Isohexid-Amine sind aufgrund ihrer vergleichsweise rigiden Struktur von großem Interessen für die Herstellung neuartiger biomassebasierter Polyamide. Jedoch ist für eine anwendungsbezogene Entwicklung die Verfügbarkeit der Monomere bisher der limitierende Faktor. Auf organisch-synthetischem Weg bzw. über teure homogenkatalytische Ansätze können sie im Labormaßstab aus den technisch verfügbaren Monomeren Isosorbid und Isomannid hergestellt werden. In einem vorausgegangenen Projekt wurden nun Reaktionsbedingungen identifiziert, die erstmalig die Aminierung von Isohexiden mit Ammoniak in wässrigen Lösungen ermöglichen. Im Rahmen dieses Vorhabens soll die heterogen katalysierte Aminierung von Isohexiden in wässriger Lösung in Richtung technischer Reife weiterentwickelt werden. Das Projekt ist in vier Teilaufgaben gegliedert: 1) Katalysatoroptimierung, 2) Kontinuierliche Reaktionsführung, 3) Stofftrennung und Produktentwicklung und 4) Verfahrensentwicklung. In (1) soll der identifizierte, kommerziell verfügbare Katalysator Ruthenium auf Aktivkohle (Ru/C) optimiert werden. Insbesondere ist das Ziel, unter Minimierung des Leachings, die Aktivität zu steigern, indem durch den Einsatz bimetallischer Ru-basierter Systeme bzw. von Promotoren die Affinität für die Bindung der gebildeten Amine gesenkt wird. In (2) wird die Möglichkeit der kontinuierlichen Prozessführung durch Einsatz eines kontinuierlichen Rührkessels untersucht. Im Fokus stehen dabei insbesondere Versuche zur Langzeitstabilität, der Reaktionskinetik sowie möglicher Massentransferlimitierungen an Formkörperkatalysatoren. In (3) wird die technisch relevante Trennung der Produkte über chromatographische Verfahren weiterentwickelt sowie als Konzeptstudie die Herstellung Isohexidamin-basierten Polyamide gezeigt. Auf dieser Basis soll in (4) über eine konzeptuelle Verfahrensentwicklung das Potential für eine technische Aufskalierung evaluiert sowie die Wirtschaftlichkeit abgeschätzt werden.
technologyComment of chlor-alkali electrolysis, diaphragm cell (RER):
In the diaphragm process, all reactions take place in only one cell. A diaphragm is used to separate the feed brine (anolyte) and the chlorine formed at the anode from the sodium hydroxide containing solution (catholyte) and the hydrogen formed at the cathode. Without the diaphragm being present during electrolysis, chlorine and hydrogen would form an explosive mixture and sodium hydroxide and chlorine would react to form sodium hypochlorite (NaOCl). Diaphragms used to be made from asbestos but up-todate technology allows for asbestos-free polymer-based diaphragms. Purified brine is fed to the anode compartment and percolates through the diaphragm into the cathode compartment. The percolation rate is controlled by a difference in liquid level between both compartments. At the anodes (metal oxide coated titanium), chlorine gas is formed which is collected and directed to further processing. Cathodes, where water decomposition takes place, are made of activated carbon steel. Catholyte leaving the cell, also called cell liquor, is a mixture of 10-12 wt.-% sodium hydroxide and 15-17 wt.-% sodium chloride in water. This solution is usually evaporated to 50 wt.-% NaOH. In this process, simultaneously most of the salt is removed by precipitation to a final residual of 1 wt.-%. The resulting salt is typically recirculated to brine preparation. The advantage of diaphragm cells is that the quality requirements for the brine and the electrical energy consumption are low. Disadvantageous are the high amount of thermal energy necessary for sodium hydroxide concentration and the comparably low quality of the produced sodium hydroxide and chlorine.
References:
Euro Chlor (2013) An Eco-profile and Environmental Product Declaration of the European Chlor-Alkali Industry, Chlorine (The chlor-alkali process).
technologyComment of chlor-alkali electrolysis, membrane cell (RER):
In the membrane cell process, the anode and cathode compartments are separated by a perfluoropolymer cation-exchange membrane that selectively transmits sodium ions but suppresses the migration of hydroxyl ions (OH-) from the catholyte into the anolyte. Saturated brine flows through the anode compartment, where chlorine gas is produced at the anode. The electric field in the electrolysis cell causes hydrated sodium ions to migrate through the membrane into the cathode compartment. The cathode compartment is fed with diluted sodium hydroxide solution. Water is electrolysed at the cathode releasing gaseous hydrogen and hydroxyl ions, which combine with the sodium ions and thus increase the concentration of sodium hydroxide in the catholyte. Typically, the outlet concentration of sodium hydroxide is around 32 wt.-%. A part of the product stream is diluted with demineralised water to about 30 wt.-% and used as catholyte inlet. In some units, a more diluted 23 wt.-% NaOH solution is produced. In these cases, the inlet concentration is adjusted to 20-21 wt.-%. Usually the NaOH solution is evaporated to the marketable concentration of 50 wt.-% using steam. Depleted brine leaving the anode compartment is saturated with chlorine and is therefore sent to a dechlorination unit to recover the dissolved chlorine before it is resaturated with salt for recirculation. The advantages of the membrane cell technique are the very high purity of the sodium hydroxide solution produced and the comparably low energy demand. Disadvantages comprise the high requirements on brine purity, the need for sodium hydroxide evaporation to increase concentration, and the comparably high oxygen content in the produced chlorine. In general, multiple cell elements are combined into a single unit, called electrolyser, of whom the design can be either monopolar or bipolar. In a monopolar electrolyser, the anodes and cathodes of the cells are connected electrically in parallel, whereas in the bipolar design, they are connected in series. Therefore, monopolar electrolysers require high current and low voltage, whereas bipolar electrolysers require low current and high voltage. Since bipolar systems allow higher current densities inside the cells, investment and operating costs are usually lower than for monopolar systems.
References:
Euro Chlor (2013) An Eco-profile and Environmental Product Declaration of the European Chlor-Alkali Industry, Chlorine (The chlor-alkali process).
technologyComment of chlor-alkali electrolysis, mercury cell (RER):
The mercury cell process comprises an electrolysis cell and a decomposer. Purified and saturated brine (25-28 wt.-% NaCl in water) is fed to the electrolysis cell on top of a film of mercury (Hg) flowing down the inclined base of the cell. The base of the cell is connected to the negative pole of a direct current supply forming the cathode of the cell. Anodes consisting of titanium coated with oxides of ruthenium and titanium are placed in the brine without touching the mercury film. At the anodes, chlorine gas is formed which is collected and directed to further processing. Due to a high overpotential of hydrogen at the mercury electrode, no gaseous hydrogen is formed; instead, sodium is produced and dissolved in the mercury as an amalgam (mercury alloy). The liquid amalgam is removed from the electrolytic cell and fed to a decomposer, where it reacts with demineralised water in the presence of a graphite-based catalyst to form sodium hydroxide solution and hydrogen. The sodium-free mercury is recirculated back into the cell. Cooling of hydrogen is essential to remove any water and mercury. The sodium hydroxide solution is very pure, almost free from chloride contamination and has usually a concentration of 50 %. Further treatment comprises cooling and removal of catalyst and mercury traces by centrifuges or filters. Advantages of the mercury cell process are the high quality of chlorine and the high concentration and purity of sodium hydroxide solution produced. The consumption of electric energy for electrolysis is, however, higher than for the other techniques and a high purity of the feed brine is required. Inherently, the use of mercury gives rise to environmental releases of mercury.
References:
Euro Chlor (2013) An Eco-profile and Environmental Product Declaration of the European Chlor-Alkali Industry, Chlorine (The chlor-alkali process).
technologyComment of potassium hydroxide production (RER):
Potassium hydroxide is manufactured by the electrolysis of potassium chloride brine in electrolytical cells. Hydrogen and chlorine are withdrawn from the cell. The rest of the reaction mixture contains KOH, water, and unreacted potassium chloride. This reaction mixture is then concentrated in an evaporator. Most of the potassium chloride crystallizes by evaporation, and is recycled. After evaporation, the potassium hydroxide is precipitated. Potassium hydroxide, chlorine and hydrogen are obtained from potassium chloride brine according to the following reaction:
2 KCl + 2 H2O -> 2 KOH + Cl2 + H2
Reference:
Jungbluth, N., Chudacoff, M., Dauriat, A., Dinkel, F., Doka, G., Faist Emmenegger, M., Gnansounou, E., Kljun, N., Schleiss, K., Spielmann, M., Stettler, C., Sutter, J. 2007: Life Cycle Inventories of Bioenergy. ecoinvent report No. 17, Swiss Centre for Life Cycle Inventories, Dübendorf, CH.
technologyComment of sodium chloride electrolysis (RER):
Sodium chloride electrolysis