Application practice of 13X molecular sieve in LPG drying

In the process of liquefied petroleum gas (LPG) refining, moisture removal is the core link to ensure product quality and process safety. Traditional desiccants are limited by adsorption capacity and regeneration performance, and it is difficult to meet the needs of modern petrochemical industry for low dew point and long cycle operation. With its unique pore structure and surface chemical properties, 13X molecular sieve has become an innovative material in the field of LPG drying, and its technical advantages have been fully verified in industrial applications.

Analysis of the framework structure and water absorption mechanism of sodium-type molecular sieves

13X molecular sieve belongs to sodium-type X-type molecular sieve (NaX), and its crystal structure is formed by silicon-oxygen tetrahedron and aluminum-oxygen tetrahedron through sharing oxygen atoms to form a three-dimensional framework. The sodium ions (Na⁺) in the framework maintain structural stability through electrostatic action, while forming uniform pores with a diameter of about 10Å. The polarity of water molecules makes it easy to form hydration with sodium ions. Experimental data show that at 25°C and relative humidity of 50%, the static water adsorption of 13X molecular sieve can reach more than 26.5wt%, which is significantly better than 3A molecular sieve (20wt%) and 4A molecular sieve (22wt%).

The adsorption of water molecules in the pores of 13X molecular sieve presents multi-level adsorption characteristics:

Weak adsorption: Hydrogen bonds are formed between hydroxyl groups on the pore surface and water molecules, and the adsorption energy is about 49.5kJ/mol;

Strong adsorption: Sodium ion hydration is dominant, and the adsorption energy reaches 80-100kJ/mol;

Co-adsorption effect: Hydrogen sulfide (H₂S) and carbon dioxide (CO₂) in LPG can be adsorbed simultaneously to achieve deep purification.

Process advantages of 13X molecular sieve in LPG drying

1. Efficient dehydration performance

The actual application of a petrochemical enterprise shows that the LPG drying tower using 13X molecular sieve can reduce the water content of the raw gas from 150ppm to below 5ppm, and the dew point temperature is stably controlled below -70℃. Its adsorption kinetic characteristics show that the desorption amount in the first 5 minutes accounts for 65% of the total water, and 90% of the dehydration task is completed within 30 minutes, which is significantly better than the 2-hour equilibrium time of activated alumina.

2. Anti-pollution and long-term operation

The compressive strength of 13X molecular sieve reaches 80N/particle, and the abrasion rate is ≤0.1wt%. It can operate stably for more than 3 years in the LPG high-speed fluidized bed. After a certain device has been running for 20 consecutive water absorption-desorption cycles, the equilibrium adsorption amount has only decreased by 12%. After ion reinforcement treatment with 0.5mol/L sodium chloride solution, the cycle stability has increased by 30%.

3. Gradient regeneration process optimization

In view of the analytical characteristics of 13X molecular sieve, a variable temperature-variable pressure combined regeneration mode is adopted in industrial practice:

The first stage: 180℃, atmospheric pressure desorption of main water, desorption rate 85%;

The second stage: 150℃, 5kPa vacuum treatment of residual water, total desorption time shortened by 40%.

This process reduces steam consumption by 28%, and the regeneration cycle is compressed from 24 hours to 14.4 hours.

Industrial application cases and economic benefits

Case 1: LPG desulfurization unit

A refinery applies 13X molecular sieve to the LPG desulfurization process. Under the conditions of inlet H₂S content of 800ppm and water content of 120ppm, the outlet H₂S drops to below 5ppm, and the water content is stable at 3ppm. After the equipment transformation, the annual consumption of molecular sieves is reduced by 40%, and the energy consumption of drying tower regeneration is reduced by 35%.

Case 2: Cryogenic air separation raw gas purification

In the air separation unit, 13X molecular sieve removes hydrocarbons such as H₂O, CO₂ and C₃H₆ from LPG at the same time, increasing the purity of the raw gas to 99.999%. Compared with the 3A/4A molecular sieve series process, the equipment investment is reduced by 25% and the operating flexibility is increased by 40%.

13X molecular sieve has become the mainstream technology in the field of LPG drying due to its excellent adsorption performance and process adaptability. With the continuous innovation of material modification and process integration, this technology will play a greater role in the process of clean energy and industrial low-carbonization.