In the third project year, numerous glycerol reforming experiments were performed to determine the dependence of glycerol conversion on the process conditions. Furthermore, it is important to obtain insight in the composition of the gas produced as a function of process conditions, because it is subsequently used in the methanol synthesis process and methanol yields depend strongly on its composition.
Another important aspect which was investigated is the influence of alkali in crude glycerol on the gas compositon. This was done by comparing the results of reforming experiments with pure and crude glycerol.
An overview of the influence of the process conditions on the glycerol conversion is shown in the table below. The experiments were performed in the following ranges: T = 500 – 700 ºC, [feed] = 5 – 20 wt%, t = 7 – 173 s, P = 250 – 280 bar.
The pure and crude glycerol conversion correlates positively with the temperature and the residence time. The feed concentration has a negligible effect in the conversion of pure glycerol, while it has a negative influence on the conversion of crude glycerol at higher concentrations. The conversion is independent of the pressure within the range studied. The effect of alkali on the conversion is rather small, except for high feed concentration.
Contrary to the conversion, the gas composition was strongly dependent on the process conditions. The concentration ranges of main components are shown in the table below. It is obvious that a wide variety of gas compositions can be obtained. The influence of alkali is more pronounced on the gas composition than on the conversion. The presence of alkali promotes the water-gas shift reaction and leads to higher H2 and CO2 concentrations and a lower CO concentration, which is unfavorable for methanol synthesis. The presence of alkali has no influence on the hydrocarbon concentration.
The research on glycerol RSCW will be continued by optimizing the process conditions to produce a gas with an attractive composition for methanol synthesis.
Furthermore, a process to remove the alkali from the crude glycerol will be investigated.
Removal of salt from glycerine waters
Crude glycerol consists of glycerol, water and salts (mainly NaCl). Crude glycerol as feedstock for reforming in supercritical water (RSCW) needs a pretreatment step to remove its salt content. The solubility of NaCl is very low at reforming conditions (T = 650 ºC and P = 250 bar) and may cause reactor plugging.
There is a limited amount of data available on the two-component system of aqueous salt solutions at supercritical conditions (for water). Baierlein (PhD thesis, 1962) reports experimental work which shows that aqueous salt solutions form two separate phases when heated to supercritical conditions, viz. a supercritical steam phase, containing some salt, and a liquid brine phase. According to Baierlein, the steam phase at 420 ºC and 274 bar, contains 0.11 wt.% salt and the brine contains 20.1 wt.% salt.
No data were found on the three-component system water-glycerol-salt.
A process flow diagram was constructed for salt separation from crude glycerol by treatment at 420 ºC and 274 bar.
The following assumptions were made:
- the system forms a liquid and a supercritical steam phase;
- glycerol is volatilized completely and mixed with supercritical steam;
- glycerol is thermally stable at this temperature and pressure;
- the salt content of the supercritical steam and the brine is not effected by glycerol;
The supercritical steam phase, upon condensation, forms an aqueous solution with the following composition: 82.5 wt.% water, 17.3 wt.% glycerol, 0.091 wt.% salt
Modelling supercritical reforming of crude glycerol
A conceptual process flow diagram (PFD) was constructed with crude glycerol as feedstock. In the PFD, the salt separation was integrated and effluent water was recycled. The reforming chemistry was assumed not to be affected by salt and equal to the reforming chemistry of pure glycerol.
The results from these calculations will be verified by experiments in the coming period focusing on:
- Salt removal by pretreatment of crude glycerol;
- RSCW of crude glycerol and crude glycerol after salt separation;
- Consequences of effluent water recycle
Removal of CO2 from glycerol derived gas
The product gas, after glycerol reforming, needs conditioning to optimize methanol synthesis, i.e. decreasing the CO2 content to a few vol.%.
Three conceptual process flow diagrams (PFD) were constructed for CO2 separation from the gas at 274 bar, each with different absorbent, viz. aqueous mono-ethanol-amine (MEA), methanol and water.
MEA is very selective and absorbs only CO2; the other components remain in the gas phase. CO2 is recovered by distilling the solution at 80 bar. The vapour, consisting of steam and CO2, is condensed and CO2 is recovered as a high purity liquid. Depending on further use, additional purification – especially the removal of residual dissolved MEA – may be needed.
As opposed to MEA, methanol is a system-friendly component. Methanol is less selective for CO2 than MEA. At 274 bar and 23 ºC, methanol absorbs CO2 almost completely, but also some H2, CO, CH4, and C2H6 are absorbed. The purity of the CO2 recovered is only 50 %. This flow is therefore mixed with the hydrocarbon-containing effluent gas from the methanol reactor and fed to a (high pressure) reformer. The reformer gas is recycled to the methanol reactor.
Another solvent option is water at 274 bar. Water shows strong selectivity towards CO2. The high pressure of the system enhances this effect. Because of its good performance and low cost, high pressure water absorption is – at this moment in time – the preferred process for methanol gas conditioning.