Removal of heat stable salts from N-methyldiethanolamine wastewater by anion exchange resin coupled three-compartment electrodialysis

Highlights

• HSS from MDEA wastewater were successfully removed by RTED method.

• Less fouling of the anion exchange membranes occurs in RTED process.

• RTED is a cost-effective and environmentally friendly method for HSS removal.

Abstract

Heat stable salts (HSS) are well-known degradation products of gas desulfurization using N-methyldiethanolamine (MDEA) solvents. Excessive HSS content causes a decrease in desulfurization efficiency, a decrease in the active ingredients of MDEA, and the corrosion of pipelines. In this paper, a self-designed two-compartment electrodialysis (ED) stack, a three-compartment electrodialysis (TED) stack, and an anion exchange resin coupled three-compartment electrodialysis (RTED) stack were assembled to remove the HSS from spent MDEA wastewater. The HSS desalination mechanisms of the three electrodialyzers were investigated, and their HSS removal efficiencies and losses of MDEA were investigated and compared. According to the results, approximately 93.84% of HSS were removed from the spent MDEA wastewater in the RTED, 7.88% higher than those in the TED and 28.57% higher than those in the ED. The loss of MDEA for the RTED was 3.78%, which is 1.78% lower than that for the TED and 17.29% lower than that for the ED. A relatively high sodium hydroxide (NaOH) concentration in the NaOH compartment, but no more than 3%, is beneficial for both improving the HSS removal efficiency and decreasing the loss of MDEA in TED and RTED. Much less fouling of the anion exchange membranes occurs in the RTED process than in the ED process because of the effect of the filled resin and the NaOH compartment. The cost of RTED is estimated to be US $0.88/kg HSS, which is far lower than that of both ED and TED.

Introduction

The Gas Exporting Countries Forum expects global production and consumption of natural gas to increase from 3.9 trillion to 6 trillion m³ by 2050. This growth will boost the role of gas in the global energy balance from 27% to 34% in the same period [1]. In the process of natural gas exploitation and coal synthesis gas production, some hydrogen sulfide (H₂S), carbon dioxide (CO₂), and other organic sulfides, such as carbon disulfide (CS₂) and carbonyl sulfide (COS), will inevitably be mixed. The high content of sulfides will not only corrode equipment and pipelines but also pollute the environment and threaten human health and safety. Further, a large amount of CO₂ will reduce the calorific value of both natural gas and synthesis gas (syngas), and CO₂ is also one of the main gases responsible for the greenhouse effect.

In industry, N-methyldiethanolamine (MDEA) is the most commonly used solvent for removing H₂S, CO₂, and other acidic compounds from natural gas and coal syngas because of its highly selective removal of acidic gases and its high capture efficiency. However, chemical degradation, thermal degradation, and oxidative degradation gradually occur during long-term desulfurization, along with the formation of heat stable salts (HSS). The formation of HSS is caused by the chemically irreversible reactions between these degradation products and other impurities, such as sulfur dioxide (SO₂), nitrogen dioxide (NO₂), CO₂, and oxygen (O₂), in the feed gas stream [2]. The composition of HSS is very complex; it includes organic acids, such as acetic and oxalic acid, as well as chloride and a variety of sulfates, sulfites, and nitrates [3]. The accumulation of these salts and other impurities can cause problems, such as excessive foaming and the capacity reduction of MDEA solution. Corrosion of the operating units, which reduces the equipment’s lifetime, has also been reported [4], [5]. The traditional method used to preserve the absorbent activity of MDEA is to partially remove degraded MDEA from the system and replace it with fresh MDEA solution. However, this leads to the discharge of a large amount of wastewater and an increase in the total cost of acidic gas removal of at least 10% [6]. Other options for preserving the absorbent activity include distilling the free amine away from the HSS and neutralizing MDEA with sodium or potassium caustic to reclaim MDEA from the HSS [7], [8]. However, because of a phase transition that occurs in the distillation process, the high energy consumption is a practical shortcoming of this method. Neutralization only solve amine loss, without getting rid of HSS from the spent MDEA wastewater. In addition, the number of sodium ions introduced negatively affects the absorption of H₂S by the MDEA solution [9].

In recent years, several efforts have been made to develop new technologies to solve this issue concerning HSS and reclaim MDEA. The two most prominent and widely used types of technique among these are electrodialysis (ED) [6], [10], [11], and ion exchange resin techniques [12], [13]. ED has the advantages of having a low energy consumption, having a small area, and not requiring additional chemicals [14], [15]. However, because of the hydrolysis of MDEA in aqueous solution, MDEAH⁺ inevitably migrates to the concentrate compartment under the direct-current (DC) electric field, resulting in a 10–30% loss of MDEA during the ED process [10], [11]. Ion exchange resin techniques provide an effective and economical way to reduce MDEA loss with lowering energy consumption [16]. However, a low efficiency (approximately 50–65%) in the regeneration of exhausted exchange resin means that the process requires a high consumption of sodium or potassium hydroxide. Additionally, the alkaline wastewater produced by regeneration causes a secondary pollution that places a heavy burden on the environment [10], [13], [17].

From the above, it can be seen that the neutralization method, ED method, and ion exchange resin method all have advantages and disadvantages. In this study, we attempted to combine them to unite their advantages and eliminate their disadvantages. The first step was to reduce the loss of MDEA in the ED process by combining the ED and neutralization methods. To achieve this goal, a sodium hydroxide (NaOH) compartment was added to an ED stack to create a three-compartment electrodialysis (TED) stack. To further accelerate the HSS removal efficiency of the ED process, especially for the MDEA solution with low HSS concentration, an anion exchange resin was added into the dilute compartment of the TED stack to form an anion exchange resin coupled three-compartment electrodialysis (RTED) stack, which finally combined the ED process, neutralization method, and anion exchange resin method. Altogether, the aims of this research were to (1) investigate and compare the desalination mechanism, HSS removal efficiency, and MDEA loss of ED, TED, and RTED processes; (2) investigate the effect of NaOH concentration on the HSS removal efficiency and the MDEA loss of TED and RTED; (3) investigate the anion exchange membrane fouling of ED and RTED; and (4) investigate the economic viability of ED, TED, and RTED.