Development and design of the in-situ regeneration section of Vitrisol®, a novel, highly
selective desulphurization process
Wermink, Wouter Nicolaas
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2019
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Wermink, W. N. (2019). Development and design of the in-situ regeneration section of Vitrisol®, a novel, highly selective desulphurization process. Rijksuniversiteit Groningen.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
101
Chapter 6: Sulphur solubilities in toluene, o-xylene, m-xylene and p-xylene at
temperatures ranging from 303.15 K to 363.15 K
Reproduced in part with permission from J. Nat. Gas Eng. 2018, 3, 2, accepted for publication. Copyright 2018 Scrivener Publishing.
Abstract
The solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene was investigated at temperatures ranging from 303.15 K to 363.15 K. It was determined that, for experimental conditions studied, o-xylene exhibited the highest sulphur solubility and toluene the lowest sulphur solubility. The enhanced sulphur solubility of xylene might be attributed to the molecular structures of o-xylene and orthorhombic sulphur. Because of the configuration of the carbon atoms in o-o-xylene, they are in closer proximity to the sulphur atoms of orthorhombic sulphur resulting in stronger Van der Waals interactions. Because of the smaller distance between the carbon and sulphur atoms, the p orbitals of the sulphur atoms have an increased interaction with the aromatic ring of o-xylene. The sulphur solubility data reported in this study were in good agreement with sulphur solubility data in toluene, m-xylene and p-xylene published in open literature. Sulphur solubility data in o-xylene was not published in open literature. The temperature dependencies of the solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene were determined from solubility equilibria. The enthalpies of dissolution ΔHo of sulphur in toluene, o-xylene, m-xylene and p-xylene were determined to be 27.93 kJ/mol, 27.95 kJ/mol, 26.98 kJ/mol and 27.70 kJ/mol, respectively.
6.1 Introduction
Sulphur solubilities in toluene, o-xylene, m-xylene and p-xylene were investigated for the design of the regeneration steps of the novel Vitrisol® desulphurization process.1,2 The Vitrisol® process removes H2S by precipitation with copper sulphate (CuSO4) in an aqueous, acidic solution. Copper
sulphide (CuS) and sulphuric acid (H2SO4) are formed in the gas treating process:1-8
( ) ( ) (6.1)
The current status of the Vitrisol® process is a scavenger-like application. To reduce operational costs for large scale H2S removal operations, a regeneration section was designed to replenish Cu2+.2 In the
regeneration section sulphur is formed and dissolved in a nonpolar solvent for further processing and purification.
In this study the solubility of sulphur in the nonpolar solvents toluene, o-xylene, m-xylene and p-xylene were investigated for the design of the regeneration section of the novel Vitrisol® desulphurization process. Solvents were chosen with respect to their effect on health and environment as well as on the ease of recovery of sulphur.
6.2 Literature review
The solubility of sulphur in toluene, m-xylene and p-xylene has previously been investigated by several authors. The solubility of sulphur in o-xylene however was not reported in open literature. Cossa9 investigated sulphur solubility in various nonpolar solvents. Cossa did not describe the experimental setup and method used for experimentation. A sulphur solubility of 1.479 g per 100 g of toluene at a temperature of 296.15 K was reported.
102
Hildebrand and Jenks10 studied solubility of orthorhombic sulphur in various nonpolar solvents. Sulphur solubility in toluene was measured at temperatures ranging from 273.15 K to 356.65 K. Sulphur solubility in m-xylene was measured at temperatures ranging from 298.15 K and 353.15 K. Sulphur was purified by sublimation. Toluene was purified by the method used by Richards and Coombs.11 m-Xylene was purified by oxidizing o- and p-xylenes present in the m-xylene. Afterwards, m-xylene was washed to remove o- and p-toluic acids, and further purified. The purified m-xylene boiled at 412.15 K. Solubility experiments were performed with an experimental setup as described by Hildebrand and Jenks.12 The amount of sulphur dissolved for temperatures up to 327.15 K was determined in two manners; first the solvent was evaporated by using a hotplate. Afterwards solvent was evaporated by introducing a flow of air. The amount of sulphur dissolved at temperatures above 327.15 K was determined by preparing a solution of known composition and noting the temperature at which a minute residual crystal neither increased or diminished in size when observed through a microscope.
Tuller13 reported sulphur solubilities in various pure and mixed solvents. Sulphur solubilities in toluene and m-xylene were reported for temperatures ranging from 214.9 K to 356.65 K and 298.15 K to 353.15 K, respectively, as published by Seidell.14 Orthorhombic sulphur and monoclinic sulphur solubilities in p-xylene were reported for temperatures ranging from 358.15 K to 373.65 K and 371.15 K to 380.15 K as published by Hammick and Holt.15 They purified sulphur by two recrystallizations from toluene and heating before use in a steam oven. p-Xylene was purified by freezing out and melting at 286.35 K. Hammick and Holt determined the temperatures at which predetermined percentages of solid sulphur just dissolved in p-xylene in sealed glass tubes.
Jay et al.16 studied the solubility of elemental sulphur in toluene at temperatures ranging from 267.15 K to 313.15 K. Solubility experiments were performed in a cylindrical glass stirred vessel connected to a thermostatted bath. An excess of sulphur was added to the reactor, next the solvent was added. To assist the dissolution of sulphur in the solvent the temperature was increased 10 K above the temperature of the experiment. Afterwards the temperature was gradually decreased to the temperature of the experiment. The solution was stirred for 12 h at fixed temperature. Then the stirring was stopped, and after 30 min a liquid sample was taken which was analyzed by GC. An empirical correlation was fitted on their sulphur solubility data at varying temperatures.
Ren et al.17 measured the solubility of elemental sulphur in pure organic solvents and organic solvent–ionic liquid mixtures at temperatures ranging from 293.15 K to 353.15 K. Solubility experiments were performed in a jacketed glass cell connected to a thermostatted bath. Sulphur was slowly added to the solvent in the cell at constant stirring and temperature. The sulphur solubility was measured by cloud-point method using laser beam scattering. Experimental data was reported only in the form of a figure. The temperature dependence of the sulphur solubility in different solvents was fitted with a modified Apelblat equation.18,19
Wang et al.20 measured the solubility of elemental, rhombic sulphur in various organic solvents at temperatures ranging from 298.15 K to 363.15 K. Solubility experiments were performed in a jacketed glass vessel connected to a thermostatted bath. Sulphur was slowly added to the solvent in the vessel at constant stirring and temperature. The sulphur solubility was measured by cloud-point method using laser beam scattering. The temperature dependence of the sulphur solubility in different solvents were fitted with the Van ‘t Hoff equation and a modified Apelblat equation.
Experimental mole fraction solubilities x of sulphur S8 in toluene, m-xylene and p-xylene as reported
103 Table 6.1: Experimental mole fraction solubilities x of sulphur S8 in toluene, m-xylene and p-xylene at
temperature T reported previously.
Solvent Reference T/K x Solvent Reference T/K x
Toluene Cossa9 296.15 5.28 x 10-3 Toluene Wang et al.20 298.15 7.82 x 10-3 303.15 9.27 x 10-3 Toluene Hildebrand 273.15 3.21 x 10-3 308.15 1.10 x 10-2 and Jenks10 298.15 7.20 x 10-3 313.15 1.30 x 10-2 308.15 9.68 x 10-3 318.15 1.54 x 10-2 318.15 1.28 x 10-2 323.15 1.82 x 10-2 327.15 1.71 x 10-2 328.15 2.15 x 10-2 356.65 4.01 x 10-2 333.15 2.54 x 10-2 338.15 2.99 x 10-2 Toluene Tuller13 214.9 2.84 x 10-4 343.15 3.53 x 10-2 232.9 6.08 x 10-4 348.15 4.16 x 10-2 239.4 7.70 x 10-4 353.15 4.89 x 10-2 244.65 1.07 x 10-3 358.15 5.75 x 10-2 252.15 1.37 x 10-3 363.15 6.75 x 10-2 255.4 1.57 x 10-3 257.15 1.73 x 10-3 m-Xylene Hildebrand 298.15 8.08 x 10-3 263.15 2.08 x 10-3 and Jenks10 318.15 1.47 x 10-2 273.15 3.34 x 10-3 353.15 4.08 x 10-2 286.15 5.50 x 10-3 288.65 5.99 x 10-3 m-Xylene Tuller13 298.15 8.24 x 10-3 293.15 6.64 x 10-3 318.15 1.52 x 10-2 296.15 6.87 x 10-3 353.15 4.53 x 10-2 298.15 7.34 x 10-3 308.15 9.95 x 10-3 p-Xylene Tuller13 358.15 4.87 x 10-2 a 327.15 1.80 x 10-2 365.65 6.29 x 10-2a 356.65 4.52 x 10-2 373.65 8.25 x 10-2a 371.35 7.43 x 10-2b Toluene Jay et al.16 267.15 2.80 x 10-3 371.15 7.46 x 10-2b 274.95 3.73 x 10-3 376.45 8.25 x 10-2b 284.15 4.53 x 10-3 379.15 9.58 x 10-2b 293.25 6.05 x 10-3 380.15 1.03 x 10-1b 303.35 7.56 x 10-3 312.95 9.96 x 10-3 a: Orthorhombic sulphur b: Monoclinic sulphur
The solubility of sulphur in o-xylene was not reported in open literature.
6.3 Materials and methods
6.3.1 Setup
Sulphur solubility experiments were performed in two experimental setups.
Sulphur solubility experiments with toluene, o-xylene, m-xylene and p-xylene were performed in a 0.25 l three-necked glass reactor. The reactor was partially immersed in an oil bath. The temperature
104
of the reactor was regulated by a temperature-controlled hot plate. A PT1000 thermometer controlled the temperature of the oil bath. The temperature of the solution was measured with a PT100 thermometer. The solution was stirred. A condenser was placed on top of the reactor. The temperature of the condenser was regulated by a Julabo thermostatted bath. The reactor was in open connection to the surroundings. The experimental setup is shown in Figure 6.1.
At high temperatures, the reactor temperature was a few degrees lower compared to the temperature of the oil bath. Condensed solvent entering the reactor caused a small decrease of the reactor temperature. Therefore an additional sulphur solubility experiment was performed in an experimental setup based on a different principle to investigate whether the temperature difference between oil bath and reactor influenced the sulphur solubility measurements.
Figure 6.1: Experimental setup with condenser.
A sulphur solubility experiment with toluene was performed in a 0.5 l glass reactor fully immersed in a water bath. The reactor was operated batchwise with regard to the liquid phase, and continuous with regard to the gas phase. Nitrogen was fed through a solvent saturator, fully immersed in the water bath, to saturate the gas phase and maintain the solvent balance. The temperature of the water bath was regulated by a Julabo heater and monitored with a PT100 thermometer. The solution was stirred. The glass reactor contained glass baffles. The experimental setup is shown in Figure 6.2. Because the solvent saturator and reactor were immersed in the water bath, no condenser was required to maintain the solvent balance. Therefore this experimental setup did not result in a temperature difference between reactor and water bath.
Sampling was performed with syringes. Samples were collected in Petri dishes. After evaporation of the solvent, the amount of sulphur was determined gravimetrically with a Denver Instruments s-114 mass balance.
6.3.2 Materials
Purum p.a. sulphur [7704-34-9], anhydrous 99.8 % toluene [108-88-3], IUPAC name methylbenzene, ReagentPlus® m-xylene [108-38-3], IUPAC name 1,3-dimethylbenzene, ReagentPlus® p-xylene [106-42-3], IUPAC name 1,4-dimethylbenzene and reagent grade o-xylene [95-47-6], IUPAC name 1,2-dimethylbenzene were used as supplied from Sigma-Aldrich. Sample information is given in Table 6.2.
105 Figure 6.2: Experimental setup with immersed reactor and solvent saturator.
Table 6.2: Sample information.
Chemical CAS Source Purification method Purity w
Sulphur 7704-34-9 Sigma-Aldrich None 0.999
Toluene 108-88-3 Sigma-Aldrich None 0.9992
m-Xylene 108-38-3 Sigma-Aldrich None 0.997
p-Xylene 106-42-3 Sigma-Aldrich None 0.998
o-Xylene 95-47-6 Sigma-Aldrich None 0.995
6.3.3 Procedures
Sulphur solubility experiments were performed in two experimental setups.
A volume of 200 ml of solvent was added to a 0.25 l three-necked glass reactor. An excess of sulphur, approximately 50 g, was added to the reactor. The reactor was partially immersed in the oil bath, i.e. the solvent content in the reactor was immersed in the oil. Stirring was initiated and the condenser was cooled down to a temperature of 278.15 K to maintain the solvent balance. The reactor was heated to the required temperature. The solution was stirred at the required temperature for two hours, and afterwards sulphur particles in the solution were allowed to settle for 20 min.
A volume of 400 ml of solvent was added to a 0.5 l glass reactor. An excess of sulphur, approximately 100 g, was added to the reactor. Nitrogen was supplied through a gas saturator immersed in the water bath containing the solvent to maintain the solvent balance. The reactor was fully immersed in a water bath. Stirring was initiated and the reactor was heated to the required temperature. The solution was stirred at the required temperature for two hours, and afterwards sulphur particles in the solution were allowed to settle for 20 min.
Sampling was performed carefully with a 10 ml syringe; after a settling time of 20 min a sample without sulphur particles was taken. At the moment of sampling the temperature was measured. The sample was collected in a Petri dish. The sample was left overnight in a fume hood to evaporate the solvent. The amount of sulphur was determined gravimetrically.
106
6.3.4 Consideration
First, the influence of stirring (or equilibrating) time of the solution before taking a sample, the use of a filter and settling time were investigated.
Experiments were performed with o-xylene at temperatures of approximately 303.15 K, 313.15 K, 323.15 K, 333.15 K, 343.15 K, 353.15 K and 363.15 K to study the effect of stirring time on determining the sulphur solubility gravimetrically. Stirring times of 1 h, 2 h, 3 h and 16 h were investigated. For all the used stirring times, the determined sulphur solubilities did not deviate. Therefore a stirring time of 2 h was chosen to determine sulphur solubilities. Furthermore, it was observed that approximately 25 min of dissolution time was required at a temperature of 363.15 K to fully dissolve sulphur and obtain the maximum sulphur solubility.
Initially, filters were used to obtain samples in Petri dishes. However, at increased temperatures, i.e.
T > 343.15 K, the filter induced substantial crystal formation and samples could not be ejected.
Therefore the use of filters was not continued. By introducing a settling time, samples without sulphur particles could be taken. Sulphur solubilities determined at lower temperatures with and without filters resulted in the same determination of the sulphur solubility.
Because filters were not used to obtain samples, the solution was allowed to settle after stirring. Samples were taken from a sulphur solubility experiment with o-xylene after a settling time of 20 min, 60 min, 120 min and 240 min at temperatures of 313.15 K and 353.15 K. The determined sulphur solubilities did not deviate. Therefore a settling time of 20 min was chosen during experimentation.
Solubility experiments were conducted to rule out the possible occurrence of hysteresis: 1) starting at the lowest temperature and step-wise increasing the temperature, 2) starting at the highest temperature and step-wise decreasing the temperature, 3) by randomly increasing and decreasing the temperature. It was concluded that the method of (randomly) heating and/or cooling did not affect the determination of the sulphur solubility.
Experiments were performed at temperatures of approximately 303.15 K, 313.15 K, 323.15 K, 333.15 K, 343.15 K, 353.15 K and 363.15 K, respectively. At one temperature at least two samples were taken and analyzed.
6.3.5 Uncertainties
The accuracy of the PT100 thermometer, used to measure the temperature in one significant number, is temperature dependent. E.g., at temperatures of 303.15 K and 363.15 K the accuracies and standard uncertainties are ± 0.21 K and ± 0.33 K, respectively, and 0.12 K and 0.19 K, respectively. The accuracy and standard uncertainty of the reference thermometer with two PT100 thermometers, used to measure the temperature in two significant figures, are ± 0.03 K and 0.017 K, respectively. The accuracy and standard uncertainty of the mass balance are ± 1 x 10-4 g and 5.8 x 10
-5
g, respectively. At least two consecutive samples were taken with repeatability within 2 %.
In this study the solubility measurements are associated with two types of experimental uncertainties: the gravimetric determination of the solubility u(x,grav) and the repeatability of the solubility measurements u(x,rep). The solubility is determined according to:
(6.2) With
107
(6.3)
And
(6.4)
MW,S8 is the molar mass of the orthorhombic sulphur molecule (256.52 g/mol).
To determine the solubility gravimetrically, first the solution with dissolved sulphur is measured, and after evaporation of the solvent the sulphur content is measured. The uncertainty in the gravimetric determination of the solubility uR(x,grav) is determined from the standard uncertainty in the
calibration of the mass balance u(m,cal) = 5.8 x 10-5 g according to:
( ) √ ( ) ( ) (6.5) And ( ) √ ( ) ( ) ( ) (6.6) Results in: ( ) ( ) √ ( ( ) ) ( ( ) ) (6.7)
The relative standard uncertainty in determining the solubility gravimetrically is uR(x,grav) = 0.0083
%.
The uncertainty of repeatability of measurements to determine the sulphur solubility at one temperature for one solvent is determined according to:
( ) √∑ ( )
(6.8)
With ∑
(6.9) Since the solubility measurements are performed with the same experimental procedure, the relative uncertainty of repeatability for all solubility measurements can be determined according to:
( ) ( ) √∑ ( ( )
)
(6.10) The relative standard uncertainty of repeatability is uR(x,rep) = 0.99 %.
The relative combined standard uncertainty UR(x) can be determined according to:
108
The relative combined expanded uncertainty with coverage k = 2 is UR(x) = 2.0 %.
6.4 Results
Sulphur solubilities were investigated with different experimental methods. Temperature dependencies of sulphur solubilities in toluene, o-xylene, m-xylene and p-xylene were determined from all the experimental points obtained from the different experimental methods and setups. The experimental data was compared to data available in literature.
6.4.1 Sulphur solubilities
The solubility of sulphur in toluene was determined with different experimental methods, i.e. by increasing the temperature, by decreasing the temperature and by randomly changing the temperature. Two types of experimental setup were used to determine sulphur solubilities in toluene. One type was a 0.25 l glass reactor heated by an oil bath (Figure 6.1). The other type was a 0.5 l glass reactor immersed in a water bath (Figure 6.2). Table 6.3 reports the experimental values of the sulphur solubilities in toluene.
Table 6.3. Experimental mole fraction solubilities x of sulphur in toluene at temperature Ta.
Solvent T/K x Remarks
360.55 0.05251 Method: decreasing temperature 360.25 0.05214 Setup: reactor in oil bath
350.35 0.03840 350.55 0.03910 340.65 0.02818 341.15 0.02864 Toluene 331.65 0.02169 331.65 0.02167 322.15 0.01595 322.15 0.01600 312.35 0.01188 312.35 0.01193 302.95 0.008901 302.95 0.008847
302.95 0.008629 Method: increasing temperature 302.95 0.008629 Setup: reactor in oil bath
312.65 0.01175 312.65 0.01168 321.95 0.01563 321.95 0.01594 Toluene 331.65 0.02118 331.65 0.02118 340.35 0.02777 340.35 0.02772 350.35 0.03830 350.35 0.03840 360.15 0.05326 360.15 0.05276
351.35 0.03868 Method: random temperature change 351.35 0.03874 Setup: reactor in oil bath
109 312.32 0.01209 312.35 0.01185 341.3 0.02819 341.3 0.02908 Toluene 302.57 0.008908 360.23 0.05388 360.23 0.05381 321.3 0.01633 321.3 0.01622 330.98 0.02132 330.98 0.02170
362.25 0.05841 Method: decreasing temperature 362.4 0.05817 Setup: reactor immersed in 352.55 0.04179 water bath 352.52 0.04146 342.63 0.03078 342.57 0.03075 Toluene 332.62 0.02285 332.57 0.02269 322.69 0.01711 322.64 0.01724 312.67 0.01281 312.63 0.01308 302.79 0.009734 302.75 0.009790 a
Standard uncertainty for temperature is dependent on the temperature. The standard uncertainty for temperature between 302.75 K and 362.40 K varies between 0.12 K < u(T) < 0.19 K. The relative combined standard uncertainty for solubility is UR(x) = 0.99 %.
The solubility of sulphur in o-xylene was determined with two experimental methods, i.e. by increasing the temperature and by decreasing the temperature. Experiments were performed in a 0.25 l glass reactor heated by an oil bath (Figure 6.1). Table 6.4 reports the experimental values of the sulphur solubilities in o-xylene.
Table 6.4: Experimental mole fraction solubilities x of sulphur in o-xylene at temperature Ta.
Solvent T/K x Remarks
360.84 0.06852 Method: decreasing temperature 360.99 0.06855 Setup: reactor in oil bath
350.53 0.04900 350.56 0.04884 340.83 0.03606 340.74 0.03674 o-Xylene 331.16 0.02714 330.24 0.02711 320.68 0.02021 320.42 0.02012 311.05 0.01512
110
311.02 0.01512 302.09 0.01123 302.05 0.01040
303.15 0.01166 Method: increasing temperature 303.15 0.01147 Setup: reactor in oil bath
303.15 0.01155 312.75 0.01531 313.05 0.01536 313.05 0.01537 312.95 0.01522 322.75 0.02042 322.35 0.02038 o-Xylene 331.75 0.02720 331.75 0.02732 332.15 0.02733 341.75 0.03711 341.75 0.03716 341.85 0.03702 351.15 0.04997 350.15 0.04857 359.15 0.06803 358.15 0.06614 a
Standard uncertainty for temperature is u(T) = 0.017 K. The relative combined standard uncertainty for solubility is UR(x) = 0.99 %.
The solubility of sulphur in m-xylene was determined with one experimental methods, i.e. by decreasing the temperature. Experiments were performed in a 0.25 l glass reactor heated by an oil bath (Figure 6.1). Table 6.5 reports the experimental values of the sulphur solubilities in m-xylene. Table 6.5: Experimental mole fraction solubilities x of sulphur in m-xylene at temperature Ta.
Solvent T/K x Remarks
358.24 0.05617 Method: decreasing temperature 358.42 0.05584 Setup: reactor in oil bath
349.07 0.04185 349.08 0.04187 339.42 0.03161 339.44 0.03134 m-Xylene 330.26 0.02395 330.37 0.02388 320.66 0.01810 320.51 0.01781 311.17 0.01364 311.25 0.01358 301.83 0.01017 301.85 0.01015 a
Standard uncertainty for temperature is u(T) = 0.017 K. The relative combined standard uncertainty for solubility is UR(x) = 0.99 %.
111 The solubility of sulphur in p-xylene was determined with two experimental methods, i.e. by increasing the temperature and by decreasing the temperature. Experiments were performed in a 0.25 l glass reactor heated by an oil bath (Figure 6.1). Table 6.6 reports the experimental values of the sulphur solubilities in p-xylene.
Table 6.6: Experimental mole fraction solubilities x of sulphur in p-xylene at temperature Ta.
Solvent T/K x Remarks
359.15 0.05448 Method: decreasing temperature 359.15 0.05426 Setup: reactor in oil bath
350.25 0.04049 350.25 0.04059 340.85 0.03024 340.85 0.03018 p-Xylene 331.95 0.02307 331.95 0.02290 322.15 0.01720 322.15 0.01719 312.55 0.01296 312.55 0.01290 303.15 0.009669
302.72 0.009695 Method: increasing temperature 311.99 0.01291 Setup: reactor in oil bath
321.53 0.01745 331.52 0.02303 331.21 0.02303 p-Xylene 341.24 0.03057 340.56 0.03054 350.94 0.04040 350.22 0.04113 360.68 0.05634 359.78 0.05655
360.33 0.05530 Method: decreasing temperature 360.30 0.05524 Setup: reactor in oil bath
351.37 0.04145 351.23 0.04143 341.64 0.03085 p-Xylene 341.50 0.03075 331.68 0.02301 331.42 0.02309 321.56 0.01716 321.58 0.01723 311.42 0.01301 302.72 0.009690
a Standard uncertainty for temperature is u(T) = 0.017 K. The relative combined standard uncertainty
112
Figure 6.3 shows the sulphur solubilities in toluene, o-xylene, m-xylene and p-xylene as a function of temperature with error bars displaying standard error:
Figure 6.3: Experimental mole fraction solubilities x of sulphur in toluene, o-xylene, m-xylene and p-xylene at temperature T.
From Figure 6.3 it can be concluded that of the solvents investigated, o-xylene exhibits the highest solubility for sulphur and toluene the lowest. Furthermore, it can be concluded that the solubility of sulphur is significantly increased with temperature for the aromatic solvents.
From the experimental values of the solubility of sulphur in toluene it can be concluded that the type of experimental setup, as well as the experimental method, did not influence the experimental determination of the sulphur solubilities in toluene. No hysteresis was observed.
6.4.2 Temperature dependence
The solubility equilibrium for sulphur in solvents can be expressed by:
( ) ( ) (6.12)
With the solubility equilibrium constant assumed to be:
(6.13)
The solubility equilibrium constant is related to the change in Gibbs free energy G according to:
(6.14)
At equilibrium, the change in Gibbs free energy equals zero. Therefore, Equation 6.14 can be rewritten as:
(6.15)
Where the standard Gibbs free energy Go equals 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 300 310 320 330 340 350 360 370 So lu b ili ty x T/K Toluene o-Xylene m-Xylene p-Xylene
113 (6.16) And therefore (6.17) The enthalpy change can be determined from the slope of a figure of lnKs versus T-1 (Van ‘t Hoff plot).
The entropy change can be determined from the y-intercept.
Figures of lnKs versus T-1 of the sulphur solubility in toluene, o-xylene, m-xylene and p-xylene with
error bars displaying standard error are shown in Figures 6.4, 6.5, 6.6 and 6.7, respectively.
Figure 6.4: Temperature dependence of the solubility equilibrium of sulphur in toluene.
Figure 6.5: Temperature dependence of the solubility equilibrium of sulphur in o-xylene. y = -3359.2x + 6.337 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0
2.7E-03 2.8E-03 2.9E-03 3.0E-03 3.1E-03 3.2E-03 3.3E-03 3.4E-03
ln Ksp ( -) T-1 (K-1) y = -3361.9x + 6.5804 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0
2.7E-03 2.8E-03 2.9E-03 3.0E-03 3.1E-03 3.2E-03 3.3E-03 3.4E-03
ln
Ksp
(
-)
114
Figure 6.6: Temperature dependence of the solubility equilibrium of sulphur in m-xylene.
Figure 6.7: Temperature dependence of the solubility equilibrium of sulphur in p-xylene.
The temperature dependencies of the solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene were determined with 55 data points, 33 data points, 14 data points and 36 data points, respectively.
The enthalpy and entropy changes of the solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene are summarized in Table 6.7.
y = -3245x + 6.126 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0
2.7E-03 2.8E-03 2.9E-03 3.0E-03 3.1E-03 3.2E-03 3.3E-03 3.4E-03
ln Ksp ( -) T-1 (K-1) y = -2825.3x + 6.6901 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
2.7E-03 2.8E-03 2.9E-03 3.0E-03 3.1E-03 3.2E-03 3.3E-03 3.4E-03
ln
Ksp
115 Table 6.7: Enthalpy and entropy changes of the solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene.
Solvent ΔH°/kJ.mol-1 ΔS°/J.mol-1.K-1
Toluene 27.93 52.69
o-Xylene 27.95 54.71
m-Xylene 26.98 50.93
p-Xylene 27.70 52.48
6.4.3 Comparison with literature
The sulphur solubilities in toluene, m-xylene and p-xylene were compared with solubilities published in literature. The solubility of sulphur in o-xylene was not reported in open literature.
Figure 6.8 shows the sulphur solubility in toluene reported in this study compared to sulphur solubilities in toluene reported previously.
Figure 6.8: Experimental mole fraction solubilities x of sulphur in toluene at temperature T reported in this study and previous studies.
From Figure 6.8 it can be concluded that previously reported solubilities of sulphur in toluene agree well with most of the sulphur solubilities reported in this study. The data point reported by Hildebrand and Jenks10 at a temperature of 356.65 K is slightly lower than the sulphur solubility reported by Tuller13 and the solubilities reported in this study. The data points reported by Wang et al.20 are slightly higher than the sulphur solubilities reported in this study and previously.
Figure 6.9 shows the sulphur solubility in m-xylene reported in this study compared to sulphur solubilities in m-xylene reported previously.
From Figure 6.9 it can be concluded that previously reported solubilities of sulphur in m-xylene agree well with the sulphur solubilities reported in this study. The data point reported by Hildebrand and Jenks10 at a temperature of 353.15 K is slightly lower than the sulphur solubility reported by Tuller13 and the solubilities reported in this study.
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 200 250 300 350 400 So lu b ili ty x T/K Cossa, 1868 Hildebrand and Jenks, 1921 Tuller, 1954 Jay et al., 2009 Wang et al., 2018 This study
116
Figure 6.9: Experimental mole fraction solubilities x of sulphur in m-xylene at temperature T reported in this study and previous studies.
Figure 6.10 shows the sulphur solubility in p-xylene reported in this study compared to sulphur solubilities in p-xylene reported previously.
Figure 6.10: Experimental mole fraction solubilities x of sulphur in p-xylene at temperature T reported in this study and previous studies.
From Figure 6.10 it can be concluded that previously reported solubilities of sulphur in p-xylene seem to agree well with the sulphur solubilities reported in this study, though most of the points reported by Tuller13 were determined at higher temperatures. Tuller reported experimental values for two types of sulphur, i.e. orthorhombic and monoclinic sulphur. At temperatures below 368.65 K orthorhombic sulphur is the stable allotrope of sulphur, at temperatures above 368.65 K monoclinic sulphur is the stable allotrope of sulphur.
0.00 0.01 0.02 0.03 0.04 0.05 0.06 290 300 310 320 330 340 350 360 370 So lu b ili ty x T/K Hildebrand and Jenks, 1921 Tuller, 1954 This study 0.00 0.02 0.04 0.06 0.08 0.10 0.12 290 310 330 350 370 390 So lu b ili ty x T/K Tuller, 1954, orthorhombic sulphur Tuller, 1954, monoclinic sulphur This study
117
6.5 Discussion
6.5.1 Previous studies
The data points reported by Hildebrand and Jenks10 for the solubility of sulphur in toluene and m-xylene up to a temperature of 327.15 K are in good agreement with the data points reported in the present study. However, the points reported by Hildebrand and Jenks at temperatures above 327.15 K are slightly lower compared to the sulphur solubilities reported in this study and previously.
A possible explanation could be the experimental method used to determine the sulphur solubility. The amount of dissolved sulphur was determined gravimetrically via sampling and solvent evaporation for data points up to a temperature of 327.15 K. At temperatures above 327.15 K, the amount of sulphur dissolved was determined by preparing a solution of known composition and noting the temperature at which a minute residual crystal neither increased or diminished in size when observed through a microscope. It can be concluded that the experimental method of observing a change in crystal size through a microscope is not an accurate method of measuring the sulphur solubility in toluene and m-xylene.
Ren et al.17 reported sulphur solubilities in their Figure 1. The sulphur solubilities in toluene and p-xylene, shown in their Figure 1, are much higher than the sulphur solubilities for the respective solvents shown in Figure 6.3 in the present study. A possible explanation could be that Ren et al. used the molar mass of sulphur, i.e. 32.065 mol/g, instead of the molar mass of orthorhombic sulphur S8, i.e. 256.52 g/mol. However, at temperatures of approximately 330 K and above their
Figure 1 shows a significant difference in sulphur solubility between toluene and p-xylene. Figure 6.3 in this study, on the contrary, shows a relatively small difference in sulphur solubility between p-xylene and toluene for all temperatures studied.
Ren et al. fitted the solubilities according to a modified Apelblat equation, and reported the correlated parameters in their Table 2. The term ln(x) in their Equation 1 is approximately 4.9 x 103 and 6.4 x 103 for toluene and p-xylene, respectively, at the investigated temperatures. Therefore calculated sulphur mole fractions in the solvents toluene and p-xylene result in infinite high numbers according to their Equation 1. Their correlated parameters for the modified Apelblat equation for sulphur solubilities in solvents are not applicable.
Wang et al.20 reported sulphur solubilities in toluene that are slightly higher than the data points reported in this study and previously. It is remarkable that the sulphur solubilities, shown in their Figure 3, agree well with the data points reported by Ren et al.17 according to Wang et al. The sulphur solubilities in toluene reported by Ren et al. (in their Figure 1) are much higher than the sulphur solubilities reported by Wang et al. in their Table 3 and in this study.
6.5.2 This study
From the experimental values of the sulphur solubility in toluene it can be concluded that the type of experimental setup, as well as the experimental method, did not influence the experimental determination of the sulphur solubilities in toluene. No hysteresis was observed. Previously reported solubilities of sulphur in toluene, m-xylene and p-xylene agree well with the sulphur solubilities reported in this study.
This study clearly illustrates, for experimental conditions studied, the increased sulphur solubility of o-xylene compared to p-xylene, m-xylene and toluene. The differences in solubility might be attributed to the molecular structures of the solvents and the sulphur molecule (orthorhombic α-sulphur).
Orthorhombic α-sulphur is the stable form of sulphur at temperatures below 95.5 °C, consisting of a puckered 8-membered ring of sulphur atoms in a crown arrangement (Figure 6.11a). As can be seen in Figure 6.11a, orthorhombic α-sulphur is not a flat molecule like e.g. benzene. Therefore the p orbitals of orthorhombic α-sulphur cannot form pi bonds.
118
Toluene, o-xylene, m-xylene and p-xylene are shown in Figure 6.11b.
Figure 6.11a: Molecular structure of orthorhombic sulphur.
Figure 6.11b: Molecular structures of toluene, o-xylene, m-xylene and p-xylene.
From Figures 6.11a and 6.11b it can be concluded that because of the configuration of the carbon atoms in o-xylene, they are in closer proximity to the sulphur atoms in orthorhombic sulphur compared to the other aromatic hydrocarbons.
The hypothesis is made that because the carbon and sulphur atoms are in closer proximity, the Van der Waals interactions are stronger. The stronger Van der Waals interactions decrease the distance between the carbon atoms and sulphur atoms and therefore the p orbitals of orthorhombic sulphur have an increased interaction with the aromatic ring of o-xylene, resulting in an increased solubility. Based on this hypothesis cyclohexane should exhibit a higher sulphur solubility than hexane. The configuration of the carbon atoms in cyclohexane should result in a better overlap with the sulphur atoms in orthorhombic sulphur. The sulphur solubility in cyclohexane is approximately a factor 3 larger compared to the sulphur solubility in hexane,13 which is in agreement with the hypothesis. Furthermore, according to the hypothesis, cyclohexane should exhibit a lower sulphur solubility than benzene. The orbitals of the carbon atoms in cyclohexane are sp3 hybridized, there is no aromatic ring. The sulphur solubility in benzene is approximately a factor 2 larger compared to the sulphur solubility in cyclohexane,13 which is in agreement with the hypothesis.
6.6 Conclusion
The solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene was investigated at temperatures ranging from 303.15 K to 363.15 K. It was determined that, for experimental conditions studied, o-xylene exhibited the highest sulphur solubility and toluene the lowest sulphur solubility. The enhanced sulphur solubility of xylene might be attributed to the molecular structures of o-xylene and orthorhombic sulphur. Because of the configuration of the carbon atoms in o-o-xylene, they are in closer proximity to the sulphur atoms of orthorhombic sulphur resulting in stronger Van der
119 Waals interactions. Because of the smaller distance between the carbon and sulphur atoms, the p orbitals of the sulphur atoms have an increased interaction with the aromatic ring of o-xylene. The sulphur solubility data reported in this study were in good agreement with sulphur solubility data in toluene, m-xylene and p-xylene published in open literature. Sulphur solubility data in o-xylene was not published in open literature. The temperature dependencies of the solubility of sulphur in toluene, o-xylene, m-xylene and p-xylene were determined from solubility equilibria. The enthalpies of dissolution ΔHo of sulphur in toluene, o-xylene, m-xylene and p-xylene were determined to be 27.93 kJ/mol, 27.95 kJ/mol, 26.98 kJ/mol and 27.70 kJ/mol, respectively.
6.7 Nomenclature
G change in Gibbs free energy [kJ/mol]
G° change in standard Gibbs free energy [kJ/mol]
H° change in standard enthalpy [kJ/mol]
KS solubility equilibrium constant [-]
m mass [kg]
MW molecular mass [kg/kmol]
R universal gas constant [J·mol-1·K-1
]
S° change in standard entropy [J·mol-1·K-1
]
T temperature [K]
U, u uncertainty
w mass fraction [-]
x mole fraction [-]
Subscripts and superscripts
C combined
Petri Petri dish
R relative
S8 orthorhombic sulphur
Solvent solvent
Syringe syringe
6.8 References
[1] Versteeg, G.F. and Ter Maat, H. Method and system for selective removal of contamination from gas flows. WO patent 1998055209 A1, assigned to Procede Twente B.V., priority date June 2, 1997.
[2] Wermink, W. N., Ramachandran, N. and Versteeg, G. F. Vitrisol® a 100% selective process for H2S
removal in the presence of CO2. J. Natural Gas Eng. 2017, 2, 50-83.
[3] Ter Maat, H., Hogendoorn, J.A. and Versteeg, G.F. The removal of hydrogen sulfide from gas streams using an aqueous metal sulfate absorbent. Part I. The absorption of hydrogen sulfide in metal sulfate solutions. Sep. Purif. Technol. 2005, 43 (3), 183─197.
[4] Ter Maat, H., Al-Tarazi, M., Hogendoorn, J.A., Niederer, J.P.M. and Versteeg, G.F. Theoretical and experimental study of the absorption rate of H2S in CuSO4 solutions. The effect of enhancement
of mass transfer by a precipitation reaction. Chem. Eng. Res. Des. 2007, 85 (1), 100─108.
[5] Wermink, W.N. and Versteeg, G.F. The oxidation of Fe(II) in acidic sulphate solutions with air at elevated pressures. Part 1. Kinetics above 1 M H2SO4. Ind. Eng. Chem. Res. 2017, 56 (14), 3775–
120
[6] Wermink, W.N. and Versteeg, G.F. The oxidation of Fe(II) in acidic sulphate solutions with air at elevated pressures. Part 2. Influence of H2SO4 and Fe(III). Ind. Eng. Chem. Res. 2017, 56 (14),
3789–3796.
[7] Wermink, W.N., Spinu, D. and Versteeg, G.F. The oxidation of Fe(II) with Cu(II) in acidic sulphate solutions with air at elevated pressures. Chem. Eng. Commun. 2018, accepted.
[8] Wermink, W.N. and Versteeg, G.F. The dissolution of CuS with Fe(III) in acidic sulphate solutions.
Ind. Eng. Chem. Res. 2018, submitted.
[9] Cossa, A. Ueber die Löslichkeit des Schwefels. Ber. Dtsch. Chem. Ges. 1868, 1 (1), 138-139.
[10] Hildebrand, J.H. and Jenks, C.A. Solubility VII. Solubility relations of rhombic sulfur. J. Am. Chem.
Soc. 1921, 43 (10), 2172─2177.
[11] Richards, T.W. and Coombs, L.B. The surface tensions of water, methyl, ethyl and isobutyl alcohols, ethyl butyrate, benzene and toluene. J. Am. Chem. Soc. 1915, 37 (7), 1656─1676. [12] Hildebrand, J.H. and Jenks, C.A. Solubility. IV. Solubility relations of naphthalene and iodine in the
various solvents, including a method for evaluating solubility data. J. Am. Chem. Soc. 1920, 42
(11), 2180─2189.
[13] Tuller, W.N. The sulphur data book. McGraw-Hill, New york, 1954.
[14] Seidell, A. Solubilities of inorganic and metal organic compounds. D. van Nostrand Company, Inc., New York, 1940.
[15] Hammick, D.L. and Holt, W.E. CCLXVII. – Pseudo-ternary systems containing sulphur. Part I. Sulphur and quinolone, pyridine, and p-xylene. J. Chem. Soc. 1926, 129, 1995─2003.
[16] Jay, S., Céraz, P., Serin, J-P, Contamine, F., Martin, C. and Mercadler, J. Solubility of elemental sulfur in toluene between (267.15 and 313.15) K under atmospheric pressure. J. Chem. Eng. Data
2009, 54 (12), 3238─3241.
[17] Ren, Y., Shui, H., Peng, C., Liu, H. and Hu, Y. Solubility of elemental sulfur in pure organic solvents and organic solvent-ionic liquid mixtures from 293.15 to 353.15 K. Fluid Phase Equilib. 2011, 312, 31─36.
[18] Apelblat, A. and Manzurola, E. Solubilities of o-acetylsalicylic, 4-aminosalicylic, 3,5-dinitrosalicylic, and p-toluic acid, and magnesium-DL-aspartate in water from T = (278 to 348) K. J. Chem.
Thermodyn. 1999, 31 (1), 85─91.
[19] Zhao, J-H, Wang, L-C, Xu, H-S, Song, C-Y and Wang, F-A. Solubilities of p-aminophenol in sulfuric acid + water from (286.15 to 362.80) K. J. Chem. Eng. Data 2005, 50 (3), 977─979.
[20] Wang, R., Shen, B., Sun, H. and Zhao, J. Measurement and correlation of the solubilities of sulfur S8 in 10 solvents. J. Chem. Eng. Data 2018, 63 (3), 553─558.
