Contents - Akademi Sains Malaysia

ASM Science Journal, Volume 7(1), 2013during that temperature, all pre-asphaltene and asphaltenewere converted into lighter molecular components ofoil+gas. Therefore, it was definitive that the coal extractiontended to pre-dominate at lower liquefaction temperature,with radical reaction which started to occur at liquefactiontemperature at or above 400ºC. According to Prasassarakichet al. (2007), increasing the temperature above 410ºC willcause a decrease in liquid yield. This was because whenusing high reaction temperature, the liquid was fragmentedand formed free radicals and was stabilized by hydrogento form the gas phase. However, further increasing of thetemperature would result in more gas formation and lessoil. Hence, it was suggested that 420ºC was the optimumtemperature which resulted the high coal conversion andoil yield.Effect of reaction time. The effect of reaction time oncoal conversion and product yields of MB coal was alsoinvestigated and the results are shown in Figure 4 andTable 2. Apparently, the percentages of coal conversion ateach different reaction time were relatively the same. Therewas a slight difference in the oil+gas yield as the reactiontime increased. The data indicated that the reaction timeaffected only a little in enhancing the total conversion.A fair amount of coal conversion and oil+gas yield wereobserved with the lesser amount of reaction time whichalso could be improve by the introduction of catalyst in theliquefaction process. The less reaction time also meant lesssolvent used which is economical compared to the amountof solvent used with a longer reaction time.Effect of solvent flow-rate. Another significant parameterthat could improve in the development of coalliquefaction process is solvent flow-rate. In an attemptto achieve high coal conversion and oil+gas yield, it isimportant to ensure that enough fresh solvent is presentduring the extraction process. One option is by providingsolvent into the reactor continuously during the liquefactionprocess. The effects of solvent flow-rate are shown inFigure 5 and Table 2. Generally, the coal conversions forthree different flow-rates were relatively the same. Thepercentage of asphaltene increased while the percentage ofpre-asphaltene was found to decrease with the increasingof solvent flow-rate.These results indicated that by enabling the solvent toflow during liquefaction process, it was possible to extractmore components out of the coal with high percentage ofcoal conversion (~80%) regardless of the different flowrateof the solvent used. Furthermore, the presence offresh hydrogen-donor solvent during the reaction processstabilized most of reactive radical species to form lowmolecular weight products such as asphaltene and preasphaltene,and also the light products of oil+gas. In otherword, with a sufficient supply of fresh donor solvent duringcoal liquefaction, most of the asphaltene and pre-asphaltenewere converted into oil+gas.Similar results for the amount of solvent on coalconversion were reported by Simsek et al. (2001), wherethe extent of conversion of coal depends on how the radicalsare being stabilized. Thus, the liquefaction yield would behigh if there was sufficient hydrogen available, while theliquefaction yields would be low if there was insufficientavailability of hydrogen which would cause the radicals toundergo re-polymerisation. The liquefaction performed witha high solvent flow-rate produced a low percentage of oilyield and a high percentage of asphaltene+pre-asphaltene.This might be due to the extraction during liquefactionwhich only dissolved, due to the less time, for the solventto fully interact with the coal radical. In other words, lesshydrogen was transferred during the liquefaction process,thus resulting in higher molecular weight compounds,which in this case is asphaltene and pre-asphaltene. Hence,more extensive formation of extractable materials wasobtained at a lower solvent flow-rate.Liquefaction Using Two-stage Solvent Flow ReactorSystemThe two-stage reactor was introduced to increase thepercentage of oil yield. The second reactor was filled withNiSiO 2 as the liquid tar passed through after the extractionprocess in the first reactor. In order to maintain the conditionfor both reactors, several parameters in the second reactorsuch as temperature, pressure and solvent flow-rate werefixed at 420ºC, 4 MPa and 1 ml/min, respectively. Theeffects of Ni loading on the product distribution yieldsare shown in Figure 6. From Figure 6, it was clear that theoil+gas yield increased as the percentage of Ni loadingincreased. Higher molecular weight compounds such as preasphalteneand asphaltene decreased and were convertedinto lower molecular weights compound, that is, oil+gas.The second reactor did not affect the total conversion,thus, there was no difference in the total conversion of thecatalyst.Liquefaction was also conducted isothermally for aspecific duration of time. Using the two-stage reactorsystem, the percentage of asphaltene and pre-asphaltenedecreased and this correlated with the increased oil+gasyield. The assumption was that both asphaltene and preasphaltenehave been converted into oil+gas. Generally,with the sufficient amount of hydrogen available, these freeradical species generated from the coal cracking processcombined with hydrogen to yield stable species such asasphaltene, pre-asphaltene and oil. The asphaltene and preasphaltenewould further fragment to smaller molecularweight radical species, and then the hydrogenation processstabilized it to form oil+gas, a light molecular weightproduct.The use of catalyst also increased the oil+gas yield.Both the hydrogenation reaction and hydrocracking ofcoal molecule occurred with the availability of catalyst.12