EFFECT OF RETENTION TIME

The effect of retention time on bio-crude oil characteristics is shown in Figure 9-5 and Figure 9-6. The relative concentrations of different major compound classes are represented by the percentage of their peak areas to the total peak area.

Figure 9-5 Effect of retention time on bio-crude oil characteristics (Chlorella, 240°C)

At the two temperature levels, fraction of organic acids decreased with the increase of retention time, while fraction of amides and N&O heterocyclic compounds increased with retention time increasing. On the other hand, effect of retention time on fraction of cyclic oxygenates was not pronounced. As defined previously, cyclic oxygenates include phenols, phenol derivatives and fused ring compounds. It was already found that the fraction of cyclic oxygenates was increasing with reaction temperature (Figure 9-3). Therefore, the relative concentrations of the cyclic oxygenates were less influence by retention time compared to reaction temperature.

Figure 9-6 Effect of retention time on bio-crude oil characteristics (Chlorella, 280°C)

The N&O heterocyclic compounds, such as pyrazine and derivatives of pyrrolidine are produced from the recombination or repolymerization of the small molecules produced from degradation of protein, lipid and carbohydrates. Under the two temperature levels, the fraction of pyrazine, which represents the product produced from recombination of amine and straight oxygenates with a short carbon chain; and the fraction of derivatives of pyrrolidine, which represents the product produced from recombination of pyrrolidine and fatty acid with a long carbon chain, were both increased. Meanwhile, higher relative concentrations of complicated compounds produced from recombination also implies that more small compounds were produced via degradation of protein, lipid and carbohydrates. For example, the scission of C-C bond in amino acids and the recombination of amino acid with organic acids could happen simultaneously. Degradation pathways of amino acids, including decarboxylation and deamination, could occur simultaneously under hydrothermal conditions with elevated temperature and prolonged retention time. Additionally, the hydrolysis of amino acids under hydrothermal conditions may result in interconversion among amino acids (Sato et al., 2004).

Figure 9-7 Effect of retention time on TIC of aqueous product at 240°C (Chlorella)

Figure 9-7 shows the effects of retention time on the aqueous product produced at 240°C with different retention times. For the two detected major peaks, which eluted time are 21.99 minute and 28.44 minute, respectively, there are two molecules associated with each peak. The

240 C, 0min 240 C, 10min

240 C, 30min 240 C, 60min

240 C, 120min 1

1

1 1

1 2

2

2 2

2 3

3

3 3

3

4 4

4

4

4 5

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5 5

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peak with 21.99 minute eluted time represented both phosphoric acid and glycerol; and the peak with 28.44 minute eluted time represented both proline and pyroglutamic acid. Other than the two major peaks, it was found the relative concentrations of five major compounds (Peak No.1-5 in Figure 9-7) changed with prolonged retention time. The No.1-5 peaks represent alanine, glycine, pyrimidine, adenine and inositol, respectively. Their structures and compound formulas were already summarized in Table 9-3. Figure 9-8 shows the changes of relative concentrations of alanine, glycine, pyrimidine, adenine and inositol with increase of retention time at 240°C.

Figure 9-8 Effect of retention time on relative concentrations of 5 compounds in aqueous product (Chlorella, 240°C)

It was found the relative concentrations of alanine and glycine first increased with retention time, and then their fractions decreased with further increase of retention time. The highest fraction of glycine (5.51%) and alanine (3.74%) was obtained at 10 and 60 minutes retention time, respectively. Alanine and glycine are two of the simplest amino acids produced from protein hydrolysis. The results in Figure 9-8 indicate the increase of retention time could benefit protein hydrolysis to produce amino acids first, but increasing additional retention time could lead to amino acid degradation or reacting with other intermediates.

Fractions of pyrimidine and adenine were both decreased with retention time. It implies both of the two compounds either degraded or reacted with other intermediates by prolonging retention time. The instability of adenine under hydrothermal conditions with increasing retention time found in this study is consistent with results in the literature (Franiatte et al., 2008).

Inositol (peak No. 5 in Figure 9-7), which is not a common sugar, belongs to carbohydrates. Its fraction decreased first with retention time and then started to increase with the further increase of retention time. Since the carbohydrates contribute about 22.0% of the total weight of C.

pyrenoidosa (Table 5-1), it could be released from hydrolysis of carbohydrates under hydrothermal conditions. The decrease of the inositol fraction could confirm the Maillard reaction occurrence when microalgae are converted under hydrothermal conditions.

Figure 9-9 shows the effect of retention time on the aqueous product TIC at 280°C. The relative concentrations of five major compounds (Peak No.1-5 in Figure 9-9) changed with prolonged retention time. The No.1-5 peaks represent alanine, (2Z)-(Hydroxyimino) acetic acid, glycine, adenine, and inositol, respectively. Their structures and compound formulas were summarized in Table 9-3. The fraction of pyrimidine detected at 240°C was negligible at 280°C.

The trend of the alanine fraction change was found similar to that at 240°C: it first increased with retention time, and then started to decrease after its relative concentration reached the maximum. However, the fraction of glycine decreased with the increase of retention time all the time. At 280°C with 120 minutes retention time, the relative concentrations of alanine and glycine were both negligible, which indicates both of the two amino acids have degraded or reacted with other intermediates. This result is consistent with that obtained at 240°C but it might be concluded that the production of glycine was already completed even with 0 minute retention time at 280°C. (2Z)-(Hydroxyimino) acetic acid is a derivative probably produced from acetic acid and ammonia. The increasing of its fraction implies some interactions between reaction intermediates and ammonia could be promoted by prolonging retention time. Since the ammonia is the inorganic nitrogen in the aqueous product produced from deamination of amino acids, this finding suggests that the inorganic nitrogen in the aqueous product could be converted into organic nitrogen under certain hydrothermal conditions.

Figure 9-9 Effect of retention time on TIC of aqueous product at 280°C (Chlorella)

Figure 9-10 shows the changes of relative concentrations of alanine, (2Z)-(Hydroxyimino) acetic acid, glycine, adenine, and inositol with increase of retention time at 280°C. The fraction

280 C, 0min 280 C, 10min

120 minutes, the relative concentration of adenine in the aqueous product was negligible. At the same time, the trend for the inositol fraction change was different from that at 240°C. The fraction of inositol decreased first with prolonging of retention time and then started to increase at 240°C, probably due to the production rate of inositol was faster than the consumption rate at 240°C. However, the consumption rate could exceed the production rate at 280°C due to the more intensive reaction conditions. No matter under which assumption, as soon as the fractions of carbohydrates and amino acids decreased, it might be concluded the interaction between them could happen, which confirms the Maillard reaction pathway is highly possible under experimental conditions.

Figure 9-10 Effect of retention time on relative concentrations of 5 compounds in aqueous product (Chlorella, 280°C)