Method for removing organic sulfur contained in liquefied petroleum gas

With the development of modern petrochemical technology, many high-efficiency catalysts have more and more stringent requirements on the sulfur content in the raw materials.

On the other hand, environmental protection standards are getting stricter and stricter requirements are imposed on the total sulfur content of LPG.

Sulfur in LPG can be divided into organic sulfur and inorganic sulfur.

Inorganic sulfur is mainly hydrogen sulfide, which can be easily removed by using different desulfurization agents and corresponding desulfurization processes. Organic sulfides include polar sulfide mercaptans and non-polar sulfides (or neutral sulfides) such as sulfides, carbon sulfides, carbon disulfides, and disulfides.

At present, the commonly used methods for removing organic sulfur include hydrodesulfurization and catalytic oxidation. The hydrodesulfurization method is to remove the above-mentioned organic sulfur in the liquefied petroleum gas by hydrogenation into inorganic sulfide, and the cost of this method is relatively high.

The catalytic oxidation method converts mercaptans into odorless disulfides and extracts them with solvents, but the precision of desulfurization cannot meet the requirement of low sulfur content in raw materials.
Molecular sieves have many uses in physical, chemical and chemical processes, and are most suitable as selective adsorbents to complete the separation of components in mixtures and as catalysts.

The use of X-type molecular sieves to remove H2S and mercaptans in natural gas has been applied, but it cannot be used to remove neutral organic sulfides.
Copper and lanthanum were successively used as modified cations of 13X molecular sieve, the contact time for ion exchange was 24 and 48 hours, and the temperature of ion exchange was room temperature and 75-80°C.

The molecular sieve obtained after the exchange is used for the removal of total sulfur, including hydrogen sulfide, of flowing hydrocarbons, which can be removed by refluxing for 1 hour at a temperature of about 250° C. 60% total sulfur.

This kind of desulfurization adsorbent adopts chemical adsorption method for desulfurization, and adopts corresponding oxidation regeneration method for regeneration, so the regeneration is complicated and the service life is short.

The liquefied petroleum gas after removing hydrogen sulfide passes through the desulfurizing agent bed in the organic sulfur removal device.
Under the action of the desulfurizing agent, the organic sulfides in the liquefied petroleum gas are directly adsorbed by the desulfurizing agent by physical adsorption.

The used desulfurizer is a modified X-type or Y-type molecular sieve whose main component is exchanged with high-valent metal cations, the high-valent metal cations are lanthanum, cerium or mixed light rare earth elements, and the exchange degree is 70% to 95%.

The desulfurizer adsorbs the organic sulfides of liquefied petroleum gas at normal temperature and operating pressure of 0.6-2.0MPa; the space velocity of the liquefied petroleum gas passing through the desulfurizer bed is 1-10h-1.

It can desorb and regenerate the desulfurizer in the desulfurizer.
When it is detected that the stream outlet of the standby organic sulfur removal tank contains sulfur, the operation of removing organic sulfur compounds in the liquefied petroleum gas is switched to the organic sulfur removal tank.

At this time, the desulfurization agent in the spare organic sulfur removal tank can be desorbed and regenerated.
The process of alternately adsorbing and desorbing the desulfurizing agent in the organic sulfur removal tank and the standby organic sulfur removal tank can maintain the continuous operation of the organic sulfur removal operation in LPG.

Use nitrogen or catalytic dry gas to regenerate the above-mentioned desulfurizer; the regeneration temperature is 250-400°C, the pressure is normal pressure or negative pressure, and the space velocity is 100-1000h-1; the regeneration gas carrying sulfide flows out of the organic sulfur removal tank deal with it separately.

The above-mentioned desulfurizer can also be regenerated with superheated steam at 260-290°C, with a space velocity of 100-1000h-1; the regeneration gas carrying sulfides flows out of the organic sulfur removal tank for additional treatment.

The preferred exchange degree of the above-mentioned modified X-type molecular sieve is 82% to 95%. A further preferred modified X-type molecular sieve has an exchange degree of 82% to 86%.
The preferred exchange degree of the above-mentioned modified Y-type molecular sieve is 70% to 75%. The exchange degree of the further preferred modified Y-type molecular sieve is 70% to 73.5%.

Molecular sieve is also called zeolite. The original unit of zeolite is SiO4 and AlO4 tetrahedron, of which SiO4 tetrahedron has no charge, and AlO4 tetrahedron has a negative charge. These tetrahedral units directly form secondary units with oxygen atoms, which are composed of di The interconnection of the level units forms three levels or polyhedrons, and finally the polyhedral units form various characteristic zeolite crystal structures.

This crystalline aluminosilicate has certain uniform cavities and channels. After dehydration, substances of different molecular sizes can pass or not pass through, and play the role of screening different substances. Therefore, zeolite is called “molecular sieve”.

Since the cations in the molecular sieve are not on the framework of the zeolite, but attract each other with [AlO4]-anions outside the framework, they are more mobile and can be exchanged by other ions.

Through exchange-calcination-exchange, an exchange degree of more than 80% can be achieved.

After being modified by nearly 100% ion exchange, it has greater adsorption capacity and adsorption capacity.

People can change the physical and chemical properties of molecular sieves by exchanging different ions, thereby expanding their application in the petrochemical field. In addition to water and H2S, liquefied petroleum gas also contains about 100ppm of mercaptan sulfides. All thiol sulfur and neutral sulfides such as sulfide and thiophene cannot be removed by conventional molecular sieves.

However, after the molecular sieve modified with different metal ions, after the central electric field strength is greatly enhanced, the neutral sulfide can be polarized and adsorbed by the molecular sieve in its cavities. As long as the pores are large enough, the liquefied petroleum gas can be completely removed. in organic sulfides.

Based on the characteristics of molecular sieves that can carry out physical adsorption, in order to overcome the shortcomings of their low adsorption capacity for neutral organic sulfides.

On the other hand, the present invention controls and selects the degree of ion exchange to reach the degree of ≥70%, which also greatly improves the adsorption capacity and adsorption efficiency of the molecular sieve for neutral organic sulfides.

The experimental data of the present invention shows that the adsorption breakthrough sulfur capacity of the modified molecular sieve for thiol sulfur has reached 5.3%. Modified molecular sieves are used to efficiently remove sulfides in liquefied petroleum gas, so that liquefied petroleum gas can be free of any sulfides and become a real sulfur-free liquefied gas.
Experiments show that under the condition that the water content in the liquefied gas is 100ppm, the sulfur content can reach 5.3%.

When the water content is 500ppm, the penetration sulfur capacity can reach 4.6%, indicating that the water content affects the sulfur capacity. In the actual production of the device, attention should be paid to the water content, and the moisture in the liquefied gas should be removed as much as possible.

The liquefied petroleum gas after the removal of hydrogen sulfide can be directly removed from the organic sulfide, and the fixed bed molecular sieve desulfurizer can be used to directly remove the water and sulfide in the liquefied gas, including the direct removal of the mercaptan in the liquefied petroleum gas. , so the process of the present invention is simple.

Molecular sieve desulfurizer is easy to regenerate. The molecular sieve after penetrating the adsorbed sulfide can be regenerated by heating at 300℃ to desorb the sulfide. After more than 200 adsorption, regeneration, adsorption and regeneration tests, the strength of the molecular sieve remains unchanged, and its activity reaches 95% of the new agent, which can be used continuously and has a long service life.

Molecular sieve desulfurizer can be regenerated by steam, nitrogen, air, water and other media, and can be desorbed and regenerated under normal pressure and reduced pressure, and desorption under reduced pressure makes the regeneration more thorough and complete.

For the regeneration of the molecular sieve desulfurizer bed in industrial production, it can be purged with nitrogen or catalytic dry gas heated to 200-300℃, or it can be purged with 1.0MPa superheated steam to regenerate the molecular sieve desulfurizer.

The organic sulfur desorbed from the molecular sieves carried away by nitrogen or catalytic dry gas or superheated steam can be incinerated to produce sulfur. It can also be cooled, so that the organic sulfur with a higher boiling point is condensed and recovered, and the rest is produced by incineration.

The organic sulfur compounds that are cooled to liquid state can be separated and recovered into high value-added organic sulfur products according to different boiling points.