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Erschienen in: International Journal of Energy and Environmental Engineering 3/2017

Open Access 06.06.2017 | Original Research

Flowrate and water presence effect on venting/SVE process efficiency

verfasst von: Mariem Kacem, Daoud Esrael, Belkacem Benadda

Erschienen in: International Journal of Energy and Environmental Engineering | Ausgabe 3/2017

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Abstract

The objective of this paper is to study the effect of the soil water content “SWC” and flowrate on Venting and Soil Vapor Extraction “SVE” process efficiency. Two experiments simulating venting process and SVE process were done on sand column. Two sand water contents were tested: low SWC (7.6%) and a dry one, at low flowrate (600 mL min−1) and high flowrate (3200 mL min−1). Results show that the efficiency is better at low flowrate and dry sand whatever the process used (96.7%). It decreases with the water saturation and flowrate increases to reach 92.4%. The good efficiency, when low flowrate is applied, needs more time application (500 min) than when high flowrate is applied (300 min) for about 2.5% efficiency win. The presence of water and the low flowrate become more difficult than the contaminant extraction in the tailing part. It has been demonstrated the diffusional nature of this part and then the interest to introduce, in this step, other technics like the discontinued extraction, particularly for dry soil and high flowrate, where efficiency was improved by 1.3%. The utility of the optimal treatment time determination has been demonstrated to save the application time.

Introduction

The process of soil vapor extraction Venting/SVE has the role of the depollution of the unsaturated zone (vadose zone) in the subsurface by the evaporation of the non-aqueous phase liquid “NAPL” trapped at soil residual saturation. Pollutant evaporation is making by air circulation under pressure gradient in the pores at contact with NAPL. Pollutants in this zone are found in different states: dissolved in the residual saturation water, adsorbed on the organic matter and/or clay fraction of the soil, and evaporated in gas phase. At this zone, the NAPL and the aqueous phase are considered not mobile under pressure gradient. The SVE system combines vapor extraction wells with blowers or vacuum pumps to remove contaminant vapors from permeable to airflow zones. In venting system, airflow is inducing in soil. In the two cases, the air renewal in the soil pores has for consequence to modify chemical balances between the various present phases.
This technology can rapidly and efficiently remove several types of organic contaminants from sites. Nevertheless, the efficiency control needs a finely knowledge of the influencing parameters to optimize time and energy consuming. The mass transfer coefficient between NAPL and gas phase has an important role on the SVE method. NAPL–gas mass transfer depends on the soil properties and operating conditions. Some works propose an empirical equation to evaluate the NAPL–gas mass transfer coefficient [2325]. In those equations, authors expressed a relationship between the mass transfer coefficient, the interstitial pore velocity, and the mean granular diameter of soil particles. Furthermore, many authors observed a decrease of the mass transfer coefficient with the decrease of NAPL saturation by evaporation. This is due to the decrease of the interfacial area [7, 14, 2527]. They introduce a relation between mass transfer coefficient, NAPL saturation, and extraction time. Yoon et al. [25] varied soil water content “SWC” and observed a mass transfer limitation at 61% water saturation. They proposed an empirical mass transfer coefficient linked to the saturation in aqueous phase and to the soil uniformity. A model was proposed by Yoon et al. [26] to assess the impact of the variation of SWC on NAPL mass transfer, vapor phase retardation, and slow desorption during SVE.
To study the extraction process and parameters effects, it is important to look at the curve of the remaining contaminant. A typical one is given by US Environmental Protection Agency [5] and can be divided into three parts (Fig. 1):
  • The first one called “flushing phase”, the gas concentration is high and constant, reflecting the long-term soil-gas equilibria condition, and it is a short phase often ignored.
  • The second one called “evaporation phase”, corresponding to pollutant evaporation caused by contact between the air and NAPL, the contaminant concentration in the extracted gas decreases rapidly corresponding to the contaminant remove from permeable soil.
  • The last one is “the diffusion phase” or “tailing part”, it begin when the pollutant concentration reaches a value limiting the air availability corresponding to the cleaner of the permeable channels. The concentration decrease is less important. It corresponds to the decrease of the NAPL–gas mass transfer coefficient that is attributed to the decrease of the contact surface between phases. In fact, the removal of the residual contaminants requires volatilization from the dissolved aqueous phase and desorption from solid surfaces.
This last part is affected by the SWC. Yoon et al. [26] observed that at low SWC, slow desorption controls long tailing. When SWC is higher, NAPL is trapped by water and NAPL mass transfer to the gas phase is limited by diffusion through water films. In this case, NAPL mass transfer is slow and, thus, controls concentration tailing. Alvim-ferraz et al. [3] and Alberrgaria et al. [2] showed that the remediation time and efficiency are depending on the tailing part, where remove residual contaminants becomes more difficult. They find that this was more observed when SWC is higher and flowrate is lower. Ma et al. [16] find the same observations for flowrate variation.
Qin et al. [18] studied the influence of water content and vapor flowrate on removal Chlorobenzene by SVE. They found that SWC had different impacts on the contaminant removal efficiency which related to the organic content in the soil. Soares et al. [20] found that SVE is extremely efficient for the remediation of sandy soils contaminated with toluene and is independent on the SWC. They suggested as Albergaria et al. [1] that this result is related to the low organic matter content and high porosity of these types of soils, which are not able to significantly adsorb the contaminants. Amin et al. [4] found the increase of toluene removal with the decrease of the SWC for sandy soil without the presence of organic matter. Ma et al. [17] found that moisture undermines the volatilization of contaminants.
Concerning the vapor flowrate, Qin et al. [18] and Jiao et al. [11], shows that the increase of vapor flowrate leads to higher contaminant removal efficiency for the same period time application. However, they noted that the increment of removal was not significant at higher flowrate levels. Naon et al. [13] and Amin et al. [4] observed the improvement of extraction at the evaporation phase for high flowrate application, but at the end of extraction, the difference in the pollutant removal is not significant between the high and low used flowrates.
On the other hand, Albergaria et al. [1] and Soares et al. [20] stated that the use of lower air flowrates will guarantee that the process occurs in equilibrium conditions and that slow diffusion effects are avoided, enhancing the remediation.
Flowrate as an operator condition and SWC as a soil property are studied in this paper. The objective is to study their influence on the SVE/venting processes efficiency and the extracted gas curve profile. To show their interest, the discontinued extraction and the optimal time are also studied. Four tests of venting and SVE experiments were done using dry sand and low SWC, low and high flowrates.

Materials

Toluene C7H8 was used as a model pollutant, and Hostun sand HN 0.4/0.8 (SIBELCO) was used as a model soil. Table 1 shows a chemical composition given by supplier.
Table 1
Chemical composition of the used soil (given by supplier)
Chemical element
Mass percentage (%)
SiO2
>98.94
Fe2O3
<0.07
Al2O3
<0.35
K2O
<0.16
CaO
<0.05
Apparent sand density ρ b was measured at laboratory with the average of values obtained from five tests. Each test was running with compacted sand in a column. The same compaction process was reproduced in the experimental column and maintained to avoid the compaction influence on hydrodynamic properties. Porosity was deduced from the relation between the soil particles density and the apparent density ρ b . The sand apparent density was measured with the average of values obtained from five tests. The particle size distribution was determined using the French norm X 11-507 (Normalisation Françasie [15]. Permeability was measured by two methods: a constant head permeameter and an air permeameter using Darcy’s law. Three tests were conducted for each method. With the air method, three flowrates were tested. The residual saturation S w was measured using the Hang water method. Experimentally determined properties are summarized in Table 2.
Table 2
Characteristics of the used sand
Parameters
Sand HN 0.4/0.8
Method
d 50 (mm)
0.61
Particle size distribution curve
C u (−)
1.46
Cc (−)
1.09
Classification
55% medium sand
45% coarse sand
\(\rho_{b}\) (g.cm−3)
1.53 ± 0.06
Mean of five tests
\(\rho_{s}\) (g.cm−3)
2.65
Supplier
\({\varvec{\bar{\bar{K}}}}\) (m2)
3.21 ± 0.08 10−10
Constant head permeameter
3.53 ± 0.06 10−10
Air permeameter
S w (%)
14.2
Hang water method
\(\varphi\) (−)
0.42 ± 0.023
Relation \(\varphi = 1 - \frac{{\rho_{b} }}{{\rho_{s} }}\)

Experiments

Experimental device

A steel column of 10 cm inner diameter and 15 cm of height was used. The column bottom is a steel cylindrical form of 1.3 cm of thickness welded to the column. A steel cover with the same thickness allows closing the column. An O-ring was used for sealing. The sand bed was placed between two gravel layers (diameter between 2 and 4 mm) of 1.6 cm of thickness. Metallic grids were placed between sand and gravel. The column was filled with four layers of sand. Each sand layer was compacted using the same metallic cylinder. The fixed water content and pollution quantity were introduced in each sand layer in four introduction points at the layer sand surface. The column was rotated with 45° angle before each layer feeling. The column was then hermetical closed. To unsure the homogeneous repartition of pollutant, the soil is keeping closed for 2 days.
Depending on the employed technique, i.e., soil vapor extraction or venting, the column was connected to a vacuum pump “KNF Neuberger N840 FT-18” or to a nitrogen bottle, respectively. The used pipes are in Teflon (PTFE) and/or steel to avoid the eventual adsorption phenomena of the pollutant. The column output is connected to a cyclone and volumetric flowmeter with regulation valve ensuring a flowrate between 0 and 40 L min−1. A Photo Ionization Detector analyzer (PID) (model DL102) allows the gas-phase pollutant concentration measurement in the extracted gas (Fig. 2). The PID can measure a pollutant concentration to 3000 ppm. For the concentration up to 30,000 ppm, the dilution probe is used.
Air or nitrogen was humidified before introducing in the column to avoid water evaporation (in one test of SVE and one test of venting).

Determination of the mass and the cumulated mass of the extracted pollutant

The instantaneous mass flowrate of the extracted pollutant was calculated from the pollutant concentration in the gas at the outlet and the total volumetric flow measurement of extracted gas using Eq. (1):
$$\dot m_{g,\beta}^i = C_{g,\beta}^i\;Q_g^{{\rm{NCTP}}}$$
(1)
in which \(\dot{m}_{g}^{i}\) is the mass flowrate of compound β in gas phase at time t i [M.T−1]; \(C_{g,\beta }^{i}\) is the mass concentration of compound β at time t i [M.L−3], and \(Q_{g}^{\text{NCTP}}\) is the volumetric flowrate of extracted gas at normal conditions of temperature and pressure [L3.T−1].
The cumulated mass for a known treatment time was obtained by the integration of the instantaneous mass flowrate of the extracted pollutant on treatment duration. The used method was the trapezoid method (Eq. 2):
$${\mathop M\limits^._{g,\beta}} = Q_g^{{\rm{NCTP}}}\sum\limits_{i = 1}^n {\left({\frac{{\left({C_{g,\beta}^i + C_{g,\beta}^{i - 1}} \right)\;\left({{t_i} - {t_{i - 1}}} \right)}}{2}} \right)}$$
(2)
in which \(\dot{M}_{g,\beta }\) is the cumulated mass of the compound β extracted by the gas flow [M] and n is the number of measurements on the treatment duration.
After each treatment, a soil sample was introduced in the oven at 105 °C during 24 h. The difference in the sample mass before and after introducing in the oven was considered to be the mass of pollutant for tests 1 and 3 and the sum of the mass of pollutant and water for tests 2 and 4.

SVE/venting experimental conditions

Four tests with different experimental conditions were conducted (Table 3). Water saturations were selected less than the residual saturation to be able to simulate the unsaturated zone with ensuring of phases immobility. Flowrate of venting/extraction was selected to respect the pore velocity in SVE process.
Table 3
Operating conditions
Conditions
Unity
Precision
Venting
SVE
Test 1
Test 2
Test 3
Test 4
Sand mass
(g)
±0.1
1216
1223
1218
1225
Porosity φ
(−)
 
0.415
0.412
0.415
0.411
Toluene mass
(g)
±0.0001
8.398
8.340
8.301
8.413
Toluene saturation
(−)
 
0.030
0.030
0.029
0.030
Water mass
(g)
±0.1
0
24.424
0
24.535
SWC
(%)
±0.2
7.55
7.60
Gas flowrate
(mL min−1)
±1
591
604
3183
3203
Soil temperature
(°C)
±0.6
20.8
20.3
19.8
19.3
Concentration at saturation
(mg L−1)
±0.1
114.5
111.7
108.9
106.2
The quantity of the toluene lost during test preparation was estimated at less than 2.5% of the total mass. This quantity was calculated for test 1 and test 3 using Eq. 3:
$${M_{{\rm{lost}}}} = {M_{{\rm{init}}}} - {M_{{\rm{ext}}}} - {M_{{\rm{dry}}}}$$
(3)
where \(M_{\text{lost}}\) is the mass of pollutant lost during sample preparation [M]; \(M_{\text{init}}\)  is the mass of the initial pollutant introduced during the test [M]; \(M_{\text{ext}}\) is the mass of extracted pollutant mass by SVE/Venting [M], and \(M_{\text{dry}}\)  is the residual pollutant mass after extraction, measured after drying at 105 °C [M].
A water loose was observed for tests 2 and 4 about 17.2 and 42%, respectively, caused by the evaporation of water in column.
The extraction process efficiency η was calculated by Eq. 4:
$$\eta = \frac{{{M_{{\rm{ext}}}}}}{{{M_{{\rm{init}}}} - {M_{{\rm{lost}}}}}} \times 100.$$
(4)
A second definition of the efficiency of decontamination system is used E [%] and calculated from Eq. (5) [28]:
$$E = \frac{{\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}} \over P} }}{{{t_e}}} \times 100$$
(5)
in which \(t_{e}\) is the spent time since extraction start up [T] and \(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{P}\) is the relative slope of the extraction curve [T−1] defined by Eq. (6):
$$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{P} = - \frac{{\Delta {C_{g,\beta}}}}{\Delta t}\frac{1}{{C_{g,\beta}^{{\rm{init}}}}} = - \frac{{C_{g,\beta}^{i + 1} - C_{g,\beta}^i}}{{{t_{i + 1}} - {t_i}}}\frac{1}{{C_{g,\beta}^{{\rm{init}}}}}$$
(6)
where \(C_{g,\beta }\) is the concentration of compound \(\beta\) (here is toluene) in gas phase at time t i .

Results and discussion

Extraction contaminant curve and efficiency

Figure 3a and b show the evolution of toluene concentration on extracted gas. Curves shape is similar for the four tests, as showed in Fig. 3. Curve profiles according to the flowrate are in agreement with other works results [4, 11, 18]. A dilution effect was observed in the case of high flowrates (tests 3 and 4). The concentration at the outlet in the flushing and evaporation phases was about 70% of the saturated gas concentration for tests 3 and 4, which is about 91% for tests 1 and 2 (Fig. 3; Table 4). This can be due to the increase of the contact areas between contaminant and air flow with higher flowrate [21]. In addition to this convective effect, lower increases due to the evaporation increase participate to this difference. Indeed, as expressed in the mass transfer relationship of Wilkins et al. [24] and Yoon et al. [25], when the flowrate increases—and then the pore’s velocity—the evaporation increases.
Table 4
Extraction results of the four tests
 
Unity
Venting
SVE
Test 1
Test 2
Test 3
Test 4
Concentration of evacuation
 Mean
(mg L−1)
104.3
102.6
77.8
73.8
 In  % at saturation
(%)
91.3
90.5
71.4
69.9
Pulsed concentration (restart)
(mg L−1)
18.2
16.31
7.68
6.85
Concentration in the end of extraction
(mg L−1)
0.042
0.065
0.016
0.029
Treatment time (after shut-off time)
(h)
7.5 (3)
8 (2.7)
4.6 (1.3)
5 (1.2)
Extracted mass
(mg)
7908.5
7725.2
7671
7623.3
Extracted mass after shut-off time
(mg)
62.95
51.9
106.73
83.4
Mass after drying
 Toluene/water
(mg)
210.2
21312,8a
349.7
14203.4a
 In  % (toluene/water)
(%)
2.5
82.8b
4.2
57.9b
Loosing during preparation
 Mass
(mg)
216.3
216.3c
161.7
161.7c
 In  %
(%)
2.57
2.59
1.95
1.9
Toluene remind in column
 Mass
(mg)
210.2
349.2
362.77
543.7
 In %
(%)
2.5
4.19
4.36
6.5
Extraction efficiencyd
(%)
96.7
95.1
94.3
92.4
Efficiency after shut-off time
(%)
97.4
95.74
95.6
93.4
a Sum of toluene and water remind in the column, measured after drying
b  % of remind mass relative to the initial mass
c The loose of pollutant during preparation of test 2 and test 4 is considered the same for tests 1 and 3, respectively
d Efficiency = extracted mass/(initial mass-loosed mass during preparation)
The tailing part is observed on logarithmic scale (Fig. 3b). The presence of water in the media delayed the toluene extraction, due to the interaction NAPL–water which reduces NAPL–gas mass transfer [26]. This result is similar to that obtained by Alvim-ferraz et al. [3] and Albergaria et al. [2] results. Regarding flowrate at the tailing part, the contaminant extraction becomes more difficult when flowrate is lower. This result corroborates the previous works [2, 3, 16].
The system was stopped for one night (about 10 h), and the concentration increases when the extraction starts. The concentration is about 18% for test 1 instead 0.03% in the end of extraction. During this shut-off time, toluene continues to evaporate and to diffuse in the porous media [8]. Qin et al. [18] describe this phenomenon by the fact that the distributions of the air flow channels can change in the column during shutdown period, which allow contaminants that may previously be far away from the former channel to be located closer to a new channel. This phenomenon was observed in several other works [6, 7, 12, 19]. According to those authors, the last diffusional extraction step can be optimized by the application of “pulsed extraction” or “discontinued extraction”. The principal of those methods is the intermittent system stop and start. Truex et al. [22] describe in their work parameters to taken into account to optimize the time to start and end shutdown period in the process. The pulsed extraction not improve the final contaminant removal compared with continuous extraction but has less operating time and then is more economic.
The maximum of the cumulated extracted toluene was reached in the case of higher flowrate (Fig. 4). A slight difference of the extraction efficiency was observed (Fig. 4; Table 4). The efficiency at low flowrate is slightly better than at higher flowrate. This result is not in concordance with the Qin et al. [18] and Jiao et al. [11] conclusion, where they suggest that efficiency increases with flowrate. Their suggestion is valid when efficiency calculation is done for the same experience time choosing in the diffusion phase when extraction becomes small. In this work, the extraction at low flowrate is applied for a longer time period (500 min). At high flowrate, 300 min are enough to extraction end. During diffusion phase, more quantity of pollutant is extracted at low flowrate than at high flowrate.
The efficiency is better in the case of dry sand (tests 1 and 3). It decreases with SWC (tests 2 and 4). This can be explained by the good accessibility of the air to the pores in the case of dry soil. These results confirmed those obtained by Amin et al. [4], but differs from the results of Albergaria et al. [1, 2] and Soares et al. [20]. This difference can be due to the size particles of used soils.
Application of a second extraction after shut-off time adds 0.6–0.7% to the efficiency in the case of low flowrate and 1–1.3% in the case of high flowrate. According to the previous works, this addition will be the same if continuous extraction was applied [9, 10, 18]. The discontinued extraction does not improve contaminant removal compared to the continuous extraction, but it saves a considerable time and energy.

Optimal treatment time

Zhao and Zytner [28] proposed a method estimation of the optimal stop time of extraction system based on extracted gas concentration curve. They showed that for the values of E (Eq. 5) less than 10−4 [%], the SVE treatment was not economically viable. They proposed the “optimal treatment time” when efficiency reached this value. The optimal treatment time was determined for the four tests (Fig. 5; Table 5). The efficiency gain after this time is lower than 1% of the total extracted mass during continuous extraction.
Table 5
Optimal stop time for the four tests
 
Test 1
Test 2
Test 3
Test 4
Optimal time (min)
232
227
76
73
Concentration (mg L−1)
0.264
0.331
0.192
0.223
Efficiency η (%) (optimal time)
96.5
94.8
93.7
91.6
Total time (continued treatment)
449
478
280
303
Efficiency η (%) (total time)
96.7
95.1
94.3
92.4
Results show an extraction time two times more important than the optimal treatment time in the case of low flowrate (tests 1 and 2). As discussed in the previous section, the convective effect provides a better pollutant extraction for high flowrate. The ratio is four times in the case of high flowrates tests (tests 3 and 4). Then, the energy consummation can be limited. The SWC has a little influence on the optimal time; it was little more important in the case of dry sand (5 min for tests 1 and 2 and 3 min for tests 3 and 4). Efficiency remains better with dry sand and lower rate flow with optimal time criteria. The optimal time is positioned in all the cases before the tailing part. This is expected, because the tailing part represents less than 0.5% of the remaining pollutant (Fig. 3b).

Conclusion

SVE/venting process is recognized and cost-effective technology for remediating soils contaminated with volatile and semi-volatile organic compounds. The good practice needs a good understanding of different technical parameters. Especially, the influence of SWC and extraction flowrate can be controlled. In this work, SVE and venting tests were running using dry and small saturated sands at low and high flowrates. Results show that the efficiency is better at low flowrate and dry sand for the two processes. It decreases with SWC and flowrate increase. The dilution effect was observed at high flowrates. The tailing part of the extraction curve was also affected by those parameters. The presence of water decreases the NAPL–gas mass transfer, and then, it becomes more difficult than the contaminant extraction. The low flowrate has the same effect. The application of an extraction after shut-off time decreases time application and then is more economic. The optimal treatment time proposed by Zhao and Zytner [28] was calculated and the utility of this parameter allowing the save time of operation was shown.
Using a low flowrate can improve efficiency of extraction, but make it long-lasting. The use of the optimal time principle and the “discontinued extraction” improve the diffusional step of extraction and the total efficiency with time and energy save. The optimal time gives the end of the evaporation phase and can be used to determine the start time of shutdown period.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://​creativecommons.​org/​licenses/​by/​4.​0/​), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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Metadaten
Titel
Flowrate and water presence effect on venting/SVE process efficiency
verfasst von
Mariem Kacem
Daoud Esrael
Belkacem Benadda
Publikationsdatum
06.06.2017
Verlag
Springer Berlin Heidelberg
Erschienen in
International Journal of Energy and Environmental Engineering / Ausgabe 3/2017
Print ISSN: 2008-9163
Elektronische ISSN: 2251-6832
DOI
https://doi.org/10.1007/s40095-017-0238-4

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