P hase b ehaVior of me-DoD

During the measurement of phase equilibrium, the realistic system of ME-DOD was taken as the 10.19% (tocopherol mass fraction) feed composition. Also, the ME-DOD system was the raw materials for concentrating natural tocopherols; thus, its phase behavior is discussed separately. Figure 4.17 shows the influence of pressure and temperature on the separation factor.

1.0 0.8 0.6 0.4 0.2

0.00.0 0.2 0.4

x₁ y1

0.6 0.8 1.0

Figure 4.16  Equilibrium lines at P = 20 MPa: x1’, mass fraction of methyl oleate in liquid (CO₂ free basis); y₁^’, mass fraction of methyl oleate in gas (CO₂ free basis); ⦁, experimental data at T = 353.15 K; ◾, experimental data at T = 333.15 K; ^▲, experimental data T = 313.15 K;

—, correlated at T = 353.15 K; ---, correlated at T = 333.15 K; ····, correlated at T = 313.15 K.

(From Fang, T., Goto, M., Sasaki, M. and Hirose, T., J. Chem. Eng. Data, 50, 390, 2004.

With permission.) 0.8 0.6 0.4 0.2

0.00.0 0.2 0.4

x₁ y1

0.6 0.8 1.0

Figure 4.15  Equilibrium lines at T = 313.15 K: x1’, mass fraction of methyl oleate in liquid (CO₂ free basis); y₁^’, mass fraction of methyl oleate in gas (CO₂ free basis); ⦁, experimental data at P = 10 MPa; ◾, experimental data at P = 20 MPa; ^▲, experimental data P = 29 MPa;

—, correlated at P = 10 MPa; ---, correlated at P = 20 MPa; ····, correlated at P = 29 MPa.

(From Fang, T., Goto, M., Sasaki, M. and Hirose, T., J. Chem. Eng. Data, 50, 390, 2004.

With permission.)

The trends illustrated in Figure 4.17 are similar to those in Figure 4.13 and Figure 4.14. Noticeably, the separation factors at pressures lower than 20 MPa are relatively small. For instance, at 313.15 K, the separation factor remained lower than 0.2 for all pressures lower than 15 MPa. As pressure increases, the separation factor greatly increases, reaching 0.35 at 20 MPa. The increase of temperature offsets the effect of pressure to some extents. On the basis of the property, a separation strategy seems to be reasonable and feasible. A fractionation column is necessary for the ME-DOD liquid system. First of all, low pressure (15 to 20 MPa) was used in combi-nation with a temperature distribution in the column to separate FAMEs like methyl oleate. Then the pressure was increased to separate tocopherol from other impurities.

This procedure needs to be verified by a fractionation operation in which operation parameters can be determined and optimized.

During our experiments, some phenomena of the realistic system of ME-DOD + CO2 were observed through the visual equilibrium cell. Figure 4.18 shows the liquid-gas interface changes that occur as pressure increases. The interface increasingly changes from clear to obscure, with the interface finally disappearing at high pres-sure about 29 MPa. At high pressure, a critical point probably exists that causes the whole system to become entirely miscible. In this situation, the liquid and gas com-positions are identical and the separation factor equals unity. It should be noticed that the disappearance of the interface does not happen suddenly. The change is a progressive process and an accurate endpoint is difficult to determine by visual observation. The critical pressure at 313.15 K is approximately estimated in the pressure range from 27.8 to 29.0 MPa by visual observation.

Figure 4.19 shows the changes in the feed situation when ME-DOD was charged by pump into the equilibrium cell at different pressures and at a constant 5 mL/min.

At low pressure (5 MPa), ME-DOD could smoothly flow into the equilibrium cell, but at high pressures, the charged ME-DOD resembles drops or fog. Moreover the rate for flowing downward at a high pressure was slower than that at low pressure.

0.4

0.3

0.2

0.1

10 15 20 25

p/MPa

S

Figure 4.17  Separation factor (S) of ME-DOD in supercritical CO2: ⦁, experimental data at T = 313.15 K; ◾, experimental data at T = 333.15 K; ^▲, experimental data T = 353.15 K.

(From Fang, T., Goto, M., Sasaki, M. and Hirose, T., J. Chem. Eng. Data, 50, 390, 2004.

With permission.)

We hypothesized that the main reason for this was the smaller difference in density between ME-DOD and supercritical CO2 at high pressure; for example, at 20 MPa and 313.15 K, the density of CO2 is 0.840 g/mL and that of ME-DOD is 0.865 g/mL.

Such a small difference in density is likely to cause the drop or fog phenomenon, even though the clear interface between liquid and gas remained. This phenomenon should be considered when designing a continuously countercurrent operation.

4.7 separation with superCritiCal Co₂ FraCtionation As described in section 4.1, the important step in concentrating natural tocopherols from ME-DOD is to remove the FAMEs, which contribute more than 70% of ME-DOD. FAMEs are important chemical materials in biofuel, metal-cutting oil, and cleaning agent production, as well as in the synthesis of other fatty acid products [17]. In section 4.6, the fundamental research on ternary and realistic phase equilibria has established a preliminary separation strategy, which must be tested through a supercritical CO2 fractionation operation.

A fractionation column is necessary for the ME-DOD liquid system. First, low pressure (the initial pressure) is used in combination with a temperature gradient along the column to separate the FAMEs. Then, the pressure is increased to separate the tocopherols from other impurities.

20 MPa 25 MPa 27 MPa 29 MPa

Figure 4.18  The interface between liquid and gas at T = 313.15 K. (From Fang, T., Goto M., Sasaki, M., Hirose, T., J. Chem. Eng. Data, 50, 390, 2004. With permission.)

P = 5 MPa P = 20 MPa P = 25 MPa

Figure 4.19  Feed situation of ME-DOD at different pressures (feed rate = 5 mL/min and T = 313.15 K). (From Fang, T., Goto, M., Sasaki, M. and Hirose, T., J. Chem. Eng. Data, 50, 390, 2004. With permission.)

4.7.1 fraCtionation aPParatUsanD ProCeDUre

A fractionation system was rebuilt from a supercritical CO2 apparatus [43]. The experimental setup consisted of a countercurrent contact column (2.4 m × 20 mm

Before each run, about 120 g ME-DOD was charged into the column at 40±1 g/h so that an abundance of raw material had accumulated at the column bottom. Consequently, the fresh CO2 fluid could come into contact with sufficient ME-DOD at the start of the fractionation operation. This ensures that each experi-ment began at a steady-state condition, which means both the feed and top fraction flow in a continuous situation with relatively stable flow rates. Fresh CO2 was charged into the column through valve 14 (V14), the pressures of the column and separator were adjusted by BPR 1 and BPR 2, respectively. The temperature gradient of the column was concurrently adjusted by eight proportional integral differential controllers. The separator conditions were maintained at 3.8 to 4.0 MPa and 333 K.

When pressure and temperature reached the required values, CO2 was introduced from the column bottom by opening V15 and simultaneously closing V14, indicating

(Continuous)Feed

P2

Windows Gas Meter 1

V16 Raffinate Dixon Packing

Extract

Extraction Column Heater

Cooler CO2

CO2

Figure 4.20  Schematic diagram of fractionation apparatus. (From Fang, T., Goto, M., Wang, X., Ding, X., Geng, J., Sasaki, M. and Hirose, T., J. Supercritical Fluids, 40, 50, 2007.

With permission.)

taBle 4.2 result of the scale-up pretreatment process Material and  productsMass (kg)FFa (%)aFaMes (%)bglycerides (%)atocopherols (%)csterols (%)b ContentrecoveryContentrecoveryContentrecovery DOD353.648.1027.21009.231009.45100 PME-DOD*351.51.253.5822.381.59.1999.09.51100.2 Crude sterols47.6//////54.777.9 ME-DOD310.80.771.286.119.710.1997.02.725.2 *: “Partly methyl esterified DOD,” which is obtained by methyl esterification a: Determined and calculated by the AOCS methods [18] b: Determined and calculated by GC analysis [21] c: Determined and calculated by HPLC analysis [21]

ME-DOD feed location was changed by switching on/off V11, V12, and V13. Addi-tionally, to achieve different solvent-to-feed (S/F) ratios, the flow rate of ME-DOD was maintained at a constant 40±1 g/h while the flow rate of CO₂ was varied by adjusting micrometering valve 1 (MV 1). The total maximum feed used for each run in crude sterols and ME-DOD was larger than 100% because some sterols were were methyl palmitate (14.57%), methyl linoleate (39.23%), and methyl oleate

Retention Time (min)

Abundance

5.00 10.00

11.26

9.62 11.17

10.40 16.2116.93 16.06 15.87 TIC : ME-DOD.D

28.54

21.17

26.21 29.40 29.13 31.47

30.51 27.44

23.24

15.00 20.00 25.00 30.00

Figure 4.21  GC-MS TIC chromatography of ME-DOD. (From Fang, T., Goto, M., Wang, X., Ding, X., Geng, J., Sasaki, M. and Hirose, T., J. Supercritical Fluids, 40, 50, 2007. With permission.)

(21.16%), and the three compounds made up 90.14% of all FAMEs. Because some compounds with high molecular weight, such as glycerides, sterol esters, pigments, and wax, could not be identified with the current analyses, the area percentages of compounds were not accurate and could not be used for quantification. Thus, HPLC and GC-FID were employed for determining the contents of tocopherols, sterols, and FAMEs, respectively. Table 4.3 also shows the analysis data obtained by HPLC and

Composition determined by gC-Ms peak no.

rt  (min)

area 

(%) Compounds trivial name of FaMe

1 9.62 1.08 Dodecanoic acid, methyl ester Methyl laurate (C12:0) 2 10.40 0.72 Tetradecanoic acid, methyl ester Methyl myristate (C14:0)

3 11.18 0.40 Unidentified /

4 11.26 14.23 Hexadecanoic acid, methyl ester Methyl palmate (C16:0) 5 15.87 39.23 9, 12-octadecadienoic acid, methyl ester Methyl linoleate (C18:2) 6 16.06 21.16 9-octadecenoic acid, methyl ester Methyl oleate (C18:1) 7 16.21 1.67 7-octadecenoic acid, methyl ester Methyl oleate (C18:1) 8 16.93 3.73 Octadecanoic acid, methyl ester Methyl stearic (C18:0) 9 21.17 0.37 13-docosenoic acid, methyl ester Methyl brassidate (C22:1) 10 23.25 0.57 Docosanoic acid, methyl ester Methyl behenate (C22:0)

11 26.21 3.55 Squalene

12 27.44 2.28 δ-tocopherol

13 28.54 6.98 γ-tocopherol

14 29.13 0.74 β-tocopherol

15 29.40 1.18 α-tocopherol

16 30.51 0.69 Campesterol

17 30.82 0.33 Stigmasterol

18 31.47 1.09 β-sitosterol

tocopherols (%) determined by hplC and isomers’ percentage

Tocopherols (%) α-Tocopherol β and γ-Tocopherols δ-Tocopherol

10.19 12.05 61.57 26.38

sterols (%) determined by gC-Fid and isomers’ percentage

Sterols (%) Campesterol Stigmasterol β-Sitosterol

2.71 32.11 23.18 44.71

FaMes (%) determined by gC-Fid and Main FaMes’ percentage

FAMEs C16:0 C18:2 C18:1

71.28 17.66 45.52 28.28

Source: Fang, T., Goto, M., Wang, X., Ding, X., Geng, J., Sasaki, M. and Hirose, T., J. Supercritical Fluids, 40, 50, 2007. With permission.

GC-FID, in which the contents of tocopherols, FAMEs, and sterols were 10.19%, 71.28%, and 2.71%, respectively. Noticeably, most FFA and glycerides were con-verted into FAMEs and most sterols in DOD were removed through the pretreatment

For the greatest degree of tocopherol enrichment inside the column, the initial pressure was investigated so that most of the FAMEs were extracted with little tocopherol content. In the first 2 hours, about 80 g ME-DOD was charged into the column and the operation was in continuous countercurrent fractionation mode, where CO₂ was the continuous phase and the feed oil was the dispersed phase. After feeding, the operation was changed to batch fractionation mode. The extracted frac-tions were collected in the separator. Every 30 minutes or 1 hour, the fractions were removed from the separator and weighed until the total yield from the separator reached about 70 wt.% (140 g) of the total feed (200 g). At that point, the experiment was terminated.

The phase equilibrium data in section 4.6.4 indicated that the separation factor between FAMEs and tocopherols change markedly from 15 to 20 MPa. Conse-quently, pressures of 14, 16, and 18 MPa were investigated for these experiments, while other operation parameters were kept at similar values throughout. The col-umn temperature gradient was set in a linear distribution from 313 K at the bottom to 348 K at the top, the S/F ratio was adjusted to 75 (the flow rates of CO2 and ME-DOD were 3±0.05 Kg/h and 40±1 g/h, respectively), and the feed location was V13.

Figure 4.22 shows that the extraction yield of FAMEs was greatly influenced by the initial pressures, as higher pressure led to higher solubility and faster extraction.

For instance, the extraction yield at 18 MPa reached more than 70% in 2.5 hours with about 7.5 Kg CO₂, while at 14 MPa, it took far more time (10 hours) and more CO₂ (30 Kg) for the extraction yield to reach the same level. On the other hand, higher the average oil loading (AOL) were used for evaluating the separation efficiency, with the former standing for the purity of the FAME product and the latter repre-senting the process velocity. AOL is defined as the ratio between the total oil mass collected and the CO2 consumed over a given time. Practically, AOL can be calcu-lated from the slopes of the yield curves shown in Figure 4.22, and the results are listed in Table 4.4. As pressure increased, AOL increased from 0.48 g/100g CO₂ at 14 MPa to 1.97 g/100g CO2 at 18 MPa. Similarly, ATC also increased, indicating that more tocopherols were solvated with the FAMEs in the supercritical CO2. This