Propylene Selectivity Improvement for Methanol to Olefins Process in a Fixed-Bed Reactor

As a promising technology to convert coal and nature gas into petrochemicals, methanol to olefins (MTO) process has received dramatic attentions from industry and academia. An on-purpose propylene process (MTP) based on fixed-bed reactor was developed by Lurgi Company and has been industrialized in China in 2011 with an annual capacity of about 500kTY propylene. However, the propylene selectivity turned out to be less than 60%, failed to meet the anticipated requirement of 65%¹, and the total selectivity including C₂, C₃ and C₄ olefins is much lower than the fluidized-bed MTO technology (>80%). Our theoretical study showed that the reason for low propylene selectivity is the low methanol concentration and the prolonged residence time and the key to improve propylene selectivity is to increases the methanol concentration in reactor and to reduce the residence time.

The autothermal tubular reactor is a multi-functional reactor combining reaction and heat transfer together in one apparatus, where the reaction heat is continuously removed through tubular heat exchange with the cold reactants². It is advantageous for MTP process since the temperature rise of catalyst bed can be controlled with a higher feed methanol concentration. A two-dimensional autothermal reactor model was developed in the present work. It shows that by increasing the feed methanol to 30%, the autothermal tubular reactor can increase the propylene selectivity significantly from around 60% in a conventional adiabatical reactor to 88.6% and the space velocity from 0.61 in the conventional reactor to 4.68 gMeOH/gcat/hr. By diluting the catalyst bed, the reaction temperature can be restricted between 445-480°C, an reasonable temperature window for MTP reactions, and most of the reaction takes place in 470-480°C, which indicates the temperature was successfully controlled.

References:

1. Agudamu, Sun Y, Zhang F. Product distribution of methanol to propylene reaction found in Shenhua Ningxia Coal Group. Coal Chemistry Industry (in Chinese). 2013(01):58-60.

2. Kolios G, Frauhammer J, Eigenberger G. Autothermal fixed-bed reactor concepts. Chemical Engineering Science. 2000;55(24):5945-5967.