Ind. Eng. Chem. Res. 2000, 39, 4597-4602
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Pilot Plant for Soil Remediation with Supercritical CO2 under Quasi-Isobaric Conditions M. J. Cocero, E. Alonso,* and S. Lucas Departamento Ingenierı´a Quı´mica, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, Spain
In the Chemical Engineering Department at the University of Valladolid (Spain), a pilot plant for soil remediation with supercritical CO2 extraction and subsequent adsorption on activated carbon has been developed. Soil polluted with hydrocarbons from a petrochemical plant has been treated in order to reduce the pollution level below legal limits. Operational parameters such as extraction temperature, solid particle size, and solvent ratio were optimized. Experiments were conducted at a 30-60 °C extraction temperature, combined with a temperature of 50 °C for the adsorption onto activated carbon. The solvent flow rate was varied between 5 and 15 kg/h, and the effect of particle size in the process was studied for particles with dp < 0.425 mm. Several experiments have shown that a gradient in temperature is necessary between the extraction step and the adsorption step and that extraction temperature should be lower than adsorption temperature. Results show that temperature increases improved both initial rates of extraction and total recoveries, maintaining in every case the gradient mentioned above. The solubility of the contaminant in the SCF does not limit SFE, and diffusion into the CO2 phase was found to be the limiting step in the extraction rate. Results show an optimal extraction temperature of 40 °C, particle size of 0.425 mm, and solvent ratio of 15.8 kg CO2/[(kg soil) h]. A comparison of this soil with the same soil contaminated with diesel oil in the laboratory has been made. The effect of particle size is particularly strong for the soil recently spiked in the laboratory; in the studied range of particle sizes, the bigger the particles, the better the extraction. That can be explained by the fact that, for aged soil, pollutants are more strongly adsorbed on the soil and the effect of particle size is not as important as the bond forces. Introduction Soil remediation using CO2 in a supercritical extraction process is one innovative technique currently available. This technology has been successfully applied to a variety of soil pollutants, including hydrocarbons, pesticides, phenols, etc., in a word, persistent pollutants of difficult treatment. Carbon dioxide extractions can be carried out at low temperature, and in the event of soil remediation, that means that such processes would not require as much energy as thermal treatment and that the structure and nutrients in the soil would remain relatively intact.1 Moreover, carbon dioxide does not leave any solvent residues in soil and it is nonflammable, nontoxic, nonreactive, and relatively inexpensive.2,3 Conclusions obtained from such studies suggest that, although this technology is highly valuable for specific types of soil, it is associated with a high cost of treatment, mainly due to the recompression cost of the supercritical fluid (usually CO2), because it is necessary to depressurize the fluid to CO2 gas to recirculate it free of contaminants and to operate under the most favorable operating conditions.4 Bearing in mind the objective of minimizing the cost of CO2 recompression, some authors have proposed isobaric separations among CO2 and solute.5,6 In this paper, a step forward is given through an alternative approach consisting of an isobaric extraction with subsequent adsorption of extracted pollutants on activated carbon, instead of the customary pressure reduction. Under these conditions, the plant is operating * Author to whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +34 983 42 31 74. Fax: +34 983 42 31 66.
in a quasi-isobaric state, and therefore, the depressurization-repressurization cycle is avoided, leading to energy cost minimization in running of the plant. This process allows for the concentration of the pollutant for its later destruction or disposal and for the attainment of clean soil that is suitable to return to its natural environment. It is necessary to mention the additional costs of regeneration or disposal of the spent adsorbent material, which can make the process economically not competitive against depressurization. However, the adsorption alternative becomes attractive when low pollutant concentrations are required in the soil, and that is the current trend in soil legislation. In these cases, depressurization is not sufficient to clean the solvent completely, and a subsequent treatment, such as adsorption, is needed, increasing the total cost of the process. A pilot plant for soil remediation has been designed and built in the Chemical Engineering Department at the Valladolid University (Spain). It is a two-step integrated plant comprising CO2 supercritical extraction and pollutant adsorption on activated carbon. The pilot plant would operate under quasi-isobaric conditions and with recirculation of the CO2. The pilot plant was designed to operate at P < 30 MPa, T < 80 °C, and a CO2 mass flow rate of 1-20 kg/h.7 Experimental Section The pilot plant flow diagram is schematically presented in Figure 1. It was based on a first pressurized vessel acting as an extractor (where the soil to be remediated was located), a second pressurized vessel
10.1021/ie000183y CCC: $19.00 © 2000 American Chemical Society Published on Web 11/01/2000
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Figure 1. Flow diagram of pilot plant for supercritical extraction under isobaric operating conditions.
acting as an adsorber (where activated carbon was placed), a pump for solvent supply, and auxiliary equipment such as heat exchangers, pressure, temperature, and flow meters. Pressurized Vessels. Both pressurized vessels were made of a jacketed stainless steel tube that was capped at the bottom end and had screws and a flange at the upper end. Soil was placed inside a “basket” that was adjusted to the walls by gaskets to avoid solvent bypass. Uniform distribution of solvent was achieved by a diffuser at the bottom of the chamber. When it was needed, a second extractor was placed in series. The dimensions of the baskets where the soil and the carbon are located are 4 cm i.d. and 50 cm length, and in both vessels, CO2 flows from bottom to top. To prepare synthetic soils of the desired pollutant and/ or specific concentrations, a third pressurized vessel, similar to the ones cited above, was used as a saturator, so that CO2 saturated with pollutant was desorbed over a standard soil. It was assumed that pollutant was uniformly distributed over the standard soil. The desired temperatures in the vessels were adjusted by the jacket water. Pump. A diaphragm pump head type EH1, from LEWA Herbert Leomberg (Germany), was used. The admission valve of this pump withstands pressures up to 27.5 MPa. This pump is used to recirculate CO2 during operation and to provide the initial pressurization prior to an experiment. Auxiliary Equipment. Heat exchangers were made of coils inmersed in stirred temperature-controlled baths. The mass flow meter was a RHEONIK RHM 01 GNT meter, combined with an RHE 08 electronic unit and 1/4-in. tubing. Valves and fittings from Hoke that were suitable for high-pressure processes, together with the data acquisition system, were the rest of the auxiliary equipment. Materials and Methods The pilot plant was used to study soil remediation of industrial soil polluted by hydrocarbons from a petrochemical plant located at “El Fangal” Murcia (Spain).
Table 1. Soil Characteristics size analysis % sand % silt % clay fraction organic matter (g/g) porosity humidity (%) density (g cm-3) pH
88.15% 6.8% 5.05% 0.0088 0.52 0.89% 1.24 7.61
Table 2. Activated Carbon Chemviron F300 average particle size, mm total interfacial area (N2 BET method), m2/g bed density, kg/m3
1.6 1000 460
The pollution level in the soil, prior to the experiments and afterward, was determined by measuring the total hydrocarbon content by IR spectroscopy,8 and the initial soil pollution in these tests was typically around 17 000 mg/kg (total hydrocarbons). Two different sets of experiments were carried out. One was with the soil polluted with hydrocarbons from the petrochemical plant, and this soil will be called “aged soil” as it is recorded to have been polluted for the last 20 years.9 The other is the same soil (without any contaminant) spiked in the laboratory with a known amount of diesel oil. Table 1 summarizes the properties of the soil. The industrial granular activated carbon used was from Chemviron F 300. It is claimed to be useful for water organics (Table 2). As explained in this analytical method,8 the soil samples were extracted with 1,1,2-trichlorotrifluoroethane using intense shaking over at least a 4-h time period, and the absorbances of these extracts were measured at a wavelength of 2930.06 cm-1. Moreover, soil contents were characterized by GC in an HP1 column with dimensions 25 m × 0.2 mm i.d. and with a 0.33-µm film thickness. These analyses revealed the presence of hydrocarbons in the range C5-C20, compounds such as methyl butane, n-heptane, n-octane, n-decane, and n-pentadecane were identified. Chro-
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Figure 2. (A) Chromatogram of polluted soil from petrochemical industry. (B) Chromatogram of diesel oil.
matograms for polluted soil and pure gas-oil are shown in Figure 2. Both analyses were performed under the same GC conditions, and they reveal a great similarity between soil pollutants from the petrochemical plant and gasoil. This fact supports the comparison between the two sets of experiments that is done in this work. Pilot plant operation started by loading the soil in the basket and placing it into the extractor. Previously activated carbon was placed into the adsorber, and then operational variables including pressure, flow, and extraction and adsorption temperatures were set. When
the selected temperatures were reached, CO2 circulation was initiated. The evolution of the soil total hydrocarbon concentration at the times 0, 30, and 60 min was studied. The final objective of this work is to determine the optimum process parameters (process temperature, extraction time, amount of solvent, and solvent ratio) in order to reduce pollution level bellow legal limits. Results and Discussion The desorption of organics from soil particles occurs through three consecutive mass transport steps: intra-
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Table 3. Dependence of Extraction Rate on Temperature Operating Conditions hydrocarbon-contaminated soil
a
run number size of solid particles (mm) pressure (bar) extraction temperature (°C) adsorption temperature (°C) solvent rate (kg/h) solvent ratio [kg/(kg h)]a extraction time (min)
1 0.212 260-270 30 50 5.5 5.8 60
2 0.212 260-270 40 50 5.5 5.8 60
3 0.212 260-270 50 50 5.5 5.8 60
4 0.212 260-270 55 50 5.5 5.8 60
5 0.212 260-270 62 50 5.5 5.8 60
initial hydrocarbon concentration (ppm) final hydrocarbon concentration (ppm) degree of extraction (%)
16600 2100 87
13700 450 97
16300 10450 36
16120 3200 80
15000 1550 89
Solvent ratio ) amount of solvent per unit of time and amount of solid.
Figure 3. Dependence of extraction rate on temperature. Adsorption temperature ) 50 °C.
particle diffusion from the interior to the outer surface of the particle, mass transfer of the organic from the outer surface of the particle to the gas phase, and bulk transport of the organic in the supercritical phase. Influence of Process Parameters. Extraction Temperature. In this plant, pollutant extraction and solvent separation take place at roughly the same pressure, and the driving force for adsorption is the ability of activated carbon to adsorb pollutants. The adsorption-desorption equilibrium is controlled in both steps (adsorption in the separation step and desorption in the extraction step) by temperature. Experiments were planned in two ways: on one hand, increasing extraction temperature from 30 to 50 °C were used, and on the other hand, temperatures from 50 to 62 °C were used, with the temperature being held constant at the adsorption temperature into activated carbon of 50 °C in every case. This means that, in the first case, the extraction temperature was lower than the adsorption temperature, and in the second, the extraction temperature was higher than the adsorption temperature. Operational restrictions made it impossible to increase the carbon temperature, because the cooling capacity in the plant was not enough for it to reach the liquid phase before pump recirculation. Because operation takes place at a pressure above the crossover point,5 in a general way, higher extraction temperatures causes higher extraction rates, as shown in the Table 3. One reason for this is the dependence of solvent power on temperature and the effect of this variable on contaminant vapor pressure. Another reason is related to the increase in mass transfer rates with temperature (Figure 3). A test performed at the same temperature in both steps (i.e., extraction temperature ) 50 °C, adsorption temperature ) 50 °C) showed that a difference in temperature is necessary between the extraction and
adsorption beds. Although at the beginning of operation contamination decreased in the soil, after 1/2 h of the experiment, pollutants returned to the soil from the carbon. Runs 4 and 5, for which the temperature on the activated carbon is lower than that in the soil, showed smaller extraction rates than runs 1 and 2. Similar results were found by Madras et al.10 for the extraction of naphthalene, phenanthrene, hexachlorobenzene, and pentachlorophenol. The effect of extraction temperature is a sum of the temperature effects on solute vapor pressure, on the density of the extraction fluid, and on the adsorption of the pollutant in the soil.3 In this case, the extraction is more efficient at higher temperatures, and that means that the effect of temperature on the pollutant’s vapor pressure prevails over the effect of this parameter on the CO2 density. Moreover, in this process, the adsorption equilibrium takes place in two ways: the pollutant’s desorption from the soil and the pollutant’s adsorption on the activated carbon. Several experiments have shown that a difference in temperature is necessary between the extraction and the adsorption step and that this temperature gradient is better when the temperature in the activated carbon is higher than that in the soil (Figure 3). Size of Solid Particles. The effect of soil particle size has been studied for two different sizes: 0.425 and 0.212 mm. Results showed that, in this range of particle size (dp < 0.425 mm), the extraction rate is higher with bigger particles. This effect is stronger for the “fresh” soil than for the aged soil. Because samples with smaller particles sizes have higher specific surface areas (per gram of soil), they may be expected to have more contaminant molecules adsorbed on the surface.11 Because pollutants have been in contact with the aged soil for many years, the time has been long enough for
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4601 Table 4. Dependence of Extraction Rate on Solid Size: Laboratory-Contaminated Soil with Diesel Oil Operating Conditions diesel-oil-contaminated soil size of solid particles (mm) pressure (bar) extraction temperature (°C) adsorption temperature (°C) solvent rate (kg/h) solvent ratio [kg/(kg h)]a extraction time (min)
0.212 260-270 30 50 5.5 5.8 60
0.425 260-270 30 50 5.5 5.8 60
initial hydrocarbon concentration (ppm) final hydrocarbon concentration (ppm) degree of extraction (%)
11200 3800 66
7200 350 95
a Solvent ratio ) amount of solvent per unit of time and amount of solid.
Table 5. Dependence of Extraction Rate on Particle Size: Aged Soil with Hydrocarbons Operating Conditions hydrocarbon-contaminated soil size of solid particles (mm) pressure (bar) extraction temperature (°C) adsorption temperature (°C) solvent rate (kg/h) solvent ratio [kg/(kg h)]a extraction time (min) initial hydrocarbon concentration (ppm) final hydrocarbon concentration (ppm) degree of extraction (%)
0.212 260-270 40 50 5.5 5.8 60 13700 450 97
0.425 260-270 40 50 5.5 5.8 60 20100 350 98
Figure 4. Dependence of extraction rate on particle size: (a) laboratory-contaminated soil with diesel oil, (b) aged soil with hydrocarbons.
a Solvent ratio ) amount of solvent per unit of time and amount of solid.
strong associations between contaminant molecules and soil particles to be formed, even with relatively large particles. That fact explains that, in the aged soil, the effect of particle size is weaker than in the soil spiked recently in the laboratory. In general, the extraction rate increases with decreasing particle size. However, in this case, the smaller particles could hinder fluid flow in the fixed bed with the apparition of channeling, and therefore, the mass transfer rate decreases with smaller particles, as shown in Tables 4 and 5 and in Figure 4a and b. Moreover, operation with a particle size of 0.212 mm leads to compaction of the bed, which brings an undesirable pressure drop. The same trend is noticed with dieseloil-contaminated soil and with hydrocarbon-contaminated soil. Solvent Ratio. Solvent ratio is one of the most important parameters for supercritical extraction. The influence of CO2 flow has been studied in the extraction of aged soil contaminated with hydrocarbons. With
Figure 5. Dependence of extraction degree on solvent amount.
increasing solvent ratio the extraction rate is enhanced significantly, as shown in Figure 5. When the solvent ratio increases, the final hydrocarbon concentration decreases, reaching a degree of elimination of 95% when operated with the maximum solvent flow [solvent ratio of 15.8 kg/(kg h)]. See Table 6.) Results with different solvent flows showed that extraction is better when the solvent ratio increases, and this effect could be due to mass transfer or solubility resistance.12 Figure 5 shows a higher degree of elimination when the solvent ratio increases for the same
Table 6. Dependence of Extraction Rate on Solvent Flow Rate: Aged Soil with Hydrocarbons Operating Conditions hydrocarbon-contaminated soil size of solid particles (mm) pressure (bar) extraction temperature (°C) adsorption temperature (°C) solvent rate (kg/h) solvent ratio [kg/(kg h)]a extraction time (min) initial hydrocarbon concentration (ppm) final hydrocarbon concentration (ppm) a
0.212 260-270 32 50 5 5.3 16600 3500 78
Solvent ratio ) amount of solvent per unit of time and amount of solid.
0.212 260-270 32 50 10 10.5 16600 2100 87
0.212 260-270 32 50 15 15.8 16600 800 95
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amount of CO2 used. That means that, under these operating conditions (extraction temperature ) 32 °C, adsorption temperature ) 50 °C), contaminant solubility in the supercritical solvent is not the extraction ratelimiting step, and therefore, the main resistance to the extraction process is the external diffusion of contaminants in the CO2 phase. Adsorption Capacity of Activated Carbon. The capacity of the vessels and solid characteristics determined the amount of soil to be treated and the amount of activated carbon used in each run. Experiments took place in batch with 1 kg of soil loaded in the extractor and 360 g of activated carbon in the adsorber. At 50 °C and 260 bar, the adsorption capacity of activated carbon was found to be 0.061 g pollutant/g activated carbon for this kind of pollutant (hydrocarbons in the range C5-C20). That means that, for an initial pollutant concentration in the soil of 20 000 ppm, the required amount of activated carbon is 206 g. Because this amount is lower than the carbon loaded in the adsorber in each run, we can conclude that it is possible to completely clean the soil in every run, without adsorber regeneration. Conclusions The results for a pilot plant for soil remediation with supercritical CO2 have been shown. Contaminants are continuously extracted using supercritical carbon dioxide and then deposited onto an activated carbon bed. It was found that the process would operate in the optimum mode if the desorption from the soil was performed at lower temperatures than the adsorption onto activated carbon. Always with this constraint, the higher the extraction temperature, the better the extraction rate. In the range of the tested solid size (dp < 0.425 mm), the bigger the particle, the better the extraction process. The effect of particle size was stronger for spiked soil in the laboratory than for the aged soil. Experiments with these soils showed better results when the solvent ratio was increased because the main resistance to contaminant transport in the extraction bed is in the bulk phase. Acknowledgment Financial support of this work by CICYT project QUI96-0560 (Spain) and technical information about the soil from EMGRISA (Spain) are gratefully acknowledged.
Nomenclature dp ) particle diameter, mm C ) pollutant concentration in the soil, ppm (pollutant mg /kg soil)
Literature Cited (1) Bellandi, R. Innovative Engineering Technologies for Hazardous Waste Remediation. International Thomson Publishing: Stamford, CT, 1994. (2) Tomasko, D. L.; Macnanghton, S. J.; Foster, R. R.; Eckert, C. A. Removal of Pollutants from Solid Matrices using Supercritical Fluids. Sep. Sci. Technol. 1995, 30 (7-9), 1901-1915. (3) Laitinen, A.; Michaux, A.; Aaltonen, O. Soil cleaning by carbon dioxide extraction: a review. Environ. Technol. 1994, 15, 715-727. (4) Cocero, M. J.; Alonso, E.; Fdez-Polanco, F. Nuevas Tecnologı´as para la Regeneracio´n de Suelos. Procesos de Concentracio´n y Eliminacio´n de los Contaminantes con Fluidos Supercrı´ticos. Ing. Quı´m. 2000, Enero, 175-183. (5) Brunner, G. Gas extraction: an introduction to fundamentals of Supercritical fluids and the application to separation processes; Springer: New York, 1994. (6) Madras, G.; Erkey, C.; Akgerman, A. Supercritical Extraction of Organic Contaminants from Soil Combined with Adsorption onto Activated Carbon. Environ. Progress 1994, 13 (1), 45-50. (7) Alonso, E.; Alonso, J. J.; Cocero, M. J. Soil Remediation by Supercritical under Quasi-isobaric Operation Conditions. Proceedings of the International Solvent Extraction Congress (ISEC’99), Barcelona, Spain, 1999. (8) Method 5520 C: Partition-Infrared Method. Standard Methods for the Examination of Water and Wastewater, 19th ed.; Eaton, A. D., Clesceri, L. S., Greenberg, A. E., Eds.; United Book Press: Baltimore, MD, 1995. (9) Gonza´lez Aurioles, J. M. Recuperacio´n de un suelo contaminado. Ing. Quı´m. 1996, Sept, 239-242. (10) Madras, G.; Erkey, C.; Akgerman, A. Supercritical Extraction of Organic Contaminants from Soil Combined with Adsorption onto Activated Carbon. Environ. Progress 1994, 13 (1), 45-50. (11) Bjo¨rklund, E.; Bowadt, S.; Mathiasson, L.; Hawthorne, S. B. Determining PCB sorption/desorption behaviour on sediments using selective supercritical fluid extraction. 1. Desorption from historically contaminated samples. Environ. Sci. Technol 1999, 33, 2193-2203. (12) Erkey, C.; Madras, G.; Orejuela, M.; Akgerman, A. Supercritical Carbon Dioxide Extraction of Organics from Soil. Environ. Sci. Technol. 1993, 27, 1225-1231.
Received for review February 4, 2000 Revised manuscript received July 12, 2000 Accepted July 24, 2000 IE000183Y
