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Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions
Accepted Manuscript Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions Muhammad Saad Khan, Behzad Partoon, Cornelius B. Bavoh, Bhajan Lal, Nurhayati Bt Mellon PII:
S0378-3812(17)30068-7
DOI:
10.1016/j.fluid.2017.02.011
Reference:
FLUID 11407
To appear in:
Fluid Phase Equilibria
Received Date: 27 October 2016 Revised Date:
30 December 2016
Accepted Date: 14 February 2017
Please cite this article as: M.S. Khan, B. Partoon, C.B. Bavoh, B. Lal, N.B. Mellon, Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions, Fluid Phase Equilibria (2017), doi: 10.1016/j.fluid.2017.02.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of Tetramethylammonium Hydroxide on Methane and
2
Carbon Dioxide Gas Hydrate Phase Equilibrium Conditions
b
a,b
, Behzad Partoon , Cornelius B. Bavoh Lal a,b,*, Nurhayati Bt Mellon b
a,b
, Bhajan
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a
a,b
Research Centre for CO2 Capture (RCCO2C), Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia.
Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia.
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Muhammad Saad Khan
*Corresponding author: [email protected] Telephone/Fax: +6053687684; +60103858473 /+6053656176
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Abstract
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In this experimental work, the phase boundaries of TMAOH + H2O + CH4 and TMAOH + H2O
13
+ CO2 hydrates are measured at different concentrations of aqueous TMAOH solution. The
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temperature-cycle (T-cycle) method is applied to measure the hydrate equilibrium temperature of
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TMAOH + H2O + CH4 and TMAOH + H2O + CO2 systems within the ranges of 3.5-8.0 MPa
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and 1.8-4.2 MPa, respectively. Results reveals that, TMAOH acts as a thermodynamic inhibitor
17
for both gases. In the presence of 10 wt% of TMAOH, the inhibition effect appears to be very
18
substantial for CO2 with an average suppression temperature (∆Ŧ) of 2.24 K. An ample inhibition
19
influence is observed for CH4 hydrate at 10 wt% with ∆Ŧ of 1.52 K. The inhibition effect of
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TMAOH is observed to increase with increasing TMAOH concentration. Confirmed via
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COSMO-RS analysis, the TMAOH inhibition effect is due to its hydrogen bonding affinity for
22
water molecules. Furthermore, the calculated hydrate dissociation enthalpies in both systems
23
revealed that TMAOH does not participate in the hydrate crystalline structure.
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Keywords: Ammonium based ionic liquids; COSMO-RS; gas hydrate; inhibitor; phase
25
equilibrium; TMAOH.
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1. Introduction
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Gas hydrates are crystal-like solids in which gas molecules are encased in cages formed by
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hydrogen bonded water molecules and stabilized by van der Waals forces. They are non-
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stoichiometric inclusion compounds formed under high pressures and low temperatures
31
conditions [1]. In oil and gas industry, one of the major flow assurance problems is the formation
32
of gas hydrate in pipelines, which can cause the blockage of hydrocarbon production and
33
transportation in pipelines and processing facilities [2,3]. Methane (CH4), carbon dioxide (CO2),
34
hydrogen sulfide (H2S), ethane (C2H6), propane (C3H8), propene (C3H6), and even iso-butane (i-
35
C4H10) can form gas hydrate under production and transportation condition, especially in deep
36
waters offshore operations. [4]. Gas hydrate formation could lead to catastrophic economic
37
losses and ecological risks. This problem costs the industry billions of dollars annually to
38
mitigate, with no permanent solution in focus. To avoid hydrate formation, generally four
39
methods could be applied; water removal; heating; depressurization; and chemical inhibition [4].
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However, In most cases, hydrate inhibition via chemical inhibitors is the only viable option for
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offshore gas pipelines [5–8]. Based on their inhibition mechanisms, these chemical inhibitors are
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categorized into two groups; thermodynamic hydrate inhibitors (THIs) and low dosage hydrate
43
inhibitors (LDHIs). THIs such as methanol and ethylene glycol work by shifting the hydrate
44
equilibrium curve to lower temperatures, enough to keep the system out of the hydrate formation
45
region. On the other hand, LDHIs is based on modern flow assurance strategies, like moving
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from complete hydrate avoidance towards hydrate risk management. There are two primary
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types of LDHIs – kinetic hydrate inhibitors (KHIs), generally water soluble polymers, and anti-
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agglomerates (AAs), which are usually surfactants. Unlike thermodynamic inhibitors, KHIs do
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not significantly change the hydrate equilibrium phase boundary but delay the hydrate nucleation
50
formation. Whereas, AAs principally do not inhibit hydrate formation but form a transferrable
51
slurry and prevent hydrate crystals from agglomerating to form physically superior hydrate plug
52
structures that can slab pipelines [5,8].
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Traditional THIs requires higher concentrations in the application, also, they are not
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environmentally friendly chemicals as they have toxic nature and high volatility. Therefore,
55
being committed towards the discovery of highly effective and environmentally friendly THIs,
56
Ionic Liquids (ILs) have been introduced as both green chemicals and tailor-made solvents for
57
specific application with negligible vapor pressure. Generally, for THI’s applications,
58
compounds that form hydrogen bonding with water molecules can effectively act as gas hydrate
59
inhibitors. Therefore, ILs have a considerable potential to work as gas hydrate inhibitors due to
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their ability to form hydrogen bonding with water molecules. Besides, their ionic nature makes
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them interact with water via electrostatic charges interactions that also making them gas hydrate
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inhibitors like other electrolytes. Xiao & Adhirama [9] initiated the research on ILs as gas
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hydrate inhibitors in 2009, they investigated Imidazolium-based ILs and found promising results
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for both THI and KHI inhibition. They found that ILs could show thermodynamic inhibition, and
65
at the same time delay the hydrate formation by slowing down the hydrate nucleation rate. They
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believed that this dual functional effect is due to their strong electrostatic charges and hydrogen
67
bonding with water. Xiao et al. [10] further reported that imidazolium-based ILs with halides
68
anions showed significant dual functional (i.e. kinetic and thermodynamic inhibition) inhibition
69
performance on methane hydrate formation. Several researchers have also reported on the
70
influence of imidazolium-based ILs as gas hydrate inhibitors [11–16]. Nonetheless, the state of
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art on the influence of other families of ILs as gas hydrate inhibitors are summarized in the
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review by Tariq et al. [17]. As imidazolium-based ILs have not shown significant inhibition
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strength compared to traditional inhibitors. Recent ILs hydrate based studies is moving from
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imidazolium-based ILs towards other kinds of ILs, such as pyrrolidinium [18,19], , ammonium
75
[20–22], and phosphonium [23]based ILs. However, studies on the effect of other types of ILs
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(especially ammonium based ILs) on gas hydrate formation, are still in the primary stage, as few
77
studies have been reported in literature so far.
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Govinda and coworker [24] comprehensively reviewed the applications of ammonium based ILs
79
(AILs) in numerous research areas such as; CO2 capturing, fuel cells, anti-corrosive agents, flow
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assurance as well as other technical applications. AILs have better environmental properties than
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imidazolium-based ILs which result in their use as food preservatives in food industries [25]. Li
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et al.[26], initially presented tetramethylammonium chloride (TMACl) as AIL’s gas hydrate
83
inhibitor in 2011. They found that TMACl achieved better THI impact compare to their
84
imidazolium counterparts. Subsequently, in 2013, Keshavarz et al. [27] evaluated the inhibition
85
impact of tetraethylammonium chloride (TEACl) with other imidazolium-based ILs and found
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that, all studied ILs were able to inhibit hydrate formation up to 0.9 K [27]. Recently, Tariq et al.
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[28], evaluated six AILs [tetra-alkylammonium acetate (TMAA), choline iso-butyrate (Ch-iB),
88
choline hexanoate (Ch-Hex), choline butyrate (Ch-But), and choline octanoate (Ch-Oct)] as CH4
89
hydrate inhibitors and found that they are able to increase the hydrate suppression temperature
90
(∆Ŧ). In addition, they suggested that some of the studied AILs could work as KHI inhibitors.
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Tariq et al. [28] further concluded that at lower concentration of AILs (i.e. 1 wt%), lesser
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inhibition effect was observed at a lower pressure range of 3.5-6.5 MPa; while at a higher
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pressure range of 6.6-12 MPa, all studied AILs worked as thermodynamic promoters instead of
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inhibitors. However, at a relatively higher concentration (5 wt%), inhibition effect is more
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evident at higher pressure regions [28]. Recently, Bavoh et al. [29] implemented the COSMO-
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RS (Conductor-like Screening Model for Realistic Solvents) technique to evaluate the effect of
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ILs hydrogen bonding energies (EHB) and sigma profiles on ∆Ŧ. They revealed that the ∆Ŧ
98
increases with increasing EHB and/or reduces with increments in alkyl chain lengths of ILs.
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However, the pairing of ILs cations and anions significantly disturbs the inhibition performance
100
of gas hydrates [29]. Khan and coworkers also applied COSMO-RS to describes the effects of
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different types of interaction energies, i.e., EHB, misfit energy (EMF), van der Waal energy
102
(EVdW) and total internal energy (ET) on the ∆Ŧ of ILs [30]. They also found that EHB of anion
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plays a major influential impact on hydrate inhibition [30].
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Therefore, the main objective of this study is to examine the influence of tetramethylammonium
105
hydroxide (TMAOH) on the phase equilibrium boundaries of TMAOH+ H2O+ CH4 and
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TMAOH+ H2O+ CO2 hydrate formation at different concentrations (1, 5 and 10 wt.%). The
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impact of an AIL on the CO2 hydrate phase boundary is reported for the first time in this work.
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In addition, a COSMO-RS based TMAOH/ Water system interaction analysis is performed to
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understand the THI inhibition mechanism of TMAOH.
2. Methodology
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2.1. Materials Table 1 summarizes the details of the materials used in this study. An aqueous solution of 95%
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TMAOH is purchased from Merck milli-pore company Germany. All gasses are purchased
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from Air Products Singapore Private Limited and used without any further purification.
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Deionized water (RO membrane plant TKA-LabTowe) is used to prepare desired concentration
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of aqueous TMAOH solutions
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Table 1: Material used for gas hydrate mitigation study
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No
Chemical Name
Symbol
Chemical Structure
Carbon dioxide
CO2
99.95 mole %
2
Methane
CH4
99.99 mole %
3
Tetramethylammonium hydroxide
TMAOH
95 wt %
4
Water
H2O
Deionized
Experimental apparatus
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Purity
The high-pressure stainless steel cell is employed for measuring the phase boundaries of CH4,
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and CO2 gas hydrate in the presence of aqueous TMAOH solutions. The apparatus consists of
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high-pressure equilibrium cell with a volumetric capacity of 500 cm3 and can work at
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temperature ranges from 253–523 K and a maximum operating pressure of 20 MPa. The
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temperature and pressure in the cell are recorded every second with an accuracy of ±0.1 K and
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±0.01 MPa respectively. Furthermore, the apparatus is fitted with a magnetic system containing
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2-bladed pitch impeller and a 400 rpm motor to provide adequate mixing of the sample in the
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cell under test conditions. The cell is submerged in a thermostatic bath, equipped with PID
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controller for controlling the bath temperature within ±0.3 ºC accuracy. The complete detail of
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the experimental setup can be found elsewhere [31,32].
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2.3.
Experimental Procedure
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An isochoric T-cycle method with step heating technique is employed in determining the
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phase equilibrium of CH4 and CO2 hydrate. Prior to the experimentations, the cell is
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thoroughly washed with distilled water and dried. Then 100 ml liquid sample of TMAOH is
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filled in the cell, then the system is cooled down to the chosen operating temperature. A small
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amount of CO2/CH4 is introduced into the cell, purged for three times and finally vacuumed to
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ensure that there are no traces of the air in the cell. After that, the cell is pressurized with
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CO2/CH4 to desired pressure. In these experiments, the pressure’s range for CH4 gas hydrate is
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3.5 - 8.0 MPa, while that for CO2 hydrate is 2.0 - 4.0 MPa. Once the conditions is stabilized,
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the mechanical stirrer is set at 300 rpm to provide adequate mixing and break the interface
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boundary formed at the liquid water interface during hydrate formation. Then the temperature
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of the system is reduced by applying the fast cooling method to facilitate the hydrate
146
formation. Once the desired cooling temperature is attained, the system temperature is held for
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4 to 8 hours to ensure the adequate formation of gas hydrates. Hydrate formation is observed
148
through sudden pressure drops witnessed in the logged data. Once the hydrate is fully formed
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with no further pressure drop, the system is heated step-wise, the span of each step usually
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varies from 2 to 6 hours, depending on its distance from expected equilibrium temperature.
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2.4. Theoretical calculations 2.4.1. Average Suppression Temperature (∆Ŧ)
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The average suppression temperature, ∆Ŧ, which is a measure of average inhibition ability of
155
THI is calculated for all concentrations by the equation presented by Xia et al [10]. n
∑ (T0,Pi
∑ ∆T = i =1 ∆T =
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n
− T1, Pi )
(Eq. 1)
n
where, ∆T is suppression temperature at constant pressure of Pi, T0,Pi represents the hydrate
158
equilibrium temperature of CH4 or CO2 in pure water (without TMAOH), while T1,Pi is the
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equilibrium temperature of CH4 or CO2 in the presence of TMAOH. 2.4.2. COSMO-RS Evaluation of TMAOH
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COSMO-RS is a quantum chemistry based thermodynamic model for arbitrary molecules in
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almost any form of chemicals by predicting their chemical potentials and consequently, their
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activity coefficients [33–37]. In COSMO-RS calculations, the solute molecules are created and
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calculated in a virtual conductor environment by induction of polarized charge densities over
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the interface of the conductor and the molecule. The details of the basic theory of COSMO-RS
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and their applications can be found elsewhere [35,38,39]. In COSMO-RS software, the polarize
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charge densities converts into a surface composition function pX(σ), usually called sigma profile
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[37].
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Determination of sigma profile (σ) in COSMO-RS is a two-steps method for TMAOH. Initial
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generations of geometry and electronic density for cation (TMA+) and anion (OH-) is
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conducted by TURBOMOLE 6.1 program package.
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functional theory (DFT), via the BP functional B88-86 with a triple zeta valence polarized
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basis set (TZVP) and the resolution of identity standard (RI) approximation [40]. Sigma
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profile of TMAOH is then estimated by COSMOthermX, utilizing the parameter file of
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BP_TZVP_C30_1301 [41,42].
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∑ ∑
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TURBOMOLE applies the density
(Eq. 2)
The distribution of the division given with respect to the sigma (σ) is called σ-profile (ps(σ)).
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The σ-profile of the solvent ps(σ), is defined as the mole fraction (xi) weighted sum of the σ -
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profiles of its compounds xi, pxi respectively in equation 2 [36,43].
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2.4.3. Enthalpy of Hydrate Dissociation
The dissociation enthalpies (∆H), of gas hydrates, are determined through the Clausius–
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Clapeyron equation by differentiation of the phase equilibrium data as follows;
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∆ H diss ∂ ln P =− 1 zR ∂ T
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(Eq. 3)
where P and T are the equilibrium pressure and temperature, z represents the compressibility
185
factor of respective gas for average gas temperature and pressure, R denotes the universal gas
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constant and ∆Hdiss is the dissociation enthalpy of gas hydrates.
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3. Results and Discussion 3.1.
Phase Equilibrium measurement of TMAOH/ H2O/ CH4 and TMAOH/ H2O/ CO2 gas Hydrate
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The TMAOH + H2O + CH4 and TMAOH + H2O + CO2 equilibrium curves are acquired in the
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absence and presence of various compositions of TMAOH to evaluate its thermodynamic
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inhibition impact at moderate pressures within the ranges of 3.5-8.0 and 1.8-4.2 MPa
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respectively, in Table 2.
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Table 2: Hydrate phase equilibrium data of aqueous TMAOH for CH4 and CO2 hydrates at 1, 5 and 10 wt%.
1 wt%
5 wt%
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
CH4+TMAOH
277.0
3.52
276.2
3.50
276.0
+H2O
279.5
4.85
278.7
4.85
278.6
282.7
6.45
281.9
6.50
284.8
8.00
283.9
7.90
1.85
+H2O
279.6
2.58
280.4
2.95
282.2
3.51
283.0
3.95
T (K)
P (MPa)
3.50
275.8
3.62
4.93
278.4
4.95
281.5
6.40
281.4
6.58
283.7
7.95
283.4
8.00
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277.4
P (MPa)
276.5
2.00
275.7
1.84
275.8
2.08
278.4
2.56
277.8
2.51
277.5
2.57
279.4
2.95
279.0
3.00
278.2
2.98
280.9
3.55
280.6
3.54
279.5
3.47
282.2
4.07
282.0
4.07
280.9
3.99
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10 wt%
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System
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TMAOH concentration in the solution
The measured phase equilibrium data of TMAOH + H2O + CH4 systems at different
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concentrations are presented in Figure 1(a). Results reveal that, TMAOH inhibits hydrate
200
formation at all studied concentrations as shown in Figure 1(a). Moreover, the inhibition strength
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is found to increase with increasing TMAOH concentration. The inhibition impact also seems to
202
be pressure dependent as maximum inhibition is found at around 5 MPa. At lower pressures
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conditions (P > 5 MPa), the inhibition impact of TMAOH is higher compared to higher pressure
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conditions (P < 5 MPa) for all concentrations. A similar phenomenon is also reported by Tariq et
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al. [28]. This phonemenum is illustrated in Figure 1(b) better. As shown in this figure, the
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suppression temperature shows a maximum value at 5 MPa for all TMAOH concentrations.
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Average suppression temperature (∆Ŧ) for 1, 5, and 10 wt% TMAOH aqueous solution is 0.9 K,
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1.1 K, and 1.5 K, respectively.
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Figure 1: (a) Hydrate phase equilibrium data of TMAOH + H2O + CH4 at 1, 5 and 10 wt%. (b) Suppression temperature (∆T) of TMAOH + H2O + CH4 for 1, 5 and 10 wt%.
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The inhibition performance of TMAOH + H2O + CH4 is compared with other AILs reported in
213
literature and presented in Figure 2. TMAOH is found to show better inhibition strength than
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studied AILs reported by Tariq et al. [28] at 1 wt% (Figure 2(a)). In Figure 2(b), TMAOH show
215
better inhibition effect that most AILs but are in the same range with TMAA and Ch-iBu at 5
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wt%. Similarly, the inhibition strength of 10 wt% TMAOH is comparable to TEACl reported by
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Keshavarz et al.[27], in Figure 2 (c). Moreover, the results are further compared with
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imidazolium based ionic liquids in references [9,14] and commercial THI (Polyethylene oxide
219
(PEO)) [9] at 10 wt % as shown in Figure 2(c). The results suggest that, TMAOH performed
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better than most of the imidazolium based ionic liquids and PEO [9].
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Figure 2: Comparison of the hydrate phase equilibrium data of TMAOH + H2O + CH4 with ammonium and imidazolium based ionic liquids and a traditional inhibitor (PEO) at different concentrations. (a) 1 wt%, (b) 5 wt%, and (c) 10 wt%.
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As mentioned before, a limited number of publications are available on the effect of ILs on CO2
226
hydrate inhibition [44–47]. There seem to be none on the inhibition effect of AILs on CO2
227
hydrate in open literature. The measured TMAOH + H2O + CO2 equilibrium curves at 1, 5 and
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10 wt% are presented in Figure 3(a). The presence of TMAOH inhibits CO2 hydrates in Figure 3
229
(a), as in CH4 hydrates. The inhibition effect is clearly concentration-dependent, as increasing the
230
concentration of TMAOH in the solution to 5 and 10% results in more inhibition impact, as
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shown in Figure 3(a). For a better illustration of this impact, the suppression temperatures (∆T)
232
for TMAOH + H2O +CO2 systems are plotted in Figure 3(b). As shown in this figure, the
233
suppression temperature of CO2 hydrate in the presence of 1, 5, 10 wt% of TMAOH are almost
234
constant at various pressures with an average suppression temperature of 1.2 K, 1.5 K, and 2.3 K,
235
respectively. Suggesting that, pressure does not show significant impact on the CO2 hydrates
236
suppression temperature at studied TMAOH concentrations. However, an opposite effect is
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observed in CH4 (see Figure 2(b)) as discussed earlier. This different behavior may perhaps be
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attributed to the hydrogen bonding between TMAOH and CO2 polar properties.
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Figure 3: Hydrate phase equilibrium data of TMAOH + H2O + CO2 at 1, 5 and 10 wt%. (b) Suppression temperature (∆T) of TMAOH + H2O + CO2 for 1, 5 and 10 wt%.
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Figure 4 presents the comparison of TMAOH + H2O + CO2 with imidazolium-based IL 1-butyl-
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3-metylimidazolium tetrafluoroborate (BMIM-BF4) reported by Chen et al [48] along with other
244
type of ILs families, like; N-ethyl-Methylmorpholinium bromide (EMMor-Br), N-ethyl-N-
245
methylmorpholinium tetrafluoroborate (EMMor-BF4), Nethyl-N-methylpiperidinium bromide
246
(EMPip-Br) and N-ethyl-N-methylpiperidinium tetrafluoroborate (EMPip-BF4)) investigated by
247
Cha et al [45] for 10 wt%. As shown in this figure, the inhibition impact, of TMAOH exhibits
248
better CO2 hydrate inhibition than other ILs. It seems that TMAOH works slightly better than
249
other ILs’ THI inhibitors.
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Figure 4: Comparison of the hydrate phase equilibrium data of TMAOH + H2O + CO2 with ammonium and imidazolium based ionic liquids reported by Cha et al. [45] and Chen et al. [48] at 10 wt%. The average inhibition strength of TMAOH, for CO2 hydrate (2.3 K) is higher than CH4 hydrate
256
(1.5 K) at 10 wt%. It should be noted that, due to the quadruple moment of CO2, TMAOH could
257
interact with CO2 more than CH4. This explains the higher inhibition of TMAOH in CO2 hydrate
258
than CH4 at the same concentration. The impact of pressure on ∆T could be due to the polarity of
259
TMAOH. For CH4 hydrate, increasing the pressure results in higher solubility of CH4 molecules
260
in the solution and at the same time increasing the tendency of water molecules to organize and
261
form hydrate. Therefore, pressure plays an essential role in the formation of CH4 hydrate, as
262
observed in this work as well as in literature [28]. On the other hand, the interaction of TMAOH
263
and CO2 molecules is much important compared to the impact of pressure on CO2 solubility.
264
Therefore, pressure does not show significant impact on the ∆T. It should be noted that the
265
interaction between TMAOH and CO2 might lead to the enhancement of hydrate formation
266
kinetics. However, further study is required for confirmation.
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3.2.
COSMO-RS analysis of TMAOH-H2O system
The TMAOH inhibition effect observed in this work is due to its hydrogen bonding affinity for
269
water molecules. To further understand the TMAOH/water interaction phenomenon, the sigma
270
profile and surface of TMAOH and water is generated in COSMO-RS as shown in Figure 5 and
271
6, respectively. In Figure 5, a peak at the right side indicates the most electronegative area (i.e.
272
act as H-bond acceptor), while a peak at the left side represents electropositivity (i.e. act as H-
273
bond donor) [29,39,49,50].
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Figure 5: Sigma profile of TMAOH + H2O system.
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It is observed from Figure 5 that, water have the peak intensity values of 3 and 2 at hydrogen
277
bonding donor and acceptor affinity regions respectively, this arises from the two hydrogen
278
atoms and lone pairs of the oxygen [37,50]. On the other hand, TMAOH contains tetramethyl
279
ammonium (TMA+) cation and hydroxyl (OH-) anion. However, the hydrophobicity of ILs is
280
generally due to their cation alky chain length, with shorter chains known to improve hydrate
281
inhibition strength [17]. This is evident in TMAOH as its cation (TMA+) induce high peaks in
282
the non-polar region with intensities of 24 and 17 respectively, as show in Figure 5. However,
283
this TMA+ peak is extended to the hydrogen bonding donor, due to its methyl functional group.
284
This improves its hydrophilicity which further enhances miscibility with water molecules
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resulting in hydrate inhibition [37]. The actual inhibition of TMAOH comes from its OH- anion.
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The OH- anion show a peak 4.72 in the hydrogen bond acceptor region. Which is relatively
287
higher than the water peak (1.49), resulting in strong hydrogen bonding affinity of the OH- anion
288
for water, which causes hydrate inhibition (as seen in Figures 1 to 4) via accepting hydrogen
289
atoms from water molecules [37].
290
The sigma surfaces of water, TMA+ cation, and OH- anion are presented in Figure 6 (a), (b) and
291
(c), respectively, for visual description of the TMAOH/water interaction phenomenon. In sigma
292
surface, the color changes from dark blue (highly electropositive) to blue (electropositive) and
293
finally green (non-polar), also, brown color represents the highest electronegativity, followed by
294
red colors (electronegative) [37]. In Figure 6(a), the σ-surface of water molecule is dominated by
295
blue color regions of strongly electropositive and red region of electronegative evident with the
296
two symmetric peaks in the σ-profile of water at 0.016 e/Å2 regions (see Figure 5) [37]. This
297
phenomenon additionally justifies the unique behavior of water molecule, to dissolve polar and
298
non-polar molecules and therefore used as universal solvent [29,37]. To act as a good cation for
299
gas hydrate inhibitor, the cation should have as less non-polar area (i.e. green color) as possible,
300
for a better miscibility with water. As shown in Figure 6(b), TMA+ surface show more relative
301
green color (nonpolar) fused with blue color (electropositive). This indicates that it possess a
302
relative electronegativity and will be able to share hydrogen atoms with water [51]. As shown in
303
Figure 6 (c), OH- anion exhibits the highest electronegativity, i.e. a large brown color surface
304
[52,53]. Hence, the high electronegativity of OH- anion brings about higher interaction with the
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water molecules, resulting in strong hydrate inhibition.
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Figure 6: Sigma surface of (a) Water molecule (b) TMA+ cation (c) OH- anion.
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3.3.
Enthalpy of dissociation for TMAOH + H2O + CH4 & TMAOH + H2O + CO2 Gas Hydrate
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The calculated ∆Hdiss for TMAOH + H2O + CH4 and TMAOH + H2O + CO2 system are
312
presented in Table 3. It is assumed that at equilibrium condition, the system mainly consisted of
313
only liquid and gas phases, therefore, the amount of hydrate phase is negligible [31]. The ∆Hdiss
314
of H2O + CH4 is 58.88kJ/mol, which lies within the range of CH4 hydrate enthalpy data [54].
315
The ∆Hdiss of TMAOH + H2O + CH4 systems seems to be the same as pure methane hydrate and
316
around 58.1 to 59.1 kJ/mol for different concentration. This indicated that TMAOH is not
317
participated in the hydrate crystalline network and no semi-clathrate hydrate is formed in the
318
presence of TMAOH. In addition, only sI hydrate structure is formed for this system. The same
319
argument is valid for CO2 hydrate formation in the presence of TMAOH. The enthalpy of H2O +
320
CO2 system is 64.73 kJ/mol, which is in the range of reported values in the literature [54,55].
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The enthalpies for TMAOH + H2O + CO2 revealed that the presence of TMAOH have no
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significant influence on the enthalpy of the system, therefore, it does not take any part in forming
323
hydrate cages.
TMAOH concentration in the solution 0 wt% 1 wt% 5 wt% 10 wt%
CH4 3.5 5 6.5 8 Overall
4. Conclusion
72.435 67.655 65.052 61.071 57.481 64.739
61.625 59.741 57.892 56.604 58.965
61.873 59.866 58.213 56.764 59.179
70.282 65.930 62.516 58.585 54.311 62.325
69.326 65.738 63.046 59.131 54.727 62.394
60.768 58.954 57.137 55.846 58.176
72.189 67.688 64.664 60.465 56.266 64.255
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61.482 59.659 57.902 56.486 58.882
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Pressure (MPa)
CO2 2 2.5 3 3.5 4 Overall
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Table 3: Calculated molar enthalpies of hydrate dissociation, ∆Hdiss (kJ/mol), of CO2 and CH4 hydrate in the presence of TMAOH solutions
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In this experimental work, the phase equilibrium measurement for TMAOH + H2O + CH4 and
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TMAOH + H2O + CO2 systems are reported. The obtained results revealed that the presence of
329
TMAOH disrupts the water activity in hydrate formation by decreasing the hydrate phase
330
boundary of TMAOH + H2O + CH4 and TMAOH + H2O + CO2 system up to 1.5 and 2.3 K at 10
331
wt%, respectively, which is quite substantial in ILs perspectives. Additionally, the reason for the
332
significant shift in ∆T observed for both systems is evaluated via COSMO-RS analysis.
333
Furthermore, Clausius−Clapeyron equation was used to calculate the molar enthalpies of
334
dissociation for the TMAOH + H2O + CH4 and TMAOH + H2O + CO2 hydrate systems which
335
are found to show no significant change while increase concentration of TMAOH.
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Acknowledgment
337
Authors like to thank Chemical Engineering Department, Universiti Teknologi PETRONAS for
338
providing financial facilities throughout the studies. The authors also would like to acknowledge
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and appreciate the Centre of Research in Ionic Liquids and Research Centre for CO2 Capture for
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providing laboratory and technical services.
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Nomenclature Ammonium based Ionic liquid
AAs
Anti-agglomerates
BMIM-BF4
1-butyl-3-methyl imidazolium tetrafluoroborate
BMIM-Br
1-butyl-3-methyl imidazolium chloride
BMIM-Cl
1-butyl-3-methyl imidazolium chloride
BMIM-HSO4
1-butyl-3-methyl imidazolium hydrogen sulphate
Ch-But
choline butyrate
Ch-iB
choline iso-butyrate
Ch-Hex
choline hexanoate
Ch-Oct
choline octanoate
CH4
Methane
CO2
Carbon Dioxide
EHB
Hydrogen bonding energy
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Total internal energy
Van der wall energy
EMMor-Br EMMor-BF4 EMPip-Br
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N-ethyl-N-methylmorpholinium bromide
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KHIs
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N-ethyl-N-methylmorpholinium tetrafluoroborate N-ethyl-N-methylpiperidinium bromide N-ethyl-N-methylpiperidinium tetrafluoroborate dissociation enthalpies kinetic hydrate inhibitors
LDHIs
Low dosage hydrate inhibitors
PEO
Polyethylene oxide
THI
Thermodynamic hydrate inhibitor
TMAA
tetra-alkylammonium acetate
TMACl
Tetra methyl ammonium Chloride
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Tetramethylammonium hydroxide
σ-profile
Sigma profile
∆Ŧ
Average suppression temperature
∆T
Suppression temperature
342
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343
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Research Highlights •
The phase boundary of TMAOH + H2O + CH4 is measured and reported.
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•
The phase boundary of TMAOH + H2O + CO2 is measured and reported for the first time.
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•
The enthalpy of hydrate dissociation for the studied system is revealed that TMAOH is
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not involve in hydrate crystalline structure.
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S0378-3812(17)30068-7
DOI:
10.1016/j.fluid.2017.02.011
Reference:
FLUID 11407
To appear in:
Fluid Phase Equilibria
Received Date: 27 October 2016 Revised Date:
30 December 2016
Accepted Date: 14 February 2017
Please cite this article as: M.S. Khan, B. Partoon, C.B. Bavoh, B. Lal, N.B. Mellon, Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions, Fluid Phase Equilibria (2017), doi: 10.1016/j.fluid.2017.02.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of Tetramethylammonium Hydroxide on Methane and
2
Carbon Dioxide Gas Hydrate Phase Equilibrium Conditions
b
a,b
, Behzad Partoon , Cornelius B. Bavoh Lal a,b,*, Nurhayati Bt Mellon b
a,b
, Bhajan
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a
a,b
Research Centre for CO2 Capture (RCCO2C), Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia.
Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia.
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Muhammad Saad Khan
*Corresponding author: [email protected] Telephone/Fax: +6053687684; +60103858473 /+6053656176
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Abstract
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In this experimental work, the phase boundaries of TMAOH + H2O + CH4 and TMAOH + H2O
13
+ CO2 hydrates are measured at different concentrations of aqueous TMAOH solution. The
14
temperature-cycle (T-cycle) method is applied to measure the hydrate equilibrium temperature of
15
TMAOH + H2O + CH4 and TMAOH + H2O + CO2 systems within the ranges of 3.5-8.0 MPa
16
and 1.8-4.2 MPa, respectively. Results reveals that, TMAOH acts as a thermodynamic inhibitor
17
for both gases. In the presence of 10 wt% of TMAOH, the inhibition effect appears to be very
18
substantial for CO2 with an average suppression temperature (∆Ŧ) of 2.24 K. An ample inhibition
19
influence is observed for CH4 hydrate at 10 wt% with ∆Ŧ of 1.52 K. The inhibition effect of
20
TMAOH is observed to increase with increasing TMAOH concentration. Confirmed via
21
COSMO-RS analysis, the TMAOH inhibition effect is due to its hydrogen bonding affinity for
22
water molecules. Furthermore, the calculated hydrate dissociation enthalpies in both systems
23
revealed that TMAOH does not participate in the hydrate crystalline structure.
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Keywords: Ammonium based ionic liquids; COSMO-RS; gas hydrate; inhibitor; phase
25
equilibrium; TMAOH.
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1. Introduction
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Gas hydrates are crystal-like solids in which gas molecules are encased in cages formed by
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hydrogen bonded water molecules and stabilized by van der Waals forces. They are non-
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stoichiometric inclusion compounds formed under high pressures and low temperatures
31
conditions [1]. In oil and gas industry, one of the major flow assurance problems is the formation
32
of gas hydrate in pipelines, which can cause the blockage of hydrocarbon production and
33
transportation in pipelines and processing facilities [2,3]. Methane (CH4), carbon dioxide (CO2),
34
hydrogen sulfide (H2S), ethane (C2H6), propane (C3H8), propene (C3H6), and even iso-butane (i-
35
C4H10) can form gas hydrate under production and transportation condition, especially in deep
36
waters offshore operations. [4]. Gas hydrate formation could lead to catastrophic economic
37
losses and ecological risks. This problem costs the industry billions of dollars annually to
38
mitigate, with no permanent solution in focus. To avoid hydrate formation, generally four
39
methods could be applied; water removal; heating; depressurization; and chemical inhibition [4].
40
However, In most cases, hydrate inhibition via chemical inhibitors is the only viable option for
41
offshore gas pipelines [5–8]. Based on their inhibition mechanisms, these chemical inhibitors are
42
categorized into two groups; thermodynamic hydrate inhibitors (THIs) and low dosage hydrate
43
inhibitors (LDHIs). THIs such as methanol and ethylene glycol work by shifting the hydrate
44
equilibrium curve to lower temperatures, enough to keep the system out of the hydrate formation
45
region. On the other hand, LDHIs is based on modern flow assurance strategies, like moving
46
from complete hydrate avoidance towards hydrate risk management. There are two primary
47
types of LDHIs – kinetic hydrate inhibitors (KHIs), generally water soluble polymers, and anti-
48
agglomerates (AAs), which are usually surfactants. Unlike thermodynamic inhibitors, KHIs do
49
not significantly change the hydrate equilibrium phase boundary but delay the hydrate nucleation
50
formation. Whereas, AAs principally do not inhibit hydrate formation but form a transferrable
51
slurry and prevent hydrate crystals from agglomerating to form physically superior hydrate plug
52
structures that can slab pipelines [5,8].
53
Traditional THIs requires higher concentrations in the application, also, they are not
54
environmentally friendly chemicals as they have toxic nature and high volatility. Therefore,
55
being committed towards the discovery of highly effective and environmentally friendly THIs,
56
Ionic Liquids (ILs) have been introduced as both green chemicals and tailor-made solvents for
57
specific application with negligible vapor pressure. Generally, for THI’s applications,
58
compounds that form hydrogen bonding with water molecules can effectively act as gas hydrate
59
inhibitors. Therefore, ILs have a considerable potential to work as gas hydrate inhibitors due to
60
their ability to form hydrogen bonding with water molecules. Besides, their ionic nature makes
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them interact with water via electrostatic charges interactions that also making them gas hydrate
62
inhibitors like other electrolytes. Xiao & Adhirama [9] initiated the research on ILs as gas
63
hydrate inhibitors in 2009, they investigated Imidazolium-based ILs and found promising results
64
for both THI and KHI inhibition. They found that ILs could show thermodynamic inhibition, and
65
at the same time delay the hydrate formation by slowing down the hydrate nucleation rate. They
66
believed that this dual functional effect is due to their strong electrostatic charges and hydrogen
67
bonding with water. Xiao et al. [10] further reported that imidazolium-based ILs with halides
68
anions showed significant dual functional (i.e. kinetic and thermodynamic inhibition) inhibition
69
performance on methane hydrate formation. Several researchers have also reported on the
70
influence of imidazolium-based ILs as gas hydrate inhibitors [11–16]. Nonetheless, the state of
71
art on the influence of other families of ILs as gas hydrate inhibitors are summarized in the
72
review by Tariq et al. [17]. As imidazolium-based ILs have not shown significant inhibition
73
strength compared to traditional inhibitors. Recent ILs hydrate based studies is moving from
74
imidazolium-based ILs towards other kinds of ILs, such as pyrrolidinium [18,19], , ammonium
75
[20–22], and phosphonium [23]based ILs. However, studies on the effect of other types of ILs
76
(especially ammonium based ILs) on gas hydrate formation, are still in the primary stage, as few
77
studies have been reported in literature so far.
78
Govinda and coworker [24] comprehensively reviewed the applications of ammonium based ILs
79
(AILs) in numerous research areas such as; CO2 capturing, fuel cells, anti-corrosive agents, flow
80
assurance as well as other technical applications. AILs have better environmental properties than
81
imidazolium-based ILs which result in their use as food preservatives in food industries [25]. Li
82
et al.[26], initially presented tetramethylammonium chloride (TMACl) as AIL’s gas hydrate
83
inhibitor in 2011. They found that TMACl achieved better THI impact compare to their
84
imidazolium counterparts. Subsequently, in 2013, Keshavarz et al. [27] evaluated the inhibition
85
impact of tetraethylammonium chloride (TEACl) with other imidazolium-based ILs and found
86
that, all studied ILs were able to inhibit hydrate formation up to 0.9 K [27]. Recently, Tariq et al.
87
[28], evaluated six AILs [tetra-alkylammonium acetate (TMAA), choline iso-butyrate (Ch-iB),
88
choline hexanoate (Ch-Hex), choline butyrate (Ch-But), and choline octanoate (Ch-Oct)] as CH4
89
hydrate inhibitors and found that they are able to increase the hydrate suppression temperature
90
(∆Ŧ). In addition, they suggested that some of the studied AILs could work as KHI inhibitors.
91
Tariq et al. [28] further concluded that at lower concentration of AILs (i.e. 1 wt%), lesser
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inhibition effect was observed at a lower pressure range of 3.5-6.5 MPa; while at a higher
93
pressure range of 6.6-12 MPa, all studied AILs worked as thermodynamic promoters instead of
94
inhibitors. However, at a relatively higher concentration (5 wt%), inhibition effect is more
95
evident at higher pressure regions [28]. Recently, Bavoh et al. [29] implemented the COSMO-
96
RS (Conductor-like Screening Model for Realistic Solvents) technique to evaluate the effect of
97
ILs hydrogen bonding energies (EHB) and sigma profiles on ∆Ŧ. They revealed that the ∆Ŧ
98
increases with increasing EHB and/or reduces with increments in alkyl chain lengths of ILs.
99
However, the pairing of ILs cations and anions significantly disturbs the inhibition performance
100
of gas hydrates [29]. Khan and coworkers also applied COSMO-RS to describes the effects of
101
different types of interaction energies, i.e., EHB, misfit energy (EMF), van der Waal energy
102
(EVdW) and total internal energy (ET) on the ∆Ŧ of ILs [30]. They also found that EHB of anion
103
plays a major influential impact on hydrate inhibition [30].
104
Therefore, the main objective of this study is to examine the influence of tetramethylammonium
105
hydroxide (TMAOH) on the phase equilibrium boundaries of TMAOH+ H2O+ CH4 and
106
TMAOH+ H2O+ CO2 hydrate formation at different concentrations (1, 5 and 10 wt.%). The
107
impact of an AIL on the CO2 hydrate phase boundary is reported for the first time in this work.
108
In addition, a COSMO-RS based TMAOH/ Water system interaction analysis is performed to
109
understand the THI inhibition mechanism of TMAOH.
2. Methodology
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2.1. Materials Table 1 summarizes the details of the materials used in this study. An aqueous solution of 95%
113
TMAOH is purchased from Merck milli-pore company Germany. All gasses are purchased
114
from Air Products Singapore Private Limited and used without any further purification.
115
Deionized water (RO membrane plant TKA-LabTowe) is used to prepare desired concentration
116
of aqueous TMAOH solutions
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Table 1: Material used for gas hydrate mitigation study
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No
Chemical Name
Symbol
Chemical Structure
Carbon dioxide
CO2
99.95 mole %
2
Methane
CH4
99.99 mole %
3
Tetramethylammonium hydroxide
TMAOH
95 wt %
4
Water
H2O
Deionized
Experimental apparatus
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Purity
The high-pressure stainless steel cell is employed for measuring the phase boundaries of CH4,
125
and CO2 gas hydrate in the presence of aqueous TMAOH solutions. The apparatus consists of
126
high-pressure equilibrium cell with a volumetric capacity of 500 cm3 and can work at
127
temperature ranges from 253–523 K and a maximum operating pressure of 20 MPa. The
128
temperature and pressure in the cell are recorded every second with an accuracy of ±0.1 K and
129
±0.01 MPa respectively. Furthermore, the apparatus is fitted with a magnetic system containing
130
2-bladed pitch impeller and a 400 rpm motor to provide adequate mixing of the sample in the
131
cell under test conditions. The cell is submerged in a thermostatic bath, equipped with PID
132
controller for controlling the bath temperature within ±0.3 ºC accuracy. The complete detail of
133
the experimental setup can be found elsewhere [31,32].
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2.3.
Experimental Procedure
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An isochoric T-cycle method with step heating technique is employed in determining the
136
phase equilibrium of CH4 and CO2 hydrate. Prior to the experimentations, the cell is
137
thoroughly washed with distilled water and dried. Then 100 ml liquid sample of TMAOH is
138
filled in the cell, then the system is cooled down to the chosen operating temperature. A small
139
amount of CO2/CH4 is introduced into the cell, purged for three times and finally vacuumed to
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ensure that there are no traces of the air in the cell. After that, the cell is pressurized with
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CO2/CH4 to desired pressure. In these experiments, the pressure’s range for CH4 gas hydrate is
142
3.5 - 8.0 MPa, while that for CO2 hydrate is 2.0 - 4.0 MPa. Once the conditions is stabilized,
143
the mechanical stirrer is set at 300 rpm to provide adequate mixing and break the interface
144
boundary formed at the liquid water interface during hydrate formation. Then the temperature
145
of the system is reduced by applying the fast cooling method to facilitate the hydrate
146
formation. Once the desired cooling temperature is attained, the system temperature is held for
147
4 to 8 hours to ensure the adequate formation of gas hydrates. Hydrate formation is observed
148
through sudden pressure drops witnessed in the logged data. Once the hydrate is fully formed
149
with no further pressure drop, the system is heated step-wise, the span of each step usually
150
varies from 2 to 6 hours, depending on its distance from expected equilibrium temperature.
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2.4. Theoretical calculations 2.4.1. Average Suppression Temperature (∆Ŧ)
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The average suppression temperature, ∆Ŧ, which is a measure of average inhibition ability of
155
THI is calculated for all concentrations by the equation presented by Xia et al [10]. n
∑ (T0,Pi
∑ ∆T = i =1 ∆T =
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n
− T1, Pi )
(Eq. 1)
n
where, ∆T is suppression temperature at constant pressure of Pi, T0,Pi represents the hydrate
158
equilibrium temperature of CH4 or CO2 in pure water (without TMAOH), while T1,Pi is the
159
equilibrium temperature of CH4 or CO2 in the presence of TMAOH. 2.4.2. COSMO-RS Evaluation of TMAOH
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COSMO-RS is a quantum chemistry based thermodynamic model for arbitrary molecules in
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almost any form of chemicals by predicting their chemical potentials and consequently, their
163
activity coefficients [33–37]. In COSMO-RS calculations, the solute molecules are created and
164
calculated in a virtual conductor environment by induction of polarized charge densities over
165
the interface of the conductor and the molecule. The details of the basic theory of COSMO-RS
166
and their applications can be found elsewhere [35,38,39]. In COSMO-RS software, the polarize
167
charge densities converts into a surface composition function pX(σ), usually called sigma profile
168
[37].
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169
Determination of sigma profile (σ) in COSMO-RS is a two-steps method for TMAOH. Initial
170
generations of geometry and electronic density for cation (TMA+) and anion (OH-) is
171
conducted by TURBOMOLE 6.1 program package.
172
functional theory (DFT), via the BP functional B88-86 with a triple zeta valence polarized
173
basis set (TZVP) and the resolution of identity standard (RI) approximation [40]. Sigma
174
profile of TMAOH is then estimated by COSMOthermX, utilizing the parameter file of
175
BP_TZVP_C30_1301 [41,42].
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∑ ∑
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=
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TURBOMOLE applies the density
(Eq. 2)
The distribution of the division given with respect to the sigma (σ) is called σ-profile (ps(σ)).
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The σ-profile of the solvent ps(σ), is defined as the mole fraction (xi) weighted sum of the σ -
179
profiles of its compounds xi, pxi respectively in equation 2 [36,43].
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2.4.3. Enthalpy of Hydrate Dissociation
The dissociation enthalpies (∆H), of gas hydrates, are determined through the Clausius–
182
Clapeyron equation by differentiation of the phase equilibrium data as follows;
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∆ H diss ∂ ln P =− 1 zR ∂ T
183
(Eq. 3)
where P and T are the equilibrium pressure and temperature, z represents the compressibility
185
factor of respective gas for average gas temperature and pressure, R denotes the universal gas
186
constant and ∆Hdiss is the dissociation enthalpy of gas hydrates.
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3. Results and Discussion 3.1.
Phase Equilibrium measurement of TMAOH/ H2O/ CH4 and TMAOH/ H2O/ CO2 gas Hydrate
191
The TMAOH + H2O + CH4 and TMAOH + H2O + CO2 equilibrium curves are acquired in the
192
absence and presence of various compositions of TMAOH to evaluate its thermodynamic
193
inhibition impact at moderate pressures within the ranges of 3.5-8.0 and 1.8-4.2 MPa
194
respectively, in Table 2.
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195 196 197
Table 2: Hydrate phase equilibrium data of aqueous TMAOH for CH4 and CO2 hydrates at 1, 5 and 10 wt%.
1 wt%
5 wt%
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
CH4+TMAOH
277.0
3.52
276.2
3.50
276.0
+H2O
279.5
4.85
278.7
4.85
278.6
282.7
6.45
281.9
6.50
284.8
8.00
283.9
7.90
1.85
+H2O
279.6
2.58
280.4
2.95
282.2
3.51
283.0
3.95
T (K)
P (MPa)
3.50
275.8
3.62
4.93
278.4
4.95
281.5
6.40
281.4
6.58
283.7
7.95
283.4
8.00
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277.4
P (MPa)
276.5
2.00
275.7
1.84
275.8
2.08
278.4
2.56
277.8
2.51
277.5
2.57
279.4
2.95
279.0
3.00
278.2
2.98
280.9
3.55
280.6
3.54
279.5
3.47
282.2
4.07
282.0
4.07
280.9
3.99
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10 wt%
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System
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TMAOH concentration in the solution
The measured phase equilibrium data of TMAOH + H2O + CH4 systems at different
199
concentrations are presented in Figure 1(a). Results reveal that, TMAOH inhibits hydrate
200
formation at all studied concentrations as shown in Figure 1(a). Moreover, the inhibition strength
201
is found to increase with increasing TMAOH concentration. The inhibition impact also seems to
202
be pressure dependent as maximum inhibition is found at around 5 MPa. At lower pressures
203
conditions (P > 5 MPa), the inhibition impact of TMAOH is higher compared to higher pressure
204
conditions (P < 5 MPa) for all concentrations. A similar phenomenon is also reported by Tariq et
205
al. [28]. This phonemenum is illustrated in Figure 1(b) better. As shown in this figure, the
206
suppression temperature shows a maximum value at 5 MPa for all TMAOH concentrations.
207
Average suppression temperature (∆Ŧ) for 1, 5, and 10 wt% TMAOH aqueous solution is 0.9 K,
208
1.1 K, and 1.5 K, respectively.
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Figure 1: (a) Hydrate phase equilibrium data of TMAOH + H2O + CH4 at 1, 5 and 10 wt%. (b) Suppression temperature (∆T) of TMAOH + H2O + CH4 for 1, 5 and 10 wt%.
212
The inhibition performance of TMAOH + H2O + CH4 is compared with other AILs reported in
213
literature and presented in Figure 2. TMAOH is found to show better inhibition strength than
214
studied AILs reported by Tariq et al. [28] at 1 wt% (Figure 2(a)). In Figure 2(b), TMAOH show
215
better inhibition effect that most AILs but are in the same range with TMAA and Ch-iBu at 5
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wt%. Similarly, the inhibition strength of 10 wt% TMAOH is comparable to TEACl reported by
217
Keshavarz et al.[27], in Figure 2 (c). Moreover, the results are further compared with
218
imidazolium based ionic liquids in references [9,14] and commercial THI (Polyethylene oxide
219
(PEO)) [9] at 10 wt % as shown in Figure 2(c). The results suggest that, TMAOH performed
220
better than most of the imidazolium based ionic liquids and PEO [9].
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Figure 2: Comparison of the hydrate phase equilibrium data of TMAOH + H2O + CH4 with ammonium and imidazolium based ionic liquids and a traditional inhibitor (PEO) at different concentrations. (a) 1 wt%, (b) 5 wt%, and (c) 10 wt%.
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As mentioned before, a limited number of publications are available on the effect of ILs on CO2
226
hydrate inhibition [44–47]. There seem to be none on the inhibition effect of AILs on CO2
227
hydrate in open literature. The measured TMAOH + H2O + CO2 equilibrium curves at 1, 5 and
228
10 wt% are presented in Figure 3(a). The presence of TMAOH inhibits CO2 hydrates in Figure 3
229
(a), as in CH4 hydrates. The inhibition effect is clearly concentration-dependent, as increasing the
230
concentration of TMAOH in the solution to 5 and 10% results in more inhibition impact, as
231
shown in Figure 3(a). For a better illustration of this impact, the suppression temperatures (∆T)
232
for TMAOH + H2O +CO2 systems are plotted in Figure 3(b). As shown in this figure, the
233
suppression temperature of CO2 hydrate in the presence of 1, 5, 10 wt% of TMAOH are almost
234
constant at various pressures with an average suppression temperature of 1.2 K, 1.5 K, and 2.3 K,
235
respectively. Suggesting that, pressure does not show significant impact on the CO2 hydrates
236
suppression temperature at studied TMAOH concentrations. However, an opposite effect is
237
observed in CH4 (see Figure 2(b)) as discussed earlier. This different behavior may perhaps be
238
attributed to the hydrogen bonding between TMAOH and CO2 polar properties.
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Figure 3: Hydrate phase equilibrium data of TMAOH + H2O + CO2 at 1, 5 and 10 wt%. (b) Suppression temperature (∆T) of TMAOH + H2O + CO2 for 1, 5 and 10 wt%.
241
Figure 4 presents the comparison of TMAOH + H2O + CO2 with imidazolium-based IL 1-butyl-
243
3-metylimidazolium tetrafluoroborate (BMIM-BF4) reported by Chen et al [48] along with other
244
type of ILs families, like; N-ethyl-Methylmorpholinium bromide (EMMor-Br), N-ethyl-N-
245
methylmorpholinium tetrafluoroborate (EMMor-BF4), Nethyl-N-methylpiperidinium bromide
246
(EMPip-Br) and N-ethyl-N-methylpiperidinium tetrafluoroborate (EMPip-BF4)) investigated by
247
Cha et al [45] for 10 wt%. As shown in this figure, the inhibition impact, of TMAOH exhibits
248
better CO2 hydrate inhibition than other ILs. It seems that TMAOH works slightly better than
249
other ILs’ THI inhibitors.
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Figure 4: Comparison of the hydrate phase equilibrium data of TMAOH + H2O + CO2 with ammonium and imidazolium based ionic liquids reported by Cha et al. [45] and Chen et al. [48] at 10 wt%. The average inhibition strength of TMAOH, for CO2 hydrate (2.3 K) is higher than CH4 hydrate
256
(1.5 K) at 10 wt%. It should be noted that, due to the quadruple moment of CO2, TMAOH could
257
interact with CO2 more than CH4. This explains the higher inhibition of TMAOH in CO2 hydrate
258
than CH4 at the same concentration. The impact of pressure on ∆T could be due to the polarity of
259
TMAOH. For CH4 hydrate, increasing the pressure results in higher solubility of CH4 molecules
260
in the solution and at the same time increasing the tendency of water molecules to organize and
261
form hydrate. Therefore, pressure plays an essential role in the formation of CH4 hydrate, as
262
observed in this work as well as in literature [28]. On the other hand, the interaction of TMAOH
263
and CO2 molecules is much important compared to the impact of pressure on CO2 solubility.
264
Therefore, pressure does not show significant impact on the ∆T. It should be noted that the
265
interaction between TMAOH and CO2 might lead to the enhancement of hydrate formation
266
kinetics. However, further study is required for confirmation.
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3.2.
COSMO-RS analysis of TMAOH-H2O system
The TMAOH inhibition effect observed in this work is due to its hydrogen bonding affinity for
269
water molecules. To further understand the TMAOH/water interaction phenomenon, the sigma
270
profile and surface of TMAOH and water is generated in COSMO-RS as shown in Figure 5 and
271
6, respectively. In Figure 5, a peak at the right side indicates the most electronegative area (i.e.
272
act as H-bond acceptor), while a peak at the left side represents electropositivity (i.e. act as H-
273
bond donor) [29,39,49,50].
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Figure 5: Sigma profile of TMAOH + H2O system.
276
It is observed from Figure 5 that, water have the peak intensity values of 3 and 2 at hydrogen
277
bonding donor and acceptor affinity regions respectively, this arises from the two hydrogen
278
atoms and lone pairs of the oxygen [37,50]. On the other hand, TMAOH contains tetramethyl
279
ammonium (TMA+) cation and hydroxyl (OH-) anion. However, the hydrophobicity of ILs is
280
generally due to their cation alky chain length, with shorter chains known to improve hydrate
281
inhibition strength [17]. This is evident in TMAOH as its cation (TMA+) induce high peaks in
282
the non-polar region with intensities of 24 and 17 respectively, as show in Figure 5. However,
283
this TMA+ peak is extended to the hydrogen bonding donor, due to its methyl functional group.
284
This improves its hydrophilicity which further enhances miscibility with water molecules
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resulting in hydrate inhibition [37]. The actual inhibition of TMAOH comes from its OH- anion.
286
The OH- anion show a peak 4.72 in the hydrogen bond acceptor region. Which is relatively
287
higher than the water peak (1.49), resulting in strong hydrogen bonding affinity of the OH- anion
288
for water, which causes hydrate inhibition (as seen in Figures 1 to 4) via accepting hydrogen
289
atoms from water molecules [37].
290
The sigma surfaces of water, TMA+ cation, and OH- anion are presented in Figure 6 (a), (b) and
291
(c), respectively, for visual description of the TMAOH/water interaction phenomenon. In sigma
292
surface, the color changes from dark blue (highly electropositive) to blue (electropositive) and
293
finally green (non-polar), also, brown color represents the highest electronegativity, followed by
294
red colors (electronegative) [37]. In Figure 6(a), the σ-surface of water molecule is dominated by
295
blue color regions of strongly electropositive and red region of electronegative evident with the
296
two symmetric peaks in the σ-profile of water at 0.016 e/Å2 regions (see Figure 5) [37]. This
297
phenomenon additionally justifies the unique behavior of water molecule, to dissolve polar and
298
non-polar molecules and therefore used as universal solvent [29,37]. To act as a good cation for
299
gas hydrate inhibitor, the cation should have as less non-polar area (i.e. green color) as possible,
300
for a better miscibility with water. As shown in Figure 6(b), TMA+ surface show more relative
301
green color (nonpolar) fused with blue color (electropositive). This indicates that it possess a
302
relative electronegativity and will be able to share hydrogen atoms with water [51]. As shown in
303
Figure 6 (c), OH- anion exhibits the highest electronegativity, i.e. a large brown color surface
304
[52,53]. Hence, the high electronegativity of OH- anion brings about higher interaction with the
305
water molecules, resulting in strong hydrate inhibition.
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Figure 6: Sigma surface of (a) Water molecule (b) TMA+ cation (c) OH- anion.
307 308
3.3.
Enthalpy of dissociation for TMAOH + H2O + CH4 & TMAOH + H2O + CO2 Gas Hydrate
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The calculated ∆Hdiss for TMAOH + H2O + CH4 and TMAOH + H2O + CO2 system are
312
presented in Table 3. It is assumed that at equilibrium condition, the system mainly consisted of
313
only liquid and gas phases, therefore, the amount of hydrate phase is negligible [31]. The ∆Hdiss
314
of H2O + CH4 is 58.88kJ/mol, which lies within the range of CH4 hydrate enthalpy data [54].
315
The ∆Hdiss of TMAOH + H2O + CH4 systems seems to be the same as pure methane hydrate and
316
around 58.1 to 59.1 kJ/mol for different concentration. This indicated that TMAOH is not
317
participated in the hydrate crystalline network and no semi-clathrate hydrate is formed in the
318
presence of TMAOH. In addition, only sI hydrate structure is formed for this system. The same
319
argument is valid for CO2 hydrate formation in the presence of TMAOH. The enthalpy of H2O +
320
CO2 system is 64.73 kJ/mol, which is in the range of reported values in the literature [54,55].
321
The enthalpies for TMAOH + H2O + CO2 revealed that the presence of TMAOH have no
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significant influence on the enthalpy of the system, therefore, it does not take any part in forming
323
hydrate cages.
TMAOH concentration in the solution 0 wt% 1 wt% 5 wt% 10 wt%
CH4 3.5 5 6.5 8 Overall
4. Conclusion
72.435 67.655 65.052 61.071 57.481 64.739
61.625 59.741 57.892 56.604 58.965
61.873 59.866 58.213 56.764 59.179
70.282 65.930 62.516 58.585 54.311 62.325
69.326 65.738 63.046 59.131 54.727 62.394
60.768 58.954 57.137 55.846 58.176
72.189 67.688 64.664 60.465 56.266 64.255
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61.482 59.659 57.902 56.486 58.882
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Pressure (MPa)
CO2 2 2.5 3 3.5 4 Overall
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Table 3: Calculated molar enthalpies of hydrate dissociation, ∆Hdiss (kJ/mol), of CO2 and CH4 hydrate in the presence of TMAOH solutions
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In this experimental work, the phase equilibrium measurement for TMAOH + H2O + CH4 and
328
TMAOH + H2O + CO2 systems are reported. The obtained results revealed that the presence of
329
TMAOH disrupts the water activity in hydrate formation by decreasing the hydrate phase
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boundary of TMAOH + H2O + CH4 and TMAOH + H2O + CO2 system up to 1.5 and 2.3 K at 10
331
wt%, respectively, which is quite substantial in ILs perspectives. Additionally, the reason for the
332
significant shift in ∆T observed for both systems is evaluated via COSMO-RS analysis.
333
Furthermore, Clausius−Clapeyron equation was used to calculate the molar enthalpies of
334
dissociation for the TMAOH + H2O + CH4 and TMAOH + H2O + CO2 hydrate systems which
335
are found to show no significant change while increase concentration of TMAOH.
336
Acknowledgment
337
Authors like to thank Chemical Engineering Department, Universiti Teknologi PETRONAS for
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providing financial facilities throughout the studies. The authors also would like to acknowledge
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and appreciate the Centre of Research in Ionic Liquids and Research Centre for CO2 Capture for
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providing laboratory and technical services.
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Nomenclature Ammonium based Ionic liquid
AAs
Anti-agglomerates
BMIM-BF4
1-butyl-3-methyl imidazolium tetrafluoroborate
BMIM-Br
1-butyl-3-methyl imidazolium chloride
BMIM-Cl
1-butyl-3-methyl imidazolium chloride
BMIM-HSO4
1-butyl-3-methyl imidazolium hydrogen sulphate
Ch-But
choline butyrate
Ch-iB
choline iso-butyrate
Ch-Hex
choline hexanoate
Ch-Oct
choline octanoate
CH4
Methane
CO2
Carbon Dioxide
EHB
Hydrogen bonding energy
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Total internal energy
Van der wall energy
EMMor-Br EMMor-BF4 EMPip-Br
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EMPip-BF4
N-ethyl-N-methylmorpholinium bromide
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EVdW
KHIs
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ET
∆H
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EMF
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N-ethyl-N-methylmorpholinium tetrafluoroborate N-ethyl-N-methylpiperidinium bromide N-ethyl-N-methylpiperidinium tetrafluoroborate dissociation enthalpies kinetic hydrate inhibitors
LDHIs
Low dosage hydrate inhibitors
PEO
Polyethylene oxide
THI
Thermodynamic hydrate inhibitor
TMAA
tetra-alkylammonium acetate
TMACl
Tetra methyl ammonium Chloride
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Tetramethylammonium hydroxide
σ-profile
Sigma profile
∆Ŧ
Average suppression temperature
∆T
Suppression temperature
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References
343
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Graphical Abstract
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Research Highlights •
The phase boundary of TMAOH + H2O + CH4 is measured and reported.
501
•
The phase boundary of TMAOH + H2O + CO2 is measured and reported for the first time.
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•
The enthalpy of hydrate dissociation for the studied system is revealed that TMAOH is
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not involve in hydrate crystalline structure.
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