liquid phase processes are proprietary, licensed technology.
This is an overview paper, providing background information for the succeeding papers in this session. The following papers will cover in more detail some experiences and know-how with acetylene and/or C3 selective hydrogenation within ethylene plants.
Acetylene and C3 hydrogenation units are purification steps for ethylene and propylene. Trace levels of acetylene, methyl acetylene (MA), and propadiene (PD) are formed in the steam cracking furnaces. The amount of acetylene and MAPD formed is a function of the furnace feed, along with furnace design and operating conditions. Acetylene will fractionate with ethylene; MAPD will fractionate with propylene. These components are only tolerable at < 1 ppm in polymer grade ethylene and propylene.
The overwhelmingly dominant method for removal of acetylene and MAPD is through selective hydrogenation. With few exceptions, acetylene hydrogenation is carried out, in vapor phase, adiabatic fixed bed reactors. The number of beds ranges from 1 to 4, with intercooling. Acetylene hydrogenation is also done with isothermal tubular reactors.
Liquid phase C3 hydrogenation became the "process-of-choice" around the mid-70' s. The dominant process is an adiabatic, fixed-bed, liquid phase system. Some C3 liquid phase hydrogenation units have isothermal, tubular reactors. Many vapor phase C3 hydrogenation units are still in service. During past Ethylene Producers Conferences (EPC), there have been several papers that deal with acetylene and C3 hydrogenation and the impact of contaminants on these operations. These are good reference documents. They are given in the "List Of References" for this paper. This paper will cover the following items"
- Options for locating hydrogenation units within ethylene plant sequences
- Pros/Cons for Each Location
- Operating Strategy
- General Design Comments
- Promoted Catalysts
- Theory of Acetylene Hydrogenation
- Green Oil
- Comments on Contaminants
- Regeneration Considerations
General Comments on C3 Hydrogenation
Link to paper