Development of a New Process for Palm Oil Refining Based on

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Development of a New Process for Palm Oil Refining Based on Supercritical Fluid Extraction Technology Zainuddin A. Manan,* Lim C. Siang, and Ana N. Mustapa Department of Chemical Engineering, Faculty of Chemical Engineering and Natural Resources Engineering, UniVersiti Teknologi Malaysia, Skudai 81310 Johor, Malaysia

This work describes the conceptual development of a new process for palm oil refining using supercritical fluid extraction (SFE) technology. The first step was to model the phase equilibrium behavior of a crude palm oil (CPO)-supercritical CO2 mixture. Next, a new flowsheet structure was synthesized to recover highpurity palm oil and its minor components. The new SFE process was finally simulated using Aspen Plus commercial process simulator version 10.2.1 based on the Redlich-Kwong-Aspen (RKA) thermodynamic model. The results obtained were in good agreement with the pilot plant data reported in the literature. It is envisioned that the development of a new, intensified palm oil refining process that is based on SFE technology can make the refining process drastically simpler and overcome the limitations of the existing technology for palm oil refining. 1. Introduction Palm oil belongs to the family of natural oils and fats and primarily consists of lipid components such as free fatty acids (FFA), monoglycerides (MG), diglycerides (DG), and triglycerides (TG). In addition to the lipid components, oils and fats also contain minor components such as vitamins, sterols, pigments, and hydrocarbons, some of which have received increased attention due to the health benefits they bring. The removal of undesirable impurities such as free fatty acids (FFA), phospholipids, and waxes from oils and fats is an important part of vegetable oil processing prior to its use as food ingredients. Crude palm oil (CPO) is currently refined via physical or chemical refining. Physical refining, which is the more popular and cheaper technique, consists of three major processes (see Figure 1). The first is the degumming step to remove undesired gum, that is, phosphatides; the second is the decolorization or bleaching step to extract color pigment in crude palm oil; and the final step is the deodorization process to get rid of unpleasant odor and taste due to the presence of aldehyde and ketone. Deodorization removes FFA by vacuum steam-distillation at 270 °C to produce refined, bleached, deodorized palm oil (RBDPO) at the deodorizer bottoms’ stream as the final product with less than 0.1% FFA content. During the removal of FFA, valuable nutrients such as tocopherol and carotenes present in palm oil are also destroyed.1 The present technology for palm oil refining applied commercially involves energy and capital-intensive operations. In addition, it faces serious limitations in terms of recovery of valuable minor components in palm oil such as tocopherols, tocotrienols, and carotenoids. Known methods such as liquidliquid extraction and trans-esterification of palm oil to recover carotenoids and tocotrienols typically do not separate the different classes of carotenoids and tocotrienols. For example, carotenoids have been extracted from palm oil, but tocotrienols typically are lost during the extraction process. Likewise, carotenoids are lost during extraction of tocotrienols from oils using liquid hexane as a solvent. Hence, there is a need for the * To whom correspondence should be addressed. Tel.: +6075535609. Fax: +607-5581463. E-mail: [email protected].

development of a new process for selective extraction of highpurity palm oil and its minor components. Application of supercritical fluid technology for extraction and purification of oils and fats has a number of processing advantages over conventional processing methods. These include low-temperature operation, inert solvent, selective separation, and the extraction of high-value product or new product with improved functional or nutritional characteristics. Supercritical fluid extraction potentially forms an interesting alternative to palm oil refining as it combines simultaneous refining and recovery of valuable minor components in palm oil. Removal of FFA from palm oil is relatively easy and can be completed with practically no loss in triglycerides in a countercurrent column.2 Extraction can be accomplished in a few simple steps at relatively low temperature. Note that the operating temperature for an SFE process is determined by the critical temperature of the solvent (e.g., for CO2, the critical temperature is 31.1 °C). Supercritical fluid extraction (SFE) of natural products using CO2 has been studied as an alternative to conventional processes in the oils and fats industry. The use of supercritical fluids in vegetable oil refining dates back to the patents registered by Zosel3 and Coenen and Kriegel,4 who proposed the removal of FFA from oil and a simultaneous improvement of organoleptic characteristics. Ooi et al.1 proved that it was feasible to produce refined quality palm oil by removing FFA in a pilot-scale countercurrent packed column using CO2 as supercritical solvent. Drescher et al.5 investigated the deacidification of palm oil using supercritical fluids with dimethylether as cosolvent and concluded that supercritical CO2 (SC-CO2) was suitable for the removal of FFA in palm oil. Gast et al.6 demonstrated the feasibility of palm oil deacidification and vitamin recovery using supercritical CO2 for a continuous pilot-plant packed column, and studied the effect of reflux ratio and solvent-to-feed ratio on the columns’ operating conditions. Simo˜es et al.,7 who worked on refining of olive oil, concluded that extraction with supercritical fluid did not alter the nutritional quality of the vegetable oil. Other examples on the use of supercritical fluids in vegetable oil refining include the refining of palm oil,1 refining of olive oil,8-10 degumming of soybean oil,11 deacidification of rice bran oil,12,13 and extraction-fractionation and deacidification of

10.1021/ie801735y CCC: $40.75  2009 American Chemical Society Published on Web 04/21/2009

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Figure 1. Process flow diagram for a conventional palm oil refinery.

wheat germ oil.14 In addition to that, the SFE technique has also been investigated to recover oil fractions with high concentrationsofvitamins,especiallycarotenesandtocopherols.6,15-17 To date, commercial application of the SFE technology has been relegated to only special applications due to the high equipment capital cost required. Besides that, the development of processes for natural oils and fats extraction based on supercritical fluid technology was hindered by the lack of suitable design tools and reliable thermodynamic data and models at high pressure. This gap has been bridged by the advances in process equipment design and the development of robust simulation tools and optimization methods for process development. Even though simulation and optimization tools have been widely used for the design of chemical processes, their application has been very limited in vegetable oil processing particularly at supercritical conditions. De Moraes18 simulated and optimized the supercritical extraction of carotene from esterified palm oil using commercial simulator HYSIS. They found that it was possible to obtain high yields of carotenes via two cycles of supercritical extraction. They also obtained ethyl ester (biodiesel) as a byproduct of the proposed process, which can be used as an alternative fuel. Lim et al.19 successfully established the phase behavior of crude palm oil (CPO) in SCCO2 using an Aspen Plus commercial simulator. This Article focuses on the conceptual development of a new process for palm oil refining using SFE technology to recover high-purity palm oil and its minor components. We begin by describing the steps involved in modeling the phase equilibrium properties of the palm oil-SC-CO2 system. This is followed by a description of the newly developed flowsheet that is based on SFE technique. Next, a rigorous steady-state equilibriumstage model for supercritical fluid extraction of refined quality palm oil using CO2 as a solvent is proposed and simulated using an Aspen Plus process simulator. It is envisioned that the development of the new, intensified palm oil refining process that is based on SFE technology can make the palm oil refining process drastically simpler and overcome the limitations of the existing refining technology.

Table 1. Predicted Palm Oil Component Thermophysical Parameters component tripalmitin triolein oleic acid R-tocopherol β-carotene

TB (K)

Tc (K)

Pc (kPa)

864.21a 947.10a 396.82a 879.92a 954.10a 360.15a 646.52a 813.56a 1250.2a 794.52a 936.93a 838.45a 908.58a 1031.1a 678.41a

ω

VL20 (m3/kmol)

1.6500b 1.8004b 0.8104b 1.1946b 1.6255b

0.8906c 0.9717c 0.3172c 0.4533c 0.5348c

a Estimated by the method of Dohrn and Brunner.21 b Estimated using the Pitzer method.22 c Liquid molar volume data at 20 °C obtained from open literature: tripalmitin, triolein, oleic acid;23 R-tocopherol;21 β-carotene.24

2. Methodology 2.1. Thermodynamic Modeling. To design an SFE process, a reliable phase equilibrium database for crude palm oil (CPO) with SC-CO2 must be established. A detailed description of the methodology for development of pure component property database for SC-CO2 and the phase equilibrium model for SCCO2 with CPO and fatty acid components can be found in our previous work.19 2.1.1. Palm Oil Characterization. Palm oil contains various components such as monoglycerides (90%), free fatty acids (3-5%), phospholipids, pigmented compounds, as well as several nutritionally beneficial bioactive compounds.20 In this study, CPO was modeled as a mixture consisting of principally TG (46.5% tripalmitin and 48.8% triolein), FFA (4.5% oleic acid), and valuable minor components (540 ppm of β-carotene and 1000 ppm of R-tocopherol), with an average molecular weight of 846.7 kg kmol-1. 2.1.2. Physical Property Estimation. Estimation of the pure components’ physical properties required by the physical property models was completed by Lim et al.19 using the Property Constant Estimation System (PCES) built-in Aspen Plus prior to simulation modeling of the SFE process. The thermodynamic and physical property parameters for palm oil components employed in this work are summarized in Table 1. 2.1.3. PhaseEquilibriumCalculations.TheRedlich-KwongAspen (RKA) thermodynamic model implemented in Aspen

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Plus was chosen to calculate the phase equilibrium between the palm oil components and the supercritical CO2. The RKA equation of state employs in the following form: P) a)

a RT Vm - b Vm(Vm + b)

∑ ∑ x x √a a (1 - k

b)

a,ij)

(2)

(bibj) (1 - kb,ij) 2

(3)

i j

i

i j

j

∑ ∑xx

i j

i

(1)

j

For ai, an extra polar factor (ηi) fitted from experimental data is used: ai ) f(T, Tci, Pci, ωi, ηi)

(4)

bi ) f(Tci, Pci)

(5)

The RKA thermodynamic model requires the binary interaction parameters (ka,ij and kb,ij) to be specific for the mixture of components treated to account for interaction between the mixture of components. The RKA thermodynamic model made use of temperature-dependent interaction parameters to improve the predicting capability of the model: T (6) + ka,ij ) 1000 T kb,ij ) k0b,ij + k1b,ij (7) 1000 The CPO-SC-CO2 mixture was treated as a pseudobinary system for the phase equilibrium calculations. The RKA model parameters were estimated on the basis of binary phase equilibria information for CPO-SC-CO2 system. The regressed parameters presented by Lim et al.1 were used to predict the phase equilibrium behavior for the system in this work (see Table 2). 2.2. SFE Process Synthesis. A two-step countercurrent SCCO2 extraction process was modeled to remove FFA from CPO and simultaneously enrich tocopherols present in CPO. Separation of FFA in the extract phase from TG was conducted in a countercurrent column (deacidification column). In the countercurrent extraction scheme, the column works as a stripping section where SC-CO2 is the continuous phase entering the bottom of the column and feed oil is the dispersed phase entering at the top of the extraction column. External reflux and multistage countercurrent extraction were used to maximize the recovery of refined palm oil in the raffinate phase. Figure 2 shows the basic flow scheme of a countercurrent extraction column consisting of contacting zones modeled as equilibrium stages. Separation of the highly volatile component (free fatty acids (FFA) and R-tocopherol) from the less volatile component (TG and β-carotene) fractions was attempted first. FFA and tocopherols were then separated from one another. For the largescale continuous SFE process, an external reflux is a more effective way to increase process efficiency.25 The method requires that sufficient extract leaving the cascade is returned as reflux with the remainder being withdrawn from the process as top product. Raffinate is withdrawn from the cascade as k0a,ij

k1a,ij

Figure 2. SFE process flow diagram for deacidification column.

bottoms product, while fresh solvent is fed directly at the bottom of the cascade as shown in Figure 2. 2.3. Process Simulation. SFE process simulation modeling was based on the concept of theoretical stages in which the extractor was modeled as a cascade of flash modules. Each stage of the column was assumed as a single flash at fixed temperature and pressure. The flash separator (FLASH2) unit available in Aspen Plus version 10.2.1 was used to model the cascade of flash modules. The process flow diagram of the SC-CO2 extraction column is shown in Figure 3. Using the developed RKA thermodynamic model, the equilibrium-stage extractor model was simulated to explore the feasible operating conditions for SFE of refined quality palm oil using SC-CO2. A 12-stage deacidification column was simulated on the basis of the equilibrium stage model. Table 3 shows the input and results of the simulation. Sensitivity analysis runs were done to investigate the effect of the number of theoretical stages and solvent flow rate on product purity at different operating conditions. The simulation results were compared to pilot plant data published in the literature to validate the developed process model. In the simulation study, the temperature range of 353-373 K and pressure range of 20-30 MPa were considered. Density differences of the coexisting phases of CPO-CO2 system determined by Machado26 at a temperature range of 333-373 K are shown in Figure 4. 3. Results and Discussion 3.1. Solubility of CPO in Supercritical CO2. Solubility in the supercritical phase is most important in assessing the performance of an SFE process. Figure 5 shows the calculated solubility of palm oil components in supercritical CO2 (under equilibrium condition) at various operating conditions. It was observed that at all temperatures, the solubilities of palm oil

Table 2. Temperature Dependence of Parameters for the RKA Equation of State polar factor, η tripalmitin triolein oleic acid R-tocopherol β-carotene

0.0225T - 9.8778 0.0185T - 9.7834 0.00004T - 1.1515 0.0224T - 7.9068 0.0225T - 8.4999

binary interaction parameters ka ka ka ka ka

) ) ) ) )

0.0007T 0.0005T 0.0004T 0.0009T 0.0001T

+

0.1942, 0.1428, 0.0391, 0.2286, 0.0920,

kb kb kb kb kb

) ) ) ) )

-0.0006T + 0.1952 0.0004T - 0.1635 0.0005T - 0.1655 0.0009T - 0.3305 -0.0016T + 0.4417

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Figure 3. Schematic diagram for SFE process: (a) countercurrent with reflux; (b) equilibrium-stage model. Table 3. Simulation Results of Countercurrent Deacidification Column Based on Equilibrium Stage Model mass frac

PALM-OIL

SC-CO2

EXTRACT

RAFINATE

tripalm triolein tocophrl carotene FFA CO2 total flow, kmol/s total flow, kg/s total flow, cum/s temperature, K

0.4652885 0.4881715 1.00000 × 10-3 5.40000 × 10-4 0.0450000 0.0 0.0774187 60.00000 0.0438504 373.0000

0.0 0.0 0.0 0.0 0.0 1.000000 118.1555 5200.000 9.976305 373.0000

0.4542821 4.95739 × 10-3 4.61564 × 10-3 1.47698 × 10-4 0.5359971 0.0 0.0122001 4.925500 4.5787 × 10-3 383.2644

0.4662703 0.5313843 6.76626 × 10-4 5.75082 × 10-4 1.09366 × 10-3 0.0 0.0652195 55.07478 0.0385093 369.4678

were found to increase with pressure, consistent with the findings of Markom et al.27 Generally, the solubility of palm oil in the equilibrium state is higher than that under continuous processing conditions.1 Thermodynamic equilibrium cannot be reached in an industrial extractor due to the finite contact time between the solvent and the solute. Therefore, the empirical extraction stage efficiency (ηE) must be used to correct the departure of oil solubility from equilibrium under continuous operating conditions:

study, extraction efficiency was defined as a function of solventto-feed (S/F) ratio. Figure 6 shows the computed extraction efficiency based on the experimental solubility data of Simo˜es et al.30 for the edible oil-CO2 system. An increase in the S/F ratio (higher gas flow rates and/or lower liquid feed flow rates) caused a decrease in the extract phase loading due to the lower

solubility (continuous processing) × 100% (8) solubility (equilibrium condition) Reverchon and Osseo28 and Stahl et al.29 suggest a constant extraction efficiency of 60% with respect to equilibrium value in their work involving a soybean oil-SC-CO2 system. In this ηE )

Figure 4. Density difference between CPO and supercritical CO2 phases.

Figure 5. Effect of temperature and pressure on the solubility of palm oil in supercritical CO2 (at S/F ratio of 40).

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Figure 8. Countercurrent extraction (without reflux) of crude palm oil: experimental (Gast et al., 2001) and simulation results (this work).

Figure 6. Loading of oil in CO2 (Simo˜es et al., 2001) and calculated extraction efficiency (this work) as a function of S/F ratio.

extraction efficiency of 58% was assumed. In the simulation run, the FFA content of CPO is assumed to be 2.35 wt % to obtain conceptually consistent and reliable results for comparison. Table 4 shows the comparison between simulation results and experimental work by Ooi et al.1 The calculated maximum deviation from the experimental data was at 0.12% for the predicted FFA content in raffinate phase and 6.19% for yield. These results show that a rather low raffinate recovery is obtained using a simple countercurrent scheme without reflux stream. The results are in good agreement with experimental data except at 27.4 MPa and 323 K. This might be due to the flooding in the studied packed column as the required density difference of the coexisting phases for the CPO-supercritical CO2 system was not complied above a pressure of 25 MPa at temperature of 323 K, as shown in Figure 4. Gast et al.6 performed a pilot study on the separation between the palm oil components in a pilot-plant countercurrent packed column, which consists of both enriching and stripping section. In the experiment carried out by Gast et al.,6 CPO feed flux was kept between 71 and 82 g/h, and the mass flow rate of supercritical CO2 was varied between 2.0 and 4.5 kg/h. The same operating conditions as those studied by Gast et al.6 were rigorously simulated for the extractor. A comparison of countercurrent extraction (without reflux) process simulation and experimental results as those obtained by Gast et al.6 at 370 K and 25 MPa is shown in Figure 8. The content of high volatile components (i.e., FFA and tocochromanols) in the extract and raffinate phases is shown as a function of the solvent-to-feed ratio, respectively. Fatty acids-rich fraction was recovered from the top of the column, and deacidified palm oil was obtained from the bottom of the column. The average deviations of calculated FFA concentration in the extract and raffinate phases to the experimental data were 8.83% and 8.36%, respectively. The simulated behavior of the SFE process under study agrees well with the previous pilot-plant study for the palm oil processing. For the extraction column used in the work of Ooi

Figure 7. Effect of pressure on the yield of raffinate and solubility of palm oil in supercritical CO2 at 50 °C (S/F ratio of 40 ( 5).

amount of liquid available for complete saturation of gas phase.30 Figure 7 compares the experimental solubility data of CPO in supercritical CO2 determined by Ooi et al.1 to that predicted in this work. Extraction efficiency of 60% was assumed for countercurrent extraction process with S/F ratio of 40. Supercritical CO2 flow rate required under continuous operation was less, that is, only 60% of the solvent required under equilibrium conditions. 3.2. Palm Oil Deacidification: Comparison to Pilot Plant Results. Ooi et al.1 adapted the simple countercurrent extraction scheme in an attempt to produce refined palm oil. In the countercurrent process, CPO was continuously fed to the top of the extractor at a rate of 60 g/h, and supercritical CO2 was pumped into the bottom of the column at a rate of 2400 g/h. A simple extractor was rigorously simulated for the conditions of pilot plant studies reported by Ooi et al.1 An

Table 4. Comparison of Experimental (Ooi et al., ref 1) and Simulation Results (This Work) for Countercurrent Palm Oil Processing at 50 °C FFA content of the raffinate pressure (MPa) S/F ratio

27.4 62.8

24.0 58.2

13.7 88.7

10.3 40.0

experimental data simulation experimental data simulation experimental data simulation experimental data simulation FFA (%) carotene (ppm) yield (%) extraction efficiency (%)

0.25 72.0

0.13 865 50.45 57

0.19 ( 0.01 690 ( 10 68.5 ( 3.0

0.23 735 64.26 58

1.21 98.0

1.43 549 98.2 53

2.02 99.5

2.33 540 99.98 63

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Figure 11. Conceptual process flowsheet for the integrated deacidificationenrichment process with supercritical CO2.

Figure 9. Effect of stage number on the recovery of FFA.

3.4. Final Flow Scheme of an Integrated SFE Process for Palm Oil Processing. Summarizing the results of the previous section, the basic conceptual structure of an integrated extraction-separation process can be outlined as presented in Figure 11. CPO (FFA content of 4.5 wt %) was introduced into deacidification column with partial extract reflux. The raffinate phase from deacidification column consists of refined palm oil with FFA content less than 0.1 wt %. The extract phase, which contains mainly FFA and tocopherols, was treated in the enriching column to produce enriched tocopherol fraction. 4. Conclusions

Figure 10. Effect of S/F ratio on the product recovery.

et al.,1 a 4-stage column was used in the simulation, while a column height of 0.61 m was used in the experiment. It was estimated that the HETS (height equivalent to the theoretical stage) for extraction column was around 0.15 m. In the work of Gast et al.,6 an 8-stage was required for the simulation, which corresponds to a HETS of 0.5 m, as a column height (stripping section) of 4 m was used in the pilot study. 3.3. Evaluation of Separation Performance. Once the feasible operating conditions phase equilibria were determined, preliminary evaluation of separation process parameters such as number of stages, solvent-to-feed ratio, product purity, and yield was performed using the Aspen Plus 10.2.1 process simulator. The primary function of the deacidification column is to produce a refined quality palm oil (in the raffinate phase) with FFA content of less than 0.1 wt %. Separation analysis generally resulted in a preliminary estimation for optimum operating condition taking into account that a higher solvent density often yields a higher loading and a decreased selectivity. The proposed SFE process was rigorously simulated at 370 K and 20-30 MPa. Figure 9 shows the effect of stage number on the separation behavior for the CPO mixture for the production of refined palm oil. CPO feed was introduced into the top of the column. For a given operation, the separation performance of FFA removal was improved with increase in the stage number. Figure 10 shows the effect of S/F ratio on the product recovery in the raffinate phase. Recovery of refined palm oil was found to gradually decrease with an increase in the S/F ratio.

The development of a new flowsheet model for palm oil refining using SFE began with modeling of the phase equilibrium behavior of a crude palm oil (CPO)-SC-CO2 mixture. Next, a new flowsheet structure was synthesized to recover highpurity palm oil, which includes its minor components. Using an Aspen Plus steady-state process simulator, a new process for the production of refined palm oil based on SFE technology was successfully designed, simulated, and analyzed. A twostage process was found to be suitable for enriching tocopherols and to remove the FFA from CPO using supercritical CO2 as solvent. In the first extraction column, R-tocopherol and FFA were separated from palm oil TG and β-carotene. The content of FFA of palm oil was reduced from 4.5% to 0.1%. β-Carotene generally remains in the oil phase. The extract phase consisting mainly FFA and R-tocopherol was found to be suitable for additional processing. The steady-state simulation successfully demonstrated the conceptual feasibility of the alternative scheme for the recovery of refined quality palm oil while maintaining the high-value β-carotene and, at the same time, recovering tocopherols. Acknowledgment We would like to thank Universiti Teknologi Malaysia (UTM) for providing the UTM Industrial and Technology Development Fellowship Award. We also acknowledge the provision of Aspen Plus (Release 10.2.1) by Aspen Technology Inc. for research and documentation purposes. Nomenclature AbbreViations EOS ) equation of state FFA ) free fatty acids PFAD ) palm fatty acid distillate RKA ) Redlich-Kwong-Aspen S/F ) solvent-to-feed ratio Symbols a ) cross-energy parameter for a mixture in cubic EOS

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ai ) partial parameter, cubic EOS a(1), a(2) ) coefficients for EOS partial parameter ai b ) co-volume parameter for a mixture in cubic EOS bi ) partial parameter, cubic EOS b(1), b(2) ) coefficients for EOS partial parameter bi ka,ij ) EOS binary interaction parameter ka,ij0, ka,ij1 ) functions in EOS interaction parameter correlation kb,ij ) EOS binary interaction parameter kb,ij0, kb,ij1 ) functions in EOS interaction parameter correlation xi ) composition of component i in a condensed/liquid phase yi ) composition of component i in a supercritical fluid phase P ) absolute pressure PC ) critical pressure R ) universal gas constant ) 8.314 J mol-1 K-1 T ) absolute temperature in Kelvin TC ) critical temperature Ri ) generalized temperature-dependent parameter Rij ) separation factor between components i and j ηE ) extraction efficiency ω ) acentric factor υ ) molar volume Subscripts i,j,k ) denotes pure components i, j, and k ij ) denotes mixture of components i and j

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ReceiVed for reView November 13, 2008 ReVised manuscript receiVed March 27, 2009 Accepted April 1, 2009 IE801735Y