Infinite dilution activity coefficient and vapour liquid equilibrium measurements for dimethylsulphide and tetrahydrothiophene with hydrocarbons

doi:10.1016/j.fluid.2010.03.027

Fluid Phase Equilibria

Volume 295, Issue 1, 15 August 2010, Pages 17-25

https://doi.org/10.1016/j.fluid.2010.03.027 Get rights and content

Abstract

The activity coefficients at infinite dilution (γ^∞) of dimethylsulphide (DMS) in four hydrocarbon solvents were measured using the dilutor technique at temperatures between 288 K and 303 K. The four hydrocarbons were hexane, 1-hexene, 2,2,4-trimethylpentane and 2,4,4-trimethyl-1-pentene. The dilutor technique is based on the stripping of the highly diluted solute, i.e. DMS, by a constant flow of inert gas. The gas composition was analysed by gas chromatography and the rate of solute removal was calculated from the area of the peaks.

In addition, a static total pressure apparatus was used to measure the vapour–liquid equilibrium of the binary systems of propane + DMS and propane + tetrahydrothiophene at 293 K and 313 K. In the static total pressure method, the analysis of the constituent phases is avoided. The systems’ components were injected to the equilibrium cell in known amounts. The composition of the liquid and vapour phase was calculated from the measured temperature and total pressure. The parameters for the Wilson activity coefficient model were regressed. When possible, a comparison between our experimental results and data found in the literature was performed.

Introduction

The refining industry is forced to manufacture cleaner and more ecological products in a greener way due to tightening demands in legislation and environmental concerns [1]. Methyl-Tert-Butyl-Ether (MTBE) has been and still is a very important component in petrol. MTBE's use was banned in California in 2004 since it was suspected to be responsible for ground water contamination [2]. Since then, in the US and Canada, it has been a common practice to substitute MTBE with a less environmentally sensitive compound having similar octane enhancing characteristics. MTBE can be replaced with isooctane (2,2,4-trimethylpentane) [3]. Several former MTBE units have been converted to produce isooctene (2,4,4-trimethyl-1-pentene) through dimerisation of isobutene, which was previously used to produce MTBE. Isooctene can then be further hydrogenated to isooctane.

The isooctene unit feed can originate from the fluid catalytic cracking (FCC) unit and it comprises of a range of hydrocarbons. This feed stream also contains mercaptans, THT, thiophene along with its light alkyl derivatives, benzothiophene and sulphides like DMS as impurities [4]. Commercial petrol is composed of different fractions originating from reforming, isomerisation and (FCC) units [5]. However, up to 85–95% of the sulphur present in the petrol pool comes from FCC petrol. Phase equilibrium data for determining the behaviour and distribution of sulphur compounds in the refinery streams is extremely important, since the sulphur content of petrol and diesel is strictly regulated. The sulphur content must be reduced without decreasing the octane and cetane number or losing fuel yield [6]. The USA EPA Tier II regulations impose maximum sulphur content in fuel of 30 ppm [7]. The EU has laid down a directive [8], which states that petrol and diesel fuels must have a maximum sulphur content of 10 ppm.

Propane is present in natural gas, liquefied petroleum gas (LPG), and hydrocarbon streams in petroleum refineries. Natural gas and LPG are colourless and odourless [9]. To detect the presence of natural gas and LPG in case of leaks, it is marked with compounds that have a very low odour threshold, like tetrahydrothiophene (THT) or dimethylsulphide (DMS) [10]. The vapour–liquid equilibrium (VLE) data for THT and DMS is required for estimating the concentration of the sulphur compounds in the gas/vapour phase. As THT and DMS are also present in the hydrocarbon streams of the petroleum refineries, the VLE data is required for modelling the behaviour of the organic sulphur compounds in the oil refining processes, especially in the production of sulphur-free fuels.

The relevance of this work comes from the need of good quality data to enable the accurate modelling of the distribution of organic sulphur impurities in natural gas, LPG and refinery streams and refinery unit operations. The limiting activity coefficient for DMS in four solvents was measured over a range of temperature from 288 K to 333 K. The experimental apparatus was earlier tested with toluene in water [11]. Isothermal VLE for binary systems consisting of DMS + propane and tetrahydrothiophene + propane was measured with a static total pressure apparatus at 293 K and 313 K.

Section snippets

Materials

The materials used and their purities are shown in Table 1. The chemicals were used as received except for the sulphur compounds, which were dried over molecular sieves.

Apparatus and procedure

The dilutor technique used to determine the infinite dilution activity coefficients was already described in detail in a previous paper [11]. The schematic picture of the experimental setup is shown in Fig. 1.

The principle of the measurement is that the highly diluted component is stripped out of the solvent by a constant inert

Infinite dilution activity coefficient measurements

Leroi et al. [15] described the method to measure activity coefficient at infinite dilution by inert gas stripping and GC. The thermodynamic basis of the procedure has been described by Krummen et al. [16]. The method consists of measuring the rate of elution of a solute as an entraining inert gas is passed through a highly dilute solute. It is assumed that the vapour leaving the dilution cell is in equilibrium with the liquid phase. The limiting activity coefficient can be evaluated from the

Results and discussion

The dilutor measurement method was evaluated with the toluene–water system already in our previous paper [11]. The γ^∞ values determined for DMS + hexane, DMS + 1-hexene, DMS + 2,2,4-trimethylpentane, DMS + 2,4,4-trimethyl-1-pentene in this work in the temperature range from 288 K to 303 K are presented in Table 3.

In this work, the measured γ^∞ values for DMS varied depending on the solvent, from 2.16 in hexane to 1.40 in 2,4,4-trimethyl-1-pentene. The γ^∞ for DMS decreased with increasing temperature in

Conclusions

The importance of the component pairs measured in this work evolves from the need of accurately modelling the sulphuric traces especially in the processes related to the petroleum industry. This need is coming from the ever tightening environmental regulations concerning sulphuric traces in petrol and diesel fuels.

In this paper the new measurements of γ^∞ for DMS in hydrocarbons between 288 K and 303 K and VLE for DMS + propene and tetrahydrothiophene + propene at 293 K and 313 K are presented. This

List of symbols

slope (1/s) of ln(A/A₀) as a function of time, determined from the chromatogram area decrease against time

A_i

peak area of component i at time t

A₀

peak area of component i at beginning of the measurement

F_in

flow rate of the stripping gas entering the measurement cell

F_N

flow rate of the stripping gas

F_{N}^{\exp}

measured flow rate of the stripping gas

K_i

distribution coefficient

m_solv

mass of solvent

n_solv

number of moles of solvent in the dilution cell

P_{i}^{s}

saturation vapour pressure of the pure solute i

P_{FM}

pressure

Acknowledgments

Ms. Tiina Puttonen and Ms. Minna Pakkanen are greatly acknowledged for their assistance during the measurements. The authors acknowledge Tekes—the Finnish Funding Agency for Technology and Innovation/Gasum Natural Gas Fund for financial support.

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¹: Research fellow, Academy of Finland.

View full text

Infinite dilution activity coefficient and vapour liquid equilibrium measurements for dimethylsulphide and tetrahydrothiophene with hydrocarbons

Abstract

Introduction

Section snippets

Materials

Apparatus and procedure

Infinite dilution activity coefficient measurements

Results and discussion

Conclusions

List of symbols

Acknowledgments

Fluid Phase Equilib.

Appl. Catal. A: Gen.

Catal. Today

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Hydrocarbon Process

Ind. Eng. Chem. Res.

Sittig's Handbook of Toxic and Hazardous Chemical and Carcinogens

J. Chem. Eng. Data