・Development of microporous ceramic membranes for organic acid dehydration
Membrane separation will offer us great opportunities to reduce energy demands revolutionary in future chemical and petroleum industries. While a conventional separation system, or distillation inevitably requires a large amount of energy, roughly (reflux ratio + 1) × (heat of vaporization), membrane separation can minimize the reflux ratio, resulting in a large decrease in energy consumption where relative volatilities of molecules are low. For example, separation of water/acetic acid(AcOH) mixtures, an important operation in productions of such as terephtalic acid, acetate ester, and AcOH itself, is one of the systems where membrane separation is highly desired due to the low relative volatility of water and AcOH, 1.3-2.7.
In the last decade, a great deal of studies has been devoted to the science and technologies of pervaporation(PV)-based membrane separation. Since PV-based separation requires only heat of vaporization of permeate molecules, a drastic reduction of energy demands can be possible by a complete replacement of distillation with PV. However, the complete replacement of distillation with PV requires significantly high performance of membranes and huge investments, which hinders its practical application. On the other hand, our preliminary study suggests that just installing membrane separation unit at latter part of a distillation tower is effective enough to largely reduce energy demands for separation. This distillation–membrane hybrid separation system enables a large-scale reduction in energy utilization in an existing distillation unit with minimum risk by the addition of a membrane separation unit. This is due to the following important features of membrane separation technology:
(1) Requirements for the membrane performance are not so high. The process can be flexibly modified depending on the membrane performance.
(2) Large-scale reduction in energy consumption is possible without scrapping an already working distillation unit by the addition of a membrane separation unit.
(3) The performance of the distillation–membrane hybrid separation system can be incrementally enhanced by replacing membranes as the membrane properties are improved.
Our preliminary estimation described above shows that proper combination of distillation and membrane separation units enables large-scale reduction of energy consumption without scrapping distillation towers by just installing a membrane separation unit.
Regardless of the advantages, separation of water/AcOH mixtures through existing membranes has not been realized, possibly due to a lack of thermal and organic acid stability of membranes. Since hydrophilic polymer membranes often deteriorate in high temperature and/or organic acid conditions, microporous ceramic membranes seem favorable due to their higher thermal and organic acid stability.
・Application of microporous ceramic membranes to membrane reactor for esterification
Membrane reactors (MR), combination of reaction and separation systems, offer potential applications to chemical processes, delivering more compact and less capital-intensive processes with substantial saving in energy consumption. For example, applying MR to esterification is expected to improve conversion, displacing equilibrium by selective removal of by-product, water. Compared with other competitive systems such as reactive distillation, MR seems more favorable where relative volatilities of reactants and products are low, such as esterification of acetic acid (AcOH) and ethanol (EtOH). In terms of a membrane for the esterification, microporous ceramic membranes seem favorable due to their higher thermal and organic acid stability than hydrophilic polymer membranes.