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Ethylene Glycol
Process Analysis and Modeling
Warm up Project
September 5, 2001
Benjamin Alan Grayson
Ethylene Glycol
Ethylene glycol has many interesting properties and uses. At room temperature,
ethylene glycol is clear, colorless and slightly viscous liquid. It has no smell, but has a
sweet flavor to it. It is used as antifreeze and a deicing fluid for vehicles. Also, it is used
to make polyester compounds and is used in the paint and plastic industries as a solvent.
In addition, ethylene glycol is an active ingredient in photographic developing solutions,
hydraulic brake fluids and in inks [1]. When analyzing ethylene glycol, one should begin
with how ethylene glycol is produced, why it should be modeled, and what are some
proposed alternatives to common production techniques.
The major commercial production of ethylene glycol is by the hydration of
ethylene oxide [2]. According to the assignment, the minimum requirements of an
ethylene glycol production plant would be one reactor, two separators, and a distillation
column. However, most ethylene glycol productions plants include ethylene oxide
production plants as well. In order to fully discuss the production, one should look at
how the raw material of ethylene oxide is produced [3].
Diagram 1 shows a common process flow diagram of an ethylene oxide
production plant. Beginning with ethylene and oxygen flow, which includes methane as
a dilutent, the mixture enters into the reactor. The reactor described was a shell and tube,
recycled plug flow, kerosene cooled reactor. In the reactor, the oxygen and ethylene
react to form ethylene oxide, carbon dioxide, and water. The hot effluent from the
reactor passes through a heat exchanger and is cooled down. Small amounts of methane
are added to the effluent stream at this time to dilute the mixture. The methane is mostly
inert, but some does react during the ethylene oxide production process which reduces the
inert’s concentration. The next unit operation is a water absorber. This removes the
ethylene oxide and water to be used in the production of ethylene glycol, or sold as
ethylene oxide at this point. The recycle stream is then purged of the inerts and scrubbed
to remove the carbon dioxide. Then, the recycle stream reenters the ethylene and oxygen
feed streams [4].
Diagram 1: Flow Diagram of Ethylene Oxide Production [4]
After the ethylene oxide is produced, an aqueous solution of ethylene oxide is fed
into a plug flow reactor. The reactor hydrolyses the ethylene oxide solution and forms
ethylene glycol. Common byproducts of this reaction are di-, tri-, and tetraethylene
glycols. After the solution exits the reactor, the water is removed by evaporation from
the mixture to form a recycle stream. The concentrated solution of mono, di-, tri-, and
tetraethylene glycol is fed into a stripper to remove any light impurities and the remaining
water after evaporation. The dehydrated solution is then fed into a series of distillation
columns to separate the various glycols [5].
Modeling a process such as the production of ethylene oxide and ethylene glycol
can save time and money for the companies beginning the process of construction of a
new industrial plant. Even though the process flow chart appears relatively simple, there
are many variables that need to be modeled and controlled to achieve the optimal
production of ethylene glycol. Feed streams of all of the reactors will have to be modeled
to determine optimal flow rates and ratios of reactants. One example is the ratio of
reactants, ethylene and oxygen, in the feed stream to the ethylene oxide reactor in
Diagram 1. Taking the initial step shown above in the flow chart, the ethylene, oxygen
and methane ratios can have dramatic influences on the final product. Also, one has to
weigh the product result versus the safety concerns of having potentially explosive
materials on site. Another variable is the reactor temperatures. Fuel needed to run
furnaces and reactors costs are major expenses of many industrial businesses. Modeling
a process will give a potential company an idea of how big to make the reactors, how
much fuel is required per day, and the number of operators needed to run the plant safely.
Modeling this process to maximize profit and minimize cost is the goal of all successful
businesses.
After the plant is built, modeling of the process can determine if improvements to
the production schedule can be made. Demand for ethylene glycol in the US in 1997 was
6.3 billion pounds rising to 6.5 billion pounds in 1998. Projected use in 2002 is 7.5
billion pounds. Between 1982-1997, the highest price that ethylene glycol sold for was
66 cents per pound. Current prices are approximately 20-21 cents per pound [6]. With
billions of pounds of ethylene glycol produced in the United States every year, every
penny saved by decreasing fuel costs and increasing yield means millions of dollars of
profit for the industry.
As stated earlier, one important control aspect and modeling concern of safely
working in the production of ethylene oxide, ethylene glycol and their by-products is to
know the individual properties of the components. Reactors in this process run at
approximately 250oC. This operating temperature is well above the flash point of
ethylene oxide and all of the glycols. Above the flash point temperature, vapor-air
mixtures are explosive within flammable limits as noted below. The LEL is the lower
explosive limit and the UEL is the upper explosive limit. Although, there is a slight to
moderate risk at room temperature if these chemicals are exposed to an open flame,
containers may explode if they are involved in a fire. Safely modeling a procedure can
decrease potentially hazardous conditions before production begins.
Ethylene Glycol [7]
Diethylene Glycol [8]
Triethylene Glycol [9]
Tetraethylene Glycol [10]
Ethylene Oxide [11]
Boiling Point Flash Point Autoignition Temperature
197.6
111
398
245
117
229
278
177
371
276
141
*NA
10.6
-18
429
LEL
3.2
1.7
0.9
*NA
3
UEL
15.3
10.6
9.2
*NA
100
*NA = not available
Table 1: Properties of Selected Products and Reactants
Up until this point, ethylene glycol produced from ethylene oxide is the only
production process discussed. Some alternate methods of producing ethylene glycol have
been conceived. According to one source, ethane can be reacted with water using a
hypothetical reaction to make ethyl alcohol [12]. By continuing this procedure one step
further, we can obtain ethylene glycol.
Ethylene glycol can also be readily prepared in the laboratory by refluxing ethylene
dichloride with a dilute solution of sodium carbonate [13].
One final interesting discover is that in the literature searches on the production of
ethylene glycol, most companies used a plug flow reactor to react the ethylene and
oxygen to form ethylene oxide, and the ethylene oxide and water to form ethylene glycol.
However, groups are modeling and experimenting on trying a different approach to the
production of ethylene glycol. They describe a reactive distillation column as a viable
alternative to plug flow reactors. “Reactive distillation is an innovating process which
realizes both distillation and chemical reaction into a solely unit. The underlying
motivation relies on the fact that industrial RDC [Reactive Distillation Columns]
processes may be operated at unstable operating conditions, which often corresponds to
optimal process performance. [14]” “Reactive distillation is potentially an attractive
process alternative to conventional liquid-phase chemical reaction processing for systems
that exhibit one or more of the characteristics: equilibrium limited chemical reactions,
exothermic reactions, poor raw materials usage due to selectivity losses, or excessive
flowsheet complexity. Commercial reactive distillation processes such as the Nylon 6,6
process, methyl acetate process and MTBE process have demonstrated reductions in
capital investment and/or energy consumption. [15]”
After analyzing ethylene glycol, looking at how ethylene glycol is produced, why
it should be modeled, and what are some proposed alternatives to common production
techniques, one should realize that this clear, colorless, and odorless substance is very
interesting in the eyes of researchers. Many possibilities lie in process alternatives, and
projected consumer usage makes it a profitable investment for industry. Modeling and
control systems are valuable tools and are used every day to maximize profit and
minimize costs for industry.
References
[1]
http://www.atsdr.cdc.gov/tfacts96.html
[2]
http://www.chemexpo.com/news/profile981102.cfm
[3]
http://www.us.thermal.alfalaval.com/process/ethylene_glycol.html
[4]
http://www.owlnet.rice.edu/~ceng403/ethox97.html
[5]
http://www.us.thermal.alfalaval.com/process/ethylene_glycol.html
[6]
http://www.chemexpo.com/news/profile981102.cfm
[7]
http://www.jtbaker.com/msds/e5125.htm
[8]
http://msds.pdc.cornell.edu/msds/siri/msds/h/q131/q398.html
[9]
http://www.mathesongas.com/msds/TriethyleneGlycol.htm
[10]
http://www.mathesongas.com/msds/TetraethyleneGlycolDimethylEther.htm
[11]
http://msds.pdc.cornell.edu/msds/siri/msds/h/q195/q116.html
[12]
http://wey238ab.ch.iup.edu/olccii/student/eg.html
[13]
http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/glycol.html
[14]
Monroy-Loperena, Rosenda, et al. A robust PI control configuration for a highpurity ethylene glycol reactive distillation column. Chemical Engineering Science
55 (2000) 4925-4937.
[15]
Chen, Fengrong, et al. Simulation of kinetic effects in reactive distillation.
Computers and Chemical Engineering 24 (2000) 2457-2472.