A detailed investigation of adsorption isotherms, enthalpies and

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A detailed investigation of adsorption isotherms, enthalpies and kinetics of mercury adsorption on non-impregnated activated carbon Jonas Moritz Ambrosy, Christoph Pasel, Michael Luckas, Margot Bittig, and Dieter Bathen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05932 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Industrial & Engineering Chemistry Research

A detailed investigation of adsorption isotherms, enthalpies and kinetics of mercury adsorption on non-impregnated activated carbon Jonas M. Ambrosy1*, Christoph Pasel1, Michael Luckas1, Margot Bittig2, Dieter Bathen1,2 1

Thermal Process Engineering, University of Duisburg-Essen, Lotharstraße 1, D-47057 Duisburg, Germany

2

Institute of Energy and Environmental Technology, IUTA e. V., Bliersheimer Straße 60, D47229 Duisburg, Germany

Abstract

In comparison to the adsorption of hydrocarbons the adsorption of mercury on activated carbons reveals many unexpected results. Both physisorptive and chemisorptive mechanisms play a role even in the adsorption on non-impregnated activated carbons. In this work the adsorption of Hg0 from a N2 carrier gas stream is studied on three commercial adsorbents. Single and cumulative breakthrough curves are measured in a fixed bed at temperatures of 25 °C to 100 °C and mercury concentrations of 50 to 1000 μg∙m-3. From the measured adsorption isotherms isosteric heats of adsorption are calculated. Here adsorption enthalpies in the range of 50 % of the vaporization enthalpy are determined. In addition, desorption experiments are conducted to distinguish the contributions of physisorption and chemisorption. A dynamic simulation of experimental breakthrough curves yields diffusion coefficients which are discussed with respect to the concentration and temperature dependence of diffusion mechanisms in mercury physisorption.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Introduction

Mercury-containing waste gases from small and medium-sized plants may have strongly changing mercury concentrations and discontinuous volume flows. Well-established large-scale processes (absorptive gas scrubbing, flue gas adsorption) are rather suited for the separation of mercury from large, continuous volume flows with uniform loading. For the separation of mercury from fluctuating waste gas streams fixed-bed adsorption is a proper method, which has been investigated in many scientific studies1–34. For economic and technical reasons, particularly suitable adsorbents for Hg0 adsorption are non-impregnated1–5,9–14,35,36 and impregnated activated carbons2,3,13,22–24 , the development and investigation of which make up the largest part of the literature1–3,5,6,9–14,24. Most references focus on specific technical applications and compare breakthrough curves on different adsorbents in a small temperature and concentration range. Therefore, despite the large number of publications, there is no deep scientific understanding of adsorption mechanisms. The current state of knowledge on mechanisms of mercury adsorption is described below. Krishnan et al. investigated the adsorption and desorption of Hg0 on commercially available activated carbons in the concentration range from 30 to 60 ppb at 23 and 140 °C. For raw activated carbons, an increase in the adsorption temperature results in a reduction of the adsorption capacity, whereas an increase in the mercury concentration leads to a linear increase in the capacity. A distinction is made between physisorptive and chemisorptive interactions, whereby chemisorption is attributed to functional oxygen groups with very slow reaction kinetics.1,2 The group of Ho et al. published experimental data on the adsorption capacity of raw activated carbon at 25 °C and mercury concentrations of 25.3 μg∙m-3 in a fixed bed adsorber. The results were used along with data from Korpiel and Vidic3 and Krishnan et al.1,2 to fit isothermal parameters.4–8


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Karatza et al. published single breakthrough curves and Langmuir isotherms of Hg0 adsorption on a commercially available raw activated carbon at 90 to 150 °C and mercury concentrations of 0.3 to 3 mg∙m-3.9,10 Skodras et al. determined parameters of Langmuir and Henry isotherms for Hg0 adsorption in the range of 50 to 200 °C at mercury concentrations of 350 μg∙m-3

11–14

. From TPD experiments

(temperature programmed desorption) they concluded that functional oxygen-containing groups (especially lactone and carbonyl groups) increase the chemisorptive adsorption capacity. The authors assume that the positive partial charge on the carbon atom of the functional group supports an electron transfer from mercury to the surface and thus facilitates the oxidation of mercury in the formation of a covalent bond with the surface14. Kim et al.17 and Zhou et al.18 also concluded from their experiments that the presence of oxygen-containing groups on the surface leads to a chemisorption of mercury. Sun et al.16 and Li et al.19,20 investigated the influence of functional oxygen groups on the adsorption of elemental mercury by modifying activated carbons in different ways and determined the oxygen-containing groups by Boehm titration. Subsequently, the activated carbons were loaded with mercury and TPD experiments were performed. From the measured concentration curves of mercury desorption it was deduced that besides physisorption chemisorptive interactions with ester and carbonyl groups occur. Zhang et al. confirmed these results by similar experiments21. Li et al. propose that the adsorbent surface acts as an electron acceptor in the chemisorption of mercury. One idea is that the good displaceability of delocalized π-electrons in the aromatic carbon network leads to the formation of positive partial charges that take up electrons from mercury. Another suggestion is that quinone-like structures on the surface are reduced to hydroquinone in the


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presence of protons, as in the case of the quinhydrone electrode, and thereby take up electrons of the mercury. Similarly, carbonyl groups could be reduced to hydroxyl groups.14,19,20 Liu et al. used density functional theory (DFT) to estimate adsorption enthalpies in the deposition of Hg0 on the carbonaceous surface and at functional groups of activated carbons. The adsorption enthalpy with the carbon surface is dominated by physical interactions. In contrast, the interaction with lactone, carbonyl and semichinone groups resulted in higher adsorption enthalpies close to chemisorption. Phenol and carboxyl groups, on the other hand, reduced the adsorption enthalpy. According to the authors, mercury does not interact directly with the oxygen-containing groups, but with the adjacent carbon atoms, which are positively polarized by electron-withdrawing inductive effects of the oxygen-containing groups15. Padak et al. performed similar calculations regarding the interactions of elemental mercury with the graphite surface of activated carbons and confirmed the findings of Liu et al. for the physisorption of mercury on the carbon surface.33,34 Dynamic modeling of breakthrough curves on the basis of transport equations and mass balances requires comprehensive calculations to simultaneously solve a set of partial differential equations. The method provides local and temporal concentration and loading profiles. Furthermore, fitting of the model to experimental data delivers numerical values for transport coefficients. In order to avoid comprehensive calculations, many authors fit formal kinetic equations or physically founded transport equations to experimental uptake curves to determine kinetic parameters and transport coefficients. Uptake curves show the loading of the adsorbent as a function of time, which is determined by monitoring the adsorption process with thermogravimetric analysis equipment or by evaluating experimental breakthrough curves. Particularly for chemisorption processes, the pseudo-first and pseudo-second-order model are used. Cai et al.25 and Jang et al.26 adapted the pseudo-second-order model to uptake curves of adsorption of Hg0 on activated carbons


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impregnated with KI and KBr and KI, HCl and S. Several authors compared the fitting of uptake curves to the pseudo-first-order model and the pseudo-second-order model. According to Skodras et al.12, Zhou et al.27 and Hsi et al.29 adsorption on activated carbon impregnated with bromine or sulphur could be better represented by the pseudo-second-order model. Fitting to uptake curves of activated carbon by Zhang et al.21 and Fuente-Cuesta et al.28 and to oxygen-modified activated carbon by Zhang et al. provided good results for both approaches. Another formal approach for the modelling of chemisorptive adsorption processes is represented by the Elovich equation 37 which was successfully used by Skodras et al.12 to describe experimental uptake curves. Starting from second Fick's law, Boyd38 and Barrer39 developed an equation for the fractional reaching of equilibrium capacity. Fuente-Cuesta et al.28 and Skodras et al.12 observe a good agreement in fitting this equation to experimental measurements of mercury adsorption on activated carbons. Compared to the pseudo-first-order model and the pseudo-second-order model, fitting to Fick's diffusion model turned out be less accurate. Ho et al.4–7, Meserole et al.30 and Karatza et al.9,10,31 performed dynamic simulations of adsorption processes on activated carbons using a kinetic approach and incremental solving of mass balances. Ho et al. and Meserole et al. used the LDF approach for the mapping of kinetics. The kinetic approach of Karatza et al. assumes a second-order reaction of mercury from the gas phase with the adsorption site. Chung et al.32 used a kinetic approach implying the product of two terms which describe two adsorption sites of different accessibility. For both adsorption terms, the approach of Karatza et al. is used, neglecting the desorption rate. They assume that in the case of chemisorptive interactions there is an irreversible reaction and no desorption takes place. Skodras et al.23 used a model for


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two-dimensional simulation considering radial dependencies to describe the dynamic adsorption process of elemental mercury. Literature shows that both physisorptive and chemisorptive steps play a role in the adsorption of mercury on activated carbons. Equilibrium data of Hg0 are fragmentary and the interaction between physisorption and chemisorption was studied only to a small extent. Therefore, the Chair of Thermal Process Engineering at the University of Duisburg-Essen is systematically investigating the adsorption of Hg0 on activated carbons in the temperature range of 25 to 100 °C and concentrations of 50 to 1000 μg∙m-3. In this study we will focus on physisorption. Based on adsorption and desorption experiments combined with simulation we will present valid data and systematically discuss adsorption equilibrium (isotherms, enthalpies) and kinetics (diffusion coefficients).

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2.1

Experimental section

Materials

Three commercial activated carbons AC 01, AC 02 and AC 03 of granular form with particle size from 1.6 to 2.0 mm were used for the experiments. The materials were delivered by Carbon Service & Consulting GmbH & Co. KG. Table 1 summarizes data on material properties relevant for adsorption. Table 1: Data and chemical compositions of the adsorbents AC 01, AC 02 and AC 03 activated carbon

raw material

activation method

dry residue

ash content

C

[weight%]


S

N

H

O

[weight-% of dry mass]

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AC 01

anthracite

steam

100

10.7

87.4

0.24

0.32

0.53

0.8

AC 02

charcoal

acid

99.6

2.9

90.4

0.44

0.23

0.51

5.5

AC 03

hard coal

steam

100

13.8

73.5

0.40

0.62

1.3

10.4

The activated carbons AC 01 and AC 03 were prepared by steam activation of hard coal while AC 02 was made by chemical activation of charcoal with phosphoric acid. In all adsorbents the major element is carbon accompanied by small amounts of hydrogen, nitrogen, and sulfur. The oxygen content of the three activated carbons differs considerably. Written in ascending order we find: AC 01

A detailed investigation of adsorption isotherms, enthalpies and

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