The 3a in there is redundant.
13X 4X8 Molecular Sieve Desiccant is a multiple purpose, highly porous, high capacity alkali metal alumino- silicate in the spherical form. It is the sodium form of the Type X crystal structure with pore diameters of approximately 10A°. It can adsorb all molecules that can be adsorbed by 3A, 4A, and 5A molecular sieve. Type 13X can also adsorb molecules such as aromatics and branched-chain hydrocarbons, which have large critical diameters.