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Methyl mercaptan absorption study into a hybrid solvent mixture composed by Diethanolamine + Methanol + Water at temperatures from 313.9 K to 353.0 K Iran D Charry Prada, Rodrigo Rivera-Tinoco, and Chakib Bouallou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02712 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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Industrial & Engineering Chemistry Research

Methyl mercaptan absorption study into a hybrid solvent mixture composed by Diethanolamine + Methanol + Water at temperatures from 313.9 K to 353.0 K Iran D. Charry Prada; Rodrigo Rivera-Tinoco; Chakib Bouallou* MINES ParisTech, PSL – Research University, CES – Centre d'Efficacité Energétique des Systèmes, Z.I. Les Glaizes – 5 rue Léon Blum, 91120 Palaiseau, France

ABSTRACT Methyl mercaptan absorption kinetics was studied for a hybrid solvent mixture consisting of diethanolamine/water/methanol for concentrations of 40/40/20 wt% and 40/20/40 wt% respectively, and for temperatures between 313.9 and 353.0 K. is usually found in sour gases and is removed along with other acid gases. Physico-chemical characterization is performed for the treatment of with the hybrid solvent which is up-to date lacking of chemical absorption data. A systematic methodology is proposed to assess the effect of each mass transfer, thermo-physical properties, as well as the chemical reactions in the system. An empirical novel equation is proposed for the Henry’s law constant solubility of with respect to the methanol in solution. A chemical kinetic mechanism based on the formation of a Zwitterion complex is proposed for the chemical species in liquid phase. Results validated with experimental data enabled all the reaction constants to be herewith parameterized in terms of the Arrhenius law.

1. INTRODUCTION Methyl mercaptan (or methanethiol, ) is a highly reactive volatile compound, part of the quartet of the total reduced sulfurs (TRS) along with hydrogen sulfide ( ), dimethylsulfide ( ) and dimethyl disulfide ( ). These compounds are known for being toxic and for causing malodorous air pollution. They are produced either naturally or by industrial processes, for instance, from oil refineries, pulp and paper mills and sewage treatment facilities as from bogs and marshy areas1. Methyl mercaptan is of special interest for petrochemical industry as it is commonly present in both gaseous streams (natural gas, synthesis gas and refinery streams) and in liquid streams (liquid fuels and liquefied petroleum gas - LPG). Its typical content can vary from several parts-per-million to 50 % by volume, depending upon the extent of ethane, propane and butane removal in preceding liquid recovery steps throughout the gas production line2–5. threshold exposure limit for the general public in order to produce serious and long-lasting adverse health and sensory effects is from 40 to 7.3 ppm during 8 hours to 10 minutes respectively, and three times these concentrations may lead to life-threatening health effects or death6. Moreover, it presents acid properties leading to corrosion issues in process equipment. Methanethiol is usually accompanying the and therefore removed along with it. In wake of this, absorption processes are matured technologies in the chemical industry used for the removal of acid gas and sulfur compounds. These processes allow the separation of specific components in the gas stream either by solubility with a solvent (physical absorption), or by formation of reversible chemical bonds (chemical absorption). Physical absorption is achieved by using organic solvents, for instance, carbonate propylene (Fluor® solvent), DMPEG (Selexol® solvent), NMP (Purisol® solvent), methanol (Rectisol® solvent), morpholine derivates (Morphysorb® solvent), among others. Chemical absorption is in contrast done by using aqueous amine solutions (i.e., mono-, di-, or tri- alkanolamines) or aqueous solutions of alkaline salts (alkali hydroxides or carbonates, ammonium, etc)2,7. In fact, for 1 ACS Paragon Plus Environment


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the natural gas production, acid gas and methanethiol removal is frequently done in amine absorption units and eventual polishing by may be done a tail gas unit. The removal efficiency of organosulfur compounds decreases as the hydrocarbon-chain length increases. In the case of reported 2 removal efficiency can be around 45-55 % . Since chemical solvents have proven sufficient efficiency and sharp separation at moderate pressure and temperature conditions but limited by reactivity and regeneration energy demands; whereas physical absorbents have proven selective separation not limited to reactivity but proportional to partial pressures, the use of hybrid solvents capable to meld the advantages of both systems have been attractive for the purification of gas containing methanethiol2,8. Among different hybrid absorption processes reported in the literature, the process using a mixture of an alkanolamine and methanol in aqueous phase (reported as Hybrisol® solvent8) has been of special interest for the gas purification industry. This process minimizes the solvent recirculation and reduces energy demands for a wide range of acid gas components partial pressure8,9. To date, only K. N. Tounsi et al.8 have reported thermodynamic modeling and experimental data for the and absorption in a Hybrisol®-type solvent mixtures, but there is not known literature concerning these properties for the methyl mercaptan absorption in this type of solvents. The complexity with the absorption lays into its chemically weak acidity that renders difficult its ionization in weak alkaline media such as ethanolamine solutions10. Moreover, the scarcity of literature data related to the solubility of the thiols compounds at temperatures above 248.0 K in alcohol solutions, due to the high vapor pressure of the solvent renders absorption studies to become very challenging. Only MonteCarlo simulations have been reported for obtaining solubility data of in pure methanol, which 11 at 248 K is equal to a Henry constant of 51 kPa (0.51 bar) . Therefore, there are neither accurately reported reaction constants values nor solubility constant estimations, for the range of temperature and solvent concentrations of application for this process related to methanethiol. The present work studied the absorption of methyl mercaptan into a solvent mixture consisting of diethanolamine (DEA), methanol ( ) and water ( ) at temperatures between 313.9 K to 353.0 K and solvent concentrations of 40/40/20 wt% and 40/20/40 wt% for respectively. As a result, this study first presents experimental absorption data for the mentioned system; secondly, it proposes a detailed kinetic mechanism for the chemical absorption based on the chemical properties of the system; thirdly, it proposes a van’t Hoff’s equation type for the Henry’s law solubility constant of methanethiol in methanol; and lastly, it presents a systemic mathematical analysis using the stochastic optimization method of Genetic Algorithms (GA) for determining the kinetic reaction constants in the form of the Arrhenius law. The obtained results of this study allow the complete characterization of the mentioned absorption system and further direct use of the parameters during modeling and design stages of this absorption process. This work opens the possibility to refining the modeling of absorption phenomena by considering the methanethiol as one chemical species in the system, rather than as a part of the bulk of sour gases to be removed. This represents a major contribution for the modeling and design of units exploiting gas containing high levels of methanethiol.

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2. EXPERIMENTAL SECTION 2.1. REAGENTS AND EXPERIMENTAL PROCEDURES The experimental data and thermo-physical properties considered on this work were obtained in our laboratory according to the procedure and the experimental protocol established by F. Amararene and colleagues12,13. The experimental setup is described in detailed by F. Amararene13. The solvent mixtures consisting of diethanolamine (DEA), methanol ( ) and water ( ) were prepared using distilled and vacuum-degassed water, Methanol (99.8 wt%, Merk®), and DEA (99.0 wt% purity, Aldrish®). Absorption measurements were carried out by loading the solvent mixtures into a thermostated Lewistype reaction cell with a volume of and thereafter injecting pure during very short time from the upper part of the reactor, denoting this volume of the gas as . Continuous stirring of the liquid and gas phases was maintained by using a Rushton turbine, while controlling the constant gas-liquid interface equal to . Experiments were performed at constant temperatures of K, K, and K for nominal solvent concentrations of 40/40/20 wt% (named as “Solvent Mixture No 1”) and 40/20/40 wt% (named as “Solvent Mixture No 2”) for respectively. DEA concentration was fixed to 40 wt% as suggested within the normal range of aqueous solution concentrations for industrial absorption processes with this amine14; besides the fact that previous works have reported independence between the thiol species solubility and the DEA solution concentrations used15. For every solvent mixture experiment at specific temperature and concentration, up to three gas injections (or gas absorption experiments) were performed. The data for each experiment were collected as pressure-time values until the system reached equilibrium. The data acquisition interval was . The maximum experimental error in the gas pressure values during absorption was estimated at 8%. 2.2. CHEMICAL KINETICS AND MASS TRANSFER MODELING The use of a hybrid solvent requires extensive data analysis in order to differentiate the effects of the reactions between methanethiol, diethanolamine and water (chemical absorption), and the total solvent solubility effect, mostly coming from the alcohol and water (physical absorption). The objective is to account for both liquid and gas phases non-idealities contributing to the non-intuitive behavior of the system. A detailed chemical reaction mechanism based upon the acid-base interactions for the system consisting of , (also noted as ), and is proposed in this work. This mechanism considers the protonation of the amine and the further formation of an intermediate complex or Zwitterion. This is in agreement with a solution ionization reaction presented by Bedell and Miller16 for methanethiol in aqueous amines. Furthermore, because of their acidity properties, mercaptans have been reported to react with alkali bases or amines to form mercaptide salts 2,16 similarly considered in the here proposed mechanism. The reversible reaction, shown in eq 1, describes the Zwitterion intermediate formation followed by the deprotonation reactions with the water, eq 2, and with the amine itself, eq 3. For the described mechanism, represents the constant of the ith-reaction. Reversible water equilibrium contribution was considered negligible as the equilibrium constant, , is the order of magnitude of ; assumption hereafter confirmed based on the obtained results.



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(1)

(2) (3)

Similarly to previous works, it is considered that only the acidic gas at the interface participates in the chemical reactions through its absorption into the liquid phase, as described in the eq 417,18. Moreover, the absorption rate from the gas to the liquid phase only depends on the physicochemical properties of mass transfer coefficient, the partial pressure and its solubility in the aqueous-amine mixture expressed in terms of the Henry’s law constant, as described in the eqs 4 and 5.

(4) (5)

Where is the interfatial surface area the gas volume

,

, , the enhacement factor ,

, the liquid volume

the methanethiol partial pressure in the gas phase

the methanethiol concentration in the liquid phase by the chemical reaction expressed in eq 1 (mol.m-3.s-1).

, and

,

, ,

, and rr1, the methanethiol consumption

Differential concentration equations for all chemical species in the liquid phase were simultaneously considered as shown below. The diffusion and parallel reactions between the DEA and water are neglected. Once considering the Zwitterion’s mechanism, it was assumed a semi-steady state for the Zwitterion’s complex concentration. After mathematical arrangements, the forward-rates of the ithreaction ( ) were therefore defined as following:

(6) (7) (8) (9) (10)


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(11)

(12)

(13)

Parameterization of the chemical reaction constants can be expressed in terms of the Arrhenius law, eq 14, where is the pre-exponential constant and is the activation energy (

(14)

In order to define the mass transfer to the liquid phase, was calculated using the established correlations for the used experimental setup, Lewis cell, in function of the dimensionless Sherwood number , Reynolds number and Schmidt number expressed as follows13,19:

(15) (16)

(17) (18)

Where, refers to the Lewis cell internal diameter , , the density of the solution , , the dynamic viscosity of the solution , , the diffusion coefficient of methyl mercaptan in the solution , , the Rushton turbine’s agitation speed , and , the liquid’s phase stirrer diameter . As assumed in previous works from Bouallou and colleagues18,20,21, at anytime was obtained from the total measured pressure and the inert measured pressure values. The enhancement factor at diluted initial absorption conditions was afterwards estimated from eqs 19 to 21 in function of the time of the analysis .

(19)



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(20)

(21)

As shown in eqs 20 and 21, factor corresponds to the slope of the linear representation of the gas phase absorption data in the first moments of the absorption phenomenon or in the surface layer21. In wake of this, close to the fluid phase equilibrium behavior, with no driving force for further absorption, becomes non-linear and does not represent the instantaneous absorption velocity as the reaction effect takes over the gas to liquid mass transfer phenomenon. 2.3. NUMERICAL MODEL AND OPTIMIZATION METHOD In order to mathematically describe the physico-chemical system, all the numeric subscripts of the equations in this work correspond to the system components, otherwise indicated, as for methyl mercaptan (1), diethanolamine (2), water (3) and methanol (4), respectively. The subscript refers to the solvent mixture. A diagram illustrating the main steps of the overall proposed solution methodology is shown in figure 1. Data fitting for the hybrid solvent absorption by the model described in the previous section aims at determining the chemical kinetic parameters (Arrhenius laws) and the gas solubility values. Such data fitting is carried out by the minimization of an objective function expressed in terms of the total sum of the square of the errors between experimental and modeled (or calculated) values of the gas pressures on each time step, as shown in the eq 22.

(22)

As the proposed absorption model involves multiple non-linear and differential equations, the discretization solution used was the 4-th order Runge-Kutta method and the used optimization method for was the Genetic Algorithms (GA) coded in VBA language. Succinctly described, GA is a heuristic solution search technique initially motivated by the Darwinian principle of evolution through genetic selection, which mathematically translated allows producing a set of possible solutions from a set of a randomly generated population, which through a fitness-based selection with respect to the , evolves through successive generations by increasing the average fitness till reaching the optimization criterion17,22. GA method resulted especially advantageous for this study because it allows converging to a set of possible local solutions rather than a single local solution; the total set of possible solutions were considered for all the experimental results in order to generate mathematical correlations in function of temperature and/or solvent volumetric concentrations for the reaction constants and the solubility parameter respectively. Among the parameters decided to be used along the GA method are: a one-point crossover operator, a uniform mutation operator with a crossover probability of 0.85 for a constant population of 500 individuals, and limited to 50 generations. Variables considered as independent and impacting the values of the function f(x) are 6, specifically the kinetic constant values for Kr1, Kr1*, Kr2, Kr3, and the parameters and H1/m.


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As described in Figure 1, mathematical determination of the Henry’s solubility and the kinetic parameters were performed in two-stage iteration loops for every experimental injection dataset, followed by a global refining iteration loop for optimizing the kinetic parameters and solubility values in function of temperature and solvent concentration. The first single-injection iteration loop aimed to determine the optimal and ; whereas the second loop aimed to determine the optimized dataset values for . The final overall optimization loop validated the proposed values for in terms of the Arrhenius law parameters, eq 14. For the single-injection first iteration loop, due to the lack of solubility data for methanethiol neither in some of the used pure solvents nor in their mixtures as for the analyzed hybrid solvent, iterations allowed calculating the optimal Henry’s law constant value of in the solution . These iterations aim at further facilitating to minimize the function , eq 22 in the second step of iteration, and simultaneously assess results by a statistical analysis. This analysis enables only values of Henry constant leading to a deviation lower than 1 % between the calculated and the experimentally obtained equilibrium pressures to be retained. Nevertheless, an additional parameter considering the changes in the values was calculated for the set of values. These variations in the values respond to the observed homologous changes for the obtained through different gas injections, for a constant solvent loading and experimental temperature. On the other hand, for the second single-injection iteration loop, optimum values were obtained by minimizing the function , eq 22, for the total acquired pressure data points, along with a statistical significant t-test analysis for a p-value of 0.05. This test is carried out in order to verify that there is not statistical probability of difference between the two set of resulting values with difference variance, that is, between and for the total injection time. General data analysis exhibited limited effect of the and optimization procedure over the initially optimized values of Finally, once obtained the locally optimized solubility parameters and the mathematical solutions for and values for injection datasets at different temperatures and solvent concentration, a generalized equation for each was formulated in terms of the Arrhenius law, eq 14. The final optimization routine allowed refining the obtained values for the kinetic and solubility parameters in function of temperature. Multiple-stages iteration procedure enables discriminating between the solubility and kinetic effects. An advantage of this approach is that it reduces the overall optimization time by reducing the number of variables and their respective ranges considered in the GA for every injection dataset.



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Figure 1. Numerical solution methodology 3. RESULTS AND DISCUSSIONS 3.1. THERMO-PHYSICAL PROPERTIES OF THE SOLVENTS AND GAS SOLUBILITY In terms of the solvent properties, firstly, since the density of the solvent mixture is a required parameter in the described solution methodology, it was parameterized as a function of temperature and solvent concentration. Experimental density measurements have been previously reported for the two solvent mixture compositions in the range of temperature of interest on this work12, as shown in table 1. Along with the reported experimental density data, an estimation model for the density of the mixture, based on the excess volume of the ternary liquid system, was proposed12. However, in response to the scarcity of the binary excess volume parameters, a simplified mean value calculation, in terms of the mass fraction, , was used on this work, as given by eq 23. Results from this approach exhibited sufficient fitness with a deviation lower than 1.4 % compared to the experimental data, as it is also shown in table 1. This low deviation may be attributed to the similarity of density for the pure 8 ACS Paragon Plus Environment

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species. After successive absorption/desorption cycles, a simplified mean density equation allows considering the variation of the solvent concentrations efficiently through the mathematical modeling. The polynomial coefficients for the density of the pure solvent species are tabulated in table 2.

(23)

Table 1. Density of hybrid solvent mixtures of Solvent Mixture No 1 model12

Experiments12

Deviation

This work

Deviation

308.15

0.4

0.2

1000.4

1.2

1000.2

0.02%

987.2

1.32%

323.15

0.4

0.2

990.1

1.2

991.1

0.10%

977.7

1.25%

333.15

0.4

0.2

982.9

1.2

983.0

0.01%

971.0

1.21%

343.15

0.4

0.2

975.4

1.2

976.0

0.06%

964.0

1.17%

353.15

0.4

0.2

967.6

1.2

966.9

0.07%

956.6

1.14%

Deviation

This work

Deviation

Solvent Mixture No 2 model12

Experiments12 313.15

0.4

0.4

947

1.2

948.1

0.11%

939.8

0.76%

323.15

0.4

0.4

938

1.2

940.5

0.27%

932.3

0.61%

333.05

0.4

0.4

929

1.2

932.5

0.37%

924.5

0.48%

343.05

0.4

0.4

921

1.2

924.4

0.37%

916.5

0.49%

352.95

0.4

0.4

912

1.2

915.8

0.42%

908.3

0.41%

Table 2. Coefficients of the polynomial density equation, Component

Range

Ref.

0.999

298-372

23,24

1297.5

-0.677

974.53

-0.312

-0.0011

1.000

181-373

25

765.33

1.814

-0.0035

1.000

278-368

26

As a second physico-chemical property required in the proposed methodology, the Henry’s law constant solubility parameter of methanethiol in the mixture was estimated from iterative calculations of the equilibrium pressure between experimental measurements and the calculations resulting from the described kinetic and physico-chemical model. is the key parameter affecting the calculation over the equilibrium pressure. The results are shown in table 3, along with the overall coefficient of the mass transfer equation for methyl mercaptan, .



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Table 3. Physical absorption properties of hybrid solvent mixtures of Injection No.

(t-test)

No of data points

t-Stat /

/

/

/

Solvent Mixture No 1 1

0.00

1.13

10-7

-0.147

229.5

1.437

6750

5.22

10-8

-0.146

114.8

-0.974

7900

1.50

10-7

-0.154

300.3

-1.037

5370

6.33

10-8

-0.153

150.1

-0.972

6920

2.43

10-4

-0.214

424.4

-4.711

3735

184.74 2

0.24

1

0.00 181.45

2

0.20

1

0.00

179.17

Solvent Mixture No 2 1

0.00

2

0.22

3

0.51

1

0.00

183.34

4.63

10-7

-0.146

137.4

0.471

1200

3.08

10-7

-0.145

91.4

0.568

1370

1.87

10-7

-0.144

55.6

1.199

1140

3.87

10-7

-0.149

380.0

0.403

3000

181.52 2

0.23

1.65

10-7

-0.148

155.0

-0.300

2600

1

0.00

5.03

10-7

-0.178

454.2

-1.294

1830

177.47 2

0.15

3.64

10-7

-0.177

226.5

0.970

2180

3

0.26

2.31

10-7

-0.177

113.2

1.530

1285

Calculated values showed a decrease after successive gas injections for fixed experiment temperature and concentration, that is, an increase in the solubility of the gas into the mixture after successive absorption cycles, likely resulting from a change in the pH of the solution due to the presence of solubilized thiol species. for the pristine solutions was however parameterized using the estimation proposed by Praunitz and coworkers27,28 for the Henry’s law constant of a liquid mixture, eq 24. This equation is based on the Wohl type expansion of the excess molar Gibbs energy, at constant temperature, and it is expressed in terms of the volume fractions as defined in the eq 25, and the Henry’s law constant of in pure solvents . In overall, Henry’s law constant depends upon the solvent composition, temperature and pressure of the absorbed gas; it was however assumed that the pressure is low enough to consider the gas phase as an ideal gas. For the working solvent mixture, the presence of stronger hydrogen bonds coming from the water interactions, than the Van de Waals forces, leads to the formulation of the thermodynamic excess quantity, or excess Henry coefficient, . This quantity describes the deviations from the ideal solution; for which, negative values, as shown in table 3 for the mixture of interest, denotes higher solubilities than for the ideal mixture, and therefore positive deviations from Raoult’s law; whereas, positive sign for may 28 describe negative deviations of the Raoult’s law . (24)


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(25) (26) (27)

Wohl type mathematical expansion, eq 24, is based on the physical characteristics between the molecules, which so far, cannot be determined quantitatively; therefore, empirical expressions from experimental data and the physico-chemistry of the mixture are usually proposed. In wake of this, is expressed in terms of the two-body interaction coefficients and the three-body interactions , eqs 26 and 27. Approximation to the overall three-body interactions term as a contribution of the total content of the solvent mixture was previously suggested by Y. W. Wang et al.28. The polynomial equation used to describe the different interaction parameters, is shown in the eq 28, for which parameters are presented in table 4. The empirical second-order polynomial equation for the interactions was proposed and experimentally validated by Yaghi and Houache29 for temperatures from 303.15 K to 348.15 K. The other interaction parameters have been established from a statistical classification presented by Renon and Prausnitz30 from binary and ternary vapor-liquid equilibrium data, in function of the chemical properties of the species. As a result, the system has been described as a type Ic on the mentioned classification after being a mixture of polar liquids in which the effect of non-randomness on the shape of the excess Gibss energy curve is not strong. The system was attributed to a type IV after being a mixture of self-associated substance, a reduced polarity substance and an alcohol mixture, with a high degree of nonrandomness. Finally, the three-body interaction of was attributed to a type VII of mixture representing a mixture of water and polar self-associated substances with a high degree of randomness30. In consequence, all the coefficients used for the equation are shown in table 4.

(28)

Table 4. Two- and three- body interaction parameters of the Wohl type expansion for calculating eqs 27 and 28 Mixture

,

Ref. 56.8

-3.53

10

-1

5.49

10

29

-4

0.30

0

0

0

30

0.40 - 0.55

0

0

0

30

0.47

0

0

0

30

For the case of Henry’s law constant of in pure solvents , different approaches were explored for its estimation in order to parameterize it into a temperature dependent expression, eq 29, with the first term solely referring to the vant Hoff’s type equation with equal to the enthalpy of solution of the system divided by the universal gas constant. First, solubility Henry’s law constant of methanethiol in water was parameterized from diverse available experimental data31,32. Second, 11 ACS Paragon Plus Environment


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solubility Henry’s law constant of methanethiol in pure ethanolamine was estimated from the analogy with respect to the solubility of , eq 30, as a consequence of the fast reactivity of methanothiol and alkanolamines solutions; for which solubilities of in water and in ethanolamine are reported for temperatures between 293.15 K to 358.15 K29. Previously reported data from Y. W. Wang et al.28 for the solubility of in function of the enthalpy of solution, was corrected by Yaghi 29 and Houache after being insufficient to describe the system at temperatures above 303.15 K. Coefficient’s values used in eq 29 for the mentioned pairs of components are presented in table 5.

(29) (30)

Table 5. Parameters of the Henry’s law constant

, eq 29

System

Ref. 3.526

10

5

2009

0

0

31,32

1.086

107

2372

0

0

29

4.710

104

0

0.5337

29

On the other hand, since there is not available data for the solubility of in pure , a new van Hoff’s equation-type is proposed for its Henry’s law constant, as presented in the eq 31. This formulation is proposed in function of the alcohol concentration in the mixture, for the temperature range of interest. The results were obtained from the estimated and the mentioned applied mixture properties. The obtained mathematical regression analysis is plotted in the figure 2. It is however recommended to pursue further specific absorption analyses in order to confirm the formulation for different scenarios. (31)

Figure 2. Henry’s law constant estimations for methyl mercaptan in methanol 12 ACS Paragon Plus Environment

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Based on the determined Henry’s solubility constant for the in the pure solvents and its pair mixtures, it is noticed an improvement in the absorption capacity of the as compared to water, DEA and its mixture. This enhancement is coming from the positive effect of the physical solvent, methanol, in the mixture. Improvement estimated of at least 50-59 % in the absorption capacity as compared to the DEA aqueous solutions with concentrations from 30 to 50 vol%.

3.2. Absorption kinetics Once all the mass transfer properties are determined, reaction constant values were obtained and parameterized in terms of the Arrhenius law, as shown in the eq 14. The optimized parameters are presented in table 6. Up to present, early reports have mentioned that elimination of methanethiol in aqueous ethanolamine solutions was either only limited by physical solubility and reactions were not feasible given either the high viscosity of the solution, or the high reaction rates that may inhibit its measurement by experimental procedures2,10. Nevertheless, it is only known to the authors, a reported order of magnitude for the global reaction constant of methanethiol in diethanolamine aqueous solutions given by F. Rahmani et al.10 with a value equal to . This is in agreement with the obtained values in this study. The obtained high values for reaction rate constants, compared to the water dissolution constant, confirmed the initial assumption of neglecting the effect of water dissociation in the species concentration balances.

Table 6. Arrhenius law coefficients for the methyl mercaptan reactions Reaction Constant

Pre-exponential factor,

Activation energy, /

The final results for the absorption profiles of methyl mercaptan in the hybrid solvent consisting of at different temperatures and solvent concentrations are presented in figures 3 and 4. These figures display the changes of methanethiol pressure profile in the gas phase in function of time, due to the combined effect of its chemical reactivity and its physical absorption in the mixture. Statistical t-test analysis, performed in order to evaluate the statistical difference between the experimental and calculated pressures, is shown in table 3 (as t-Stat), for with . According to the t-test conditions, for a null hypothesis of statistically different dataset, if either or the null hypothesis is rejected. It was therefore confirmed statistical equality between the experimental measurements and the optimized calculations for almost all the conditions; only the experiment at for the Solvent Mixture 13 ACS Paragon Plus Environment


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No 1 showed statistical difference. The mentioned difference could be attributed either to errors in the experimental procedure, or to unknown chemical interactions between the species at those conditions.

a).

b).

).

c).

Figure 3. Absorption of methyl mercaptan in hybrid solvent at a) K; b)


K; c)

K

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a).

b).

c).

Figure 4. Absorption of methyl mercaptan in hybrid solvent at a) K; b)

K; c)

K

Concentration profiles for all the species present in the absorption of methanethiol in the hybrid solvent were also obtained by considering the Arrhenius law constants herewith presented. For instance, figure 5 shows the results for the Solvent Mixture No 1 at 333.4 K in function of time. These results justify the formation of ionic species through the absorption while the concentrations of diethanolamine irreversibly decrease. Similar profiles were obtained for all the evaluated experimental conditions.



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Figure 5. Modeled concentration profiles of methyl mercaptan, diethanolamine and ionic species resulting from the absorption in a hybrid solvent of at K.

4. CONCLUSIONS Methyl mercaptan absorption was studied in a hybrid solvent mixture, similar to the commercially reported being used in the Hybrisol® process. The analyzed hybrid solvent consists of diethanolamine (DEA), methanol ( ) and water ( ). absorption is of special interest as it is usually found in sour gas streams and none detailed physico-chemical property dataset has been reported before for a treatment with chemical or hybrid solvents. Since general modeling, simulation and design stages of sour gas treatment with this type of technology require specifications of all the physico-chemical and reaction parameters for all compounds, the novel proposed methodology allowed characterizing the full system for being used in engineering design applications afterwards. Experimental absorption data are provided for two different hybrid solvent concentrations and three different temperatures ranging from 313.9 K to 353.0 K. Along with the data, the proposed systematic methodology allowed determining the mass transfer properties of the mixture, reporting for the firsttime a new correlation for the Henry’s law constant of pure gas in methanol in the form of the van’t Hoff equation-type, and determining the chemical kinetic parameters of the considered reactions between the gas and, the water and the DEA in the solution. Henry’s law constant of methanethiol in solution was modeled using the Wohl’s type expansion of excess molar Gibbs energy, showing in overall negative deviations from Raoult’s law or higher solubilities for the working mixture than for the homologous ideal solution. Noteworthy, estimated Henry’s solubility constant in the hybrid solution exhibited an improvement in the solubility as compared to the DEA aqueous solutions of approximately 50 % positive deviation. Furthermore, from the obtained reaction rate constants, Arrhenius law parameters are proposed for the considered reactions in the range of temperature of the analyses. The determined reaction constants demonstrate the relevance of the ions dissociation of methanethiol into the amine aqueous solutions through the possible formation of a 16 ACS Paragon Plus Environment

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Zwitterion complex intermediate and therefore eventual absorption capacity degradation of the mixture caused by cumulated ionized species in the solution.

AUTHOR INFORMATION

Corresponding Author *Phone: +33-1-69-191700, +33-1-40-51-91-11. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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