Elsevier

Chemical Engineering Science

Volume 55, Issue 23, December 2000, Pages 5857-5872
Chemical Engineering Science

Benzene adsorption and hot purge regeneration in activated carbon beds

https://doi.org/10.1016/S0009-2509(00)00189-5Get rights and content

Abstract

Experimental and theoretical studies were performed on adsorption of benzene from nitrogen gas stream and thermal regeneration by hot nitrogen purge, in fixed beds charged with activated carbon. System temperature and effluent concentration data were collected during adsorption-regeneration runs. A mathematical model was developed to simulate temperature and concentration data of adsorption and regeneration. The model developed was based on non-equilibrium, non-isothermal and non-adiabatic conditions. Three heat transfer resistances were considered in interfaces of gas–solid, gas–wall, and wall–atmosphere. A linear driving force mass transfer model with a variable lumped-resistances coefficient was found to provide an acceptable fit to the experimental data. Experimental and modelling results were used to study the effects of adiabatic and non-adiabatic operations, contact time, gas velocity, regeneration temperature and initial bed loading on the regeneration efficiency. Besides, the specific energy requirement and purge gas consumption were evaluated to discuss the process efficiency.

Introduction

The organic solvents have been known as the source of about 35–40% of VOCs (volatile organic compounds) emissions to the atmosphere. Their contribution to VOCs is similar in magnitude to all the VOC arising from fuelling and the use of motor vehicles. Since well-developed catalytic converters and other major modifications in the use and distribution of motor fuel substantially reduce the latter source, it is not surprising that increasing pressure will be brought to bear on solvent users to cut the harm done to our environment by their discharges. In this regard, the emission control of organic solvents by promising technologies has attracted special interest as means of protecting the environment and human and health from air pollution (Smallwood, 1993). Among the feasible technologies for VOC abatement, adsorption has been widely recognised as an effective mean of controlling emissions to the atmosphere and, in some applications, of recovering recyclable materials from process exhaust streams. A particularly common application of adsorption for VOC control is solvent recovery. In general, it has been recognised that activated carbons are the most suitable adsorbents for this application because it possesses a high surface area, an intricate pore structure, and a hydrophobic nature (Ruhl, 1993).

The economic feasibility of many adsorption-based separation processes greatly depends on efficient removal of adsorbates from the adsorbent used. Among the several techniques to regenerate the saturated adsorbent particles, thermal swing adsorption (TSA) and pressure swing adsorption (PSA) schemes have been generally employed for this application. A thermal swing cycle operates between two different temperatures, but at same pressure. In the meanwhile, pressure swing cycles operate between two pressures without the change in temperature, except for that caused by the heat of adsorption and desorption. The choice should always be dictated by economics and sometimes these techniques can be employed together (the so-called PTSA) to increase the regeneration efficiency. In the adsorption systems designed for solvent recovery, thermal swing regeneration has been commonly used since the solvent molecules show high affinity on the solid surface even at very low partial pressures. Essential information to design the TSA-solvent recovery process involves the adsorption equilibrium relationships within the operating temperature range and mass- and heat transfer mechanisms around the system.

The primary objective of this study is to obtain better understandings of TSA-solvent recovery process through experiments and mathematical modelling. This includes the development of a realistic model of adsorption and thermal regeneration in non-isothermal and non-adiabatic fixed beds, and the collection of the pertinent experimental data for high boiling solvent. Through the present study, the experimental and theoretical works were made on the adsorption of benzene from a nitrogen gas stream using a fixed bed packed with activated carbon. Regeneration of fixed beds was performed by hot nitrogen purge.

Relevant heat transfer coefficients for the apparatus used were estimated by modelling of the temperature data obtained from several experimental runs of nitrogen heating. Schork (1986) and Schork and Fair (1988) extensively reviewed such estimation procedures for heat transfer coefficients in fixed adsorption bed. Besides, a variable, lumped-resistances linear driving force mass transfer coefficients were employed to simulate the adsorption and regeneration data.

Section snippets

Theoretical

A fixed-bed adsorption model was developed to analyse and simulate column dynamics of temperatures and concentration for adsorption and thermal regeneration. The model developed is based on non-equilibrium, non-isothermal and non-adiabatic conditions as well as the axial dispersion and the heat conduction. Heat transfer resistances are considered in gas–solid, gas-column and column-atmosphere. A linear driving force (LDF) expression with a variable, lumped-resistances coefficient represents

Apparatus and procedure

A bench scale fixed-bed adsorption equipment was used to study non-isothermal and non-adiabatic adsorption of benzene vapour onto activated carbon, regenerated by hot nitrogen purge. Sorbonorit B was chosen as a representative carbon adsorbent and its 12–14 mesh fraction was used. The physical properties of adsorbent and adsorbate can be found elsewhere (Yun, Choi & Kim, 1999). The nitrogen used for regeneration and as carrier gas during adsorption was 99.9% purity. The physical properties of

Estimation of heat transfer coefficients

Four experimental runs were additionally performed in which clean beds of activated carbon were heated with nitrogen purges of varying temperature and flow rate. All the experimental conditions for the nitrogen heating runs are summarised in Table 3. The purpose of these experimental works was to study the energy balance around the equipment, uncoupled from the adsorbate mass balance.

Three heat transfer coefficients, hs,hw, and U are adjustable parameters in the energy balance equations of a

Parametric analysis of thermal regeneration by model simulation

In this study, extensive computer simulations were conducted to explore the effects of various operating parameters on the shape of the depletion curves and on the process efficiency of the thermal regeneration. For this purpose, the experimental conditions of run dbc0v02 were taken as the base case. Variations from the experimental conditions are listed in Table 5.

For examining the regeneration efficiency of TSA process, two parameters have been usually considered as the most important

Conclusions

Fixed-bed experiments were successfully conducted to obtain the pure benzene adsorption and thermal regeneration data. Non-equilibrium, non-isothermal and non-adiabatic fixed-bed model was employed to simulate the concentration and temperature profiles for both adsorption and regeneration runs. In addition, several nitrogen heating experimental runs were performed to estimate the relevant heat transfer parameters around the equipment. Based on analysis of the experimental and modelling results,

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