Thermal Decomposition of Nitrosamines in Amine Scrubbing

Thermal Decomposition of Nitrosamines in Amine Scrubbing

Introduction

Unlike other carbon capture processes, amine scrubbing is a chemical absorption process which directly contacts the contaminated flue gas with a solvent.  The contaminants in the flue gas can react with the solvent, forming dangerous byproducts under the high temperatures in the stripper.  Nitrosamines, a family of organic compounds that form from secondary amines reacting with nitrosating agents, are one of the largest environmental issues associated with amine scrubbing.  Over 80% of nitrosamines are carcinogenic to humans at low concentrations so it is important to understand and then control their accumulation in amine scrubbing.  This work will describe thermal decomposition, the most straightforward techniques for controlling nitrosamine accumulation

Thermal Decomposition Kinetics

Recent research has shown that N-nitrosopiperazine (MNPZ) will decompose under stripper conditions (Fine & Rochelle, 2013).  Decomposition is first order in MNPZ and half-order in the amine concentration.  This work has been expanded to include the thermal decomposition of n‑nitrosodiethanolamine (NDELA), nitrosopiperidine (NPIP), and nitrosomorpholine (NMOR) to understand how nitrosamine structure effects thermal degradation.  Furthermore, the role of the solvent has been tested on both MNPZ and NDELA.  Nitrosamine decomposition was found to be base catalyzed with a Bronsted slope of 0.5 (Figure 1).  Base catalysis may be useful in amine scrubbing since reclaiming often runs under strong basic conditions.  Nitrosamine decomposition was also found to be highly temperature dependent with an activation energy of approximately 100 kJ/mol across several nitrosamines.  Thus, it is important to have a thermally stable solvent so that the stripper can run at higher temperature to increase nitrosamine decomposition. 

Figure 1: Base Catalysis in Nitrosamine Thermal Decomposition Thermal Decomposition Products

Samples of thermally decomposed MNPZ were analyzed using a previous HPLC method (Fine, Goldman, Nielsen, & Rochelle, 2013) as well as a group analysis method for total nitrosamines. (Dreshcer & Frank, 1978; Walters & Smith, 1983).  Both methods agreed within the precision of the instruments, proving that MNPZ does not transnitrosate into another nitrosamine (Figure 2).  In another experiment the decomposed nitrosamine was analyzed for total aldehydes.  Aldehyde concentration increased stoichiometrically as the nitrosamine decomposed; the final sample was analyzed using high resolution mass spectrometry and the aldehyde was determined to be piperazin-2-ol (Figure 3). 

Figure 2: MNPZ Decomposition into Non-nitrosamine species

High Resolution Mass Spectrometry of Piperazin-2-ol in Decomposed MNPZ Sample Thermal Decomposition Mechanism

We have now proposed a mechanism for nitrosamine thermal decomposition using the empirical kinetics and decomposition products found (Figure 4).  The hypothesized formation of nitrous oxide will be studied using a cycling reactor. The piperazine imine form is readily hydrolyzed to form the piperazin-2-ol seen on the high resolution mass spectrometry.