Gas–liquid partition coefficients and Henry's law constants of methyl mercaptan in aqueous solutions of Fe(II)–CDTA chelate complex

https://doi.org/10.1016/j.fluid.2007.02.008Get rights and content

Abstract

Utilization of ferric chelate complex of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) for the oxidative scrubbing of H2S and CH3SH in Kraft mill streams can be beneficial from the standpoints of iron protection against precipitation and oxygen-mediated regenerative oxidation of the ferrous chelate CDTA. The physical solubility of methyl mercaptan in CDTA–Fe(III) complex cannot be measured directly because of oxidation of the sulfur-bearing gaseous species with the ferric chelate. Therefore, this investigation was carried out to determine the gas–liquid partition coefficients and Henry's law constants of methyl mercaptan in aqueous iron-free CDTA solutions and non-reacting ferrous chelate solutions (CDTA–Fe(II) complex), using the static headspace method with an estimated accuracy of about 2%. Experiments with aqueous solutions of chelate concentrations varying between 38 and 300 mol m−3 were carried out at temperatures between 298 and 333 K and atmospheric pressure. It was shown that the methyl mercaptan solubility decreases with increasing temperature for all systems but was not much influenced, in the studied conditions, by the chelate concentration especially at larger temperatures.

Introduction

Total reduced sulfurs (TRS) quartet contain hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3) and dimethyl disulfide (CH3S2CH3). These sulfur compounds, especially responsible for olfactory nuisances, are part of a well-known environmental problem afflicting pulp mills exploiting the Kraft mill sulfate-pulp process. The origin of these odors is partly ascribable to pulp production equipments such as boilers, blow tanks, and washers, or to black liquor recovery equipment. Because of their toxic and corrosive character, they must be removed down to very low concentration levels. Since the early 1990s, several Canadian provincial governments and the United States promulgated a number of regulations upon the Kraft pulp manufacturers to collect and treat their total reduced sulfurs emitting vents. The olfactory threshold of TRS for human beings is four orders of magnitudes below the regulated emission level which is approximately 10 ppm in Canada and the US. This has given rise to very strict regulations in order to reduce their emissions from specific sulfate pulp process equipments such as kilns, evaporators, washers, etc. Considering the progressive nature of legislations, it is anticipated that increasingly tighter regulations will be applicable in the near future especially in North America where about 15% of the world Kraft mills are in operation.

Utilization of ferric chelate complex of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) for the oxidation of two of these sulfur compounds (H2S and CH3SH) is a favorable process from the standpoint of iron-sequestration and protection against precipitation in the alkaline environments characteristic of the Kraft mill streams [1], [2], [3], [4], [5], [6], [7], [8]. Like the hydrogen sulfide, the methyl mercaptan is absorbed in ferric chelate solutions but is oxidized into dimethyl disulfide (DMDS) and adds up to the remaining dimethyl sulfide (DMS) and DMDS coming from the air effluent undergoing physical absorption in the aqueous iron chelate solutions. Due to the propensity of the bare ferric and ferrous ions to precipitate as hydroxides within neutral to alkaline media, the use of organic chelate agents, which have the capacity to bond with cationic ions, proved to be efficient to prevent iron precipitation. Another advantage of ferric chelate oxidation rests on the possibility to regenerate the ferrous chelate product into the active ferric chelate in the presence of dissolved oxygen.

The physical solubility of these compounds in chelate-containing solutions is therefore a key parameter needed for the design of absorption scrubbing equipments in many technical applications. However, when the compound reacts within the aqueous solutions, as in the case of H2S and CH3SH with Fe(III) chelates, the genuine physical solubility cannot be measured directly. This interfering oxidation is absent when H2S or CH3SH is contacted with Fe(II) chelates.

In some recent papers [9], [10], [11], we have studied the solubility of three of these sulfur compounds in chelate-containing solutions. In addition, the availability of oxygen along with hydrogen sulfide in the Kraft process atmospheric air emissions was found to be propitious for the simultaneous redox regeneration from ferrous to ferric chelate for perpetuating the scrubbing cycle so that the oxygen solubility was also investigated [12]. As a continuation of our research program, the solubility of methyl mercaptan in aqueous solutions in which it does not react (Fe(II) complex of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) or iron-free CDTA) was studied here.

The aim of this investigation was to determine the gas–liquid partition coefficients and Henry's law constants of methyl mercaptan (MM) in iron-free CDTA solutions and in CDTA–Fe(II) complex using a static headspace method based on measuring chromatographically the equilibrium headspace peak areas of MM. The static headspace method is well known as a very effective tool for analyzing volatile organic compounds present in contaminated condensed samples. The method has therefore found a swide-ranging use in environmental analysis and was chosen in this study because of the very corrosive character of the methyl mercaptan for the solubility apparatus used in the previous works [9], [12]. The method was successfully used in the case of DMS and DMDS [10], [11]. New experimental data over a chelate concentration ranging from 38 to 300 mol m−3 are reported at different temperatures between 298 and 333 K. To the best of our knowledge, no similar data exist in the literature.

Section snippets

Static headspace method

The complete description and procedure are presented in the previous publications [10], [11]; therefore, only a few essentials are repeated here.

The headspace sample is prepared in a sealed vial by introducing a known volume of sample of initial concentration CS. If we consider this system at equilibrium consisting of the liquid phase (L) with concentration CL and the headspace gas phase (GH) with concentration CGH, a mole balance between the total moles of VOC in the sample, the moles of VOC

Chemicals

Trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) with a minimum purity of 99.3 wt%, iron (II) chloride tetrahydrate with a minimum purity of 99 wt%, sulfuric acid with a purity of 95–98 wt%, and HPLC distilled water were purchased from Aldrich Chemical Co. Sodium methanethiolate with a minimum purity of 95 wt% was purchased from Fluka.

Sample preparation

In order to prepare the standard CDTA solutions, the CDTA was stoichiometrically added to a sodium hydroxide aqueous solution to obtain a solution with pH 7. The

Results and discussion

The accuracy of the measuring method was already demonstrated previously [10], [11]. In addition, the gas–liquid partition coefficients and Henry's law constants of methyl mercaptan in distilled water at different temperatures between 298 and 333 K were obtained. The results are given in Table 1 and compared with those reported by Przyjazny et al. [15]. Because, the authors did not give the raw experimental data, the values of the gas–liquid partition coefficients were calculated from the

Conclusion

Using the static headspace method with an uncertainty estimated to about 2%, new experimental results for the solubility of MM in water, CDTA and ferrous CDTA complex solutions in the temperature range of 298–333 K and atmospheric pressure are presented. The chelate concentration was varied between 38 and 300 mol m−3. MM solubility decreases with increasing temperature for all systems studied. At a constant temperature, the difference in MM solubility in CDTA and CDTA–Fe(II) complex is more

Acknowledgement

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Grant Program Environment & Sustainable Development is gratefully acknowledged.

References (14)

  • K.S. Cho et al.

    J. Ferment. Bioeng.

    (1992)
  • M.C. Iliuta et al.

    Fluid Phase Equilib.

    (2004)
  • I. Iliuta et al.

    Chem. Eng. Sci.

    (2003)
  • A. Przyjazny et al.

    J. Chromatogr.

    (1983)
  • M. Järvensivu et al.
  • M. Järvensivu et al.
  • B. O’Connor et al.
There are more references available in the full text version of this article.

Cited by (0)

View full text