Nanofiltration for the concentration of heat stable salts prior to MEA reclamation

Highlights

• Nanofiltration used to concentrate the heat stable salts in contaminated MEA.

• Greater than 80% of the heat stable anions can be rejected.

• Rejection of the MEA is less than 7%.

• The CO₂ loading must be low and the solution must be neutralized upstream.

Abstract

While monethanolamine has shown great potential as a solvent for the capture of carbon dioxide, impurities can build within the solution over time, leading to increased viscosity and corrosivity. Classically, these impurities are removed by a combination of neutralization and either thermal reclamation, ion exchange or electrodialysis. In this work, we evaluate the use of nanofiltration to concentrate the heat stable salts within the solution prior to such reclamation. This allows the recirculating solvent to operate with low concentrations of these impurities, while providing a low volume, concentrated solution for reclamation. Results show that nanofiltration can reject greater than 80% of the heat stable anions, while allowing the monoethanolamine to permeate through the membrane, for return to the process. Rejection of the MEA itself is less than 7%. The nanofiltration operation is only effective on lean solvent with CO₂ loadings of less than 0.2 and neutralization would be required upstream to deprotonate the amine. The two membranes tested (Koch MPF-34 and MPF-36) appeared stable to exposure to the solvent for over four months.

Introduction

In spite of the emergence of greener energy resources, fossil fuel combustion remains a major source for energy production in the 21st century due to the well established knowledge of the operation and the number of existing plants. With the amount of CO₂ generated, the operation is also related to increasing global warming and the greenhouse effect. In the past few decades, a number of approaches have been considered to mitigate these greenhouse gas emissions by implementing post-combustion CO₂ capture at fossil fuel power plants.

Among all the post-combustion capture techniques under consideration (Stewart and Hessami, 2005, Yang et al., 2008), the most feasible and widely accepted approach is to use an amine such as monoethanolamine (MEA) as a solvent to selectively absorb CO₂ from the flue gas (Lepaumier et al., 2009, Plaza et al., 2010, Strube et al., 2011). MEA is capable of providing high levels of CO₂ purity and recovery with relatively fast kinetics. This performance is strongly related to the bonding efficiency between MEA and CO₂ to form carbamate anions and protonated amines (Scheme 1).

However, parasitic reactions can occur during post-combustion capture due to the presence of impurities in the flue gas. The reaction between MEA and these impurities may result in the formation of heat stable salts (HSS), which are difficult to regenerate and cause an increase in the viscosity of the solvent and corrosion to the operating units (Rooney et al., 1996, Tanthapanichakoon and Gale, 2003, Verma and Verma, 2009). These heat stable salts can include organic acids such as acetic and oxalic acid, as well as sulfates, sulfites, nitrates, nitrates which form by irreversible reactions with free amine (Dumée et al., 2012) (Scheme 2).

Additionally, neutral species and oligomers can also be formed through thermal degradation and polymerization of amine species (Chi and Rochelle, 2002, Davis and Sexton, 2008). These contaminants reduce the amount of active amine available for CO₂ capture.

To overcome these issues, a fraction of degraded MEA needs to be purged and replaced with fresh MEA thus increasing the maintenance cost for all unit operations and the operating cost for solvent replacement (Verma and Verma, 2009). For example, the financial loss due to ethanolamine degradation, evaporation at the top of the stripper column and HSS concentration build-up may reach up to US$ 8 M/year for a 1 Mm³/year CO₂ capture plant (Steeneveldt et al., 2006, Pires et al., 2012). Alternatively, the trapped amine species can be released by neutralizing the acidic contaminants using a carbonate salt, sodium hydroxide or potassium hydroxide (Rooney, 1999). However, neutralization alone can lead to precipitation of less soluble salts, causing fouling or foaming in the downstream processes (Verma and Verma, 2009). Thermal reclamation can also be used, whereby MEA is distilled from the unwanted salts and oligomers, leaving a waste sludge. Nonetheless, this approach leads to significant MEA losses and a large volume of waste sludge requiring disposal (Thitakamol et al., 2007).

A membrane-based separation technology, electrodialysis, has also been considered as a method to purify contaminated MEA solvent (Kohl and Nielsen, 1997, Burns et al., 1995). A patent for recovering alkanolamines was issued to the Dow Chemical Company in the 1980s (Bedell and Tsai, 1989) and an on-line purification process for MEA using electrodialysis (also known as UCARSEP^® process) was developed by Union Carbide in the late 80s to early 90s. The method can provide high HSS removal and high amine recovery (Burns et al., 1995, Cohen and Gregory, 1999), However, the solution must still first be neutralized, and the application is limited to the removal of charged contaminants only. Additional treatments are required to remove the neutral species and oligomers, which are also generated during the MEA scrubbing process. Ion exchange is another approach that can be used in a comparable approach.

Approaches such as thermal reclamation and electrodialysis are most effective when the total concentration of contaminants is relatively high (>1 wt% or 10,000 ppm (Burns et al., 1995, Gregory and Cohen, 1999)). For this reason, reclamation is often carried out in a batch mode, with a reclamation unit brought on site on a temporary basis to improve solvent quality. However, the presence of only 500 ppm (mg/liter) of amine degradation products in the circulating solvent may increase corrosion rates by fifty-fold (Rooney et al., 1996, Rooney et al., 1997). Solution viscosity also increases as the concentration of contaminants increases, which in turn increases the energy demand and reduces the carbon dioxide recovery due to reduced kinetics.

In this paper, nanofiltration (NF) is investigated as a mechanism for concentrating the heat stable salts within an amine solution, following neutralization. If effective, such an approach would allow the carbon capture process to operate with low HSS concentrations (e.g. 500–1000 ppm) while continuously delivering a lower volume and more concentrated solution to a downstream thermal reclamation or electrodialysis process. A schematic of the proposed flowsheet is provided as Fig. 1.

Due to the simultaneous improvement of the membrane chemistry and the substantial decrease in the membrane production costs, NF is progressively emerging as an integrated purification solution for many industrial applications (Van der Bruggen et al., 2008, Mänttäri et al., 2006a, Gozálvez-Zafrilla et al., 2008, Lau and Ismail, 2009). The method utilizes a pressure drop across the membrane (trans-membrane pressure) as a driving force to separate ions and uncharged molecules based on their charge and size difference. With a typical molecular cut-off membrane between 200 and 1000 Daltons, NF has the potential to prevent most macro-molecules from permeating through the membrane. The application of NF as an efficient and low cost technique in water softening (Low et al., 2008) and acid removal (Listiarini et al., 2010, Visser et al., 2001, Chalatip et al., 2009) has also been demonstrated. Additionally, it has been used in the past decade for the treatment and purification of organic co-solvents (Volkov et al., 2008).

The approach taken in this study involves testing a series of model solutions with different HSS on a NF lab-scale rig using two commercially available membranes. The investigation focuses on 30 wt% MEA/water solutions, which represents the typical composition of industrial solvent. The concentration of the HSS salts was selected to be 500 ppm for each salt.

The aim of the study is to find a process that concentrates heat stable salts and thus reduces the viscosity and corrosivity of the circulating MEA solvent. In turn, this should reduce the maintenance costs and the energy input of the carbon capture process.