PRODUCTION OF ACTIVATED CARBON FROM ENUGU COAL FOR THE BLEACHING OF PALM OIL
1L. E. Aneke and *2U.S.C. Echegi
1Department of Chemical Engineering, Federal University of Technology, Owerri
2Department of Chemical Engineering, Institute of Management and Technology, Enugu.
Accepted: 11/02/2015
*Corresponding author: [email protected]
Abstract
The bleaching capacities of activated carbons produced from Enugu coal were investigated. Activated charcoal from Coconut shell was used in the bleaching for comparison. The activated carbon precursors were carbonized prior to activation, with phosphoric acid, sulphuric acid and zinc chloride as activating agents. The extent of bleaching was monitored spectrophoto-metrically. The bleaching capacity of the activated carbon was found to be a function of the activation temperature, nature of the activating agent, adsorbent particle size and adsorbent dosage. An optimum activation temperature of 6500C was observed. Activated carbon from phosphoric acid was observed to be most effective in colour reduction followed by zinc chloride and sulphuric acid. It was also established that the essential physical characteristics of the oil were not impaired by the bleaching process.
Keywords: Charcoal, coconut, coal, palm oil, H2PO4
Introduction
Activated carbon has long been a substance of great importance in removal of small concentrations of dissolved substances from liquids. Its application ranges from the treatment of domestic and industrial waters to the production of refined sugar, pharmaceuticals, decolourization of petroleum products, purification of vegetable oils, etc. Recent research interest has ranged from advanced water treatment to use in direct contact with blood as an artificial kidney among others [1].
The refining of vegetable oils in Nigeria is dominated by the application of imported fuller’s earth and other assorted activated clay products. Undoubtedly, the over-dependence on these clay products has hampered the search for alternative source of adsorbents.
Ironically, there are abundant deposits of coal which have the potential to achieve a comparable efficiency as fuller’s earth in the refining of vegetable oils.
However, coal like most products has to be subjected to carbonization and activation processes in order to obtain activated carbon with well developed adsorptive properties [2].
The removal of colouring matter called caroteniod substances from crude palm oil by activated carbon is an adsorption process. The efficacy of an activated carbon in achieving this objective depends on the type of precursor raw material, mode of activation, temperature and time of contact in the adsorption process [2-3-4]. For coal-based activated carbon, one of the most important factors for obtaining high values of adsorptive properties for bleaching process is the mode of activation [1]. The variables for consideration in the mode of activation of a coal are temperature of activation, type of acid employed as activating agent and the concentration of activating agent [2- 5].
In this work, the effects of these variables on the bleaching capacity of activated carbon produced from Enugu coal were investigated. Activated carbon and charcoal from coal and coconut shell samples respectively were used to bleach palm oil and the adsorption efficiency evaluated from the result of the bleaching process.
Materials and Methods Pre-activation Treatment
Enugu coal: The coal samples were obtained from Onyeama mine. 50g of the sample was ground with a shock crusher and grinder. The ground coal was dried in an oven at 900C for 24 hours and later sieved to mesh sieve 500μm.
Coconut shell: The shell was sourced from Eke Market in Ede-Oballa, Nsukka Local Government Area of Enugu State. The carbonization was carried out in line with the procedure described by Smisek et al [4]. 30g of the sample was subjected to carbonization at a temperature of 3000C for 1 hour in a muffle furnace. The carbonized product was dried in an oven for 12 hours.
Carbonization of Coal
10g of the coal sample was further crushed to a size of 200-400μm and placed in retort vessel and put into a heating furnace. It was heated in absence of air at a temperature of 6000C for 1½ hours. The sample was allowed to remain in the furnace for 4 days where it was cooled to room temperature [4-6].
Activation of Coal and Coco-nut Samples
The carbonized coal sample was recrushed to 150- 200μm particle sizes and placed in a galvanized vessel. 0.1M H3PO4 was added to a coal sample in the ratio of 0.15 to 1 part of coal sample and to the mixture was added 10ml of water in similar ratio. The
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mixture was stirred vigorously for 20 minutes using a magnetic stirrer.
The mixture was then heated to 6000C under high pressure in a furnace for a period of 5 hours. The sample was allowed to cool to room temperature and washed with distilled and de-ionized water to remove excess acid. The pH was adjusted between 6 to 8 using 0.5M NaOH. The product sample was dried in an oven at a temperature of 1050C for 1½ hours and stored in an air-tight container for usage. The same procedure was repeated by using 0.25-1.50M H3PO4. Similarly the same method of activation was carried out using activating agents of H2SO4 and ZnCl2.
The activation process was subsequently conducted at varying temperatures of 4500, 5000, 5500, 6000, 6500, 7000 and 7500C at a given concentration of each of
the three activating agents. The same procedure was repeated using carbonized charcoal from coconut shell sample.
Finally, the adsorptive properties of both samples of raw and activated carbon were evaluated as recommended in Huffman et al. and Mattson et al [5- 7]. The surface areas of both samples were determined using the methylene Blue Absorption Test (MBT) method [8].
Bleaching
Prior to the bleaching of the oil, the saponification value, iodine value and acid value of the oil were determined in line with the methods described by Anderson [9].
The bleaching was carried out by weighing out 40ml of degummed palm kernel oil into a conical flask placed in a thermostatic bath. The initial colour absorbance of palm kernel oil was measured with unicam 500 series UV visible spectrophotometer at a maximum wavelength of 400nm.
1.0g of adsorbent (2.5 weight percent of adsorbent) was measured out and mixed with 40ml of palm oil.
The resulting mixture was heated and thoroughly stirred until a temperature of 800C was attained. The temperature was maintained for 40 minutes accompanied by uniform stirring. The spent adsorbent was subsequently filtered out using a fine filter paper and the filtrate, the bleached oil, collected in a beaker.
The final colour of the bleached oil was measured with spectrophotometer and the degree of bleaching of the oil evaluated.
The experimental procedure was repeated by using 2.0g, 4.0g, 6.5g, 9.0g, 12.0g, 15.0g and 20.0g of adsorbent (representing weight percent of 5, 10, 15, 20, 25, 30 and 35 of adsorbent respectively). For each adsorbent dosage, the test was conducted at temperatures of 900, 1000, 1100 and 1200C for constant particle sizes of 150µm and time intervals of 40 minutes.
Finally, the effect of particle size on adsorption was studied by conducting the bleaching procedure with the particle sizes of 200, 180, 150 and 75μm at constant temperature of 110˚C and the same time interval of 40µm.
In this work, two types of adsorbents were studied.
These are coal-based activated carbon (CAC) and activated charcoal from coconut shell (NAC).
Results and Discussion
The results of the analysis of Enugu coal and coconut shell (raw and activated) are given in Tables 1 and 2.
The results show that the physical properties of the adsorbent changed significantly after the activation process. Moisture content, ash content and volatile matter were reduced whereas specific surface area, fixed carbon and porosity increased.
Table 1. Properties of Coal and Coal-based Activated Carbons
Paramet er
Raw coal
Activate d carbon from H3PO4
Activated carbon from H2SO4
Activate d carbon from ZnCl2
Moisture content,
%
9.88 3.53 4.40 5.16
Ash (%
d.b)
16.44 8.37 11.92 10.88 Vol.
matter (% d.a.b)
34.41 20.24 20.24 22.10
Fixed carbon (% d.b)
41.27 67.86 63.44 61.84
Surface area, m2/g
506 745 683 729
Porosity 0.441 0.536 0.503 0.511 Density,
g/cm3
0.944 0.905 0.851 0.911
Table 2: Properties of Raw Coconut Shell and Activated Charcoal from Coconut Shell.
Parame ter
Raw cocon ut shell
Activated Charcoal from H3PO4
Activated Charcoal from H2SO4
Activat ed Charco al from ZnCl2
Surface area, m2/g
611 967 910 948
Porosity 0.493 0.567 0.614 0.553 Density,
g/cm3
0.919 0.861 0.889 0.869
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For instance, the surface area and porosity increased from 506 to 745m2/g and 0.441 to 0.536 respectively for activated carbon produced with phosphoric acid.
The same trend was obtained for the other activating agents investigated. The results from coconut shell sample recorded relatively higher values of these parameters aforementioned. Similar results have been reported by Ukwuoma, and other workers [10-11].
Hence the values of physical parameters are strongly dependent on the mode of activation. This is consistent with previous work [2-6]. The activation process aims at improving the adsorptive properties of adsorbent such as surface area, porosity among others.
Figures i-vi depict the absorbance of the bleached oil.
The results show that there was a relatively rapid adsorption earlier in the run, as measured by percentage colour reduction. This was followed by a decreased rate of adsorption and finally a slow approach to maximum. This trend concurred with the natural phenomenon observed in adsorption [1].
In figure i, the result revealed that there is high percentage colour reduction by all the activated carbon samples produced from different modes of activation. For instance, in the adsorption process where phosphoric acid was used as activation agent, colour reduction rose to a maximum value of 87.36%
at concentration of 0.8M after which it fell gradually with the concentration. Also the performance of activated carbon produced from ZnCl2 in the bleaching process is highly appreciable. The value increased to a maximum of 78.04% but at high concentration of 1.25M. Remarkably, at high concentration of 1.00-1.50M, the degree of bleaching decreased progressively.
The result shows that phos-phoric acid is more effective than zinc chloride for sub-bituminous brand of coal. This is consistent with the work of Hassler, and Mattson, et al [2-5]. The H+ ions of H3PO4 have greater tendency to increase the rate of reaction of volatile matter and the subsequent creation of
porosity. However, it was observed that the result from sulphuric acid H2SO4 deviated from this analogy, and recorded the least value (71.42%) of colour reduction.
The variance in the results obtained from H2SO4 may be attributed to its degradation effect on the structure of coal. Apart from being a ladder to achieve a more effective spacing and structural arrangement of the carbon atoms, some non carbon elements become part of the molecular architecture of the activated carbon.
In addition, they provide a skeleton on which the carbon is deposited, the freshly formed carbon becoming bonded by adsorption forces of the mineral elements. However, ZnCl2 at high concentration causes hydrogen in the source materials to be stripped away as water rather than hydrogen. As reported by Hassler [2].
This reaction with sub-bituminous coal at concentration above 1.20M is highly intense leading to total alteration in size, shape and structural distortion in the arrangement of crystallites.
Figure ii shows the effect of temperature of activation on the bleaching capacity of the activated carbon. In all the cases investigated, it was observed that the bleaching capacity of activated carbon depends on the temperature of the activation as reported by Mattson et al [5].
Table 3: Characterization value of Palm Oil
Character Saponification Value (S.V)
Acid value (V.A)
Iodine Value (I.V)
Raw oil 201.11 7.16 46.71
Oil treated with coal based
activated carbon
197.33 6.18 45.44
Oil treated with
coconut based activated carbon
198.45 7.20 44.92
Legend
H3PO4
H2SO4
ZnCl2
Concentration of acid (M)
Fig. 1: Percentage colour reduction of coal based activated carbon versus concentration of different acids
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At the optimum temperature of 6500C, the activated carbon produced by using H3PO4 recorded the highest value of colour reduction of 90.21%.
However, the values obtained at the optimum temperatures for the other activated carbons were relatively less. A similar trend was maintained as both passed through a maximum and thereafter decreased with temperature. These results are consistent with reports given by Heal and Smisek et al [3-4]. The activated carbon from ZnCl2 gave 80.34% as the highest value of colour reduction at temperature of 6500C. Above this temperature, there was a slight reduction in value recorded. The least value of 71.90% was obtained from the activated carbon produced by using H2SO4 at its optimum temperature of 6000C. Coal prior to activation contains hydrogen in the form of hydrocarbon chains and rings attached to border atoms of the hexagon plates. Much of this hydrogen is removed during activation at higher temperatures, the evolution of this latter portion of hydrogen is paralleled by a simultaneous decrease in adsorptive power. Secondly, during pyrolysis activated carbon crystallites may split into fragments which group to form the thermo-stable aromatic structure existing in the hexagon. At high temperatures, the crystallites are propelled into clusters to form secondary structures which have differences in adsorptive characteristics and other related properties of the carbon.
Figures 3 – 6 show the effect of adsorbent dosage on the bleaching capacity of the adsorbents.
Colour Reduction (%)
4500 5000 5500 6000 6500 7000 7500 8000
Temperature (oC)
Fig. 2: Percentage colour reduction versus temperature of activation.
Legend
H3PO4
H2SO4
ZnCl2
Colour Reduction (%)
Adsorbent dosage (Percentage weight of adsorbent) Fig. 3: Percentage colour reduction of coal based activated carbon versus percentage weight of adsorbent
Adsorbent dosage (Percentage weight of adsorbent) Fig. 4: Percentage colour reduction of coconut based activated carbon versus percentage weight of adsorbent
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The effect was studied by keeping all other experimental conditions constant while varying the mass of adsorbent. The results show that the extent of bleaching of palm oil increased with the quantity of absorbent, but the amount of adsorbate adsorbed per unit mass of adsorbent decreased consider-ably. The decrease in unit adsorption with increasing quantity of adsorbent was basically due to adsorption sites remaining available and unsaturated as adsorption progressed.
The extent of removal of carotenoid substances from palm oil between zero and 20 percent weight of adsorbent dosage in all the parameters investigated indicates the occurrence of maximum adsorption capacity. Above 25% of the adsorbent dosage, the reduction in colour pigment decreased progressively.
This means that bleaching adsorbent dosage above 25% will produce insignificant change in colour and that any further increase in adsorbent dosage will result in un-economical process. Similar results were obtained by previous workers, including Yagwerea, [12] and Dedrick et al [1]. Remarkably, in vegetable oil plants, the bleaching of oils requires considerably less adsorbent than bleaching of the same oil under similar laboratory conditions [10-13]. This is mainly due to effective design of the equipment and process conditions of the plant which create the necessary condition for the optimum utilization of all the contending variables.
Also the analysis of the effect of the particle size on adsorption revealed that the degree of adsorption varies with the particle size of the activated carbon.
The extent of adsorption increased with decreasing particle size of the adsorbent. In general, the intra- particle mass transfer effect increases with decreasing particle size [9-14]. In this work, the highest adsorption capacity was recorded at particle size of 150μm. The value decreased at 75μm particle size, a result that may be attributed to the effect of agglomeration of the particles which is mostly prevalent at very minute particle sizes [12-15].
The results thus indicate that activated charcoal from coconut shell exhibited higher performance in colour reduction of palm oil for all the parameters investigated when compared to coal based activated carbon. These results are in agreement with those earlier reported by Dedrick et al [1].
Table 3 depicts characterization values of the raw and the refined oils. The results show that the oil characteristics as measured by saponification value, acid value, and iodine value were not seriously affected by the bleaching operation. Onukwuli et al [16], obtained similar results for acid activated clay.
This factor is very important in order to ascertain the safety of the oils for human consumption.
Conclusion
Carbon adsorbents produced from Enugu coal and coconut shell were activated using phosphoric acid, sulphuric acid and zinc chloride as activating agents.
Phosphoric acid was found to be the most effective for bleaching of palm oil, while the optimum activation temperature was found to be 6500C.
Adsorbent dosage (Percentage weight of adsorbent) Fig. 5: Percentage colour reduction of coconut based activated carbon versus adsorbent dosage
Colour Reduction (%)
200µm 180µm 150µm 75µm
Adsorbent dosage (Percentage weight of adsorbent) Fig. 6: Percentage colour reduction of coal based activated carbon versus adsorbent dosage
1 2 3 4
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The physical characteristics of palm oil were not affected by the bleaching process. This is an indication that the mode of activation may be employed for commercial processing of palm oil.
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