Liquid–liquid equilibria for ternary systems polyoxymethylene dimethyl ethers + para-xylene + water

Highlights

• The LLE for ternary systems (PODE_n + PX + H₂O) was measured at T = 293.15 K, 313.15 K.

• PX were studied as an effective solvent to extract PODE_n from aqueous solution.

• The NRTL and UNIQUAC models were used to correlate the experimental data.

• The consistency of the LLE data were tested by Hand correlations and Othmer–Tobias.

Abstract

Polyoxymethylene dimethyl ethers (PODE_n) are promising diesel additives which can be synthesized from formaldehyde and methanol. In this work, the ternary liquid–liquid equilibria (LLE) were analytically determined at 293.15 K and 313.15 K for the following systems: PODE₁ + para-xylene (PX) + water, PODE₂ + PX + water, PODE₃ + PX + water, PODE₄ + PX + water, respectively. The isothermal experimental results have shown a good linear fit in Hand plots and the Othmer–Tobias. The design of the solvent extraction equipment for extracting PODE_n from aqueous solution requires an appropriate model for the liquid–liquid phase equilibrium. The well-known NRTL and UNIQUAC thermodynamic models were applied to correlate the experimental data, and the results indicate that the UNIQUAC model gives a better agreement.

Introduction

As environmental issues become more urgent, the development of efficient diesel additives for reduction of emissions of particular materials (PM) and NO_x is highly desirable. Oxygenated compounds such as methanol, dimethyl ether (DME) and methylal are known to reduce soot formation during combustion when they are added into diesel fuels [1], [2]. However, due to the lower cetane number and high vapor pressure of DME [4], methanol [5], and methylal [6], they are not suitable for incorporation into diesel directly without modification of the engine’s infrastructure. To overcome these disadvantages, a more attractive solution polyoxymethylene dimethyl ethers (PODE_n, CH₃O(CH₂O)_nCH₃) is being used as a diesel fuel additive. PODE₃₋₄ are believed to be the most appreciate constituents as diesel fuel additives among all PODE_n compounds [2], [3], which can be blended into diesel without structural changes of diesel engine.

Generally, PODE_n are synthesized from two routes [7]: one is methylal + trioxane (or paraformaldehyde) and the other is methanol + formaldehyde (FA). However, the first route is believed to be not appropriate for PODE_n synthesis in industrial production due to the high cost of raw materials; therefore, the second route has been found as an alternative way for the production of PODE_n and it is of great importance from the industrial view due to its lower raw materials’ cost. However, this reaction system brings serious problems in separation process.

It is difficult to obtain highly purified desired products (e.g., PODE₃₋₄) from the reaction products in the presence of water [7], [8], [9], [10], [11]. Above a certain temperature, water reacts with the PODE_n in an acid-catalyzed reaction forming alcohols, such as the reaction of PODE₁ with water to form hemiformal, i.e., CH₃OCH₂OH (or poly-hemiformals, i.e., CH₃O(CH₂O)_nCH₂OH) and methanol [8], [9], [10], [11].

$${\text{PODE}}_1+{\text{H}}_2\text{O}\leftrightarrow {\text{HF}}_1+\text{ME}$$

$${\text{HF}}_{n-1}+\text{FA}\leftrightarrow {\text{HF}}_n$$

During the PODE_n synthesis reactions process, based on the report of Burger et al. [8], high concentration formaldehyde solution is always used to improve the selectivity of PODE₃₋₄, according to the following reactions:

$$2\text{ME}+\text{FA}\leftrightarrow {\text{PODE}}_1+{\text{H}}_2\text{O}$$

$${\text{PODE}}_{n-1}+\text{FA}\leftrightarrow {\text{PODE}}_n$$

Reactions (3), (4) are reversible in acidic conditions and lead to an equilibrium chain distribution which mainly depends on the formaldehyde concentration in the reaction. Increasing the formaldehyde concentration shifts the distribution to longer chains, but the formaldehyde solution with high concentration is easy to be polymerized, which forms paraformaldehyde as white solid blocking pipelines.

These problems have brought great difficulty in the distillation operation, and made it hard to obtain pure products in PODE synthesis process. In addition, how to recycle massive formaldehyde in educts as raw material for reaction should also be considered.

Solvent extraction process was therefore developed by introducing para-xylene as an extracting solvent. In proper solvent extraction, water could be separated from reaction products to inhibit Reactions (1), (2). On the other hand, massive formaldehyde could be recycled in water-rich phase, which associates strongly with water in oligomers as followings [10], [12]:

$${\text{CH}}_2\text{O}+{\text{H}}_2\text{O}\leftrightarrow {\text{HOCH}}_2\text{OH}$$

$$\text{HO}{({\text{CH}}_2\text{O})}_{n-1}\text{H}+{\text{HOCH}}_2\text{OH}\leftrightarrow \text{HO}{({\text{CH}}_2\text{O})}_n\text{H}+{\text{H}}_2\text{O}n⩾2$$

LLE data are valuable in studies of the applicability of activity coefficient. A reliable thermodynamic model for a good system description is important for equipment modeling, development and design [13]. Although there is a growing industrial interest in the separation processes associated in PODE_n production, there are few experimental LLE data on these systems. Albert et al. [14] measured the LLE of (water + methylal(PODE₁)) and (water + methanol + methylal), and related data were correlated well using the UNIQUAC model. Kuhnert et al. [10] modeled the LLE of ternary system (formaldehyde + water + methylal) well by revised UNIFAC. No experimental data of PODE_n>1 could be obtained from current literatures. Some investigations have been carried out on hydrophobic solvents which influence separation effectiveness, immiscibility region, and LLE behavior of ternary systems. Several aromatic compounds have been chosen because of their low solubility in aqueous phase [15], [16], [17], [18].

Additionally, due to the complexity of compositions in PODE_n reaction system, the boiling point of the lightest component PODE₁ is only 315.15 K [14] in ambient pressure. To recycle raw material PODE₁ as much as possible, the actual extraction temperature should not exceed the highest temperature limit of 315.15 K, while the lower operation temperature will lead to much higher extraction cost. The operation temperature range between 293.15 K and 313.15 K in PODE_n extraction plant is more economical.

As no available literature data on liquid–liquid phase equilibrium of multicomponent mixtures (PODE_n + water) and extraction solvent para-xylene, this contribution reported experimental results of the phase equilibrium in such mixtures: liquid–liquid equilibrium of ternary systems (PODE₁ + PX + water), (PODE₂ + PX + water), (PODE₃ + PX + water) and (PODE₄ + PX + water) at temperatures 293.15 K and 313.15 K. All experimental liquid–liquid phase equilibrium data were correlated by applying the NRTL [19] and UNIQUAC [20] models to fit the extraction process.