Comparing SiCHA and 4A Zeolite for Propylene/Propane Separation using a Surrogate-Based Simulation/Optimization Approach

Abstract

Propylene/propane separation is one of the most energy-intensive in the chemical industry. Adsorption may offer a low-energy alternative process. In this work, we compare 4A zeolite and a new 8-ring silica chabazite zeolite (SiCHA) for separating these mixtures in a pressure vacuum swing adsorption (PVSA) process. We base our assessment on a five-step PVSA cycle with concurrent pressurization, high pressure adsorption, rinsing with the heavy component (i.e., heavy reflux), forward blowdown, and reverse evacuation, which we simulate rigorously using a nonisothermal isobaric micropore diffusion model with concentration-dependent diffusivity developed by Khalighi et al. [Ind. Eng. Chem. Res. 2012 51, 10659–10670] We develop fast neuro-fuzzy surrogates for these simulations and estimate minimum energy consumptions per tonne of propylene using a genetic algorithm (GA). We show that the blowdown step, although widely used in the literature for 4A zeolite, is redundant for both 4A zeolite and SiCHA. While 4A zeolite requires lower separation energy per tonne of propylene, it admits lower throughputs due to lower diffusivities. Since energy costs outweigh capital costs for this separation, a comparison based on approximate total annualized cost also confirms that a process using 4A zeolite is cheaper than that using SiCHA. Between the two industrial propylene/propane feeds of 50:50 and 85:15 mol:mol, the latter requires lower energy and cost per tonne of propylene than the former for separation into two high-purity products.