Grantee Research Project Results
Final Report: A Liquid Absorbent for In-Process Recycling of Ethylene from Purge Streams
EPA Contract Number: 68D98142Title: A Liquid Absorbent for In-Process Recycling of Ethylene from Purge Streams
Investigators: Lyon, David
Small Business: Bend Research Inc.
EPA Contact:
Phase: I
Project Period: September 1, 1998 through March 1, 1999
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (1998) RFA Text | Recipients Lists
Research Category: SBIR - Pollution Prevention , Pollution Prevention/Sustainable Development , Small Business Innovation Research (SBIR)
Summary/Accomplishments (Outputs/Outcomes):
This summarizes work completed by Bend Research, Inc., on a Phase I program for the Environmental Protection Agency on Contract No. 68D98142, titled "A Liquid Absorbent for In-Process Recycling of Ethylene from Purge Streams." The goal of this program is to develop a liquid absorbent for the selective recovery of ethylene from ethylene purge streams.Ethylene surpasses all other organic petrochemicals in production and is the primary raw material in a wide variety of synthetic organic chemical products, including polyethylene, various polyethylene copolymers, synthetic ethanol, ethylene oxide, and acetaldehyde.
The largest use of ethylene is in the production of the various forms of polyethylene. About 20 billion pounds of polyethylene are produced annually in the U.S. alone. Low-density polyethylene (LDPE) is typically polymerized at high pressures in the presence of a free-radical initiator. In the polymerization process, which requires significant monomer recycle because of low per-pass conversion, about 1% to 2% of the ethylene feed is purged from polyethylene reactors to minimize accumulation of feed-stream impurities such as methane and ethane.
In most plants, this ethylene-containing purge stream is flared, resulting in the loss of valuable ethylene monomer and generation of significant process pollution. The EPA recognizes that in-process recycling reuse of a potential waste within existing operations not only reduces discharges to the environment but also conserves raw materials, reducing the environmental impact of their production. This serious processing and environmental problem represents a major in-process recycling opportunity, based on the enabling technology that is the focus of this program.
Several existing processes such as cryogenic distillation and polymeric membranes can be used to recover ethylene from purge-gas streams. However, low process temperatures and high capital costs (for cryogenic distillation) and modest selectivities for purge-stream components (for commercial polymeric membranes) lead to significant capital and operating costs, which can largely negate the benefits of in-process recycle.
The focus of this program is the development of an in-process recycle technology for recovering ethylene from purge-gas streams. Key to this process is development of a liquid absorbent that will allow the selective separation of ethylene from the purge-gas stream.
A schematic of the proposed in-process ethylene-recycle process is shown in Figure 1. In this process, ethylene is recovered from the purge stream using an absorbent solution that has high affinity for ethylene and but little or no affinity for the other components of the feed stream (primarily the paraffins ethane and methane). The ethylene-containing stream is brought into contact with the liquid absorbent using a conventional packed absorption column under conditions that favor absorption. The liquid absorbent is then be pumped to a flash tank, where the paraffins are recovered. The ethylene does not desorb in this step. Instead, the ethylene-rich liquid absorbent is pumped to a second tank (i.e., the desorption column), where it is heated to about 100oC at 50 to 100 psia. Under these conditions, the ethylene is removed from the absorbent solution, exits the desorption column, and can be recycled back to the polymerization reactor. This step regenerates the liquid absorbent, which is then cooled and pumped back to the packed absorption column for reuse.
Because the engineering methods for this type of process are well-developed (the process is analogous to natural-gas sweetening processes that employ organic amines), the key to developing the proposed process for in-process ethylene recycle is development of a liquid absorbent that
(1) selectively absorbs ethylene over the paraffin components in the
purge-gas stream, and
(2) is suitable for use in the process described above.
During Phase I, we successfully demonstrated the feasibility of our approach by preparing three absorbent solutions with high selectivities for ethylene over paraffins and rapid ethylene-absorption rates.
Based on the absorbent performance achieved in Phase I and the improved absorbent stability we expect to attain in Phase II, we estimate processing costs of $0.15/lb of ethylene for the in-process ethylene-recycle process a cost that is lower than the current price of ethylene ($0.19/lb). We believe that by optimizing the absorbent performance during Phase II, we can reduce processing costs to approximately $0.06/lb, paving the way for commercialization of the process in Phase III.
In addition to our experimental work, during Phase I we worked with Robert Anderson of Foresight Technologies, a commercialization consultant working with the EPA. He prepared a technology-assessment report assessing the commercial potential for the ethylene-recycle technology, which shows strong commercial promise.
Supplemental Keywords:
Scientific Discipline, Water, Sustainable Industry/Business, cleaner production/pollution prevention, Chemistry, Technology for Sustainable Environment, New/Innovative technologies, Engineering, Engineering, Chemistry, & Physics, in-process recycling, polyethylene, waste reduction, liquid absorbent, in process recycling, recycling, innovative technology, pollution preventionThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.