Molecular Sieve 13X

The Widest Pore in the A and X Family

SorbiTech 13X is a sodium form X type zeolite with the empirical formula Na₂O · Al₂O₃ · 2.8 SiO₂ · 6 H₂O. The X framework is more open than the A family, and the sodium cation leaves the pore window at approximately 10 Ångström. That width gives 13X its two defining properties: the highest equilibrium loading for water and carbon dioxide of the common sieves, and access to larger molecules such as mercaptans, carbonyl sulphide, and aromatics that a 3 or 4 Ångström pore turns away.

It is this dual capability, deep drying together with sulphur and carbon dioxide removal in one bed, that places 13X at the front of cryogenic air separation plants and at the inlet of natural gas treating trains. SorbiTech manufactures the grade as 1.6 to 2.5 mm and 2.5 to 5.0 mm beads with a controlled particle size distribution.

Adsorbing Water, Carbon Dioxide, and Mercaptans Together

The adsorption sequence inside a 13X bed reflects molecular polarity and size. Water binds most strongly to the sodium cations and is held at the inlet of the bed. Carbon dioxide is held next, in a separate zone further down. Mercaptans, hydrogen sulphide, and heavier aromatics occupy the most open part of the pore network and break through last.

A single SorbiTech 13X layer can therefore handle a combined duty that would otherwise require two or three different beds in series, which is why operators specify it over 4A whenever the carbon dioxide or sulphur removal alone would justify a dedicated unit.

Cryogenic Air Pre Purification Duty

In a cryogenic air separation unit every part per million of water or carbon dioxide reaching the cold box freezes against a heat exchanger surface and starves the column of flow. The pre purification bed must therefore deliver outlet water below 0.1 ppmv and outlet carbon dioxide below 1.0 ppmv through the entire adsorption cycle.

A graded SorbiTech 13X bed, with optional activated alumina at the inlet for bulk water, holds those specifications for adsorption half cycles of 4 to 12 hours at 5 to 10 barg and 10 to 25 °C. The wide pore also adsorbs C₂ and C₃ hydrocarbons, protecting the cold box from the slow hydrocarbon accumulation that would otherwise force a defrost.

Cycle Selection: TSA versus PSA

In natural gas treating, 13X is run on a temperature swing cycle, regenerated with hot purge gas at 200 to 350 °C. The long adsorption stage and the heavier hydrocarbon coadsorption favour temperature swing. In air pre purification it may be run on either temperature swing or pressure swing, depending on plant heat integration. Pressure swing avoids a regeneration heater at the cost of a larger sieve charge.

• Water capacity: ≥ 24 percent by weight
• Regeneration temperature: 200 to 350 °C (TSA) or ambient (PSA)
• Adsorption pressure: up to 100 barg (gas treating), 5 to 10 barg (ASU)
• Purge ratio: 5 to 15 percent depending on cycle

Layered Bed Configurations and Sizing

In several duties the SorbiTech 13X is run as part of a layered bed. For air pre purification the most common arrangement is a thin activated alumina layer at the inlet, which absorbs the bulk water at near isothermal conditions, followed by the main 13X layer that holds the residual water and the carbon dioxide. The alumina protects the 13X from hydrothermal damage during an upset.

Bed sizing starts from the equilibrium isotherm at the operating partial pressure and temperature. The Langmuir constants from the SorbiTech data sheet are validated by the SorbiTech engineering team against an in house database of plant performance, returning a bed length to diameter ratio between 1.5 and 3, a face velocity between 0.1 and 0.3 m/s, and a margin of 15 to 25 percent on equilibrium capacity for feed upsets.

Forms and Supply

Manufactured to ISO 9001:2015 with a certificate of analysis per batch and supplied through the SorbiTech sales channel as standard media for an existing front end, as the charge in a SorbiTech pre purification package, or as a guard layer ahead of a downstream catalyst. The same 13X material is delivered with the engineered vessels for a complete cryogenic feed train, with SorbiTech available for full EPC. The 13X grade is the front end adsorbent for cryogenic and PSA separation; SorbiTech delivers it in the on site gas generation package, with sector context under industrial gas and air separation and the parent molecular sieves family.