Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-01-20
2001-10-30
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S935000
Reexamination Certificate
active
06308532
ABSTRACT:
BACKGROUND OF THE INVENTION
Petrochemical offgases, such as refinery offgases from Fluid Catalytic Cracker Units (FCCU) or Coker units, generally comprise an offgas mixture which comprises: hydrogen; nitrogen; carbon monoxide; ethane; ethylene; argon; propylene; as well as, butane and pentanes. In particular, the refinery offgasses from a FCCU contain olefin components, up to about 20 percent by volume ethylene and up to about 11 percent by volume propylene, which components normally are not recovered from the offgases, but which components may have value to warrant recovery and use in other petrochemical processes or uses in downstream processing. The typical range of components is listed in Table 1.
Normally, such refinery offgases are burnt and used as fuel. When the refinery FCCU is located near an ethylene or propylene plant, then recovery and downstream use of the recovered ethylene and propylene or propylene is economical. In such recovery processes, ethylene and propylene are recovered together and then directly or separately recovered from the mixture.
In the publication, Chemical Engineering, May 1999, pages 30-33, “Refiners Get Cracking on Petrochemicals” hereby incorporated by reference, the attributes of Fluid Catalytic Cracker (FCC) technology are explained. Propylene is increasing in demand, since it is a feedstock for polypropylene. About 30 percent (30%) of the global demand is a byproduct of FCC operations for producing gasoline. Over the next five years, the growth in demand is anticipated at over 5 percent (5%) per year. In response to this demand, FCC process technology is being modified to enhance olefin's production. Thus, the current yield of 5 percent (5%) of the feedstock converted to propylene is being increased to about 35 percent (35%), while the ethylene yield is increased from about 1 to 4 percent (1-4%).
Another option is the reaction of ethylene with 2-butene to produce propylene, termed metathesis. Thus, the recovery of these components from a FCC provides the opportunity for further propylene yields.
U.S. Pat. No. 5,546,764, issued Aug. 20, 1996, and incorporated herein by reference, discloses an absorption process for recovering ethylene from a feed gas stream in an absorber stripper employing a heavy hydrocarbon absorption solvent, the absorber stripper bottoms stream is then fractionated to produce an overhead, ethylene product stream.
The recovery of valued olefin components from offgasses may be accomplished by partial condensation from the offgas feed stream; however, such recovery requires low temperatures, e.g., about —150° F. or lower. These low temperatures require refrigeration supplied by turboexpander processes and heat exchangers with multi-pass plate fin heat exchangers. This type of equipment is not used extensively in oil refineries. Furthermore, the use of such low temperatures increases the possibility of gum formation by the reaction of NOX compounds with butadiene, which gums may be explosive when the plant is shut down and warmed up for restart.
U.S. Pat. No. 5,345,772, issued Sept. 13, 1994, shows a single column process with Natural Gas Liquid (NGL) recycled to the upper zone of the column. This process is applied to the recovery of the paraffinic components of propane or ethane from gas streams with a high CO
2
content. These gases are encountered in enhanced oil recovery projects where the CO
2
content of the gas stream may be 83 percent (83%), as shown in the patent example.
It is desirable to provide a system and process for the recovery of olefins from offgases at warmer temperatures, such as propane refrigeration levels, to avoid the disadvantages of prior art recovery processes and systems and to provide other economic and process efficiency advantages.
SUMMARY OF THE INVENTION
The invention relates to a system and process for the recovery of olefins from petrochemical offgases. In particular, the invention concerns the recovery of ethylene and propylene or propylene from refinery offgases from a Fluid Catalytic Cracker Unit (FCCU) of a refinery at propane refrigeration temperatures.
The process provides for maintaining the overhead condenser temperature by the use of the liquid bottoms recycle stream from the single column. Generally, the overhead condenser is maintained at above −140° F., but depending on the olefin component to be recovered, the overhead condenser temperature may range, for example, from above about −114° F. for ethylene recovery or above about −35 to −40° F. for propylene recovery, or within a range of about −20° F. to −114° F. for a mixture of ethylene and propylene.
The system and process generally comprises a single distillation column to receive an olefin-containing, hydrocarbon gas stream and maintaining the overhead condensation temperature at a warmer temperature than −150° F., such as, at a refrigeration temperature level of above −140° F., by recycling a heavy C
3
+ bottoms stream from the single column, to the overhead condenser or upper section of the distillation column, and withdrawing a liquid bottoms stream; a vapor side stream rich in the selected olefin; and an overhead vapor stream lean in the olefin components.
The system and process of the invention comprises a system and process for the recovery of olefin components, such as: ethylene; propylene; or combinations thereof from a refinery offgas, such as: a FCCU; a reformer; a Coker offgas; or other olefin gas source, which process comprises: introducing the gas feed stream into a recovery distillation column with an overhead condenser; withdrawing a liquid bottoms product stream from the recovery column; recycling at least a portion of the liquid or lower section bottoms product stream into the overhead condenser or into an upper tray section, e.g., top 5 to 10 trays of the recovery column, to warm the overhead condenser; maintaining with the recycled product stream, the overhead condenser at selected propane, propylene, or ethylene refrigeration levels of above −140° F. and generally above −114° F.; withdrawing a heavy liquid product stream from the recovery column; withdrawing a vapor phase side downstream from the column, which side downstream is rich in the olefin components to be recovered; and condensing and recovering the selected olefin components from the vapor phase side downstream, for downstream processing use; and withdrawing an olefin lean, overhead vapor stream from the overhead condenser or an upper section of the recovery column; and optionally, employing the lean overhead vapor stream as regenerated gas in the dehydration of the feed gas stream, fuel use, or other use.
Typically, and optionally, the refinery offgas feed stream process includes: pretreating steps of water washing to remove impurities; removing acid gases, like carbon dioxide and hydrogen sulfide; and dehydrating the offgas feed stream. The process may be directed to the recovery of propylene or ethylene-propylene, generally, for downstream processing use, such as, where an ethylene-propylene petrochemical plant is located near the offgas source.
The system and process provide multiple economic and process advantages over prior art systems and processes to include, but not be limited to: eliminating the requirement of a separate column to produce the C
4
+ recycle stream, while only propane level refrigeration is required in some instances for the system and process. The system and process eliminate the formation of dangerous and explosive prior art gum formation, because of the relatively warm refrigeration level temperatures employed.
When propane refrigeration alone is used, the process permits the use of a carbon steel column and carbon steel shell and tube heat exchangers, due to the propane or higher temperature refrigeration levels and no expensive turboexpanders are employed. While in one embodiment illustrated, acid gas removal occurs in the offgas feed stream, the acid gas need not be removed upstream of the recovery column, because of potential freezing
Calfee Halter & Griswold LLP
Capossela Ronald
Chart Industries, Inc.
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