Novel off-Gas Treatment Technology To Remove Volatile Organic

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A novel off-gas treatment technology to remove volatile organic contaminants with high concentration Hong Sui, Tao Zhang, Jixing Cui, Xiqing Li, John Crittenden, Xingang Li, and Lin He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02662 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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Industrial & Engineering Chemistry Research

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A novel off-gas treatment technology to remove volatile

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organic contaminants with high concentration

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Hong Suia,c,d, Tao Zhangc, Jixing Cuic, Xiqing Lib, John Crittendene, Xingang Li a,c,d, Lin He a,d*

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a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

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b

College of Urban and Environmental Sciences, Peking University, Beijing 10087, China

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c

National Engineering Research Centre for Distillation Technology, Tianjin 300072, China

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d

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 300072, China

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e

School of Civil and Environmental Engineering and the Brook Byers Institute for Sustainable

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Systems, Georgia Institute of Technology, Atlanta, Georgia, 30332-0595, United States

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*

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EMAIL: [email protected]

Corresponding author: Lin He

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ACS Paragon Plus Environment


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Graphic abstract:

2 Clean gas

Flesh solvent (or absorbents)

Regeneration column

Absorption column

Gas stream with high concentrations of VOCs

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Solvents enriched with VOCs

Recovered VOCs

Recycled solvents

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Abstract：：

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The triethylene glycol (TEG) was used to treat the off-gas stream containing 16

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volatile organic chemicals (VOCs) with high concentration (up to 15000 ppm) by

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experimenal solvent absorption and ASPEN Plus computational simulation,

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respectively. This solvent absorption technology (SAT) does not only purify the

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off-gas, but also recover the VOCs. The effects of some typical operational

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parameters, including temperature, pressure, liquid loading rate, ratio of absorption

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liquid to gas flow rates, and absorption column height, on the absorption efficiency

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have been systematically investigated. Simulation results show that the optimal

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operational conditions are determined as: 30 ℃, 0.5 MPa (absolute), recycling TEG

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(with a purity of 99.9 %) to gas ratio of 4:1 by weight, and 8 theoretical stages. In

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addition, two stages of thermal coupling are applied to minimizing the energy

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consumption of off-gas absorption process, leading to a reduction of 89.77 % in

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energy consumption. Laboratory experiments and field tests show that both VOCs

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absorption efficiency (using TEG as absorbent) and regeneration efficiency are highly

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consistent with those of ASPEN Plus simulation, suggesting the feasibility of

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simulation. Based on pilot-scale experiments, the equipment investment and

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operational costs of SAT have been calculated and analyzed. Results show that the

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SAT is comparable with the traditional thermal treatment, but with the advantages of

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no secondary pollution. These findings indicate that solvent absorption method is a

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promising green technology for treating high VOCs off-gas.

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Keywords ： off-gas, volatile organic chemicals (VOCs), solvent absorption, 3



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triethylene glycol, ASPEN Plus simulation

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1. Introduction

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Volatile organic chemicals (VOCs) exist in many places or gas streams, such as oil or

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gas holders which are used to hold gasoline, jet fuel, heating oil, and other industrial

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compounds found in soil and groundwater. The discharge of VOCs to the air without

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treatment is unacceptable due to their toxicity to humans (e.g., carcinogenicity) and

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ecosystems (e.g., tropospheric ozone)1 Therefore, some

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standards have been set to restrict their discharge by many countries during the past

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decades.

stringent professional

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Two popular methods for VOCs removal from soil and groundwater are soil

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vapor extraction (SVE) and air stripping (AS)1. In these technologies, further

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treatments are still required to purify the off-gas before its final discharge to the air,

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which could be categorized into four groups: thermal treatment, absorption method,

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biological process and emerging technologies (e.g., photocatalysis and non-thermal

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plasma treatment)1. Recently, condensation technology has been applied to recovered

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VOCs from the off-gas vapor stream of SVE or dual phase extraction (DPE) systems2.

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Among the above technologies, adsorption3, 4 and Biofiltration5, 6 are mainly used for

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VOCs treatment with low concentrations, typically less than 5,000 and 1,500 ppmv

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for total VOCs, respectively. However, during these processes, the widely used

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adsorbent, granular activated carbon (GAC), works poorly when treating VOCs with

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high polarity or high vapor pressures (i.e., highly volatile compounds)7. While,

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Biofiltration is found to be sensitive to the variations of operational parameters, such

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as moisture content, temperature, pH, and nutrient levels1. Although the thermal 5



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method is able to be used to treat the off-gas with a broad range of concentration, it is

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not suitable to process SVE off-gas with halogenated compounds due to the second

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pollution by the newly generated acid gases8, 9. Additionally, incomplete combustion

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would occur if the oxygen was not sufficient or the “three Ts (temperature, time, and

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turbulence)” were not addressed adequately. Other off-gas treating technologies, such

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as photocatalytic and non-thermal plasma, are also not being widely commercialized

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due to their complexity and high cost during operation10-12. The Vapor Condensation

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(VC) method sometimes works poorly in purifying off-gas containing materials with

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low boiling point, although it may perform well in treating off-gas with high VOC

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concentrations at low gas flow rates. In addition, VC would also be difficult in

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meeting air emission standards due to the constraints of vapor liquid phase

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equilibrium2.Therefore, due to the unavoidable disadvantages and limitations for the

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above treatment methods, it is urgent to develop a green process which is capable of

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removing high-content VOCs from off-gas streams in a cost-effective way.

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Gas absorption technology has been considered as a promising method for

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off-gas purification, which was reported in only a few studies13. In some cases, the

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contaminants (e.g., acid gases) in vapor stream can react with a component in the

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absorbent solution (e.g., caustic solution). While in some other cases, the

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contaminants are able to be dissolved into the absorbent solution, which requires

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further treatment for purifying the rich absorbent and recycling use1. During the past

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decades, many solvents have been investigated such as water, mineral oils, biodiesel,

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ionic liquids, vegetable and lubricant oils, and non-volatile petroleum oils14-16. Vuong 6


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et al.17 used Henry’s constant to characterize the absorption capacity of various

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VOCs/solvent systems, and used the absorption rate and the overall liquid mass

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transfer coefficient to evaluate absorption selectivity. Unfortunately, most solvents

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only work well in absorbing a few kinds of specific VOCs, not all VOCs. Therefore, it

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is important to screen appropriate solvents which are effective in removing a much

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wider range of VOCs compositions, and find a reliable way to optimize the

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operational conditions (a balance of best treat effect and low cost).

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ASPEN Plus, a process simulation software possessing a powerful database, has

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been widely used in chemical engineering research and made significant contributions

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in many fields, such as oil refinery18, 19, off-gas and CO2 absorption from power

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plant20, 21, coal chemical industry22, 23 and biomass power generation systems24, etc.

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Here, a potential sight was brought out whether we could combine the process

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simulation with experiments together, which reduces the operational cost and time for

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design.

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Accordingly, the aims of this study are to: a) evaluate VOCs removal efficiency

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using ASPEN Plus simulation software, b) choose one environmentally friendly

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absorbent (solvent), which can effectively remove VOCs in off-gas; c) understand the

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effect of temperature, pressure, the ratio of recycling solvent flowrate to gas flowrate

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(L/G ratio), absorption tower height and liquid and gas loading rate, on VOCs

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removal efficiency, d) validate the industrial application; and evaluate the economics

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of solvent absorption technology using a pilot-scale experimental set.

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2. Materials and Methods 7



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2.1 Materials

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All chemicals for bench scale test were purchased at analytical grade from Tianjin

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Jiangtian Technology Co. Ltd., China. The benzene series (a series of aromatic

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hydrocarbons derived from benzene by replacing one or more of the hydrogen atoms

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with one or more methyl groups), including benzene, toluene, xylenes and

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haloalkanes (e.g., trichloroethylene, 1,1,1-trichloroethane, and methylene chloride),

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were stored in two 100 ml flasks at the volume ratios, shown in Table 1. Triethylene

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glycol (TEG) was chosen as the absorbent, because the simulation results suggested

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that it was the optimal absorbent.

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2.2 Experimental Solvent Absorption and Regeneration

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The solvent absorption and regeneration experimental set-up is shown in Figure 1.

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VOCs were placed in a closed container according to the proportions that were shown

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in Table 1. An air blower was used to enhance the volatilization and delivery of the

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VOCs into the absorption column (Φ32 mm × 1 m). The compositions of VOCs in the

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gas stream were controlled by adjusting the flowmeter according to the quantitative

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detection by GC with FID detector. The total flowrate of the gas stream was 1 m³•h-1.

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TEG was sprayed from the top of the column through a sprayer with the flowrate of

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90 mL•min-1. After absorbing the VOCs, the absorbent enriched with VOCs was

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collected at the bottom of the absorption column and fed to the regeneration column

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(Φ 32 mm × 0.6 m height). Gas chromatography (GC) was used to analyze the gas

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samples from the column when the absorption was equilibrium (~20 minutes later).

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To determine the industrial and economical feasibility, a pilot plant study with a 8


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treatment capacity of 500 m3•h-1 was conducted in a chemical plant in Baoding, China.

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The absorption column and regeneration column were filled with Sulzer structured

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packings (Mellapac 250Y) with the diameter of 0.35 m and 0.4 m, respectively. A

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circulating water system was used to cool the absorbent after regeneration and the hot

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heat-conducting oil which was used to heat the reboiler at the bottom of the

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regeneration column. The off-gas was extracted from 4 soil vapor extraction wells and

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was transported to the off-gas absorption column by pipeline. This system ran 20 days

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continuously.

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2.3 Simulation Models

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As shown in Figure 2, a steady state simulation model using ASPEN Plus was

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developed. Table 2 presented a brief description of each unit operation block shown

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in ASPEN Plus flowsheet. The absorption column was operated at ambient pressure

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and temperature. The NRTL thermodynamic model was used in this simulation. The

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flowrate of off-gas was controlled at 500 m3•h-1 with VOCs at 11.38 kg•h-1 (1.5 wt%

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of the off-gas b). The compositions of VOCs in off-gas are listed in Table 3.

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2.4 Analytical Method

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Total hydrocarbon concentration (without methane) was obtained using Portable

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VOCs Detector (MiniRAE 3000 PGM 7320) of RAE Systems (Beijing) Co. Ltd. The

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benzene series were analyzed by GC-FID. The injection and detector temperatures

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were both set as 250 ºC. The injection volume was 0.2 µL with split ratio of 10:1.

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VOCs were separated using a HP-5 capillary column (30 m × 0.32 mm internal

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diameter) and nitrogen was used as the carrier gas. The oven temperature was 9



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increased from 40 ºC to 120 ºC at 5 ºC•min-1 and kept at 120 ºC for 20 min.

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GC-FID was also used for chlorinated hydrocarbon analysis. The injection and

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detector temperatures were both set as 200 ºC. The injection volume was 0.2 µL with

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split ratio of 5:1. VOCs were separated in a PE-FFAP capillary column (30 m × 0.32

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mm internal diameter × 0.25 µm film thick). The nitrogen gas was used as the carrier

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gas. The oven was kept at 70 ºC for 15 min.

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3. Simulation Results and Discussion

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3.1 Different Absorbents

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To obtain the impact of the absorbent solution, TEG, methyl benzoate, butyrolactone,

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salicylic acid and N-methyl-2-pyrrolidone (NMP) were used to test their absorption

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efficiency by process simulation. Table 4 shows that the absorption efficiency of

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methyl benzoate is the highest among the tested agents. The VOCs effluent

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concentration reduced sharply to less than 3.85×10-2 (i.e., n-heptane) or even as low

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as 3.11×10-18 mg•m-3 (i.e., o-xylene) when methyl benzoate was used as adsorbent.

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However, large amount of absorbents (i.e., methyl benzoate, salicylic acid,

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butyrolactone and NMP) were found to be lost during absorption due to volatilization

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(Table 5), which limits their potential application in industry. After careful

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comparison from the absorption efficiency and long-term operation, TEG was

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considered as the optimal absorbent due to its high absorption ability to VOCs with

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the lowest entrainment (1 ppm) in the treated gas. Therefore, TEG was selected as

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adsorbent for further investigation in the following sections. 10


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3.2 Effect of Temperature and Pressure

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Temperature and pressure are important operational factors which influence the

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absorption efficiency. The lower the temperature, the higher the absorption efficiency

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is. However, considering the energy consumption, the absorption should be better

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operated at ambient temperature and pressure. Therefore, the simulation conditions

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were set at the range from 20 to 40 ℃ and 0.1 MPa. Unfortunately, the treated gas did

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not satisfy GB16297-96 limit level even at 20 ℃ (normal pressure). Take n-pentane as

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an example, the discharge concentrations at 20 ℃, 30 ℃ and 40 ℃ were 534.6, 592.3

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and 632.2 mg•m-3, respectively, as shown in Table 6, which were much higher than

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the discharge standard for total alkanes (

Novel off-Gas Treatment Technology To Remove Volatile Organic

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