Modified High Vacuum SVE Operations

In June 1996, the operating data exhibited a continuing decline in vapor recovery rates, as indicated by the corresponding increase in supplemental fuel use. During the first quarter of operation the gasoline-equivalent hydrocarbon recovery rate decreased from about 2 gallons per hour (gal/hr) to about 1 gal/hr. This decrease continued during the second quarter from about 1 gal/hr to about 0.6 gal/hr. In response, DBS&A and AcuVac began to modify (fine-tune) the SVE system using a variety of means. These included using well-specific information to change and adjust the operating

SVE well configuration and enhancing vapor flows by injecting air, first into the PSH and then into the perched groundwater.

Table 1. SVE System Gasoline-Equivalent Hydrocarbon Recovery

Quarter

Dates

Cumulative

Average

Recovery (gal)

Months

Recovery Rate (gal/hr)

Quarter

Cumulative

1

12/95-02/96

3

1.15

2,003

2,003

2

02/96-05/96

6

0.78

1,609

3,612

3

05/96-08/96

9

0.72

1,528

5,140

4

08/96-11/96

12

0.40

694

5,834

5

12/96-02/97

15

0.54

945

6,779

6

03/97-05/97

17"

0.16

185

6,964

7

06/97-08/97

20

0.43

746

7,714

8

09/97-12/97

23b

0.29

217

7,931

9

12/97-01/98

25h,c

0.25

89

8,019

a The SVE system was shut down between May 3 and June 10, 1997 b Full SVE system ran every fourth day. 1 SVE system operation ended on January 30, 1998.

a The SVE system was shut down between May 3 and June 10, 1997 b Full SVE system ran every fourth day. 1 SVE system operation ended on January 30, 1998.

9000

9000

11/22/95 1/21/96 3/21/96 5/20/BS 7/19/06 9/17196 11/16*96 1/15/97 3/16/97 5/15/97 7/14/97 9/12/97 11/11/97 1/10/96 3/11/96

Date

Note: The SVE system was shut down between May 3, ar c June 10, 1997

11/22/95 1/21/96 3/21/96 5/20/BS 7/19/06 9/17196 11/16*96 1/15/97 3/16/97 5/15/97 7/14/97 9/12/97 11/11/97 1/10/96 3/11/96

Date

Note: The SVE system was shut down between May 3, ar c June 10, 1997

Figure <3 Cumulative gasoline-equivalent hydrocarbon removal

3.4.1 Initial SVE Mini-Pilot Tests

The primary tool used to better optimize the SVE system was a short-term testing procedure to obtain well-specific information. DBS&A and

AcuVac conducted the first of these short-term SVE mini-pilot tests in late June 1996 and completed nine tests over the next 18 months. The periodic mini-pilot tests provided information about well vacuum, vapor flow rates, and contaminant concentrations. DBS&A and AcuVac used these operational data, along with PSH and water level measurements, to balance the operation in the SVE circuits, adjusting SVE operations as necessary to optimize contaminant removal.

The mini-pilot tests consisted of measuring well vacuum and airflow in each SVE well using AcuVac's portable IC unit. During the initial tests in June 1996, stepped vacuum increases were also applied to further characterize each well's response. In addition, water levels and/or PSH were measured, and individual influent vapor samples were collected and analyzed for total hydrocarbon and carbon dioxide (CO2) concentrations using a portable HORIBA gas analyzer.

The June mini-pilot tests showed that the flow from individual wells varied from less than 1 to 16 cfm at a vacuum of approximately 70 inches of water. The hydrocarbon vapor concentrations ranged from 188 to 4,630 ppmv (Table 2). Based on the June vapor flows and hydrocarbon concentrations, 13 wells were selected for continued SVE operation to maintain hydraulic control. These included wells 1-2, 1-5, and 1-7 from circuit 1, wells 2-1 and 2-6 from circuit 2, all wells except well 3-3 in circuit 3, and combination monitor/SVE well MW-9 (Fig. 6).

Table 2. Mini-Pilot Test Data Summary

Quarter of

Mini-

Date of

Maximum

Concentration Range

Remedial

Pilot

Test

PSH Thickness

Hydro

co2

o2

System

Test

in Monitor

carbons

(%)

(%)

Operations

Number

Wells (ft)

(ppmv)

3

1

6/26-27/96

-3.5 (MW-1)

188-4,630

0.60-3.08

3

2

8/24/96

1.21 (MW-1)

34-5,360

0.64-2.54

...

4

3

9/27-28/96

1.13 (MW-1)

40-2,040

0.56-3.28

4

4

11/19/96

1.30 (MW-1)

18-1,944

0.22-1.94

...

5

5

2/4/97

0.53 (MW-3)

114-7,960

0.26-2.18

...

6

6

5/1-2/97

0.36 (MW-3)

14-842

0.04-2.04

18.9-20.8

7

7

6/9-10/97

0.22 (MW-3)

22-1,672

0.08-3.36

17.2-20.2

7

8

8/23-24/97

0.24 (MW-3)

18-480

0.08-0.60

19.2-20.8

8

9

12/3/97

None

30-474

0.20-1.44

19.2-20.8

10

6/12/98

None

20-270

0.04-0.42

19.9-20.8

PSH = Phase-separated hydrocarbon — = Not measured

C02 = Carbon dioxide 02 = Oxygen

PSH = Phase-separated hydrocarbon — = Not measured

C02 = Carbon dioxide 02 = Oxygen

Satisfactory results, as indicated by relatively steady natural gas consumption and slowly increasing vacuum, were recorded until early August 1996, when flows increased and the system vacuum began to drop. A second set of mini-pilot tests was completed on August 24, 1996. The results of the August 1996 pilot tests showed that SVE well 2-6 was short-circuiting the system by providing high air flows at low hydrocarbon concentrations (12 cfm and 136 ppmv). Apparently, fresh air had found a preferential pathway into the screened interval (41 to 61 feet bgs; sand pack at 38 to 61 feet bgs), thereby leaving stagnant air in the lower section of the well (this well also contained 2.73 feet of PSH).

Based on the August vapor flow and hydrocarbon concentration results, six SVE wells were selected for continued operation: wells 1-3, 1-5, and 1-7 from circuit 1, wells 2-5 and 2-7 from circuit 2, and for downgradient hydraulic control, well MW-9. All of the SVE wells in circuit 3 were turned off due to the relatively high groundwater elevations, the high vacuums, and low air flows measured in these SVE wells during the August 1996 test. In addition, the SVE wells in circuit 3 generally had very low hydrocarbon concentrations, ranging from 34 to 249 ppmv. During this third quarter of SVE operation, the IC engine consumed another 1,500 gallons of gasoline-equivalent hydrocarbons, for a total of more than 5,000 gallons since the start of SVE system operation (Table 2; Fig. 8).

In general, air flow rates greater than 10 cfm resulted in low (500 ppm) hydrocarbon concentrations, but some moderate flow rates (5 to 10 cfm) resulted in quite high (2,000 ppm) hydrocarbon concentrations. Numerous low flow (0.0 to 3 cfm) wells also had low concentrations, while a few of these had hydrocarbon concentrations greater than 2,000 ppm. Generally, the system vacuum varied between 50 and 80 inches wc.

3.4.2 SVE with Air Bubbling into PSH

In late September 1996, the persistence of PSH was addressed by bubbling air at a low flow directly into two SVE wells (1-5 and 1-6) that contained the thickest (about 1-foot) layers of PSH. Overall, a positive vacuum was maintained on the SVE system. Although the bubbling probably diluted the overall concentration of hydrocarbon vapors recovered by the system (see Table 1, recovery rate for the fourth quarter), continued SVE operation along with the bubbling was successful in removing the PSH from these two wells.

After the adjustments made in late August 1996, the gasoline-equivalent hydrocarbon recovery rate increased from about 0.55 gal/hr to between 0.65 and 0.70 gal/hr. AcuVac conducted mini-pilot tests in the SVE wells on September 27-28, 1996. These tests showed that the flow from individual wells varied from less than 1 to 16 cfm at a vacuum of approximately 90 inches of water. Hydrocarbon vapor contents ranged from 40 to 2,040 ppmv. Based on the observed vapor flows and hydrocarbon concentrations, and with the objective of maintaining hydraulic control by applying suction to raise water levels, 10 wells were selected for continued operation. These included wells 1-5, 1-6, and 1-7 from circuit 1, wells 2-1 and 2-5 from circuit 2, wells 3-2, 3-4, 3-5, and 3-6 from circuit 3, and combination monitor/SVE well MW-9. In addition, air bubblers were installed into the PSH layers in SVE wells 1-5 and 1-6 to help remove the product that persisted in those two wells.

Mixed results, as indicated by the decreasing gasoline-equivalent hydrocarbon burn rate and increasing natural gas consumption, were observed after these adjustments to the system. The gasoline-equivalent hydrocarbon burn rate decreased from between 0.55 and 0.60 gal/hr to between 0.25 and 0.30 gal/hr. However, by the next set of mini-pilot tests, completed on November 19, 1996, the bubblers were successful in removing not only the PSH in these two wells, but also a significant portion of the PSH present in monitor wells in the vicinity.

3.4.3 SVE with Air Sparging in Groundwater

After the experience with air bubbling in SVE wells 1-5 and 1-6, operation of the AS system was initiated at two adjacent SVE wells (1-3 and 1-4) where PSH was then localized. The air flow at these two sparge points was set to a low volume (3 cfm) in an attempt to continue the PSH removal that resulted from the air bubbling in wells 1-5 and 1-6. The pressure associated with this low flow rate was about 55 pounds per square inch.

Hydrocarbon recoveries were enhanced during this (fourth) quarter of SVE system operation, and PSH thicknesses in both the monitor and the SVE wells decreased. DBS&A continued operating both the SVE and AS systems using a variety of configurations to focus on areas with the highest hydrocarbon vapor concentrations or to maintain hydraulic control. Because the systems had been constructed with separate controls located at the equipment compound and at the wellhead, the SVE and AS systems at each location could be operated independently of each other.

Similar adjustments were made during the next three quarters (5, 6, and 7) with mixed recoveries (945, 185, and 746 gallons). With SVE recovery rates declining, the schedule of operation was reduced for the eighth and ninth quarters to run the SVE system every fourth day in rotation with various AS cycles. An additional 217 gallons were recovered and destroyed during the eighth quarter and 89 gallons during the ninth quarter.

During this period of combined operation, PSH was successfully removed from all of the monitor and SVE wells (including SVE well 2-6) sometime after August 1997, and no PSH was observed for a number of months between August 1997 and December 1998. When hydrocarbon recoveries declined to approximately one tenth the rate allowed in the air permit (2 pounds per hour), the SVE system was shut down. After two years of operation, the SVE system had logged more than 13,500 hours and removed and treated more than 8,000 gallons of hydrocarbon product. The AS operation continued, but the focus of attention switched from vapor recovery and destruction to in situ biodegradation.

0 0

Post a comment