Issue 44/2012 - Analysis of isoprene production technology

Analysis of isoprene production technology

Isoprene is one of the most versatile and abundant components of cleaved C5, usually extracted
from a mixed C5 fraction of a cracked heavy liquid hydrocarbon.
There are three main production
methods of isoprene: dehydrogenation, synthesis and extraction distillation.

Dehydrogenation method

The dehydrogenation method is divided into isoamylane dehydrogenation and isoprene dehydrogenation according to raw materials, and catalytic dehydrogenation and oxidative dehydrogenation
according to the reaction mechanism.
At present, catalytic dehydrogenation has industrial production plants
in Gutrich Company in the United States, Sierou Company in the Netherlands and Russia.

Isopentane two-step catalytic dehydrogenation method The raw material isopentane comes from catalytic cracking or straight-run gasoline, first dehydrogenating isoamylene to isoprene, and then isopentene catalytic dehydrogenation to produce isoprene, and then using acetonitrile or dimethylformamide extraction and distillation to obtain high-purity isoprene products
.
The feature of this method is that the raw materials are cheap and easy to obtain, but the process flow is relatively complicated
.
Two-step catalytic dehydrogenation of isoamylene is the main method
for the production of isoprene in CIS and Eastern European countries.

Isoprene catalytic dehydrogenation The raw material isoamene is extracted
and separated from the C5 fraction of the refinery.
The catalytic dehydrogenation of isoprene to crude isoprene includes three steps: dehydrogenation, adsorption and rectification, and in order to obtain polymeric-grade products, crude isoprene must also be purified
by extraction.
This method is characterized by a wide concentration range of raw isoprene (10%~30%), but in recent years, this method has not been used to produce isoprene
.

Chemical synthesis

Chemical synthesis methods mainly include isobutylene-formaldehyde method, acetylene-acetone method, propylene dimerization method, etc
.

Isobutylene-formaldehyde method uses isobutylene and formaldehyde as raw materials to produce isoprene, so it is also called enaldehyde synthesis method
.
It can be divided into one-step and two-step methods, which are being developed by the former Soviet Union in 1964 and industrialized
by Toray Corporation in Japan in 1972.

Isobutene-formaldehyde one-step method There are two kinds of liquid phase and gas phase synthesis, and the gas phase synthesis method uses phosphate catalyst or silicon oxide and antimony oxide as catalyst, and formaldehyde and isobutylene gas are reacted to generate isoprene
.
The liquid phase synthesis method is to use tert-butanol generated by hydration of the extracted C5 fraction containing isobutylene after extraction of butadiene as raw material, with solid acid or solution acid as catalyst, excess tert-butanol (or isobutylene) and formaldehyde for liquid-phase reaction to generate isoprene
.

The process has good development prospects, but it is still in the development stage, and the reasons for the failure to industrialize are mainly the low single-pass conversion rate and selectivity
.

Isobutylene-formaldehyde two-step method isobutylene and formaldehyde react in the presence of dilute sulfuric acid catalyst to generate 4,4-dimethyl-1,3-dioxane, and then further crack to generate isoprene
in the presence of phosphoric acid catalyst.
The raw material isobutylene of the two-step method can be used as C5 fraction and/or ethylene cracked C5 fraction, and formaldehyde can be used in 37%~40% formaldehyde aqueous solution
.
The law was developed and industrialized by Japan, and industrialized
by Russia.
The disadvantages of this method are long process, high cost, low yield and poor
selectivity.

Acetylene-acetone method Isoprene
is synthesized from acetylene and acetone as raw materials.
Originally developed by the Italian company SNAM, it was industrialized in 1970 in Renenna, Italy, with a 30,000-ton industrial plant
.
The process mainly includes the alkyne reaction of acetylene and acetone in liquid ammonia catalyzed by potassium hydroxide to synthesize methylbutynol, and the yield of acetone can reach 95%; The selective hydrogenation reaction of methylbutynol hydrogenation to methbutanol by palladium catalyst has a hydrogenation yield of about 99%; The dehydration reaction of methanol dehydration to isoprene under the catalysis of aluminum chloride can reach 99.
8%.

The method is characterized by high isoprene yield, up to 89%, mild operating conditions, carbon steel can be used for equipment, but the price of raw materials acetylene and acetone is higher
.
And the acetylene used is very dangerous, so it is generally not used
.

The propylene dimerization method was developed by Goodyear and Scientific Design in the United States, and a 60,000-ton plant
was built in the United States in 1962.
The process includes propylene dimerization into 2-methyl-1-pentene, propylene conversion rate 60%~95%, selectivity 95%; The conversion rate of isomerization reaction is 70%~75%, and the selectivity is 90%~99%; Cracking (demethylation), the total yield of isoprene is 5O%~70%
of the raw material.
This method is characterized by high raw material consumption and low yield, and propylene price is the key to
whether the process is competitive.

Extractive distillation

The industrial separation method of isoprene is mainly solvent extraction distillation
.
According to the different solvents used, the separation methods mainly include acetonitrile (ACN) method, dimethylformamide (DMF) method and N-methylpyrrolidone (NMP) method, and a variety of new separation methods have been developed in recent years and gradually applied
in industrial production.

ACN method is one of
the C5 fraction separation methods widely used abroad.
Japanese synthetic rubber (JSR), ExxonMobil, Shell and other companies have successfully developed the ACN method separation process of C5 fractions and built industrial plants
.

The ACN method is characterized by abundant sources of acetonitrile, low price and low
corrosion to equipment.
It is easier to solve
the problems of low solvent viscosity, high extraction column efficiency, low operating temperature, and equipment blockage caused by material polymerization.
However, the isolated isoprene is not high purity and can only meet the requirements
of the specifications as a raw material for butyl rubber.
If you want to meet the specifications of isoamyl rubber raw materials, you must use a combination of acetonitrile method and chemical treatment, which makes the production process complex and the cost increases
.

At present, the world's largest acetonitrile method for isoprene separation plant is an 80,000-ton plant
built by Goodyear in 1977.

DMF method is successfully developed by Japanese company Ruion Company, and the process is mainly composed of
two-stage extractive rectification and two-stage ordinary rectification.

The DMF method is characterized by the fact that the raw material does not require heat treatment, the solubility of isoprene is large, the solvent selectivity is high, the operating cost is low, the equipment is not corroded, and it can by-produce dicyclopentadiene
of a certain purity.
Therefore, the product yield of this method is higher and the purity of the product is also high
.
But DMF is a very harmful carcinogen and is not good
for the health of the operator.

NMP method uses N-methylpyrrolidone as the extraction solvent to achieve isoprene separation, which was first developed by BASF in Germany and later improved
.
The improved process uses catalytic hydrogenation technology, which eliminates the second extraction unit
.

The C5 fraction enters the extraction distillation column with NMP containing 8% water (mass fraction) as the solvent
.
The diolefin-containing stream is produced from the side line of the extractive distillation column and enters the distillation column
.
Isoprene is obtained at the top of the distillation column, and metaprene, cyclopentadiene and saturated solvent are returned to the bottom
of the extraction distillation column.
The solvent containing m-prene and cyclopentadiene with a mass fraction of 95% is extracted from the bottom of the extraction distillation column, enters the hydrogenation reactor, and performs a hydrogenation reaction on a palladium-containing alumina carrier catalyst to convert meta-prene to pentene and pentane, and cyclopentadiene to cyclopentene and cyclopentane
.
The hydrogenated material returns to the top of the extractive distillation column, and the top distillate of the column is composed of pentene, pentane, cyclopentene and
cyclopentane.

C5 fractions can also be advanced to the distillation column, and then to the extraction rectification column, and isoprene is produced from the side line of the extraction and rectification column
.

The process is relatively simple, the solvent is not toxic, the NMP pre-washing method is used to remove cyclopentadiene, 1,3-pentadiene and 2-butyne, the isoprene yield can reach more than 97%, but the purity of the product is relatively low
.

azeotropic distillation The azeotropic distillation method developed by Goodyear uses the characteristics of isoprene and n-pentane to form azeotropes to separate and extract isoprene in the C5 fraction, and the resulting product is isoprene-n-n-pentane azeotrope
.
This method is suitable for n-pentane can be used as a solvent for isoprene polymerization reaction or has no effect on isoprene polymerization reaction in the presence of n-pentane
.
In general, the mass fraction of isoprene in this azeotrope is greater than 70%.

The whole process of azeotropic rectification actually requires only two columns
: the delightening column and the deweighting column.
The delightening tower removes components with a lower boiling point than the azeotrope, and the deweighting tower removes components with a boiling point higher than the boiling point of the azeotrope
.
If there is a special need, there can be multiple lightening towers and multiple weight-removal towers
.
In order to obtain high-purity isoprene, a large plate count and a large reflux ratio
are required.

The biggest disadvantage of azeotropic rectification is that the obtained product is an azeotrope of isoprene and n-pentane, and it is impossible to obtain isoprene
with higher purity.
Further purification and regeneration is cumbersome and not a very economical process
.

Other methods In addition to the above methods, a new separation method, namely chemical absorption method
, has been developed in recent years.
This method uses metal cations (such as Ag and Cu) to carry out reversible reactions with diolefins to form Ag (or Cu)-π diene electron complexes, which can be separated from
alkanes because the complex is immiscible with organic substances.
Complexation reactions are reversible reactions in which the diolefins in the complex are recovered
by changing the temperature or pressure.
Chemical absorption method has the advantages of low energy consumption, good selectivity, simple device, saving equipment investment, environmental friendliness, etc.
, and has great potential and has attracted more and more attention
.
However, no industrialization of the process has been reported
at present.

Conclusion

With the development of ethylene industry, the cracking C5 resources as ethylene by-products are becoming more and more abundant and concentrated, and the separation and utilization of C5 fractions have received more and more attention
.
Combined with the development trend of C5 fraction separation and utilization at home and abroad, the chemical utilization of C5 olefin has shifted from the initial mixed utilization to single-component utilization, and the utilization focus is mainly focused on isoprene, meta-prene and cyclopentadiene in C5 fraction, so the development of a more effective technical route for separating and cracking C5 fraction is still the focus of
rational use of C5 fraction in the future.

At present, the main method of C5 total fraction separation process is still concentrated on extractive rectification, and the solvents used are mainly ACN, DMF and NMP, and the three solvent extraction methods have been successfully industrialized
.
Whether in the separation process, energy consumption, material consumption, etc.
, various solvent extraction methods have room for
technological progress.
Existing processes can be further improved
by adopting methods such as thermal integration, process coupling, and process optimization.
At the same time, the development of new extractants, the introduction of catalytic hydrodeyne removal technology and the adoption of chemical absorption methods are also new directions for the future development of
C5 separation technology.