Measurements and modeling of LLE and HE for (methanol + 2,4,4-trimethyl-1-pentene), and LLE for (water + methanol + 2,4,4-trimethyl-1-pentene)
Introduction
Tertiary ethers are used as gasoline additives. Methyl Tertiary Butyl Ether (MTBE), Ethyl Tertiary Butyl Ether (ETBE), and Tertiary-Amyl Methyl Ether (TAME) are the main commercially used compounds. The tertiary ethers can partially be produced from bio-methanol or bio-ethanol. Recently, the European Union (EU) directives are promoting the use of biofuels instead of fossil fuels. Scientists are making efforts to develop new processes for increasing productivity of new tertiary ethers from bio based raw materials. [1]
Diisobutylene is mainly made of 2,4,4-trimethyl-1-pentene (TMP-1) and 2,4,4-trimethyl-2-pentene (TMP-2). Etherification of diisobutylene with methanol produces 2-methoxy-2,4,4-trimethylpentane (TOME). TOME can be applied in the gasoline to increase the octane number. During the etherification reaction, dehydration of excess methanol produces dimethyl ether and water. The water induces a phase split in the system.
The phase equilibria related to TOME production process is rare. Uusi-Kyyny et al. [2] reported only two sets of (vapor + liquid) equilibrium (VLE) data for the binary system of (methanol + TMP-1) (isothermal at T = 331 K, and isobaric at 101 kPa) and one set of VLE data for the (methanol + TOME) (isothermal at T = 333 K). Halttunen [3] reported the LLE for the binary mixture of (water + TMP-1). This research aims at presenting measurements and a model to deepen our knowledge on the phase equilibria in a possible process for production of TOME from TMP-1 and methanol.
The LLE for the binary system of (methanol + TMP-1) and LLE for the ternary mixture of (water + methanol + TMP-1) were measured to describe the complex phase equilibrium behavior appearing in the TOME production process; in addition the excess enthalpy (HE) was measured to enhance the temperature dependence of the model (equation (1)).where is the activity coefficient of component i; T is temperature, K; is partial excess enthalpy of component i in the solution, J · mol−1; and R is universal gas constant, J·(mol · K)-1.
Measured HE and LLE with two VLE datasets from literature were combined to regress Non-Random Two-Liquid (NRTL) [4] activity coefficient model parameters. LLE data for the binary system of (water + TMP-1) from the literature were regressed to obtain NRTL parameters for the system of (water + TMP-1), was improved by adding the measured ternary data in this regression. Finally the binary parameters were used to describe the ternary system.
This work is novel in reporting LLE and HE for the binary system of (methanol + TMP-1), and LLE for the ternary system of (water + methanol + TMP-1), in addition to simultaneous modeling of four binary system datasets including isothermal VLE, isobaric VLE, LLE and HE. This experience can show the abilities of NRTL model in describing the properties of a system in a temperature range of over 120 K.
Section snippets
Materials
Methanol and TMP-1 were dried by adding molecular sieve (0.3 nm Merck) into the chemical. Table 1 provides the CAS number, water content, purity, measured and literature densities, and suppliers of the compounds. The molar mass and extended Antoine equation parameters are available in table 2.
(Liquid + liquid) equilibrium by turbidimetry [5]
A jacketed cell was applied for LLE measurements by turbidimetry. Figure 1 demonstrates a schematic of the apparatus. ASL F 200 Series Precision Thermometer with accuracy of T = 0.02 K was applied for
(Liquid + liquid) equilibrium for binary mixture
Mixtures of compounds with known mole fractions were placed in the turbidimetric apparatus, which is a so called synthetic method [5]. In turbidimetry, the formation of two liquid phases can be observed by variation in transparency of the studied mixture. A known amount of methanol was injected into the cell and the temperature was reduced to 253.15 K. TMP-1 was added until a phase split was observed. Increasing and decreasing the temperature at constant composition shows the miscibility gap
Analysis methods and equipment
Agilent 6850 gas chromatograph GC equipped with flame ionization detector (FID) and Agilent 6890N GC equipped with both thermal conductivity detector (TCD) and FID detectors were applied to obtain compositions. For all the GC measurements, Agilent GC 2 cm3 glass vials with aluminum crimp caps and PTFE/red rubber septas were utilized. The GC settings are provided in Appendix B. Mettler Toledo DL38 volumetric Karl-Fisher Titrator (KF) was used to analyze the water content. The relative uncertainty
Modeling
The Non-Random Two-Liquid model (NRTL) can describe the activity coefficients and the measured points for the binary mixture discussed in this work. The NRTL model is embedded in ASPEN PLUS [10] in the following form (equation (2)):where: is non-randomness parameter. aij, bij, cij, dij, eij, fij are binary interaction parameters. In NRTL equation cij = cji and dij = d
Results and discussion
Results of HE is given in table 3. The analytical and turbidimetric measurement data points for the system of methanol + TMP-1 are provided in TABLE 4, TABLE 5 respectively. A polynomial was fitted to the turbidimetric measured data only to find 10 isothermal tie lines and the upper critical solution temperature, the R-squared value for the polynomial was 0.9999. These generated tie lines in addition to analytical LLE were applied in the regression of the NRTL model parameters. The average
Notes
The authors declare no competing financial interest.
Funding statement
Saeed Mardani, Petri Uusi-Kyyny and Juha-Pekka Pokki acknowledge Academy of Finland for financial support.
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Present address: National institute of applied sciences (INSA), Chemical Engineering and Fine chemistry Department, INSA de Rouen, B.P. 08, Avenue de l’université 76801 Saint Etienne du Rouvray CEDEX.