Thermophysical Properties of Fresh and

Lean Thermally Degraded N- Methyldiethanolamine

Ahmad Abukaashabeh, Emad Alhseinat, Sameer Al-Asheh, Fawzi Banat

Abstract

This paper gives a deep insight to the thermophysical properties for thermally degraded methyldiethanolamine (MDEA) solutions. Density, viscosity and surface tension of samples of fresh MDEA solutions, real lean MDEA solutions, and thermally degraded MDEA solutions were measured at different temperatures. The measured values were validated through comparison with literature data. The obtained results were used to produce correlations for MDEA solution density and viscosity as a function of initial amine concentration, degradation time, and temperature. Increasing the degradation time and temperature increased the density, viscosity and surface tension. The predicted concentrations of MDEA of real lean amine samples were compared with the measured values through acid-base titration. The relative deviation between the results of titration and the predicted ones by the developed correlations ranged from
3.5% to 7.5%.
Key Words: MDEA, thermal degradation, density, viscosity, surface tension, temperature

Introduction

Aqueous solutions of alknolamines are widely used in gas sweetening units for the absorption of sour gases. It is well known that in order to fully characterize the physicochemical behavior of theses absorbents, it is important to create a reliable body of information on their different thermophysical properties that are relevant for the design, operation, and optimization of sour gas treatment plants [1]. Solution density and viscosity are important properties in the mass- transfer rate ,modeling of absorbers and regenerators as such properties affect the liquid-film coefficient for mass transfer [2]. The surface tension is also an important property to be considered in the design of gas absorption towers, as it affects the hydrodynamics and mass transfer rate of acid gases through the absorption solution [3]. Design of related operations such
as pumps and heat exchangers would also benefit from better knowledge of the physical properties of process solutions. Furthermore, the foaming behavior of these absorbents is very much related to their thermophysical properties [4]. Indeed, the thermophysical properties of alkanolamines aqueous solution are affected by several factors, i.e. the sour gases loading, the degree of the thermal degradation, and the amount of contaminates. Very little information are available concerning the effect of acid gas loading on the physical properties of amine treating solutions. Apparently less attention has been given to understand the effect of the alknoamines degradation and effect of contaminates on the physical properties of amine treating solutions.
Several experimental studies can be found in the literature addressing the effect of alkanolamine concentration and temperature of single and blended alkanolamines aqueous solutions on their thermophysical properties. Density data of aqueous solutions of mixtures of two alkanolamines have been reported in the literature as a function of concentration and temperature. To our best knowledge, no thermophysical properties data have been reported up to date for thermally degraded alkanolamines solutions.
This study comes as a part of the ongoing project studying the deterioration of solvent quality and foaming problems in Habshan Gas Sweetening Unit (HGSU), GASCO, and Abu Dhabi (UAE). In Gasco Habshan, approximately 45-50 weight% methyldiethanolamine (MDEA) is used to remove H2S/CO2 from the feed gas. This work was undertaken to determine the effect of thermal degradations of MDEA on the density, viscosity and surface tension of the used alkanolamine solutions. Real amine samples collected from Habshan Gas Sweetning Unit and fresh solution of 46 wt% MDEA (~7 molal) loaded with low concentrations of H2S and RSH were studied. Thermal degradation was conducted at different using a Parr reactor.

Experimental Methodology

Density, Viscosity, Surface tension, and MDEA concentration measurement

The density of the samples was measured using an Anton Paar density meter (DMA 4100M). The measurement is based on the oscillating U-tube method. The accuracy of DMA 4100 M is up to 0.0001 g/cm³.
The kinematic viscosity [cSt] is a measure of the resistance of a fluid to flow under gravity. Solution viscosity was measured by Cannon-Fenske viscometer. The measuring accuracy of this viscometer is ± 0.01 %. The obtained data were then combined with the measured densities of the same solutions to determine the dynamic viscosity.
The Digital Tensiometer K9 manufactured and supplied by Kruss was used to measure the surface tensions of the tested solutions. The resolution of the Digital Tensiometer K9 is 0.1 mN/m. Thus, the main source of error in this study was assumed to be mainly due to the experimental error in solutions preparation.
In this study, the MDEA concentration of real lean MDEA solutions and thermally degraded
MDEA solutions were determined using an automatic titrator (907 Titrando, 900 touch control,
801 stirrer, 854 iConnect probe, Metrohm). Back titration has been carried out by adding 1N H2SO4 to the samples, then titrated the excess with 0.1 N NaOH.

Thermal degradation of MDEA solution using Parr reactor

Parr reactor was used for thermal degradation of MDEA solutions. Parr reactor (Parr Instrument Co., model 4560) is a 300 ml stainless steel (T316 SS) reactor equipped with a Parr 4848 reactor controller; this controller has a temperature and pressure control, data logger and a pressure gauge and a motor with speed controller, The test was carried out at two different temperatures (120 ºC and 130 ºC) where the formation of the heat stable salts (un-regenerable salts) in the regenerator column in the gas sweetening units is likely to occur. These temperatures represent the regenerator's column temperature and the reboiler temperature; respectively. Real lean MDEA solutions were collected from Habshan Gas Sweetening Unit, whereas a fresh 46 wt% MDEA solution was loaded by bubbling sour gas through it to reach a fresh 46 wt% MDEA solution loaded with 37.65 ppm of H2S and 40.24 ppm of RSH. A real lean amine sample was analyzed using ICP-OES (OPTIMA 8000, PerkinElmer) for possible elements existing in the solution to ensure that concentration of salts or contaminants would not affect the test; Table 1 lists the this analysis.

Table 1: ICP-OES analysis of lean amine samples using OPTIMA 8000

The vessel was washed with deionized water followed by ethanol before and after every run. 150 ml of the tested amine were discharged into the reactor. The system was then purged with nitrogen at 2 bars for 5 minutes and then depressurized to 0.1 bar (gauge pressure) in order to release all oxygen in the vessel. Finally, the solvent was heated to the desired temperature, i.e.
120oC stepwise, according to the heating program as displayed in Table 2.

Table 2: Amine solvent heating program in the Parr Reactor

Samples were collected at regular intervals during the heating degradation process. After that, the reactor was cooled down gradually to the room temperature, and then the system was pressurized by nitrogen gas to 1 bar (gauge pressure) to push the sample solution through the dip tube to a condenser. After sampling, the reactor was heated up according to Table 2 and left to
stabilize overnight; after which the pressure was compensated with nitrogen. Small losses (<0.1 bar) were observed during sampling as the dip tube used for sampling was fully immersed in the solution during the course of the test.

References

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2. Weiland, R.H., et al., Density and Viscosity of Some Partially Carbonated Aqueous Alkanolamine

Solutions and Their Blends. Journal of Chemical & Engineering Data, 1998. 43(3): p. 378-382.

3. Rangwala, H.A., et al., Absorption of CO2 into aqueous tertiary amine/MEA solutions. Can. J.

Chem. Eng., 1992. 70(3): p. 482-490.

4. Alhseinat, E., et al., Foaming study combined with physical characterization of aqueous MDEA

gas sweetening solutions. Journal of Natural Gas Science and Engineering, 2014. 17(0): p. 49-57.