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Evaporation of olive oil mill vegetation waters
Desalznatzon, 81 (1991) 249-259
Elsewer Science Publishers B.V., Amsterdam
Evaporation of Olive Oil Mill Vegetation Waters G. DI GIACOMO, V. BRANDANI and G. DEL RE University of L’Aquila, Deportment of Chemistry, Chemical Engineering and Materials, Monteluco dz Roio, LXquila (Itab)
67040
SUMMARY
The evaporation of olive oil mill vegetation waters permits to pull down the polluting load to an extent superior to the 90% in terms of COD. This operation can be easily done by using the industrial evaporators now existing, and it can be optimized up to a level whose value depends on the nature and age of the vegetation water in question. The concentrated solution which comes out from the evaporator can be thermally decomposed by a two stage pyrolytic process which allows to previously divide the inorganic salts, avoiding, in this way, to encrust the boiler during the combustion of the polluting organic substances. Since the thermal decomposition process of the concentrated solution is strongly exothermic, the exceeding heat can be used to ulteriorly lower the COD of the treated water to values compatible with the environmental regulations.
INTRODUCl-TON
The depuration of the aqueous effluents resulting from the transformation processes of some of the typical agricultural products of the mediterranean regions is a problem which has a particular importance, both for their quantity, and for the presence of still unsolved technical difficulties for the realization of treatment processes that are efficient and that have low costs of construction and operation. Typical examples of what has been said are represented by the depuration of the distillery vinasses and the olive oil mill vegetation waters which, in 249
250
Italy, reach the total amount of almost 3 million cubic meters per year. The peculiarities which make particularly difficult their treatment are a high COD value (over 200 g/l) the nature of the polluting organic compounds, and the high percentage of dissolved minerai salts and of solids in suspension. So far the most used industrial process for treatment of both vinasses and vegetation waters (VW), are based on a concentration section with a single or multiple effect evaporator from which a distillate is obtained, whose residual COD value is not superior to 5-7% than the initial one; at the same time 10 to 20% of the feeded liquid leaves the evaporator from the bottom as concentrated solution characterized by a water content of 30-50% by weight. The recondensed vapor can be sent, as it is, either to the biological depuration, or, as for the VW, it can be distilled in order to regain an alcoholic phlegm whose commercial value should not be completely neglected. On the contrary, it is much more difficult to work with the concentrated solution that, even if it is in smaller quantities than at the beginning, must be destroyed in some way, or made inert and then got rid of. Many systems have been proposed to treat this concentrated solution; as to spread it over the soil, or to use it as a component for the production of fodder or of agricultural amendments, or as a fuel, since its higher heating value (HHV) is calculated between 2000 and 3000 kcal/kg. The use of the concentrated solution as fuel is particularly attractive, since while it could be definitely pulled down the polluting load, on the other hand it could be a helpful low-cost alternative to the ordinary fuel for the evaporator or/and the bottom column boiler in case the distillation is considered. For this reason, in the distilleries and in the few VW treatment plants existing there are usually special boilers which should burn the mud and the concentrated solution coming from the aqueous effluent concentration. Anyhow it is well known that usually these devices don’t work as they are supposed to, since the inorganic salts present in the concentrated solution (from 5 to 10% by weight) melt during the combustion and their deposits encrust the pipes of the boiler making it shortly inefficient. Therefore this part of the plant is often unemployed and it is replaced by boilers that work with methane or oil making the depuration process much more expensive. In previous works [l-3] a process has been studied which permits to execute the combustion of the concentrated solution, avoiding the typical problems related to the high salt content. In particular the concentrated solution previously mixed with olive stone is pyrolized to separate the inorganic salts, which deposit on the charcoal bed. The vapor stream, leaving from the top of the pyrolytic reactor, can then be burned to destroy the polluting organic substances and to recover the energy required in the evaporation and distillation sections. The above described method can be also regarded as an
objective evaluation of lignocellulosic residuals, which now are not anymore used in the managements of the modern farms and are always largely present when processing agricultural products. In the following pages there will be reported and described the results of the experimental tests made in order to individuate the best concentration level in the evaporation section and to verify the technical feasibility of the pyrolytic process.
EXPERIMENTALSECTION The evaporation tests have been performed at laboratory scale with a glass discontinuous evaporator using VW coming from both traditional and continuous olive oil mills and having different ages. The results are shown in Figs. 1 and 2 where the different curves are referred to samples of water coming from the same olive oil mill and from the same lot of olives having an increasing aging from 1 to 5. An important peculiarity of these curves is the existence of an optimal value of the concentration level, which is related to the presence of a minimum that is generally localized near the COD value corresponding to a water content of 50% by weight in the concentrated solution coming out of the evaporator. The presence of this minimum is explainable since, unlike the other aqueous effluents of the agro-industrial processes, in the VW there are both compounds more volatile than water (most of them produced by the fermentation of sugary substances) and compounds less volatile than water as for instance the polyphenols. Further advantage of using evaporation instead of other depuration processes is represented by the fact that the head product is an alcoholic phlegm whose further processing in a distillery could help the economy of the VW treatment process. The pyrolytic experimental tests have been performed with a batch laboratory reactor made of AISI 304 steel heated by a stove having an automatic temperature regulator. This reactor has an useful volume of about 0.65 liters and it is provided with: a flanged cover that permits to easily get into the reactor, a hole at the top connected with the pipe that discharges the pyrolytic gases, and a filter, made by 100 mesh metallic net jointed at the cover, which is placed between the charge to pyrolize and the hole for the exit of the gases, in order to avoid that some solid particles may be dragged during the most violent phase of the pyrolysis. The pipe for the pyrolytic gas discharge, which gets off from the stove through a special hole, is connected, in some cases, to a surface glass condenser which allows, adjusting the, temperature of the cooling water, to obtain a partial condensation of the gaseous stream
25? TABLE I
Pulling down of the COD following the concentration process through evaporation lOO-(COD of the condensed solutton/COD of VW feeded) Sample VW from centrifuge concentrated solution up to 50% of water VW from press concentrated solution up to 50% water VW from centrifuge condensed at minimum COD VWfrom press condensed at minimum COD
12000
I
II
III
IV
V
97.6
96.6
95.7
94.6
93.9
96.0
95.4
94.6
93.7
93.0
97.6
96.0
94.1
93.5
93.4
95.7
95.3
94.4
93.8
92.9
0 TE!ST n. 1 Al-Egfl.2 l TESTn.3
+TESTn.4 oTESTn.5
I
0
20
WEIGHT
40
PERCENT
60
60
1
OF DISTILLATE
Fig. 1. COD of &stilIate vs the weight percent of distillate of VW, having different aging, increasing from l-5 and coming from continuous centnfugatlon oil m1lIs
253 32000
T
TEST n. 1
30000
0
26000 26000
ATEmn.2
24000 22000
+TESTn.4
n
TEST n. 3
oTEXn.5
0 20000 E v 16000 n 16000 ;\ 8 14000 12000 1oow 6000 & L 4000’
0
20
WEIGHT
40
PERCENT
60
60
100
OF DISTILLATE
Fig 2 COD of distillate vs the weight percent of distillate of VW havmg different aging, increasing from l-5 and coming from traditlonal oil mtlls
The results of the pyrolytic experimental tests done with charges of lignocellulosic material obtained mixing, in different ratios, the olive stone of exhausted husk with the concentrated solution resulting from the evaporation of the VW at 50% of water, are shown in Table 2. The yield of liquid products, AP2 and OP and that of charcoal, C, has been obtained by weighting the products, at the end of each test that lasts about 40-45 minutes. The amount of gas burned at the exit has been evaluated from the mass balance, knowing the total weight of the charge put in the reactor before the beginning of the test. The results reported in Table 2 show that the percentage of charcoal ob@nable when using only olive stone is considerably higher than the one obtainable with continuous units heated directly by the partial combustion of the charge [l] while it is considerably lower the percentage of bio-oil which is estimated to be about the 3.5% of the charge. Furthermore, as a result of the tests from number 5 to 11 we can see that the percentage of charcoal obtained decreases while R increases. Nevertheless, if the charcoal
254 obtained refers to the amount increases with R.
of olive stone charged,
the percentage
TABLE II Results of the pyrolytic experimental concentrated solutions of VW
tests obtamed
with mixtures
of olive stone and
Tn
TC,g
R
U%
MT,K
Cg
Gas,g
AP,g
OP,g
1 2 3 4 5 6 7 8 9 10 11
100.0 100.0 100.0 100.0 98.5 98.5 98.0 98.4 98.6 98.4 98.0
0.0 0.0 0.0 00 0.6 1.2 1.6 1.6 1.6 1.6 1.6
6.5 6.5 6.8 6.8 21.9 31.7 35.0 34.2 35.6 35.1 34.5
733 733 813 813 813 813 813 813 813 813 813
37.0 34.6 32.1 31.6 22.3 16.7 14.4 13.8 13.8 14.0 14.7
63.0 65.4 67.9 68.4 23.6 25.1 22.8 78.8 41.2 25 3 22.8
50.0 54.2 58.6 4.0 41.6 56.8 58.0
2.6 2.5 2.2 19 2.0 2.3 2.5
Tn = test number TC = total weight of the charge m the reactor, R = concentrated solution/olive stone (by weight); U !% = hurmdity of TC; MT = maximum temperature; C = quantity of charcoal obtained; AP = aqueous phase gathered; OP = organic phase gathered.
This shows that, during the pyrolysis, the salts deposit on the charcoal bed, while the organic substances present in the concentrated solution are partially decomposed and vaporized, and get off the reactor as a gaseous stream together with the water and the other organic products coming from the ligneous biomass decomposition. This can be indirectly confirmed comparing the different heat of combustion between the charcoal produced in test number 3 (7740 kcal/kg) and that produced in test number 10 (6200 kcal/kg). In test number 5 and 10 the heat of combustion of the gatheped organic phase has been measured, obtaining respectively 8034 and 7840 kcal/kg. Such high values compared to those of the oils obtainable from direct heating continuous processes are compatible with the hypothesis that, if the pyrolysis takes place in a partially oxidizing atmosphere, the liquid pyrolytic products are in greater quantities and richer in oxygen.
Still referring to Table 2 we can see that in correspondence with test number 8 the amount of aqueous phase is of one order of magnitude lower than all the others, while the amount of bio-oil changes much less. In this case the partial condensation of the gaseous stream coming from the reactor has been done making water circulate in the condenser at 372.6 K. This demonstrates that, for the industrial plants, it is possible to recuperate an organic phase to use as liquid fuel causing a condensation at a controlled temperature, and in any case superior to water boiling temperature, thus avoiding the production of a polluting aqueous solution. As a matter of fact the COD of the aqueous phase in test number 10 is 220 g/l. It’s evident, anyhow, that the recovery of a liquid organic phase strongly cuts down the combustible substances in the gas lowering its heat of combustion. On the other hand, it can be deduced that the gaseous stream coming out from the pyrolytic reactor has a heat of combustion between 2000 and 3000 kcal/kg, and it can be considered an excelIent gaseous fuel. The opportunity, for the industrial plants, to burn directly the gaseous stream coming out of the reactor, or to insert, between the pyrolytic reactor and the gas burner, a controlled temperature condenser for the recovery of a liquid organic phase to be used as a combustible oil, is strictly connected to other variables of the process (for instance the water content of the concentrated solution) as well as to the constructive characteristics and to the operating conditions of the pyrolytic reactor. Still referring to the pyrolytic experimental tests of Table 2, it is important to clarify that the interval between 0.6 and 1.6 chosen for R includes in practice all the possible natural mixtures, considering: the different kinds of olive oil mills existing, the different varieties of olives used for the production of virgin oil, the differences connected with the season variations and with a water content of the concentrated solution coming from the evaporator which is lower than 50% by weight. This concentration value is easily achievable with the modern industrial evaporators and, as the evaporation experimental tests show, in many cases is very close to the optimal value if pulling down of the COD values is considered the aim of the evaporation.
PLANT AND PROCESS CONSIDERATIONS
The results of the laboratory experimental tests confirmed the technical feasibility of the olive oil mill VW depuration process based on a first evaporation step and on a thermal decomposition of the concentrated solution previously mixed with olive stone. In particular, considering the concentration levels that can be reached by the industrial evaporators and
the characteristics of the different VW [4,5], it has been verified that when mixing concentrated solution coming from the evaporator with the corresponding amount of olive stone, a solid-like mixture is obtained which has a consistence and a humidity typical of fresh wood chips. The main difference between the examined mixtures and the wood chips concern: the lower content of fixed carbon, the higher content of mineral salts, and the fact that a previous drying step of the above mentioned solid-like mixture could produce polluting vapors. As a consequence it is necessary to study a specific continuous pyrolytic reactor which permits to do the pyrolysis of the examined mixtures with the same efficiency and operating facility of the modem reactors used for the pyrolysis of the wood chips. Such a task can be accomplished by proper modification of the existing industrial reactors [6,7]. About the potentiality of the depuration plants, it’s important to point out some factors, the most important of which are: the dimensions of the olive oil mills, and the forced courses that the lignocellulosic subproducts and the aqueous effluents must follow; * the investment and operating costs of each unit which form the treatment plant (concentration, distillation, pvrolysis and combustion of the pyrolytic gases) and of the whole plant; - the management of the plant. *
The two extreme cases, namely: the realization of a depuration plant for each olive oil mill (where the VW are produced) and the realization of a depuration plant for each husk extraction plant (where olive stone from exhausted husk are available), don’t seem to be feasible. In fact, in the first case the transportation would be low-cost (considering that the olive stone can be transported during the back trips from the extraction plant) but, the management of an industrial plant, as basically is the one proposed, is not affordable at the scale of typical olive oil mills. In the second case the greatest integration would be achieved between the subproduct and the aqueous effluents industrial treatment, with enormous advantages for both investment and operating costs, but transportation would be much more expensive, in order to collect all the VW to the few husk extraction plants. A possible approach for choosing the treatment plant potentiality, is to state the potentiality of the pyrolytic section, or, as an alternative, that of the evaporator.To give an example, it has been studied the potentiality of a plant that could serve the “Valle Peligna” area which is a medium olive producing area of central Italy. The total number of existing olive oil mills working is 19: of these 11 are traditional oil mills while 8 are continuous centrifugation mills. The total amount of olives worked in a season is 4130 tons: of these 2130 tons are worked by the traditional oil mills and 2000 tons by the
257
continuous ones. The total volume of the VW produced is 4504 m3: 2800 m3 by the continuous oil mills, 1704 m3 by the press oil mills. Considering a percentage of residual dry solid at 383 K of 11.6% for the VW coming from the traditional oil mills and of 9.3% for the VW coming from the continuous ones and a percentage of mineral salts equal to 1.5% and 0.6% respectively, we can estimate the following Average values for the Dry Solid Residual (ADSR) and for the Mineral Salt Content (MSC) ADSR = (11.6 - 1704 + 9.3 - 2800)/4504 = 10.2%
(1)
MSC = (1.5 - 1704 + 0.6 .2800)/4504
(2)
= 0.94%
We can also assume that the amount of olive stone resulting from the virgin olive oil production in the above mentioned area is 826 tons, corresponding to 20% of the total amount of processed olives.The amount of the concentrated solution at 50% of water content is approximately 920 tons, twice as much as the ADSR. With this assumption R = 1.1. For the energy balance we can also assume, with a good approximation, that the HHV of the olive stone is equal to 4300 kcal/kg [l] and that of the organic substances present in ADSR is not less than 5000 kcal/kg; it follows that the HHV of the Concentrated Solution (HHVCS) is given by: HHVCS = 5000 - 4504 * (0.102-0.0094)/920 = 2267 kcal/kg
(3)
The existing continuous pyrolytic reactors for industrial production of charcoal have an optimal potentiality between 1000 and 2000 kg/hr of wood chips. Therefore a pyrolytic reactor is chosen having a potentiality of 1500 kg/hr of solid-like mixture obtained by mixing 714 kg/hr of olive stone with 786 kg/hr of concentrated solution with a 50% of water content. As a consequence the Evaporator Potentiality (EP) is: EP = 786/2/0.102
= 3853 kg/hr
(4)
This value is very close to the 4 m3/hr of VW indicated by the constructors as the optimal value of the multiple effect evaporator potentiality. It is also worthwhile to point out that the solid-like mixture feeded to the pyrolytic reactor has a water content of 29% by weight, and therefore, from the humidity and granulometric point of view, it is very similar to a wood chip essence partially dried. Referring to the pyrolytic experimental tests reported in Table 2, the VW treatment plant of the considered area produces 300 kg/hr of charcoal
258
powder (350 tons/year), which is worth more than $85,000. From the energetic point of view it is also possible to demonstrate that the VW treatment plant is largely self sufficient. In fact, the energy obtained by burning 786 kg/hr of concentrated solution (1782 Mcal/hr) can be added to the net energy resulting from the olive stone pyrolysis which, in a rough approximation, is the difference between the energy corresponding to the combustion of 714 kg/hr of olive stone (3070 Mcal/hr), and the energy corresponding to the combustion of 300 kg/hr of charcoal (1860 Mcal/hr). From an enthalpic balance of the evaporation section it can be estimated that the energy required to evaporate 3076 kg/hr of water is less than 2000 Mcal/hr, even in the most conservative case of a single effect evaporator. It is therefore clear that even considering the thermal efficiency of the different units, and even in the worst case of VW coming only from continuous oil mills, a significative amount of exceeding energy is available. This could be used with profit in a distillation section to recover the organic volatile products such as alcoholic phlegm, whose concentration is related to the available excess of energy, and to the characteristics of the distillation plant. With the above considered potentialities the proposed plant is able to get rid of the total amount of VW of the considered area in about 50 days, working in continuous cycle, starting just after the beginning of the olive harvest, in order to minimize times and volumes of the VW stocking.
CONCLUSIONS
The evaporation and pyrolytic tests performed at laboratory scale with solid-like mixtures of olive stone and concentrated solution with 50% water, have demonstrated the technical feasibility of a new VW depuration process based on evaporation and thermal decomposition of the resulting concentrated solution. In particular, it has been verified that during the pyrolysis the mineral salts deposit on the charcoal, while the heavy organic compounds originally present in the VW are vaporized and leave the reactor with the gaseous stream, together with water and other volatile products resulting from the partial thermal decomposition of the olive stone present in the charge. The main subproduct of this VW depuration process is the charcoal whose commercial value can give a great help to limit the depuration costs. On the other hand, the integration of a VW depuration process with the carbonization of the olive stone from exhausted husk and/or other discharged lignocellulosic biomasses can be a real possibility to valorize these sub-
products which, in the modern agricultural managements, have lost much of their traditional value. The studied depuration process for VW is applicable with minor modifications also to other muddy or liquid effluents coming from the agro-industry.
ACKNOWLEDGEMENTS
This work was supported by the Regione Abruzzo, ERSA.
REFERENCES 1 G Dt Gtacomo, E Bonfttto, N Brunette, G Del Re, S Jacoboni, Pyrolysts of Exhausted Olive 011 Husks Coupled wtth Two-Stages Thermal Decomposttion of Aqueous Ohve 011 Mills Effluents, in’ G L Ferrero, K Mamatts, A Buenkes and A V Bndgwater (Eds ), Pyrolysts and Gastftcatton, Elsevter, New York, 1989, pp 586-590 2 G Dt Gtacomo, Research report - ERSA, April 1990 3 G Dt Gtacomo, E Bonfttto, N Brunetti, G Del Re, S Jacoboni, Procedtmento per la depuraztone delle acque dt vegetaztone effluent1 da1 frantot oleari, Submisston for Italian Patent n 48008-A/1989 4 L Dt Gtovacchmo, Sulle carattensttche delle acque dt vegetaztone delle ohve - Nota I, La Rtvtsta Itahana delle Sostanze Grasse, 62 (1985) 411-418 5 L Di Gtovacchmo, Sulle carattertsttche delleacque dt vegetaztone delle ohve - Nota II, La Rtvtsta Itahana delle Sostanze Grasse, 65 (1985) 481-488 6 D Barba, G Di Gtacomo, A. Germans, Implant0 m contmuo a svduppo ortxxontale per la ptrohst dt legno, m parttcolare legno provemente da1 taglio dt bosco ceduo, Itahan Patent No 1178178 (1987) 7 A V Bndgwater, Biomass Pyrolysts Technologtes, m G Grasst, G Gosse and G dos Santos (Eds ), Biomass for Energy and Industry, 5th E C Conference, Vo12, Converston and Utihsatton of Biomass, Elsevter, New York, 1990, pp 2 489-2 496
Elsewer Science Publishers B.V., Amsterdam
Evaporation of Olive Oil Mill Vegetation Waters G. DI GIACOMO, V. BRANDANI and G. DEL RE University of L’Aquila, Deportment of Chemistry, Chemical Engineering and Materials, Monteluco dz Roio, LXquila (Itab)
67040
SUMMARY
The evaporation of olive oil mill vegetation waters permits to pull down the polluting load to an extent superior to the 90% in terms of COD. This operation can be easily done by using the industrial evaporators now existing, and it can be optimized up to a level whose value depends on the nature and age of the vegetation water in question. The concentrated solution which comes out from the evaporator can be thermally decomposed by a two stage pyrolytic process which allows to previously divide the inorganic salts, avoiding, in this way, to encrust the boiler during the combustion of the polluting organic substances. Since the thermal decomposition process of the concentrated solution is strongly exothermic, the exceeding heat can be used to ulteriorly lower the COD of the treated water to values compatible with the environmental regulations.
INTRODUCl-TON
The depuration of the aqueous effluents resulting from the transformation processes of some of the typical agricultural products of the mediterranean regions is a problem which has a particular importance, both for their quantity, and for the presence of still unsolved technical difficulties for the realization of treatment processes that are efficient and that have low costs of construction and operation. Typical examples of what has been said are represented by the depuration of the distillery vinasses and the olive oil mill vegetation waters which, in 249
250
Italy, reach the total amount of almost 3 million cubic meters per year. The peculiarities which make particularly difficult their treatment are a high COD value (over 200 g/l) the nature of the polluting organic compounds, and the high percentage of dissolved minerai salts and of solids in suspension. So far the most used industrial process for treatment of both vinasses and vegetation waters (VW), are based on a concentration section with a single or multiple effect evaporator from which a distillate is obtained, whose residual COD value is not superior to 5-7% than the initial one; at the same time 10 to 20% of the feeded liquid leaves the evaporator from the bottom as concentrated solution characterized by a water content of 30-50% by weight. The recondensed vapor can be sent, as it is, either to the biological depuration, or, as for the VW, it can be distilled in order to regain an alcoholic phlegm whose commercial value should not be completely neglected. On the contrary, it is much more difficult to work with the concentrated solution that, even if it is in smaller quantities than at the beginning, must be destroyed in some way, or made inert and then got rid of. Many systems have been proposed to treat this concentrated solution; as to spread it over the soil, or to use it as a component for the production of fodder or of agricultural amendments, or as a fuel, since its higher heating value (HHV) is calculated between 2000 and 3000 kcal/kg. The use of the concentrated solution as fuel is particularly attractive, since while it could be definitely pulled down the polluting load, on the other hand it could be a helpful low-cost alternative to the ordinary fuel for the evaporator or/and the bottom column boiler in case the distillation is considered. For this reason, in the distilleries and in the few VW treatment plants existing there are usually special boilers which should burn the mud and the concentrated solution coming from the aqueous effluent concentration. Anyhow it is well known that usually these devices don’t work as they are supposed to, since the inorganic salts present in the concentrated solution (from 5 to 10% by weight) melt during the combustion and their deposits encrust the pipes of the boiler making it shortly inefficient. Therefore this part of the plant is often unemployed and it is replaced by boilers that work with methane or oil making the depuration process much more expensive. In previous works [l-3] a process has been studied which permits to execute the combustion of the concentrated solution, avoiding the typical problems related to the high salt content. In particular the concentrated solution previously mixed with olive stone is pyrolized to separate the inorganic salts, which deposit on the charcoal bed. The vapor stream, leaving from the top of the pyrolytic reactor, can then be burned to destroy the polluting organic substances and to recover the energy required in the evaporation and distillation sections. The above described method can be also regarded as an
objective evaluation of lignocellulosic residuals, which now are not anymore used in the managements of the modern farms and are always largely present when processing agricultural products. In the following pages there will be reported and described the results of the experimental tests made in order to individuate the best concentration level in the evaporation section and to verify the technical feasibility of the pyrolytic process.
EXPERIMENTALSECTION The evaporation tests have been performed at laboratory scale with a glass discontinuous evaporator using VW coming from both traditional and continuous olive oil mills and having different ages. The results are shown in Figs. 1 and 2 where the different curves are referred to samples of water coming from the same olive oil mill and from the same lot of olives having an increasing aging from 1 to 5. An important peculiarity of these curves is the existence of an optimal value of the concentration level, which is related to the presence of a minimum that is generally localized near the COD value corresponding to a water content of 50% by weight in the concentrated solution coming out of the evaporator. The presence of this minimum is explainable since, unlike the other aqueous effluents of the agro-industrial processes, in the VW there are both compounds more volatile than water (most of them produced by the fermentation of sugary substances) and compounds less volatile than water as for instance the polyphenols. Further advantage of using evaporation instead of other depuration processes is represented by the fact that the head product is an alcoholic phlegm whose further processing in a distillery could help the economy of the VW treatment process. The pyrolytic experimental tests have been performed with a batch laboratory reactor made of AISI 304 steel heated by a stove having an automatic temperature regulator. This reactor has an useful volume of about 0.65 liters and it is provided with: a flanged cover that permits to easily get into the reactor, a hole at the top connected with the pipe that discharges the pyrolytic gases, and a filter, made by 100 mesh metallic net jointed at the cover, which is placed between the charge to pyrolize and the hole for the exit of the gases, in order to avoid that some solid particles may be dragged during the most violent phase of the pyrolysis. The pipe for the pyrolytic gas discharge, which gets off from the stove through a special hole, is connected, in some cases, to a surface glass condenser which allows, adjusting the, temperature of the cooling water, to obtain a partial condensation of the gaseous stream
25? TABLE I
Pulling down of the COD following the concentration process through evaporation lOO-(COD of the condensed solutton/COD of VW feeded) Sample VW from centrifuge concentrated solution up to 50% of water VW from press concentrated solution up to 50% water VW from centrifuge condensed at minimum COD VWfrom press condensed at minimum COD
12000
I
II
III
IV
V
97.6
96.6
95.7
94.6
93.9
96.0
95.4
94.6
93.7
93.0
97.6
96.0
94.1
93.5
93.4
95.7
95.3
94.4
93.8
92.9
0 TE!ST n. 1 Al-Egfl.2 l TESTn.3
+TESTn.4 oTESTn.5
I
0
20
WEIGHT
40
PERCENT
60
60
1
OF DISTILLATE
Fig. 1. COD of &stilIate vs the weight percent of distillate of VW, having different aging, increasing from l-5 and coming from continuous centnfugatlon oil m1lIs
253 32000
T
TEST n. 1
30000
0
26000 26000
ATEmn.2
24000 22000
+TESTn.4
n
TEST n. 3
oTEXn.5
0 20000 E v 16000 n 16000 ;\ 8 14000 12000 1oow 6000 & L 4000’
0
20
WEIGHT
40
PERCENT
60
60
100
OF DISTILLATE
Fig 2 COD of distillate vs the weight percent of distillate of VW havmg different aging, increasing from l-5 and coming from traditlonal oil mtlls
The results of the pyrolytic experimental tests done with charges of lignocellulosic material obtained mixing, in different ratios, the olive stone of exhausted husk with the concentrated solution resulting from the evaporation of the VW at 50% of water, are shown in Table 2. The yield of liquid products, AP2 and OP and that of charcoal, C, has been obtained by weighting the products, at the end of each test that lasts about 40-45 minutes. The amount of gas burned at the exit has been evaluated from the mass balance, knowing the total weight of the charge put in the reactor before the beginning of the test. The results reported in Table 2 show that the percentage of charcoal ob@nable when using only olive stone is considerably higher than the one obtainable with continuous units heated directly by the partial combustion of the charge [l] while it is considerably lower the percentage of bio-oil which is estimated to be about the 3.5% of the charge. Furthermore, as a result of the tests from number 5 to 11 we can see that the percentage of charcoal obtained decreases while R increases. Nevertheless, if the charcoal
254 obtained refers to the amount increases with R.
of olive stone charged,
the percentage
TABLE II Results of the pyrolytic experimental concentrated solutions of VW
tests obtamed
with mixtures
of olive stone and
Tn
TC,g
R
U%
MT,K
Cg
Gas,g
AP,g
OP,g
1 2 3 4 5 6 7 8 9 10 11
100.0 100.0 100.0 100.0 98.5 98.5 98.0 98.4 98.6 98.4 98.0
0.0 0.0 0.0 00 0.6 1.2 1.6 1.6 1.6 1.6 1.6
6.5 6.5 6.8 6.8 21.9 31.7 35.0 34.2 35.6 35.1 34.5
733 733 813 813 813 813 813 813 813 813 813
37.0 34.6 32.1 31.6 22.3 16.7 14.4 13.8 13.8 14.0 14.7
63.0 65.4 67.9 68.4 23.6 25.1 22.8 78.8 41.2 25 3 22.8
50.0 54.2 58.6 4.0 41.6 56.8 58.0
2.6 2.5 2.2 19 2.0 2.3 2.5
Tn = test number TC = total weight of the charge m the reactor, R = concentrated solution/olive stone (by weight); U !% = hurmdity of TC; MT = maximum temperature; C = quantity of charcoal obtained; AP = aqueous phase gathered; OP = organic phase gathered.
This shows that, during the pyrolysis, the salts deposit on the charcoal bed, while the organic substances present in the concentrated solution are partially decomposed and vaporized, and get off the reactor as a gaseous stream together with the water and the other organic products coming from the ligneous biomass decomposition. This can be indirectly confirmed comparing the different heat of combustion between the charcoal produced in test number 3 (7740 kcal/kg) and that produced in test number 10 (6200 kcal/kg). In test number 5 and 10 the heat of combustion of the gatheped organic phase has been measured, obtaining respectively 8034 and 7840 kcal/kg. Such high values compared to those of the oils obtainable from direct heating continuous processes are compatible with the hypothesis that, if the pyrolysis takes place in a partially oxidizing atmosphere, the liquid pyrolytic products are in greater quantities and richer in oxygen.
Still referring to Table 2 we can see that in correspondence with test number 8 the amount of aqueous phase is of one order of magnitude lower than all the others, while the amount of bio-oil changes much less. In this case the partial condensation of the gaseous stream coming from the reactor has been done making water circulate in the condenser at 372.6 K. This demonstrates that, for the industrial plants, it is possible to recuperate an organic phase to use as liquid fuel causing a condensation at a controlled temperature, and in any case superior to water boiling temperature, thus avoiding the production of a polluting aqueous solution. As a matter of fact the COD of the aqueous phase in test number 10 is 220 g/l. It’s evident, anyhow, that the recovery of a liquid organic phase strongly cuts down the combustible substances in the gas lowering its heat of combustion. On the other hand, it can be deduced that the gaseous stream coming out from the pyrolytic reactor has a heat of combustion between 2000 and 3000 kcal/kg, and it can be considered an excelIent gaseous fuel. The opportunity, for the industrial plants, to burn directly the gaseous stream coming out of the reactor, or to insert, between the pyrolytic reactor and the gas burner, a controlled temperature condenser for the recovery of a liquid organic phase to be used as a combustible oil, is strictly connected to other variables of the process (for instance the water content of the concentrated solution) as well as to the constructive characteristics and to the operating conditions of the pyrolytic reactor. Still referring to the pyrolytic experimental tests of Table 2, it is important to clarify that the interval between 0.6 and 1.6 chosen for R includes in practice all the possible natural mixtures, considering: the different kinds of olive oil mills existing, the different varieties of olives used for the production of virgin oil, the differences connected with the season variations and with a water content of the concentrated solution coming from the evaporator which is lower than 50% by weight. This concentration value is easily achievable with the modern industrial evaporators and, as the evaporation experimental tests show, in many cases is very close to the optimal value if pulling down of the COD values is considered the aim of the evaporation.
PLANT AND PROCESS CONSIDERATIONS
The results of the laboratory experimental tests confirmed the technical feasibility of the olive oil mill VW depuration process based on a first evaporation step and on a thermal decomposition of the concentrated solution previously mixed with olive stone. In particular, considering the concentration levels that can be reached by the industrial evaporators and
the characteristics of the different VW [4,5], it has been verified that when mixing concentrated solution coming from the evaporator with the corresponding amount of olive stone, a solid-like mixture is obtained which has a consistence and a humidity typical of fresh wood chips. The main difference between the examined mixtures and the wood chips concern: the lower content of fixed carbon, the higher content of mineral salts, and the fact that a previous drying step of the above mentioned solid-like mixture could produce polluting vapors. As a consequence it is necessary to study a specific continuous pyrolytic reactor which permits to do the pyrolysis of the examined mixtures with the same efficiency and operating facility of the modem reactors used for the pyrolysis of the wood chips. Such a task can be accomplished by proper modification of the existing industrial reactors [6,7]. About the potentiality of the depuration plants, it’s important to point out some factors, the most important of which are: the dimensions of the olive oil mills, and the forced courses that the lignocellulosic subproducts and the aqueous effluents must follow; * the investment and operating costs of each unit which form the treatment plant (concentration, distillation, pvrolysis and combustion of the pyrolytic gases) and of the whole plant; - the management of the plant. *
The two extreme cases, namely: the realization of a depuration plant for each olive oil mill (where the VW are produced) and the realization of a depuration plant for each husk extraction plant (where olive stone from exhausted husk are available), don’t seem to be feasible. In fact, in the first case the transportation would be low-cost (considering that the olive stone can be transported during the back trips from the extraction plant) but, the management of an industrial plant, as basically is the one proposed, is not affordable at the scale of typical olive oil mills. In the second case the greatest integration would be achieved between the subproduct and the aqueous effluents industrial treatment, with enormous advantages for both investment and operating costs, but transportation would be much more expensive, in order to collect all the VW to the few husk extraction plants. A possible approach for choosing the treatment plant potentiality, is to state the potentiality of the pyrolytic section, or, as an alternative, that of the evaporator.To give an example, it has been studied the potentiality of a plant that could serve the “Valle Peligna” area which is a medium olive producing area of central Italy. The total number of existing olive oil mills working is 19: of these 11 are traditional oil mills while 8 are continuous centrifugation mills. The total amount of olives worked in a season is 4130 tons: of these 2130 tons are worked by the traditional oil mills and 2000 tons by the
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continuous ones. The total volume of the VW produced is 4504 m3: 2800 m3 by the continuous oil mills, 1704 m3 by the press oil mills. Considering a percentage of residual dry solid at 383 K of 11.6% for the VW coming from the traditional oil mills and of 9.3% for the VW coming from the continuous ones and a percentage of mineral salts equal to 1.5% and 0.6% respectively, we can estimate the following Average values for the Dry Solid Residual (ADSR) and for the Mineral Salt Content (MSC) ADSR = (11.6 - 1704 + 9.3 - 2800)/4504 = 10.2%
(1)
MSC = (1.5 - 1704 + 0.6 .2800)/4504
(2)
= 0.94%
We can also assume that the amount of olive stone resulting from the virgin olive oil production in the above mentioned area is 826 tons, corresponding to 20% of the total amount of processed olives.The amount of the concentrated solution at 50% of water content is approximately 920 tons, twice as much as the ADSR. With this assumption R = 1.1. For the energy balance we can also assume, with a good approximation, that the HHV of the olive stone is equal to 4300 kcal/kg [l] and that of the organic substances present in ADSR is not less than 5000 kcal/kg; it follows that the HHV of the Concentrated Solution (HHVCS) is given by: HHVCS = 5000 - 4504 * (0.102-0.0094)/920 = 2267 kcal/kg
(3)
The existing continuous pyrolytic reactors for industrial production of charcoal have an optimal potentiality between 1000 and 2000 kg/hr of wood chips. Therefore a pyrolytic reactor is chosen having a potentiality of 1500 kg/hr of solid-like mixture obtained by mixing 714 kg/hr of olive stone with 786 kg/hr of concentrated solution with a 50% of water content. As a consequence the Evaporator Potentiality (EP) is: EP = 786/2/0.102
= 3853 kg/hr
(4)
This value is very close to the 4 m3/hr of VW indicated by the constructors as the optimal value of the multiple effect evaporator potentiality. It is also worthwhile to point out that the solid-like mixture feeded to the pyrolytic reactor has a water content of 29% by weight, and therefore, from the humidity and granulometric point of view, it is very similar to a wood chip essence partially dried. Referring to the pyrolytic experimental tests reported in Table 2, the VW treatment plant of the considered area produces 300 kg/hr of charcoal
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powder (350 tons/year), which is worth more than $85,000. From the energetic point of view it is also possible to demonstrate that the VW treatment plant is largely self sufficient. In fact, the energy obtained by burning 786 kg/hr of concentrated solution (1782 Mcal/hr) can be added to the net energy resulting from the olive stone pyrolysis which, in a rough approximation, is the difference between the energy corresponding to the combustion of 714 kg/hr of olive stone (3070 Mcal/hr), and the energy corresponding to the combustion of 300 kg/hr of charcoal (1860 Mcal/hr). From an enthalpic balance of the evaporation section it can be estimated that the energy required to evaporate 3076 kg/hr of water is less than 2000 Mcal/hr, even in the most conservative case of a single effect evaporator. It is therefore clear that even considering the thermal efficiency of the different units, and even in the worst case of VW coming only from continuous oil mills, a significative amount of exceeding energy is available. This could be used with profit in a distillation section to recover the organic volatile products such as alcoholic phlegm, whose concentration is related to the available excess of energy, and to the characteristics of the distillation plant. With the above considered potentialities the proposed plant is able to get rid of the total amount of VW of the considered area in about 50 days, working in continuous cycle, starting just after the beginning of the olive harvest, in order to minimize times and volumes of the VW stocking.
CONCLUSIONS
The evaporation and pyrolytic tests performed at laboratory scale with solid-like mixtures of olive stone and concentrated solution with 50% water, have demonstrated the technical feasibility of a new VW depuration process based on evaporation and thermal decomposition of the resulting concentrated solution. In particular, it has been verified that during the pyrolysis the mineral salts deposit on the charcoal, while the heavy organic compounds originally present in the VW are vaporized and leave the reactor with the gaseous stream, together with water and other volatile products resulting from the partial thermal decomposition of the olive stone present in the charge. The main subproduct of this VW depuration process is the charcoal whose commercial value can give a great help to limit the depuration costs. On the other hand, the integration of a VW depuration process with the carbonization of the olive stone from exhausted husk and/or other discharged lignocellulosic biomasses can be a real possibility to valorize these sub-
products which, in the modern agricultural managements, have lost much of their traditional value. The studied depuration process for VW is applicable with minor modifications also to other muddy or liquid effluents coming from the agro-industry.
ACKNOWLEDGEMENTS
This work was supported by the Regione Abruzzo, ERSA.
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