Abstract

Volatile organic compounds such as aliphatic and aromatic hydrocarbons are carried through pipelines in oil and gas industry. The leakage of these compounds from pipelines mixes with ground water. The ground water contamination poses hazards to public health that limits the reuse. It’s essential to remove Volatile organic compounds (VOC’s) from contaminated water for a sustainable reuse of ground water resources. There are various methods for the effective removal of (VOC’s) from water. This article discusses about the optimized use of packed column aerator for high volume flow applications. Various design, physical parameters were used to evaluate and optimize the performance in an economical way are also discussed.

Keywords: Contaminated Ground water- Volatile organic compounds (VOC’s) removal-packed column air stripper/aeration-design-economic evaluations

Introduction

The presence of volatile organic compounds (VOC’s) in ground water sources reduces the possibilities of water reuse.^1-4 Moreover, when the water is reused for potable or irrigation purposes, the contamination of VOC’s from ground water sources pose hazards to public health.^5-8 This article will discuss about the design optimizations of a packed bed aerator for VOC removal in ground water. The packed tower aeration is a widely-used air stripping technology for the polluted groundwater treatment for large volumes of water at high flow rates.^9,10 Spray towers, bubble aerators, low profile aerators, and surface aerators are other air stripping technologies primarily used at low flow rates (<100 gpm). Typically, the packed air stripping system consist of a spray nozzle to distribute contaminated water from the top of the column over the packing in the tower, a fan to force air counter current to the water mass flow rate, and a sump to collect decontaminated water from the bottom of the tower. The high VOC removal efficiencies at very low pressure drops can be achieved with the packed bed aeration systems.^11,12 Despite its merits, the knowledge gap still exists in the area of air stripper performance optimizations.¹³ The optimizations of height of the packing material, the diameter of tower, the air/water loading rates and water temperature with respect to influent concentration of contaminants will improve the efficiency of VOC removal and the performance. Stripping factor is a representation of capacity for mass transfer relative to equilibrium condition in the packed bed aerator. This work provides a design and economic evaluations of using different packing materials. In this study, experiments were conducted using the contaminated ground water from Libyan oil fields. The design and economic evaluations of a packed bed aerator for VOC removal were carried out by using Air Stripper Design and Costing Program (ASDC). The design parameters such as stripping factor and type of packing material, and the physical parameters such as tower diameter air to water ratio, height of the packing material were evaluated to achieve the desired VOC removal efficiency.

Materials and Methods

For a packed air-stripping unit, the design parameters are stripping factor, pressure drop, and type of packing elements. The physical parameters to be estimated during the design are the tower diameter, air-to-water ratio, height of packing material needed to achieve targeted removal efficiency. The stripping factor (and hence air to water ratio) and the packing height can affect the VOC removal performance and the effluent quality.

The procedure of air stripper design presented herein is based on mass transfer theory:

Height of Transfer Unit (HTU): =. $\frac{L_m}{K_{L}a{C}_T}$         (1)

Number of Transfer Unit (NTU): = $\left[\frac{R}{R-1}\right]\text{ln}\left[\frac{X_{in}/X_{out}\left(R-1\right)+1}{R}\right]$        (2)

Packing Height (Z) = HTU x NTU (3)

Stripping factor (R) = $\left(\frac{G_m^{*}{\rho }_L}{L_m^{*}{\rho }_c}\right)*\frac{H_A}{P_T}$        (4)

Details of the general design procedures can be found in literature.¹⁴ The design and the operational parameters of a typical packed bed aeration system are summarized Table 1. The techno-economic study determines the trade-off between air/water ratio and tower volume as a function of air pressure losses and targeted packing system. Furthermore, the design optimization process is targeted to reduce the vessel and equipment size at a pressure drop in order to reduce the energy requirements. The required VOC removal efficiencies and the final product water quality will affect the liquid loading rate, packing dimensions, and air/water ratio.

 (↑) Increase & (↓) decrease

Air Stripper Design and Costing (ASDC) software was used for the design and cost optimizations. Some of the features of this software include modifications of some cost factors such as packing material unit cost and electricity rate.

The total annual cost is given by the following equation:

Annual cost=operating cost+ total capital cost $\left(\frac{i/100}{1-{\left(1+\frac{i}{100}\right)}^{-y}}\right)$        (5)

Total Capital cost =Total Direct Cost + Total Indirect Cost        (6)

Total Direct Cost = Process Equipment Cost + Support Equipment Cost        (7)

Where i is the interest rate in % which is assumed to be 10%, and y is the time in years also assumed to be 20 years. Details of the cost model can be found in literature (Dzombak et al. 1993)

The contaminated groundwater samples were taken from fields in Libya. The water was contaminated with BETX (Benzene, Ethylbenzene, Toluene, and Xylene) due to surrounding pipeline leaks. Samples were stored at 4^oC for packed bed aeration studies. Aerator packed with a plastic type packing material, Nor-Pac 5/8", was used for testing the polluted groundwater. For this case study the water flow rate was set to 100 gpm, ambient pressure to 1 bar and the pressure drop of 50 N/m²/m. The water quality data and desired concentration values of the VOCs are listed in Table 2.

Stripping factor (SF) is defined as the ratio of the air flow rate actually used to the minimum required for air stripping. It is a very important factor to be considered in order to minimize the capital and operating costs of a packed bed aeration system. Range of stripping factor values from 2 to 4 at water temperature 30 °C and water flow rate 100 gal/min were studied.

The influence of temperature on the VOC’s concentration removal was considered within a temperature range of 20-35°C and at constant water flow rate (100 gpm), constant stripping factor (R= 2.5).

The structured column packing consists of vertical sheets of ceramic/metal/plastic material with an open honeycomb structure providing a relatively high surface area. The performance of aerator packing bed depends on the packing shape, materials and the packing size. Selection of size and shape involves a trade-off between low packing factor, and large surface area. With a minimum head loss and high interfacial area, the overall mass transfer rate can be maximized. The total annual costs for the three types of packing materials (e.g. super Intalox, Nor-Pac 5/8, pall ring) for stripping factor range of 2 to 8 were studied. Benzene was chosen as the key design contaminant, water temperature was set at 30°C, and water flow rate was at 100 gallons per minute.

Results

Effect of stripping factor

The effluent concentrations of VOC’s for different stripping factors are shown in Table 3. Given a sufficiently tall column, stripping factors of >1 was selected to ensure there is enough gas for a complete removal of VOC’s. With the stripping factor (SF) of 2 the desired benzene concentration of 5 ppb was achieved and no further decrease was observed for SF’s 2.5, 3.0 and 4.0. For ethyl benzene, the target concentration was achieved with SF 2. Further increase in SF decreased the ethyl benzene removal efficiency. The desired toluene removal was achieved for SF’s 2 and 2.5. However, toluene concentrations were higher than the target value for SF’s 3.0 and 4.0. For 1, 2-Xylene, the target concentration was only achieved for SF 3.0. On the other hand, the target concentration of 1,3-xylene was within the range of 0.5 ppb for the SF’s 2.0, 2.5 and 3.0.

Table 4 presents the design parameters air/water ratio and overall mass transfer coefficients for each stripping factor. The air water ratio and tower diameter increases with SF factor while the packing height and tower height decreases with the SF increases.

Effect of water temperature

The results in terms of effluent concentration (ppb) of VOC are shown in Table 5. As the water temperature is increased, the better VOC the removal efficiencies were achieved. The optimum temperature for 1, 2-xylene was found to be 25^oC and as the temperature is increased no significant change in 1,2-xylene concentration was observed. Figure 1 clearly shows that K_L values increases as the water temperature is increased.

Figure 1: The effect of water temperature on VOC concentrations (primary y-axis) and KL (secondary y-axis).

The design parameters, air/water ratios and mass transfer coefficients are shown in Table 6.The change in water temperature has an effect on the design output parameters (Table 6), and overall mass transfer coefficient. Increasing water temperature leads to a decrease in packing height, tower height, and tower diameter.

Comparison of total annual cost based on different packing materials

It can be seen from Figure 2, that the sharp decrease was found in the cost for SF’s from 2 to 4 and the annual capital cost remains steady with the SF’s 5 to 8.

Figure 2: The effect of stripping factors of commercial packing materials on total annual cost (TAC).