Solid–Liquid Equilibria and Kinetics of the Solid Formation in Binary and Ternary Mixtures Containing (Formaldehyde + Water + Methanol)

Solid precipitation from aqueous formaldehyde solutions is a major technical problem, leading to troubles such as plugging and fouling. While these problems are ubiquitous in formaldehyde technology, it is hard to predict them, so their mitigation is difficult. Data on the formation of a solid phase in formaldehyde-containing systems are scarce and basically only available for the system (formaldehyde + water); even these few data are contradictory. We have tackled these problems from different sides in the present work. First, a new method for measuring the solid–liquid equilibria and kinetics of solid formation in formaldehyde-containing systems was developed, and it was shown that, for technically relevant conditions, long-term effects are essential: it may take over 1000 days until equilibrium is reached. Using this new technique, reliable data for the solid–liquid equilibrium of the system (formaldehyde + water) were obtained. The liquidus line in the phase diagram of that system has two branches meeting in a eutectic point: one where the solid is water, and one where it is formaldehyde-rich. The formaldehyde solubility is found to be much lower than in most previous works. As methanol is often used for stabilizing aqueous formaldehyde solutions, we also investigated solid–liquid equilibria in the systems (formaldehyde + methanol) and (formaldehyde + water + methanol). Furthermore, data on the kinetics of the formation of formaldehyde-rich solids were acquired for the studied systems. Based on the new data and extensive previous work on the chemical equilibria and reaction kinetics, a physico-chemical model was developed, which describes both the solid–liquid equilibria as well as the kinetics of the solid formation. It is shown that the very slow kinetic effects in the studied systems are mainly caused by the interaction of the slow liquid phase reaction kinetics and the precipitation of a single oligomer of formaldehyde with water for which the solubility limit is reached

Vapor–liquid Equilibrium and Distillation of Mixtures Containing Formaldehdye and Poly(oxymethylene) Dimethyl Ethers

Poly(oxymethylene) dimethyl ethers (OME, H3C–O–(CH2O)n–CH3) are promising synthetic diesel fuels. For designing OME production processes, a model for describing the vapor–liquid equilibrium (VLE) in mixtures of (formaldehyde + water + methanol + methylal + OME + trioxane) is needed. Building on previous work of our group, a physico-chemical model for the VLE in these mixtures is developed in the present work. For the development and the testing of the model, experiments of different types were carried out: VLE measurements in a thin film evaporator, batch evaporation experiments in an open still, and continuous distillation experiments in a laboratory column. The model predicts the results of the distillation experiments well. It is shown that OME with n ≥ 3 can be separated as bottom product from mixtures of formaldehyde, water, methanol, methylal, and OME with n ≥ 2. This separation is a critical step in a novel OME production process that increases the sustainability of OME production

Separation of Water from Mixtures Containing Formaldehyde, Water, Methanol, Methylal, and Poly(oxymethylene) Dimethyl Ethers by Pervaporation

In this work, pervaporation experiments were carried out, in which water was separated from mixtures containing formaldehyde, water, methanol, methylal, and poly(oxymethylene) dimethyl ethers (OME). This separation is interesting for new production processes for the synthetic fuel OME. Five commercial membranes were studied: two zeolite membranes (Type NaA and Type T from Mitsui & Co.) and three PVA-based polymer membranes (PERVAP 4100, PERVAP 4101, and PERVAP 4102 from DeltaMem AG). The membrane flux and the composition of the permeate have been measured. The zeolite membranes were tested at 343 K and 7 mbar permeate pressure and the polymer membranes were tested at 353 K and 2 mbar permeate pressure. The investigated mixtures are inherently reactive, as formaldehyde reacts both with water and methanol. The zeolite membranes could only be used once, whereas the polymer membranes showed no significant degradation in a repeat experiment

Conceptual Design of a Crystallization-Based Trioxane Production Process

Trioxane, a cyclic trimer of formaldehyde, is an important intermediate that is mainly used as a source of water-free formaldehyde in chemical production processes. Different routes for the production of trioxane from aqueous formaldehyde solutions have been described in the literature, which all have in common that they are energy-intensive. In this work, the conceptual design of a new production process for trioxane is presented. It is a modification of a state-of-the-art pressure swing process from the literature, where the most critical distillation step is replaced with a crystallization. The new crystallization-based process and the pressure swing process were simulated in a consistent manner based on a well-established physico-chemical model for the ternary system (formaldehyde + water + trioxane). To obtain a sound basis for the description of the crystallization, solid–liquid equilibria in the ternary system, for which previously no data was available, were measured at conditions relevant for the process. A comparison of the two processes showed that the new crystallization-based process has a considerably reduced energy demand and much smaller recycle streams. Furthermore, it was found that the crystallization can be performed efficiently at temperatures well above 273 K. All this makes the new crystallization-based process an attractive candidate for trioxane production