Design of adsorption column for reclamation of methyldiethanolamine using homogeneous surface diffusion model

Pravin Kannan,Priyabrata Pal,Fawzi Banat

doi:10.2516/ogst/2020073

Abstract

A predictive simulation model was applied to design a fixed-bed adsorber for studying the removal of Total Organic Acid (TOA) anions from lean Methyldiethanolamine (MDEA) solution using Calcium Alginate Bentonite (CAB) clay hybrid composite adsorbent. Unlike other conventional techniques typically used for packed bed design, the predictive Homogeneous Surface Diffusion Model (HSDM) does not require any test column breakthrough curves a priori. Mass transfer coefficients and isotherm model parameters are provided as input data to HSDM for simulating column breakthrough curves. Various isotherm models were fitted to batch equilibrium data for TOA adsorption on CAB composite adsorbent. Based on Akaike Information Criterion (AIC), Freundlich isotherm was selected and the model parameters were obtained by non-linear regression. Film transfer coefficients and surface diffusivities were determined using appropriate empirical correlations available in the literature. HSDM predictions were first validated using lab-scale column adsorption data generated at lower residence times. The effects of dimensionless numbers (Biot and Stanton) on breakthrough times were investigated using the dimensionless HSDM system and a suitable scale-up regime (Bi& ~& 1 and St& >& 10) was established wherein the sensitivity of mass transfer parameters would be minimal. Using similitude rules on key design parameters, a pilot-scale adsorption column was designed and breakthrough curves were generated using the validated HSDM. The appropriateness of the design technique was verified by comparing the estimated breakthrough data and column design parameters with conventional scale-up and kinetic approaches.

Highlights

Almost all natural gas has H2S, CO2 or both that needs to be removed before the gas is pumped through transmission pipelines
Many different factors influence the shape of the Breakthrough Curve (BTC), primarily the values of mass transfer coefficients employed in the Homogeneous Surface Diffusion Model (HSDM)
As shown in this study, HSDM could be employed for the rapid design of the adsorption column where the application of other techniques is severely challenged by the inherent dependency on experimental data

Summary

Introduction

Almost all natural gas has H2S, CO2 or both that needs to be removed before the gas is pumped through transmission pipelines. The sweetening process is carried out using aqueous Methyldiethanolamine (MDEA, 45–50 wt.%) in a regeneration column where heat is applied to strip the acid gas components and recover lean/aqueous MDEA solution (Keewan et al, 2018; Mehassouel et al, 2018; Younas and Banat, 2014). Contamination of industrial lean MDEA from heat stable salts such as total organic acids (produced by the reaction between aerial oxygen and CO2/H2S) and heavy metal ions like chromium, lead, etc. Partial removal of Total Organic Acid (TOA) anions from aqueous amines is crucial for avoiding some operational issues encountered during natural gas sweetening process. Different methods have been used for the removal of Heat Stable Salts (HSS) from amine solvents in natural gas sweetening units. Vacuum distillation, electrodialysis, ion exchange, and adsorption are used for the removal of HSS from lean MDEA solutions. MDEA contains both amine and hydroxyl groups that adhere strongly to these contaminants, thereby making separation by any available technique largely

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