Biomass to Ethylene (B2)
Introduction
Today ethylene is one of the most widely used organic compounds in the world with a global production of over 100 million tons per year. The widespread use of ethylene illustrates why engineers must continue to develop processes to more efficiently produce the organic compound. Currently, almost all of the commercially available ethylene is produced by steam cracking with petroleum as the starting ingredient. This process is a profitable and efficient method of making ethylene, but it is neither sustainable nor environmentally friendly. Furthermore, the volatility of the petroleum market makes the production of ethylene via petroleum cracking dependent on the petroleum market.
We investigated whether or not there was a sustainable and efficient method of producing ethylene, and we identified and developed a process that fulfills those two criteria. In this report we will provide a detailed explanation of our developed process, along with an economic analysis.
Executive Summary
The objective of this process is to create ethanol from biomass and then to convert this bio-ethanol to ethylene. The overall process feed is corn stover, which is degraded into simple sugars and fermented with the organism Zymomonas Mobilis to create ethanol. The ethanol is then converted to ethylene using packed bed reactors with an aluminum oxide on gamma-alumina catalyst. The working plant capacity is 2000 MT/day of corn stover. Refer to Appendix I for a overall process flow diagram.
The secondary objective of this process is to evaluate the feasibility of this process by considering economics and safety. All of these items will be discussed in the following sections.
Design Basis
Process Overview
Biomass to Ethanol Process
Ethanol Process Design
Ethanol to Ethylene Process
Ethylene Process Design
Design Trade-offs and Process Alternatives
While developing our process of using biomass to produce ethylene, our team had numerous design trade-offs that we had to consider to optimize our process. When designing a plant, calculated choices and sacrifices are made throughout the plant. We will discuss a few here.
A major design trade-off that our team needed to analyze was the length of the reaction train. Several sources used 3-4 reactors to react as much ethanol as possible, and in previous memos our team also used 3-4 reactors. Using a longer reaction train increases the extent of reaction and ultimately the yield of polymer grade ethylene. While optimizing our process and determining the economic viability of our process, we found that the reactor train should be shortened to 2 reactors.
Analysis of our previous HYSYS process with 4 reactors illustrated that the increased yield following reactor 2 was essentially negligible, meaning most of the reaction was complete following reactor 2. Obviously increasing product yield is of the utmost importance in all chemical processes, however, the trade-offs between yield and economic investment must be addressed. The capital costs, maintenance costs, and utility costs are doubled when comparing a 2 reactor train and a 4 reactor train. Implementing a 4 reactor train would economically cripple our plant during the early stages, while only providing a slight increase in yield. Ultimately, our process creates polymer grade ethylene at an excellent yield with only 2 reactors. Therefore, the pros of lengthening our reaction train do not outweigh the cons.
Another area where we had to analyze the pros and cons of different process techniques was in the separation/purification of our polymer grade ethylene. After our reaction section, we must purify our ethylene stream by removing water, ethane, acetaldehyde, acetic acid, and other byproducts of our reaction. We considered several different distillation techniques, but ultimately implemented a Triethylene Glycol (TEG) treatment flash separation process.
Distillation seemed like a very intuitive separation method to implement, but the design trade-offs forced us to choose TEG separation. Ultimately, TEG separation achieves a suitable separation at a much lower cost than distillation processes. For example, cryogenic distillation seemed like a very attractive process to implement due to the high purity that it provides by selectively distilling each byproduct out of our desired stream, but the economic requirements of running a cryogenic distillation system made the implementation of such a system in our process unlikely. Cryogenic distillation is extremely energy-intensive. To achieve low temperature distillation conditions, a refrigeration cycle is necessary along with an insulated enclosure covering the entire system. Maintaining the cold temperatures required for cryogenic distillation would drastically increase our utility costs and ultimately the efficiency of the plant. Of the distillation processes considered, we found that each one, like the cryogenic distillation process, was too expensive. TEG on the other hand, is a cheap and accessible material that is also somewhat sustainable. We recycle most of our TEG, and therefore do not need to continually replenish our TEG supply.