ARTICLE pubs.acs.org/IECR
Optimal Design of a Rotating Packed Bed for VOC Stripping from Contaminated Groundwater Krishna Gudena,† G. P. Rangaiah,*,† and S. Lakshminarayanan† †
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore 117576 ABSTRACT: Groundwater pollution by volatile organic compounds (VOCs) is a serious environmental concern, and several techniques have been suggested and employed to strip off these harmful compounds. Since groundwater is classified as a distributed scale system, an efficient VOC stripping system should be portable and economically competitive. The focus of the current study is to remove trichloroethylene (TCE) from contaminated groundwater in a rotating packed bed (high gravity or HiGee). Although industrial applications of HiGee do exist, studies on optimal design of the same is scarce in the literature. The present study optimizes the design of an industrial-scale HiGee stripping process with conflicting objectives such as total annual cost (TAC) and total VOC removal under consideration. The synergy effect of heating and rotation is studied, and several inferences from this study are listed. Pareto-optimal solutions obtained provide a wide range of optimized design alternatives, one of which can be chosen and employed by the designers, as per their end needs. Component-wise power consumption, uncertainty analysis, and sensitivity analysis of the involved parameters and variables are also studied to provide further insights into the process.
1. INTRODUCTION Removal of volatile organic compounds (VOCs) from groundwater and industrial wastewater streams is very important, because of its adverse effects on the environment and human health. These compounds are present in many groundwater streams, because of contamination by improper organic disposal, leakage of gasoline storage tanks, improper release of industrial wastewater, agricultural run-offs, leaching of landfills, and accidental chemical spills. The general techniques used to remove VOCs from the contaminated groundwater are air stripping, adsorption, distillation, membrane separation, ozone oxidation, UV treatment, aeration, caternary grid, and biological treatment.1 Process intensification (PI) is the strategy of making a dramatic reduction in plant size to meet a given production objective,2 resulting in the reduction of costs, hazardous inventory, and environmental impact. The benefits of using rotating packed beds (RPBs) for PI were highlighted by Ramshaw and coworkers almost three decades ago. As the name implies, RPB (also known as HiGee, for high gravity) involves the rotation of a torus packed bed (Figure 1) under high centrifugal force. Under this high force, liquid flows in the form of thin films and tiny droplets resulting in an enhancement in mass transfer of 12 orders of magnitude. The height equivalent to a theoretical plate (HETP) in HiGee is in the range of 2.58 cm, which is much smaller than that of 50100 cm in conventional columns.3 Because of their compact size, HiGee can be used for portable units and in places where space is limited. Easy startup, quick micromixing, and brief residence time makes HiGee more attractive than several other mass-transfer devices.4 With ∼30 units in operation worldwide,5 and as one of the promising PI technologies, HiGee has been explored in numerous areas such as absorption, stripping, distillation, polymerization, adsorption, extraction, etc.4 Some of the reported industrial applications of HiGee are deaeration of flooding water by Shengli Oil Field,6 stripping of hypochlorous acid by Dow Chemicals,7 isocyanate production by Wanhua Co.,8 and vacuum deaeration by GasTran Systems.9 r 2011 American Chemical Society
Despite these studies and applications, scaleup of HiGee is still underdeveloped, which results in oversizing of the bed.10 Moreover, when compared to conventional packed beds, HiGee has more design and operating variables, making it more complex to design. Clearly, an optimally designed process should lead to minimum total annual cost (TAC) and maximum VOC removal. These objectives are conflicting in nature, and multi-objective optimization (MOO) is required for obtaining many optimal solutions and for deeper understanding of the process and its design. Although MOO has found many applications in chemical engineering,11 RPBs have not been optimized for multiple objectives until now. Hence, MOO of RPBs for the important application of stripping VOCs from groundwater is studied in this work. Industrial-scale conditions, several objectives, and the option of heating the groundwater are considered. Since energy is a crucial factor in the selection of any system, power consumption due to individual operating variables on the total power consumption is discussed in detail. For this study, trichloroethylene (TCE) is considered as the model compound of VOCs. It is among the most commonly encountered chlorinated VOCs in the groundwater streams12 and is classified among 129 priority pollutants by the U.S. Environmental Protection Agency (USEPA).13 Easy dissolution of TCE during its manufacture, use, and disposal into groundwater makes the removal of this contaminant challenging, both temporally and spatially. According to the International Agency for Research on Cancer and National Toxicology Program, TCE has been suspected to be carcinogenic in nature.14 Hence, the maximum contaminant level (MCL) for TCE in potable water systems is set at 5 μg/L, as stated under the Safe Drinking Water Act (SDWA).15 Received: June 8, 2011 Accepted: November 22, 2011 Revised: November 2, 2011 Published: November 22, 2011 835
dx.doi.org/10.1021/ie201218w | Ind. Eng. Chem. Res. 2012, 51, 835–847
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Schematic of the HiGee stripping process.
The rest of the article is organized as follows. Section 2 describes the HiGee stripping process. Section 3 presents the formulation of the mathematical model for the proposed MOO approach in detail; different correlations used for this purpose are also outlined. Section 4 contains MOO results for the HiGee stripping process with and without a heater; results on uncertainty and sensitivity analysis for the HiGee stripping process without heater are also discussed. The article ends with the conclusions given in section 5.
as in single-objective optimization. These solutions are commonly known as the Pareto-optimal solutions.11 The MOO problem for VOC removal by HiGee stripping can be formulated as follows: Maximize VOC removal, f1 ð%Þ
ð1Þ
Minimize Total annual cost, f2 ð$Þ
ð2Þ
subject to
2. PROCESS DESCRIPTION A schematic of the HiGee stripping process is shown in Figure 1. TCE-contaminated groundwater, at 283 K and atmospheric pressure, is delivered by a centrifugal pump to an electric heater, where it is increased to a certain temperature. The groundwater flow rate, and concentration of TCE in it, are assumed to be 40 L/s and 500 μg/L, respectively.16 The motivation behind selecting these particular values is to perform a direct economic comparison with a real-time, commercial-scale conventional stripping design studied by Gross and TerMaath.16 The goal of the process is to achieve 99% removal of TCE in order to meet the norms laid by SDWA.15 Apart from the application of centrifugal force on the packed bed, another way of improving the separation of VOCs is to raise the temperature of groundwater to a higher level. This will result in a decrease in the solubility of VOCs in water, resulting in its easier removal. The preheated groundwater is transported to the inner radius of RPB where it is distributed in the form of jets through a coaxially placed distributer (Figure 1). Under the influence of centrifugal force, liquid travels radially outward in the form of thin films and tiny droplets. Simultaneously, air is delivered by a blower at the outer radius of RPB. Thus, the airliquid contacting, followed by stripping of VOCs, occurs within the annular region of the packed bed, and the treated liquid exits from the outer radius of the bed (Figure 1). The packing material used within the RPB is composed of stainless steel wire mesh with a specific surface area of 825 m2/m3 and a voidage of 0.95.
ri > ri, min
ð3Þ
ro > ri
ð4Þ
h hflooding > 0
ð5Þ
h 284
ð7Þ
1
