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A method of processing waste gases from the drying of olive press-cake
Biological Wastes 24 (1988) 137-145
A Method of Processing Waste Gases from the Drying of Olive Press-cake D i m i t r i s Papaioannou 36 Ebrar--115 25 Athens, Greece (Received 1 April 1987; revised version received 27 September 1987; accepted 10 October 1987)
ABSTRACT During the drying of olive press-cake, an essential preparatory stage in the extraction of olive press-cake oil, extremely pungent odours are released in the waste gases. This causes problems for the functioning of olive press-cake plants near residential areas. When samples of the concentrates from waste gases were analysed, it was found that the main pollutants were mixtures of organic acids of low molecular weight and fatty compounds, the latter being found in the vapours in the form of air colloids. For the retention of the pollutant substances, a solid-bedfilter was used with a filling material of the ash which is plentiful in olive press oil-processing plants, with a fibrous.filter of fibreglass to trap the fatty substances.
INTRODUCTION The solid residue derived from olive processing for the collection of the olive oil, known as olive press-cake (kernel), constitutes one of the main agricultural by-products in Greece and is a very important source for the production of vegetable oil (olive press-oil). Annual stocks of olive presscake a m o u n t to 250 000 tonnes, from which is derived approximately 25 000 tonnes of olive press-oil per annum, as well as approximately 150 000 tonnes of w o o d y residue, usually known as 'dry kernel'. Dry kernel is a first-class combustible fuel capable of producing 3 500 kcal per kilo. This means that its contribution to the domestic fuel balance amounts to the equivalent of approximately 50 000 tons of oil per annum. 137 Biological Wastes 0269-7483/88/$03"50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
138
Dimitris Papaioannou
A basic preliminary stage in the extraction of the olive press-oil is the drying of the olive press-cake, which has a moisture content o f 40% o f the wet weight. The removal o f the moisture unblocks the pores in the olive press-cake and at the same time releases the olive press-oil which is there in the wet stage in a superfine suspension (colloid) form. The now dry olive press-cake can then be saturated with an organic solvent, usually hexane, in order to extract the olive press oil. The drying of the olive press-cake is usually carried out in tilted rotating drums from which the moisture of the olive press-cake and the hot exhaust gases from the burning of the dry kernel in the plant flow simultaneously with the dry olive press-cake, which is carried away to the olive press-oil extraction chambers. The waste vapours, loaded with traces o f dried olive press-cake in suspension as well as other organic matter from the olive presscake, pass to the gas phase and flow towards the outlet of the drum. This organic matter, which is extremely pungent, becomes a serious nuisance to the surrounding area, and, therefore, the operation of oil presscake plants, even at distances of several kilometres from residential areas, creates m a n y problems. As far as the composition of the drying waste-gases is concerned, the facts known to date are concerned with the composition of the olive press-cake and the biological changes it undergoes during its storage. In measurements and analyses made so far on crushed dry-kernel during its microbial decomposition towards preparation for compost, an initial reduction in the pH value, due to the production of acetic, butyric and lactic acids (Golueke, 1972; Manios & Ballis, 1979) was noted. This was followed by an increase in the pH due partly to the break-down of the organic acids and to their evaporation (Finnstein & Morris, 1975), as well as the TABLE 1
Results of Dry Kernel Analysis (as % of Dry Substance)
Dry substance (% wt. wt.) Fatty substances Nitrogenous substances Sugars total Cellulose Lignin Hemi-ceilulose Ash Extracted substances Data: Manios (1979). a Source of dry kernels.
A spropyrgos ~
Herakleion ~
83'09 2.53 _+0"050 6-63 __0'050 2-23 _+0"040 37-58 _+0"987 21"56 _-4-0-460 13'07 ___1.840 2.95 __0'083 13.45
06"09 1-64 _+0"690 6.56 _+0.036 1-89 _+0'025 39"82 _+0.436 23"40 __0"420 15'13 +_0-866 3.91 __+0.226 7.65
Cleaning wastegasesfrom olivepress-cake drying
139
production of ammonia from the decomposition of the organic nitrogen (Hoyle & Mattingly, 1954). Average composition of the dry'kernel is given in Table 1. (Manios, 1979), using samples taken from different parts of Greece. Some basic indications of the composition of the waste gases can be obtained from the only known industrial processing method to be applied to date, which is based on the reaction of the pollutant substances with solutions of calcium hydroxide by which the basic and the saponifying (emulsifying) properties are most probably estimated. The objectives of the present work were to design and test a method of reducing the pollutants and the odour in gases produced by the drying of the press-cake, based on a solid-bed filter with a filling material of ash which is plentiful in olive press oil-producing plants, and which would thus have low investment and functional costs.
METHODS
Sample analyses In order to determine the polluting elements in the waste gases produced in the drying of the press-cake, 250 g of wet press-cake was dry distilled in an oil bath at a constant temperature of 106°C. Vapour temperature was kept between 98.5 ° and 100.5°C. During the first 5 min of the distillation, the release of approximately 2 ml of condensed gases with a distinctive smell of acrolein was observed. Thereafter no further presence of odiferous condensed substances was recorded. A sample of 100 ml of condensate was obtained from the initial quantity of 250 g wet press-cake which was separated into two phases: a wet one consisting of a water solution of organics; a fatty one floating upon the wet and consisting mainly of fatty acids. The wet phase was analysed for:
COD, according to the standard dichromate reflux method. BODs, by the standard method. Double bonds, by titration with a standard 0" 1N iodine solution having as co-titrant 0"IN standard sodium oxalate solution and starch as a titration and indicator. Total nitrogen, by a Kjeldahl method. Acidity, by titration with 0.1N sodium hydroxide solution to pH 8.3. Total phosphates, by the~persulphate-vanadomolybdate method. Sulphides (qualitative test) using a lead acetate paper.
Dimitr~ Papaioannou
140
Organic acids, were individually determined by high pressure liquid chromatography using T E C N I K O N T. M., M C-2 equipment. The column was a 100 x 6"5 mm ID-Chrompack 'for organic acids' and the mobile phase 0.01N H2SO4. Other conditions were: flow rate, 0.6mlmin-~; T, 45°C; P, 6MPa, Detector, UV, 210nm. The fatty phase was analysed for:
double bonds and acidity using the methods above. Retention of pollutants contained in the gases The strongly acidic composition of the pollutant substances permits, as a principle, their retention in a solid alkaline filter bed, creating alkaline salts. Of the possible alkaline beds, one made of ash was selected since this is a substance which has strong alkalinity and which is in plentiful supply in the
I
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/
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alkaline bed
lass
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waste gases
Fig. I.
Flow diagram of the filter.
Cleaning waste gases from olive press-cake drying
141
plants. Dry lime hydrate or amorphous calcium carbonate were added occasionally in order to improve the alkalinity and the porosity of the bed. The bed was based on a fibre glass bottom as below. The diameter of the bed container was 10cm and the height of the bed was 30cm. To retain the fatty substances in the waste gases, a fibre-glass-mesh filter, with a pore diameter of 6 pm, was used. This filter was installed before the alkaline-bed filter, so that the fatty substances were retained for reuse. The filter consisted of a preconstructed sheet of fibre-glass, 2 cm thick, with a specific weight of 0-25 g c m - 3. The diameter of the filter was 10 cm, as the diameter of the alkaline bed, for which the former served simultaneously as a base. For a flow diagram of the filter see Fig. 1.
RESULTS A N D DISCUSSION Earlier methods of dealing with the pollutants have so far included washing with an alkaline solution and catalytic incineration (on an experimental basis) using a system similar to the catalytic convertor used for exhaust gases from cars. Washing was based on a system in which the waste gases were sprayed with a lime hydrate solution. All the pollutant substances were absorbed into the washing liquid, and the lime hydrate neutralised the acids and saponified the fatty substances. The liquid could be recycled for reuse, and eventually discarded when it was overloaded. The drawbacks of this method were: the high water consumption (1-5-2 m 3 per tonne of press-cake); the high pollutant load in the liquid waste, equivalent to a 20000-30000 population from a production of 200-250 tonnes press-cake per day; the high energy consumption (about 2 kWh per tonne of press cake). Catalytic incineration (Budd, 1982) is based on the complete incineration of the pollutant substances in the presence of a catalyst made of platinum, iridium, etc. All the organic substances contained in the waste gases pass through the honeycomb catalyst, at temperatures of 350°C, giving off CO z and water. This method, which is widely used with many categories of waste gases, was applied on an experimental basis in January, 1984, to the VESO olive press oil-processing plant at Corinth, with good results. There were no traces of any of the pollutants in the gases once they had passed through the catalyst. The drawbacks of this method were: the high energy consumption (5 kg of fuel oil per tonne press-cake); the high investment of l ' 2 m 3 platinum catalyst for a daily production of 200-250 tonnes press cake and the short
142
Dimitris Papaioannou
life-time of the catalyst due to the presence of traces o f phosphorus in the stream o f pollutants. After all these drawbacks, it was thought that the filter absorption method o f cleaning the exhaust gases would be simpler and cheaper than either o f these methods, so this was tested. The analyses o f the condensates from the wet press-cake dry-distillation are shown in Table 2. The results show that the main pollutants are: light organic acids, 8 - 1 0 g per litre o f concentrate; fatty acids or their esters, 5-10 g per litre o f concentrate. The analyses showed a surprising presence of short-chain organic acids, but this could be explained as an intermediate product of anaerobic fermentation during the period after the press-cake had left the oliveprocessing plant and before it entered the olive press oil-processing plant. So samples of olive press-cake o f various ages were subjected to dry distillation and the active acidity of the concentrates measured. The results are shown in Table 3. The maximum concentration of acids appears a few days after collecting the olive press-cake from the processing plant (in the present case, between 4 and 16 days). Thereafter the presence of acids is drastically TABLE 2
Characteristics of the Condensate of the Wet Press-cake Dry Distillation (a) Water phase pH: 3'4-3.8 Colour: pale yellow Fatty substances (floating): 0"5-1% v/v (of the wet phase) COD: BOD:
Acidity: Double bonds: Total nitrogen: Phosphates: Sulphides:
10.000-11.000 mg litre- 1 7-000-8.000 mg litre- 1 5"000-6'000 mg litre- 1 (as --COOH) 100-120 mglitre- 1 (as__HCzCH _ ) 70-80 mg litre40-50 mg litre- 1 non-detectable (acetic lead qualitative test)
Organic acids (liquid chromatography) Folic acid 8-2% Acetic acid 81.4% Propionic acid 1-5% Butyric acid 3.6% Lactic acid 5"3% (b) Fatty phase (when separated from the water phase) Colour: bright yellow Double bonds: 30~0mgml i (as--HC~-CH--) Acidity: 10-15 mg ml- ~ (as --COOH) Colour change: bright violet, after 2 h of aeration
Cleaning waste gases from olive press-cake drying
143
TABLE 3 pH Values of Condensates Depending on the Age of the Presscake
Age of press-cake (in days)
pH of concentrate after dry distillation
4 8 12 16 24 32 40
3-8 3"4 3-4 3"6 4'0 4"2 4"4
reduced considering the difference in the pH value of the concentrate. The acids thus produced form azeotropes with the water and are distilled along with it during the drying process. Concerning the constitution of the alkaline bed, experiments have shown a pure ash bed to have the following characteristics: alkalinity, 3.24kg equivalents; pH (liquid suspension), 12-5; granule size, 0.1-0-5 m m (for 80%); porosity, 38%; specific weight, 0-70 g c m - 3 With regard to the flow of drying gases through the alkaline bed, the following measurements were recorded: specific permeability of the bed by the gases (Q) = 0.15 m 3 h - t m - 1 (for a pressure drop, Dp = 0.05 a t m - ~); retention time (t) = 2.5 s (for a removal level of over 95%, estimated by cooling the gases after passing through the filter and measuring the new organic content of the wet phase). Two decimeters (0.2 m) was found to be the minimum depth of the bed to ensure the steady flow of gases within the bed. The exhausted ash of the alkaline bed may be discarded after use. Its almost neutral composition makes it possible for it to be used as a natural fertiliser, since it is a valuable source of mineral salts for plants. The source of long-chain fatty acids and their esters is obvious since they are a constituent part of the wet olive press-cake. It is particularly interesting that they should appear in the vapours and subsequently in the concentrate. At first sight it would seem reasonable to assume that they are carried away by the current of drying gases given offby the olive press-cake. However, this does not explain the presence of the fatty stage in the concentrate despite the exceptionally mild conditions prevailing during the dry distillation in the laboratory and the absence of the current of air. The explanation that they are distilled together cannot be valid because the temperatures used in the distilling process are very low. The most possible explanation seems to be
144
Dimitris Papaioannou
that the fatty substances are recovered in the vapours in the form of air colloidal particles, which are formed during the drying process. The existence of water and oil in the pores of the olive press-cake leads us to be fairly sure that the fatty substances will, to some extent, be present in the form of colloidal or fine-grained droplets; that is, in particles less than 1/~m in diameter. At the m o m e n t when the water phase around them evaporates, and as far as the flow conditions in the immediate environment of the pores of the dried press-cake permit, these fatty particles do not form aggregates but are carried away separately by the vapours and are condensed in the cooler. It should be noted that particles which are less than 1/~m in diameter will be suspended in the air, since they are carried away by currents greater than 4 x 1 0 - 2 m m s -1 (0.15mh-1). The main evidence for the existence of fatty substances in air-colloidal suspension lies in the following facts: they are completely invisible in the vapours; when placed on a cold glass surface, they do not form droplets but a thin membrane irridescent in the light, i.e. less than I pm thick; they are retained by fibrous filters with an almost 100% efficiency. Concerning the retention mechanism, it can be assumed from the filter capacity, which was more than 30% of its free space, that most probably the retention proceeds as a dendritic accumulation of air-colloid fatty-particles (Pagiatakis, 1985). The reuse of the fibrous filter was possible after it had been rinsed with hexane exactly as in the process of extracting oil from dry olive press-cake. The availability of the use of this technique (the rinsing of filters) discouraged the application of other techniques, such as suction (Nietzhold, 1979) or compression. The ease of construction and the low cost of the glass fibre discourages experiments on other devices using synthetic or natural materials (Cheremisinoff & Young, 1977), and baffle-type mist eliminators (Theodore, 1982) because of the small diameter of the fatty particles. With this filter it is possible to make a saving in oil of about 5-10kg t o n n e - 1 of wet press-cake, or 500-1000 kg per day for a typical press-cake processing plant, with a daily output of 250 tonnes. Further data for the function of the fat retaining filter were obtained. It was estimated that the time the waste gases are in the fibreglass should be less than 0-1 s for an efficiency of the filter amounting to practically 100% (that is to say, there were no visible traces of fatty substances after the gases had passed through the filter). The pressure drop with a filter thickness of 2 cm did not exceed 1 m m of water head. REFERENCES Budd, B. (1982). Control of solvent emissions by catalytic incineration (Honeycat System). ECCA, Congress Nov. 1982, Brussels.
Cleaning waste gases from olive press-cake drying
145
Cheremisinoff, P. & Young, R. (1977). Air pollution control and design handbook, Part 1, Ed. Marcell Decker Inc., New York, 317. Finstein, M. S. & Morris, M. L. (1975). Microbiology of municipal solid waste composting. Advances in Appl. Microbiol., 19, 113-51. Golueke, C. G. (1972). 'Composting'. A study of the process and its principles. Rodate Press, Emmaus, Pensylvania. Hoyle, D. A. & Mattingly, G. O. (1954). Studies on composts prepared from waste materials. I. Preparation, nitrogen losses and changes in soluble nitrogen. J. Sci. Food Agric., 5, 54-64. Manios, V. (1979). An investigation into the possibility of producing compost from extracted olive press oil. PhD Thesis, Athens College of Agriculture. Manios, V. & Ballis, C. (1983). Respirometry to determine optimum conditions for the biodegradation of extracted olive press-cake. Soil. Biol. Biochem., 15(1), 75-83. Nietzhold, Ingo (1979). Luftfiltration (Nebelfiltration p. 104). M. Miiller-Verlag, Karlsruhe. Pagiatakis, A. (1985). Dentritic deposition of aerocoloid particles in fibrous filters. Bull. Hellenic Chemical Engineers, (48), 24-44. Theodore, L. (1982). Air pollution control equipment. Prentice Hall Inc., Engelwood Cliffs, New Jersey, 363.
A Method of Processing Waste Gases from the Drying of Olive Press-cake D i m i t r i s Papaioannou 36 Ebrar--115 25 Athens, Greece (Received 1 April 1987; revised version received 27 September 1987; accepted 10 October 1987)
ABSTRACT During the drying of olive press-cake, an essential preparatory stage in the extraction of olive press-cake oil, extremely pungent odours are released in the waste gases. This causes problems for the functioning of olive press-cake plants near residential areas. When samples of the concentrates from waste gases were analysed, it was found that the main pollutants were mixtures of organic acids of low molecular weight and fatty compounds, the latter being found in the vapours in the form of air colloids. For the retention of the pollutant substances, a solid-bedfilter was used with a filling material of the ash which is plentiful in olive press oil-processing plants, with a fibrous.filter of fibreglass to trap the fatty substances.
INTRODUCTION The solid residue derived from olive processing for the collection of the olive oil, known as olive press-cake (kernel), constitutes one of the main agricultural by-products in Greece and is a very important source for the production of vegetable oil (olive press-oil). Annual stocks of olive presscake a m o u n t to 250 000 tonnes, from which is derived approximately 25 000 tonnes of olive press-oil per annum, as well as approximately 150 000 tonnes of w o o d y residue, usually known as 'dry kernel'. Dry kernel is a first-class combustible fuel capable of producing 3 500 kcal per kilo. This means that its contribution to the domestic fuel balance amounts to the equivalent of approximately 50 000 tons of oil per annum. 137 Biological Wastes 0269-7483/88/$03"50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
138
Dimitris Papaioannou
A basic preliminary stage in the extraction of the olive press-oil is the drying of the olive press-cake, which has a moisture content o f 40% o f the wet weight. The removal o f the moisture unblocks the pores in the olive press-cake and at the same time releases the olive press-oil which is there in the wet stage in a superfine suspension (colloid) form. The now dry olive press-cake can then be saturated with an organic solvent, usually hexane, in order to extract the olive press oil. The drying of the olive press-cake is usually carried out in tilted rotating drums from which the moisture of the olive press-cake and the hot exhaust gases from the burning of the dry kernel in the plant flow simultaneously with the dry olive press-cake, which is carried away to the olive press-oil extraction chambers. The waste vapours, loaded with traces o f dried olive press-cake in suspension as well as other organic matter from the olive presscake, pass to the gas phase and flow towards the outlet of the drum. This organic matter, which is extremely pungent, becomes a serious nuisance to the surrounding area, and, therefore, the operation of oil presscake plants, even at distances of several kilometres from residential areas, creates m a n y problems. As far as the composition of the drying waste-gases is concerned, the facts known to date are concerned with the composition of the olive press-cake and the biological changes it undergoes during its storage. In measurements and analyses made so far on crushed dry-kernel during its microbial decomposition towards preparation for compost, an initial reduction in the pH value, due to the production of acetic, butyric and lactic acids (Golueke, 1972; Manios & Ballis, 1979) was noted. This was followed by an increase in the pH due partly to the break-down of the organic acids and to their evaporation (Finnstein & Morris, 1975), as well as the TABLE 1
Results of Dry Kernel Analysis (as % of Dry Substance)
Dry substance (% wt. wt.) Fatty substances Nitrogenous substances Sugars total Cellulose Lignin Hemi-ceilulose Ash Extracted substances Data: Manios (1979). a Source of dry kernels.
A spropyrgos ~
Herakleion ~
83'09 2.53 _+0"050 6-63 __0'050 2-23 _+0"040 37-58 _+0"987 21"56 _-4-0-460 13'07 ___1.840 2.95 __0'083 13.45
06"09 1-64 _+0"690 6.56 _+0.036 1-89 _+0'025 39"82 _+0.436 23"40 __0"420 15'13 +_0-866 3.91 __+0.226 7.65
Cleaning wastegasesfrom olivepress-cake drying
139
production of ammonia from the decomposition of the organic nitrogen (Hoyle & Mattingly, 1954). Average composition of the dry'kernel is given in Table 1. (Manios, 1979), using samples taken from different parts of Greece. Some basic indications of the composition of the waste gases can be obtained from the only known industrial processing method to be applied to date, which is based on the reaction of the pollutant substances with solutions of calcium hydroxide by which the basic and the saponifying (emulsifying) properties are most probably estimated. The objectives of the present work were to design and test a method of reducing the pollutants and the odour in gases produced by the drying of the press-cake, based on a solid-bed filter with a filling material of ash which is plentiful in olive press oil-producing plants, and which would thus have low investment and functional costs.
METHODS
Sample analyses In order to determine the polluting elements in the waste gases produced in the drying of the press-cake, 250 g of wet press-cake was dry distilled in an oil bath at a constant temperature of 106°C. Vapour temperature was kept between 98.5 ° and 100.5°C. During the first 5 min of the distillation, the release of approximately 2 ml of condensed gases with a distinctive smell of acrolein was observed. Thereafter no further presence of odiferous condensed substances was recorded. A sample of 100 ml of condensate was obtained from the initial quantity of 250 g wet press-cake which was separated into two phases: a wet one consisting of a water solution of organics; a fatty one floating upon the wet and consisting mainly of fatty acids. The wet phase was analysed for:
COD, according to the standard dichromate reflux method. BODs, by the standard method. Double bonds, by titration with a standard 0" 1N iodine solution having as co-titrant 0"IN standard sodium oxalate solution and starch as a titration and indicator. Total nitrogen, by a Kjeldahl method. Acidity, by titration with 0.1N sodium hydroxide solution to pH 8.3. Total phosphates, by the~persulphate-vanadomolybdate method. Sulphides (qualitative test) using a lead acetate paper.
Dimitr~ Papaioannou
140
Organic acids, were individually determined by high pressure liquid chromatography using T E C N I K O N T. M., M C-2 equipment. The column was a 100 x 6"5 mm ID-Chrompack 'for organic acids' and the mobile phase 0.01N H2SO4. Other conditions were: flow rate, 0.6mlmin-~; T, 45°C; P, 6MPa, Detector, UV, 210nm. The fatty phase was analysed for:
double bonds and acidity using the methods above. Retention of pollutants contained in the gases The strongly acidic composition of the pollutant substances permits, as a principle, their retention in a solid alkaline filter bed, creating alkaline salts. Of the possible alkaline beds, one made of ash was selected since this is a substance which has strong alkalinity and which is in plentiful supply in the
I
cleared~ l b
I
9--
/
i[!iii~iiiiiiiiililiiii!ii~iiii!iiiilililiiiiiiii!i!iii!i!i!i!i!i!ii!!ii!iii!i~
iiiililiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiilililil;iiiililil;iiii!i!i!i!i :i:i:i:~:i:i:i:?:i:i:i:i:i:i:21:i:i:ii:i:i:i:i:i:i:!:i:i:?:!:i:i:~:i:!:!:!:!: ii~iiii?iiiiiiiii!?ilili~iiiii;ii!iiiiiiiiiii~i![ii!i!i!i!ili!ili::::::::::::::
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alkaline bed
lass
sheet
waste gases
Fig. I.
Flow diagram of the filter.
Cleaning waste gases from olive press-cake drying
141
plants. Dry lime hydrate or amorphous calcium carbonate were added occasionally in order to improve the alkalinity and the porosity of the bed. The bed was based on a fibre glass bottom as below. The diameter of the bed container was 10cm and the height of the bed was 30cm. To retain the fatty substances in the waste gases, a fibre-glass-mesh filter, with a pore diameter of 6 pm, was used. This filter was installed before the alkaline-bed filter, so that the fatty substances were retained for reuse. The filter consisted of a preconstructed sheet of fibre-glass, 2 cm thick, with a specific weight of 0-25 g c m - 3. The diameter of the filter was 10 cm, as the diameter of the alkaline bed, for which the former served simultaneously as a base. For a flow diagram of the filter see Fig. 1.
RESULTS A N D DISCUSSION Earlier methods of dealing with the pollutants have so far included washing with an alkaline solution and catalytic incineration (on an experimental basis) using a system similar to the catalytic convertor used for exhaust gases from cars. Washing was based on a system in which the waste gases were sprayed with a lime hydrate solution. All the pollutant substances were absorbed into the washing liquid, and the lime hydrate neutralised the acids and saponified the fatty substances. The liquid could be recycled for reuse, and eventually discarded when it was overloaded. The drawbacks of this method were: the high water consumption (1-5-2 m 3 per tonne of press-cake); the high pollutant load in the liquid waste, equivalent to a 20000-30000 population from a production of 200-250 tonnes press-cake per day; the high energy consumption (about 2 kWh per tonne of press cake). Catalytic incineration (Budd, 1982) is based on the complete incineration of the pollutant substances in the presence of a catalyst made of platinum, iridium, etc. All the organic substances contained in the waste gases pass through the honeycomb catalyst, at temperatures of 350°C, giving off CO z and water. This method, which is widely used with many categories of waste gases, was applied on an experimental basis in January, 1984, to the VESO olive press oil-processing plant at Corinth, with good results. There were no traces of any of the pollutants in the gases once they had passed through the catalyst. The drawbacks of this method were: the high energy consumption (5 kg of fuel oil per tonne press-cake); the high investment of l ' 2 m 3 platinum catalyst for a daily production of 200-250 tonnes press cake and the short
142
Dimitris Papaioannou
life-time of the catalyst due to the presence of traces o f phosphorus in the stream o f pollutants. After all these drawbacks, it was thought that the filter absorption method o f cleaning the exhaust gases would be simpler and cheaper than either o f these methods, so this was tested. The analyses o f the condensates from the wet press-cake dry-distillation are shown in Table 2. The results show that the main pollutants are: light organic acids, 8 - 1 0 g per litre o f concentrate; fatty acids or their esters, 5-10 g per litre o f concentrate. The analyses showed a surprising presence of short-chain organic acids, but this could be explained as an intermediate product of anaerobic fermentation during the period after the press-cake had left the oliveprocessing plant and before it entered the olive press oil-processing plant. So samples of olive press-cake o f various ages were subjected to dry distillation and the active acidity of the concentrates measured. The results are shown in Table 3. The maximum concentration of acids appears a few days after collecting the olive press-cake from the processing plant (in the present case, between 4 and 16 days). Thereafter the presence of acids is drastically TABLE 2
Characteristics of the Condensate of the Wet Press-cake Dry Distillation (a) Water phase pH: 3'4-3.8 Colour: pale yellow Fatty substances (floating): 0"5-1% v/v (of the wet phase) COD: BOD:
Acidity: Double bonds: Total nitrogen: Phosphates: Sulphides:
10.000-11.000 mg litre- 1 7-000-8.000 mg litre- 1 5"000-6'000 mg litre- 1 (as --COOH) 100-120 mglitre- 1 (as__HCzCH _ ) 70-80 mg litre40-50 mg litre- 1 non-detectable (acetic lead qualitative test)
Organic acids (liquid chromatography) Folic acid 8-2% Acetic acid 81.4% Propionic acid 1-5% Butyric acid 3.6% Lactic acid 5"3% (b) Fatty phase (when separated from the water phase) Colour: bright yellow Double bonds: 30~0mgml i (as--HC~-CH--) Acidity: 10-15 mg ml- ~ (as --COOH) Colour change: bright violet, after 2 h of aeration
Cleaning waste gases from olive press-cake drying
143
TABLE 3 pH Values of Condensates Depending on the Age of the Presscake
Age of press-cake (in days)
pH of concentrate after dry distillation
4 8 12 16 24 32 40
3-8 3"4 3-4 3"6 4'0 4"2 4"4
reduced considering the difference in the pH value of the concentrate. The acids thus produced form azeotropes with the water and are distilled along with it during the drying process. Concerning the constitution of the alkaline bed, experiments have shown a pure ash bed to have the following characteristics: alkalinity, 3.24kg equivalents; pH (liquid suspension), 12-5; granule size, 0.1-0-5 m m (for 80%); porosity, 38%; specific weight, 0-70 g c m - 3 With regard to the flow of drying gases through the alkaline bed, the following measurements were recorded: specific permeability of the bed by the gases (Q) = 0.15 m 3 h - t m - 1 (for a pressure drop, Dp = 0.05 a t m - ~); retention time (t) = 2.5 s (for a removal level of over 95%, estimated by cooling the gases after passing through the filter and measuring the new organic content of the wet phase). Two decimeters (0.2 m) was found to be the minimum depth of the bed to ensure the steady flow of gases within the bed. The exhausted ash of the alkaline bed may be discarded after use. Its almost neutral composition makes it possible for it to be used as a natural fertiliser, since it is a valuable source of mineral salts for plants. The source of long-chain fatty acids and their esters is obvious since they are a constituent part of the wet olive press-cake. It is particularly interesting that they should appear in the vapours and subsequently in the concentrate. At first sight it would seem reasonable to assume that they are carried away by the current of drying gases given offby the olive press-cake. However, this does not explain the presence of the fatty stage in the concentrate despite the exceptionally mild conditions prevailing during the dry distillation in the laboratory and the absence of the current of air. The explanation that they are distilled together cannot be valid because the temperatures used in the distilling process are very low. The most possible explanation seems to be
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that the fatty substances are recovered in the vapours in the form of air colloidal particles, which are formed during the drying process. The existence of water and oil in the pores of the olive press-cake leads us to be fairly sure that the fatty substances will, to some extent, be present in the form of colloidal or fine-grained droplets; that is, in particles less than 1/~m in diameter. At the m o m e n t when the water phase around them evaporates, and as far as the flow conditions in the immediate environment of the pores of the dried press-cake permit, these fatty particles do not form aggregates but are carried away separately by the vapours and are condensed in the cooler. It should be noted that particles which are less than 1/~m in diameter will be suspended in the air, since they are carried away by currents greater than 4 x 1 0 - 2 m m s -1 (0.15mh-1). The main evidence for the existence of fatty substances in air-colloidal suspension lies in the following facts: they are completely invisible in the vapours; when placed on a cold glass surface, they do not form droplets but a thin membrane irridescent in the light, i.e. less than I pm thick; they are retained by fibrous filters with an almost 100% efficiency. Concerning the retention mechanism, it can be assumed from the filter capacity, which was more than 30% of its free space, that most probably the retention proceeds as a dendritic accumulation of air-colloid fatty-particles (Pagiatakis, 1985). The reuse of the fibrous filter was possible after it had been rinsed with hexane exactly as in the process of extracting oil from dry olive press-cake. The availability of the use of this technique (the rinsing of filters) discouraged the application of other techniques, such as suction (Nietzhold, 1979) or compression. The ease of construction and the low cost of the glass fibre discourages experiments on other devices using synthetic or natural materials (Cheremisinoff & Young, 1977), and baffle-type mist eliminators (Theodore, 1982) because of the small diameter of the fatty particles. With this filter it is possible to make a saving in oil of about 5-10kg t o n n e - 1 of wet press-cake, or 500-1000 kg per day for a typical press-cake processing plant, with a daily output of 250 tonnes. Further data for the function of the fat retaining filter were obtained. It was estimated that the time the waste gases are in the fibreglass should be less than 0-1 s for an efficiency of the filter amounting to practically 100% (that is to say, there were no visible traces of fatty substances after the gases had passed through the filter). The pressure drop with a filter thickness of 2 cm did not exceed 1 m m of water head. REFERENCES Budd, B. (1982). Control of solvent emissions by catalytic incineration (Honeycat System). ECCA, Congress Nov. 1982, Brussels.
Cleaning waste gases from olive press-cake drying
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Cheremisinoff, P. & Young, R. (1977). Air pollution control and design handbook, Part 1, Ed. Marcell Decker Inc., New York, 317. Finstein, M. S. & Morris, M. L. (1975). Microbiology of municipal solid waste composting. Advances in Appl. Microbiol., 19, 113-51. Golueke, C. G. (1972). 'Composting'. A study of the process and its principles. Rodate Press, Emmaus, Pensylvania. Hoyle, D. A. & Mattingly, G. O. (1954). Studies on composts prepared from waste materials. I. Preparation, nitrogen losses and changes in soluble nitrogen. J. Sci. Food Agric., 5, 54-64. Manios, V. (1979). An investigation into the possibility of producing compost from extracted olive press oil. PhD Thesis, Athens College of Agriculture. Manios, V. & Ballis, C. (1983). Respirometry to determine optimum conditions for the biodegradation of extracted olive press-cake. Soil. Biol. Biochem., 15(1), 75-83. Nietzhold, Ingo (1979). Luftfiltration (Nebelfiltration p. 104). M. Miiller-Verlag, Karlsruhe. Pagiatakis, A. (1985). Dentritic deposition of aerocoloid particles in fibrous filters. Bull. Hellenic Chemical Engineers, (48), 24-44. Theodore, L. (1982). Air pollution control equipment. Prentice Hall Inc., Engelwood Cliffs, New Jersey, 363.