Comprehensive Utilization of C9 Resources Has a Bright Prospect

By Yuan Xiaguang, Sinopec Corp

Pyrolysis C9 fractions, which are by-products of ethylene production, are residual fractions being separated from C5 fractions, C6-C8 fractions (producing BTX (benzene-toluene-xylene mixture) through hydrogenation) through extraction of liquid raw materials like pyrolysis naphtha or light diesel oil. The output of C9 fractions is generally 10%-20% of the total ethylene output. Pyrolysis C9 fractions contain a large amount of styrene, indene, cyclopentadiene (CPD), methyl ethyl benzene, pseudocumene, mesitylene and 1,2,3-trimethylbenzene, and are valuable resources for the development of the fine chemical industry. most of the pyrolysis C9 fractions used in China is used as blending components for gasoline, and a small amount is used in making petroleum resins and solvent oils with relatively low added value..

1 Comprehensive utilization needs to be increased

China's actual output of ethylene reached 14.2 million tons in 2010, and the output of pyrolysis C9 fractions exceeded 1.5 million tons. Experts predict that China's ethylene capacity will reach 27.0 million t/a by 2015 and the output of resources will be 3.0 million tons, guaranteeing sufficient pyrolysis C9 resources for large-scale utilization.

Output and Utilization of C9 resources by Selected Domestic Ethylene Enterprises in 2009    thousand tons

Enterprise    Output of ethylene    Output of C9     Proportion of C9 in ethylene (%)    Utilization of pyrolysis C9
A    205.8    11.7    5.68    Petroleum resins, etc.
B    760.1    88.9    11.69    Petroleum resins
C    189    12.6    6.67    DCPD, etc.
D    927.7    95    10.24    Petroleum resins
E    796    99.2    12.46    Petroleum resins
F    875.3    103    11.77    Petroleum resins
G    1 060.6    99.6    9.39    Petroleum resins
H    698.2    99.3    14.22    Solvent oils
I    225    23.5    10.44    Hydrogenated gasoline
Total    5 737.7    632.8    11.03    -
Source: CNCIC

Pyrolysis C9 fractions are mainly used to produce C9 petroleum resins, high purity dicyclopentadiene (DCPD), high quality aromatic solvents, BTX aromatics (through hydrogenation), fine chemicals (by deep processing), etc. In recent years, domestic producers of C9 petroleum resins have made great progress. However, most of them are small in scale, with outdated production technology, a single-product portfolio, unstable quality, serious pollution and excessive consumption of raw materials. Moreover, their product profit-earning ability is poor.
The process of making DCPD from pyrolysis C9 fractions can only use a single component among the fractions and its profitability is generally restricted by the purity of DCPD and the market situation of its downstream products. China's solvent oils with high aromatic content are mainly produced from reformed heavy aromatics. A considerable amount of aromatic solvent oil with a high boiling point can be produced along with petroleum resins by using cracking by-product C9 aromatics as raw materials. However, the aromatic solvent oils are deficient in smell and color indicators and so they need to be hydrotreated. It is more difficult to produce aromatic solvent oils directly using pyrolysis C9 fractions as raw material, mainly because the components of pyrolysis C9 fractions are complex. Active component aromatic olefins account for 50%-70% of the total pyrolysis C9 fractions amount, causing great difficulty for hydrogenation.
Separation and hydrogenation of pyrolysis C9 fractions for producing more BTX aromatics has goo market potential. The R&D of related technology is also relatively active, and will hopefully become an important approach for the utilization of pyrolysis C9 fractions.
Funding for research into producing fine chemicals by deep processing of pyrolysis C9 fractions is far from adequate in China.

2 The technology for producing more BTX aromatics using pyrolysis C9 fractions as raw materials attracts attention in China

Output of p-xylene (PX) produced by direct extraction and separation of reformate and pyrolysis gasoline lags far behind the constantly increasing domestic demand. At present, the increase of PX output on an industrial scale can be achieved generally through the disproportionation and transalkylation reactions between toluene and reformed C9 aromatics. However, pyrolysis C9 fractions (containing 150 kinds of chemical ingredients) have complex components and low purity. In pyrolysis C9 fractions, the content of trimethylbenzene for transalkylation reactions is low, so BTX aromatics can be produced neither by the simple one- or two-stage hydrogenation treatment nor directly, through the traditional transalkylation route. Sinopec Shanghai Research Institute of Petrochemical Technology has researched hydrogenation technology for producing BTX aromatics using pyrolysis C9 fractions as raw material. Nanjing Normal University has also developed technology for the separation and comprehensive utilization of pyrolysis C9 fractions. All these will hopefully become new approaches for producing more BTX aromatics using pyrolysis C9 fractions as raw materials.
A. Hydrogenation technology for producing light aromatics
If pyrolysis gasoline is further processed through hydrogenation treatment after two stages of hydrogenation treatment, some heavy aromatics turn into light aromatics, and non-aromatics like alkane and cyclopentane break down (by hydrocracking). The products obtained in this way mainly include light alkanes, BTX aromatics as well as a small amount of heavy aromatics. Light alkanes and BTX aromatics can easily be separated through distillation, avoiding extraction by means of a solvent. Most light alkanes can be used as light cracking raw material or liquefied petroleum gas (LPG) in addition to a small amount used as fuel gas. Therefore, further hydrogenating pyrolysis gasoline after the usual two stages of hydrogenation has many advantages and maximizes economic benefits.
Sinopec Shanghai Research Institute of Petrochemical Technology has researched hydrogenation for producing more BTX aromatics from pyrolysis C6+ pyrolysis gasoline, and successfully developed a bifunctional catalyst with high activity and high selectivity. The test results on industrial unit show that using C6+ pyrolysis gasoline as raw material after two stages of hydrogenation treatment,
* most C9+A molecules are transformed,
* the conversion rate of C9 aromatics is more than 85%,
* ethyl benzene and C6-C8 non-aromatics are converted better and
* there is less loss of aromatics.
Compared with the traditional aromatics extraction and the C9+A utilization route, the technology is more cost effective and has better market prospects.
In general, the process can produce more BTX using low-cost pyrolysis C9+A and yields good economic benefits. The process can reduce the ethyl benzene content of BTX aromatics, separate C8 aromatics with low ethyl benzene content, reduce the difficulty of PX adsorption and separation, and has great significance for increased utilization of pyrolysis C9 aromatics and the improvement of PX adsorption/separation feedstock.
B. Separation and transalkylation technology
Nanjing Normal University has developed technology for separation of pyrolysis C9 fractions. High purity, light C9 fractions are produced using the method of combining continuous multistrand side cutting discharge fractional distillation and extraction distillation. The solvent to be selected can greatly increase the relative volatility among different components. The boiling point of the solvent is very different from that of the components in hydrogenation pyrolysis C9. And the recovery of solvent is therefore relatively easy. The main process is as follows: First, hydrogenation pyrolysis C9 aromatics are pretreated through the continuous distillation to remove heavier components and the content of light C9 aromatics climbs to around 90% from around 30%. Second, indane and bicyclic pentane can be extracted by distillation, further increasing the content of light C9 aromatics to around 98%. As the feedstock for traditional toluene disproportionation and transalkylation units, light C9 aromatics with high purity react with toluene to produce PX. Indane and bicyclic pentane can be used as raw materials for new developments in the fine chemical industry. The technology will hopefully open up a new way for chemical utilization of hydrogenation pyrolysis C9 fractions.
Toluene disproportionation and C9 aromatics transalkylation technology is an important means for the conversion of toluene and C9 aromatics to benzene and xylene, and an effective way for making full use of cheap industrial toluene and C9+A to produce mixed xylene and benzene. Presently, more than 50% of the mixed xylene produced in aromatic hydrocarbon integrated units is produced using this technology. The catalyst and the technology of toluene disproportionation and transalkylation have now become relatively mature. The latest generation of catalyst from Sinopec Shanghai Research Institute of Petrochemical Technology (S-TDT process) can be used to process mixed feedstock with the mass fraction of C9+A over 70%. However, in the current production unit, C9+A raw material is often inadequate, its proportion in the raw materials of the toluene disproportionation and transalkylation processes is usually lower than 60%, and it is difficult to fully use the heavy aromatics processing ability of the industrial catalyst. Therefore, using pyrolysis C9 to produce more raw materials for disproportionation and transalkylation processes has more practical significance.