Single-step conversion of synthesis gas into formaldehyde
thesisposted on 02.03.2017 by Bahmanpour, Alimohammad
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Formaldehyde is known as the building block in many industries including resins, polymers, paints and adhesives. It is widely used in furniture and wood processing. The annual production rate of formaldehyde is in the range of 30 million tons globally and the demand of formaldehyde has grown by 2-3 % per year over the past two decades. Industrially, formaldehyde is produced via methanol partial oxidation. Methanol in turn is produced from synthesis gas which is produced via steam reforming of natural gas. Both methanol synthesis and steam reforming process suffer from high exergy loss due to high temperature processes and large purification units. Considering the large quantity of formaldehyde produced in the world, when combined with the high exergy losses, leads to high energy losses and CO2 emissions globally. Therefore, alternative method for the production process of formaldehyde is needed. This project develops a novel method of formaldehyde production by using equimolar quantities of carbon monoxide and hydrogen in a catalytic slurry phase reaction. Promoted nickel-based catalysts – Ru-Ni/Al2O3 and Pd-Ni/Al2O3 were used in this project due to their high activity in hydrogenation reactions. It was observed in this project that formaldehyde yield was significantly higher in the slurry phase (4.6 mmol.L-1.gcat -1) compared to the gas phase (8.2×10-3 mmol.L-1.gcat -1). Thermodynamics analysis showed that the reaction is equilibrium limited in the gas phase whereas it is kinetically limited in the liquid phase. It was found that in the slurry reactor increasing the temperature increases the reaction rate but the formaldehyde yield peaks at 353 K, above which the yield reduces. This is in agreement with the fact that the equilibrium constant of the reaction decreases as the temperature increases. Increasing pressure and stirring speed increased the formaldehyde yield. The reaction mechanism was investigated based on deuterium labelling technique. It was shown that the reactant gases dissolve in the solvent and are adsorbed on the catalyst surface. The adsorbed gases react on the catalyst surface to form formaldehyde which desorbs and immediately hydrates in aqueous conditions to form methylene glycol. The effect of solvents with high CO and H2 solubility was investigated in this study since it was found that increasing the solubility of the gases was important to achieve higher production. However, it was observed that in addition to the gases solubility, formaldehyde yield also depends on the reactivity of the solvents with desorbed formaldehyde. The last step is important to shift the equilibrium of the CO hydrogenation reaction. In a nutshell, higher pressures were in favour of the reaction but the operating pressure in this study was limited to 100 bar by the maximum available gas cylinder pressure. The most suitable solvent was found to be methanol since it has higher solubility of CO and H2 compared to water, and similar to water, it also reacts with formaldehyde to shift the equilibrium. Highest yield was achieved by using pure methanol as a solvent at 363 K and 100 bar, which resulted in formaldehyde yield of 15.58 mmol.L-1.gcat -1.