Regenerative CopperâAlumina H2S Sorbent for Hot Gas

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Regenerative copper-alumina H2S sorbent for hot gas cleaning through chemical swing adsorption Mehdi Pishahang, Yngve Larring, Eric van Dijk, Frans van berkel, Paul Dahl, Paul Dean Cobden, Michael McCann, and Egil Bakken Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03752 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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Industrial & Engineering Chemistry Research

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Regenerative copper-alumina H2S sorbent for hot gas cleaning

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through chemical swing adsorption

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Mehdi Pishahang 1,*, Yngve Larring1, Eric van Dijk2, Frans van Berkel2, Paul Inge Dahl1,

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Paul Cobden2, Michael McCann1, Egil Bakken1

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1

SINTEF Materials and Chemistry, P.O. Box 124 Blindern, NO-0314, Oslo, Norway.

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2

Energy Research Centre of the Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The

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Netherlands.

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* Corresponding Author: [email protected]

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Abstract:

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This article focuses on desulfurization of hot syngas produced from gasification of solid fossil

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fuels in the temperature range of 300–500 oC via copper-based adsorbents. The slip of H2S

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above the developed adsorbent materials for hot cleaning of syngas has been studied together

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with the regeneration mechanism, using thermodynamic analysis, thermogravimetry and

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packed bed reactor experiments, in order to establish an efficient approach to regenerate the

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adsorbent. Supported copper on gamma alumina H2S adsorbent used in this study shows H2S

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slips lower than 5 ppm in the temperature range 350 to 550 °C. The copper based sorbent

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shows around 2wt% sulfur sorption capacity in the temperature range of study. The kinetic

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evaluation confirms that the sorption kinetics for this sorbent yield sufficient performance for

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real process operation even at such low temperatures. Aiming at isothermal operation,

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chemical swing process is identified as an efficient way to regenerate the adsorbent. In this

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regeneration process the sulfide phase is stabilized to sulfate in air followed by a fast

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regeneration stage in the presence of a small stream of hydrogen.

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Keywords:

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Hydrogen sulfide, sulfur sorbent, desulfurization, syngas, palladium membrane, packed bed

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Introduction:

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Large scale industrial production of hydrogen is carried out primarily using hydrocarbons, and

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most notably natural gas in a process that involves two equilibrium limited reactions: the

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steam methane reforming (SMR) and the subsequent water-gas-shift (WGS). This process

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leads to formation of a gas rich of H2 and CO2 and very low concentration of CO. Separation

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of hydrogen from carbon dioxide is the next step in order to produce high purity hydrogen. In

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the quest for decreasing the cost of carbon neutral hydrogen production new technologies

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focus on separation of hydrogen and carbon dioxide. Technologies such as sorbent enhanced

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steam methane reforming1 (SE-SMR) and sorbent enhanced water-gas-shift2 (SE-WGS) have

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been investigated to improve the performance of the total process. Other technologies for

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hydrogen production are membrane enhanced water-gas-shift3 (ME-WGS) and membrane

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enhanced steam methane reforming4 (ME-SMR) which involve selective hydrogen removal

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membranes in order to increase the efficiency of the shift reaction. In these pre-combustion

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CO2 capture concepts, carbonic energy part of the primary fuel is first transferred to

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hydrogen, and hydrogen is then separated from CO2 via membrane. Utilization of cheaper

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hydrocarbon feedstocks like petcoke and coal for hydrogen production is widely focused

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globally in combination with high-temperature gas cleaning techniques. Palladium based

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membranes, whether sole or alloyed, show great promise for hydrogen purification at

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intermediate temperatures (300 to 450 °C)5, 6. Comparing the selectivity-ﬂux combination for

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hydrogen selective membranes, Pd-based membranes are ranked first of all the membrane

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classes7. It has been shown by Polytechnic University of Milan that the plant lay-out with

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integration of Pd-based hydrogen selective membranes in IGCC with CO2 capture achieves 2 ACS Paragon Plus Environment

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very high net electric efficiency of 40%8-10. However, sulfur poisoning pose a great challenge

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to the performance of Pd membranes11. In order to ensure sufficient lifetime for the Pd-

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membrane, desulfurization down to below 10 ppm H2S is required. In case of solid fuels, e.g.

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coal and petcoke, desulfurization at the temperatures comparable to water-gas-shift or Pd-

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membrane operation temperatures is beneficial. Thus H2S removal at intermediate

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temperatures suitable for application (300–600 °C) is of great interest, as this avoids energy

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losses attributed to cooling down and warming up the produced syngas just for the purpose of

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low temperature scrubbing sulfur removal. Metal oxide based solid sorbents12 are well-studied

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candidates to capture H2S at high temperature in order to avoid the efficiency-drop related to

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the cooling and reheating of the gas as is the case with conventional acid gas H2S removal

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systems.

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Candidate sulfur sorbents have been evaluated for several decades and the selected materials

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are often oxides made from alkaline earth (calcium, strontium, barium) or transition metals

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(vanadium, manganese, iron, cobalt, nickel, copper, zinc) or combinations of several of these

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elements12-18. Both unsupported and supported sorbents have been studied, and supported

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sorbents tend to perform better due to larger surface area and chemical and mechanical

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stability. RTI International has developed and patented a state of the art permanent H2S trap

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(RTI-3) based on ZnO supported by alumina19. This sorbent has been tested in a large

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transport reactor unit where the sorbent is fluidized at high temperature and high pressure. In

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general, due to suitable kinetics of sulfidation, ZnO shows high H2S removal efficiency

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resulting in low concentrations of H2S in the cleaned gas. However, the low melting point of

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zinc is challenging in reducing conditions and may cause contamination by vaporized

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elemental zinc. Although sorbents based on ZnO are non-regenerative and can only be used as

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scavenger sorbents, they can effectively be used for removal of very low concentration of

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sulfur compounds. Although ZnO efficiently removes traces of sulfur from the gas, it should 3 ACS Paragon Plus Environment


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be combined with a regenerative desulfurization technique for removal of high amounts and

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concentrations of H2S. Copper-based sorbents20, 21 received considerable attention as they are

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capable to maintain the combination of low levels of H2S in the cleaned gas and

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regenerability. One of the challenges is limiting agglomeration of fine Cu particles into larger

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and diffusion of Cu to the surface.

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First patented in 1883 by Carl Friedrich Claus, the "Claus process"22 is the most significant

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industrial desulfurization process for removing gaseous H2S through recovering elemental

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sulfur. The vast majority of the sulfur produced globally is the product of the Claus process23.

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A large portion of the elemental sulfur is used in the "contact process"24 to produce sulfuric

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acid, where elemental sulfur is first oxidized to SO2 and SO3 and further converted to sulfuric

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acid. Developing novel H2S removal techniques in which concentrated SOx is directly

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obtained (without the elemental sulfur route), has always been of interest. This can be

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achieved through regenerable sorbents.

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In this study H2S removal from coal based syngas via a regenerative solid copper based

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sorbents is investigated. The original 500–600 °C beneficial window of operation for H2S

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removal for efficient ME-WGS does not overlap with Pd membranes 400–500 °C service

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temperature. Thus the gas cleaning is placed between the membrane H2 separation step and

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after the partial WGS. The new window selected for gas cleaning was therefore 350–450°C

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and hydrogen separation with Pd membranes around 300–350°C. Several strategies have been

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tried to impregnate the support and restrict the Cu agglomeration, and Ti stabilized Cu

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impregnated on aluminate shows great promises. The slip of H2S above the developed sorbent

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materials for hot cleaning of syngas has been studied together with the mechanism around

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regeneration, using thermodynamic analysis, TGA and packed bed reactor experiments. There

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are several process configurations likely to be used for sulfur removal based on fluidized,

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moving and packed bed technologies. The reactor configuration will be adapted depending on 4 ACS Paragon Plus Environment

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suitability of implementation, cost, space, etc. In this work the implementation considerations

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in any specific reactor configuration has not been addressed. Some experiments are however

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performed in a packed bed configuration for simplicity, since material propoerties such as

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H2S slip, capacity and degradation can appropriately be investigated with such configuration.

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Experimental:

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Spherical porous γ-Al2O3 granules with average diameter of 600 µm, surface area of 1.5×105

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m2.kg-1 and a pore volume of minimum 5.3×10-4 m3.kg-1 (from Sasol) were used as support

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material. The supporting material is impregnated by copper/titanium based sorbents. Water

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solutions of copper nitrate and isopropanol based solution of titanium cation are used in the

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incipient wetness method. The incipient wetness technique is a pore filling method and the

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solution volume added matches exactly the available pore volume in the support.

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A three stage process of impregnation onto alumina spheres is followed

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1.

Copper nitrate trihydrate, Cu concentration: 5% molar (solution in water)

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2.

Titanium isopropoxide, Ti concentration: 5% molar (solution in isopropanol)

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3.

Copper nitrate trihydrate, Cu concentration: 10% molar (solution in water)

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After each impregnation, the sorbent was dried over night at 120 °C and subsequently sintered

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at 800 °C for 6 hrs. The BET analysis of the resulted sorbent shows a decrease in the surface

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area from 1.5×105 m2.kg-1 for spherical porous γ-Al2O3 balls to 7.2×104 m2.kg-1 for

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impregnated sintered sample. XRD (Siemens D5000 with Cu Kα radiation through a primary

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monochromator), and SEM were used to investigate the composition and structure of the

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sorbent before and after exposure to the reactive gas (gas containing H2S).



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Sorbent performance was studied in a Rubotherm (Germany) magnetic suspension apparatus

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(thermogravimetric analyzer). Due to safety considerations, all the experiments were

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performed under atmospheric conditions with maximum lab restricted H2S concentration of

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800 ppm. An in-house gas mixing system consisting of 7 mass flow controllers was used

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(Figure 1(a)). The high gas speeds were introduced to make sure that the limitation of the

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sorption is not gas diffusion related. However different gas flow rates were applied in the

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range of 0.5 to 0.7 l.min-1 in order to confirm that the reaction is not controlled by availability

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of H2S in the gas. All the thermogravimetric experiments were performed isothermally in the

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temperature range of 350 to 450 °C. In order to assess the kinetics, the experiments were

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performed at three different H2S concentrations (400, 600 and 800 ppm) at different

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temperatures 350, 375 and 425 °C. In order to correct for gas buoyancy effects, the

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experiments were repeated with blank samples.

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Figure 1. The gas mixing system and apparatuses used in this study. (a) magnetic suspension

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balance; (b) Packed bed reactor.

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As illustrated by Figure 1(b), a packed bed reactor was also used to study the H2S slip above

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the bed. The reactor consists of a quartz reactor connected to MS and GC for measuring the

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H2S and carrier gases concentration. Two different quartz reactors were used in this study.

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One with 4 mm ID and a bed height of 75 mm (W=0.75g, L/D=19), while the other

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measurements were done in a reactor with an annulus bed of 8 mm OD and 3 mm ID and a

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bed height of 15 mm (W=0.50 g, L/D=3 based on hydraulic diameter for the annulus of 5

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mm). Activation of the sorbent consisted of an inert flush at 25 °C, followed by heating in 4%

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O2 to 500 °C at a ramp of 10 K.min-1 and a 2 hour dwell. During cycling bed temperature and

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the total flow (150 ml.min-1) was maintained constant, i.e. both the adsorption and

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regeneration steps. H2S concentration of 400 ppm in the reducing gas (10% H2, 20% CO2, 5%

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H2O) was used.

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Results and discussions

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Thermodynamic analysis



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In this study a supported copper sorbent stabilized by Ti on gamma alumina is used.

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Unfortunately the thermodynamic diagram for Al–Cu–Ti–O is not available. Having in mind

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the limitations, the thermodynamic calculations can be simplified to Cu–O system in order to

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provide a guideline to design and validation of the sorption-regenration process. The "active"

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component of the sorbent is copper which has high sulfur affinity and high driving force for

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regeneration. The first impregnation most likely forms CuAl2O4 on the support structure.

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Titanium is added as an attempt to restrict the agglomeration of cupper ions through

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formation of Cu2TiO4 phase. Both titanium and aluminium are stabilizers due to the higher

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stability of titanium- and aluminium- based Cu-oxide in reducing atmospheres.

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The intermediate temperature H2S removal and sorbent regeneration approach of this study

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consists of three modes: first sulfurization, then oxidation, and finally regeneration of the

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sorbent, as illustrated in Figure 2. At 375 °C the following reactions are expected to occur in

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the three aforementioned modes.

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In the sulfurization mode, H2S is trapped to form Cupper-sulfide according to sulfidation

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reaction:

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Cu2O(s) + H2S(g) → Cu2S(s) + H2O(g)

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ΔH°(298K)= –146.8 kJ

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In the oxidation mode, the oxidation step transforms Cu-sulfide to Cu-sulfate:

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Cu2S(s) + 2.5 O2(g) → CuSO4(s) + CuO(s)

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ΔH°(298K)= –367.5 kJ

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In the regeneration mode, Cu-sulfate is destabilized by reduction to form gaseous SO2 and

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Cu-oxide:

&

&

(1)

ΔS°(298K)= –2.4 J/K

(2)

ΔS°(298K)= –275.7 J/K


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2CuSO4(s) + 3H2(g) → Cu2O(s) + 3H2O(g) + 2SO2(g)

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ΔH°(298K)= 50.1 kJ

&

(3)

ΔS°(298K)= 544.4 J/K

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Figure 2. Illustration of the regenerative sulfide-sulfate-oxide desulfurization process. Sorbent

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sulfurization mode: Desulfurization of H2S from syngas; Oxidation mode: Sorbent oxidized

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from sulfide to sulfate; Regeneration mode; SOx removal from sorbent.

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The thermodynamic route for the desulfurization and regeneration are illustrated by the

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arrows in the Cu-S-O-H predominant phase diagrams in Figures 3 and 4, for sorption and

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regeneration, respectively. These phase diagrams illustrate the expected trends, but will of

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course not reflect the stability regions completely since Cu is stabilized partly in the CuAl2O4

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and Cu2TiO4 structures. Both Figures 3 and 4 are calculated through FactSage 6.4 program at

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atmospheric pressure and a temperature of 375 °C. In Figure 3, the thermodynamic conditions

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and the phase changes for the process illustrated in Figure 2 are marked with the spherical

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symbols and arrows. The green, red and grey dashed arrows represent reactions (1), (2), and

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(3), respectively.



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From Figure 3, it is evident that Cu has the potential to decrease the sulfur level down 10-13

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atm at the oxygen partial pressures in the range of 10-18 to 10-21 atm. However the H2S partial

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pressure over the sorbent increases by moving to lower oxygen partial pressures, and the H2S

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slip level depends strongly on the oxygen partial pressure of the gas composition. Thus

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additional steam content will to some extent increase the oxygen partial pressure, and

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therefore stabilize the Cu2S phase, and resulting in lower H2S slip. Assuming that the desired

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H2S removal is down to 1 ppm, i.e. H2S partial pressure of 10-6 atm, a wide range of oxygen

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partial pressure (10-18 to 10-36 atm) can fulfill this criterion. This range of operation condition

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is marked on the phase diagram, by the solid red line in Figure 3. Cu2S is then oxidized during

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the oxidation mode to CuSO4 through the excessively exothermic reaction (2) without any

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release of SO2 from the reactor. Of course one could think of direct regeneration of Cu2S by

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oxidizing it with air to CuO without passing the CuSO4 region, but this condition is

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thermodynamically not viable below 800 °C, which does not fit the window of operation. At

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375 °C, mild reducing conditions (mostly steam, with marginal hydrogen content) regenerates

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the sorbent by decomposing CuSO4 to Cu2O, and releasing concentrated SOx. As Figure 4

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shows (solid red lines), the maximum obtainable SO2 percentage is 20% i.e. log(pSO2, atm)=

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–0.7 at an oxygen partial pressure of 10-17 atm.


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Figure 3. Predominant phase diagram for the Cu system at 375 °C for P(H2S) and P(O2).

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Figure 4. Predominant phase diagram for the Cu system at 375 °C for P(SO2) and P(O2). 11 ACS Paragon Plus Environment


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Sorbent characterization and morphology

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The supported sorbents were fabricated through the impregnation route starting from a high

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surface area support material (γ-Al2O3) with rather high concentration (20% molar) metal

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cations to form a metal oxide layer during calcination. Supporting active material on γ-Al2O3

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allows combination of high thermo-chemical stability with high surface area. After sintering,

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the sorbents were crushed and characterized by XRD, and the aimed spinel phases, (CuAl2O4

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and Cu2TiO4) were identified. The SEM picture presented in Figure 5 represents the cross-

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section of the sorbent after impregnation and sintering. It is clear from Figure 5 that the

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sorbent is heavily loaded and the active material has well filled the pores of the support

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resulting a well impregnated sorbent. BET measurements are in agreement with this

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observation indicating more than 50% decrease in surface area after impregnation and

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sintering. The bulk density of packed material is in the range of 970–1130 kg/m3.

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Figure 5. Cross-section SEM image of impregnated and sintered copper based sorbent used in

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this study.

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Thermogravimetric analysis

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Figure 6 illustrates an isothermal cycle of sorption-desorption as measured in the TGA. The

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experiment is performed under isothermal and isobaric condition, 375 °C and 1 atm. The 12 ACS Paragon Plus Environment

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sequence of the reactions in Figure 6 (arrows), correspond to the arrows in Figure 2. In order

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to distinguish the mass changes due to reduction and sulfidation reactions and to be able to

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evaluate the kinetics, the sorbent is firstly reduced by a reducing gas mixture, wet 10% H2 in

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inert, and subsequently exposed to the H2S containing feed (400 ppm). Extra steps were

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introduced in these measurements since it was necessary to separate several reactions that

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otherwise would occur simultaneously. As shown in Figure 6, the reaction rate of the

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reduction step is much faster than the sulfur sorption. This implies that in the application, the

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sorbent could be directly reduced by the H2S containing syngas in a single step. The sorption

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step results in the sulfidation of copper content of the sorbent, according to reaction (1). The

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sorption observed in Figure 6 corresponds to 65 ± 5 % of the copper participating in the

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sorption reaction (as active material) forming Cu2S, and the rest of the copper remains

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unaffected, as it is well stabilized to the sorbent as adhesive. Upon completion of the H2S

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sorption process, controlled oxidation is performed by the introduction of 5% O2 in the next

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step. This results in the oxidation of the Cu2S to CuSO4 and CuO according to reaction (2),

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which is accompanied by a weight increase. Although air could simply be used as the

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oxidizing agent, a lower content of O2 was used to limit possible overheating by the extremely

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exothermic nature of reaction (2). Finally, by introducing a reducing gas stream, the sorbent is

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regenerated and SO2 is released from the sorbent. As Figure 6 shows, the regeneration

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reduction of the CuO–CuSO4 is very fast. Controlled oxidation with 5% O2 in inert was

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performed at the end of the experiment, in order to evaluate the reversibility and study

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whether the sorbent has lost any of its capacity after the cycle. Upon oxidation by this step,

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the sorbent regained its original mass, indicating that the sorbents capacity is not affected by

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absorption–desorption cycle. In fact the repetition of the cycle for three times at 375 °C in

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different sorption and desorption gas conditions showed no indications of any decrease in the

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sorption capacity. The main advantage of this approach is the temperature range of



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regeneration. This technique can be used for temperatures in the range of 300 to 500 °C,

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which is also the temeprature range where Pd membranes are efficient.

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Figure 6. Thermogravimetric isothermal cycle of sorption–desorption.

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Figure 7. Effect of temperature on the sorbents capacity during sulfidation.


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Similar to Figure 6, isothermal cycles of sorption–desorption were repeated at different

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temperatures of 350, 425 and 450 °C. Figure 7 shows the effect of temperature on the sorbents

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capacity during sulfidation. As evident from Figure 8, the increase in temperature improves

257

the reaction rate. Bearing in mind that the reaction kinetics is either controlled by transport or

258

chemical reaction, in either case the temperature plays a positive role in sorption rate.

259

However the increase in temperature has a negative effect on sorption capacity. As shown in

260

Figure 7, by increasing temperature, the sorbent capacity decreases from 2.25 wt% at 350 °C

261

to less than 2 wt% at 450 °C. This is due to the sulfidation thermodynamics. As sulfidation is

262

an exothermic reaction, the conversion for the reaction decreases at higher temperatures.

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Assuming approximate 2 wt% capacity for sulfur, this sorbent has the capacity of more than

264

0.6 molS.kg-1. Unreacted Shrinking Core (USC) model is used to interpret the kinetics from

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the TGA data. The model is suitable for spherical particles of unchangeable size. The model

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was first developed by Yagi and Kunii25. The shrinking core model for spherical grains with

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chemical reaction control can be formulated by the following kinetics equation:

268

𝑡𝑡 = 𝜏𝜏 ∗ (1 − (1 − 𝑋𝑋)3 )

269

1

(4)

where 𝜌𝜌 𝑅𝑅𝑠𝑠

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𝜏𝜏 = 𝑘𝑘"𝐶𝐶𝑠𝑠

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In equation (5), 𝑘𝑘" is the first-order rate constant for the surface reaction.

272

(5)

𝐻𝐻2 𝑆𝑆,𝑔𝑔

Applying this kinetic model to the thermogravimetric measurement, results in equation (6),

273

and the kinetic model fittings in Figure 8.

274

1 ⁄ 𝜏𝜏 = ([𝐶𝐶(𝐻𝐻2𝑆𝑆)] ∗ exp(−20000/𝑅𝑅𝑅𝑅))/3850

(6)



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In equation (6), τ is in minutes, 𝐶𝐶𝐻𝐻2 𝑆𝑆,𝑔𝑔 is in vppm and T is in K. It is evident from Figure 8

277

describes the sorption reaction satisfactorily.

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that the shrinking core model with first-order chemical reaction control at the surface,

278

279 280

Figure 8. Effect of temperature on the sorbents conversion during sulfidation. Constant total

281

flow of 600 ml.min-1 and H2S concentration of 600 ppm was used.

282


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283

Figure 9. Effect of H2S partial pressure on the sorbents conversion during sulfidation. Inner

284

box: Effect of gas speed on the sorbents conversion during sulfidation, while constant H2S

285

partial pressure.

286

In Figure 9, the effect of H2S partial pressure on the sorbents conversion during sulfidation is

287

illustrated. The increase in H2S partial pressure increases the sorption reaction rate without

288

affecting the sorption capacity. In order to make sure that the availability of H2S in the gas

289

stream is not controlling the sorption, the H2S containing gas was charged into the reactor at

290

different feed flows, but at a constant H2S partial pressure. The results are presented in the

291

inner box of Figure 9. The conversion rate versus time was indifferent to the feed flow rate.

292

Packed bed H2S removal

293

Packed bed experiments were used to optimize and develop the best procedure for

294

regeneration. Figure 10 shows the mass spectrometry results for a typical cycle at 375 °C

295

performed in the packed bed reactor. The cycle showed in this figure follows the same

296

procedure as the experiments performed in the TGA, except that the sulfurization mode is

297

continued until the breakthrough of H2S was reached, and the gas compositions leaving the

298

reactor are analyzed. As the focus of Figure 10 is on the regeneration step, the graph starts

299

with the oxidation where the sorbent is in the form of CuO and CuSO4. Following a short

300

inert flush, the regeneration step starts with introducing H2. As Figure 10 shows, upon

301

introducing H2 and starting the regeneration mode, the SO2 release is very quick with a sharp

302

SO2 peak. Almost no H2 was detected to leave the reactor during this period. Therefore a

303

highly concentrated SO2 stream is achived during the regeneration mode. This indicates that

304

the reductive regeneration of the sorbent is very quick and effective. Following a short inert,

305

the sulfurization step begins. the sorbent adsorbs almost all the H2S content of the feed gas,

306

and a very low H2S slip during the H2S removal stage from the gas is achieved. 17 ACS Paragon Plus Environment


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307 308

Figure 10. Mass spectrometry results for a typical cycle at 375 °C of the packed bed reactor

309

experiments.

310

Figure 11 shows the quantified results for the packed-bed reactor experiments performed at

311

different temperatures. The temperature was varied from 375 °C to 550 °C. The sorption

312

capacity remains constant around 1.3 to 1.4 wt% for the temperature range from 375° to 500

313

°C. Only at 550 °C the capacity appears to drop somewhat to 1.2 wt%. The capacity observed

314

during these experiments are to some extent lower than what was observed with the TGA

315

experiments. This is of course due to the difference in the ways the TGA and packed bed

316

experimnets are performed. In the TGA the total sorbent capacity until saturation is measured,

317

but in the breakthrough experiments the capacity is calculated until the point of H2S

318

breakthrough (and thus not necessarily sorbents full thermodynamic capacity for sulfur).

319

Expectedly these two numbers differ significantly. Still it should be noted that the

320

breakthrough capacity is significant, when considering the subsequent regeneration step. It

321

should be noted that the slip and capacity measured at 375 °C and 400 °C represented by the

322

open symbols were measured in a 4 mm ID reactor with a bed height 75 mm (W=0.75g,

323

L/D=19), while the other measurements were done in a reactor with an annulus bed of 8 mm 18 ACS Paragon Plus Environment

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OD and 3 mm ID with a bed height of 15 mm (W=0.50 g, L/D=3 based on hydraulic diameter

325

for the annulus of 5 mm). Since identical slips were measured, this hints towards the absence

326

of diffusion limitations, meaning that the measured slips would represent the equilibrium H2S

327

content for the sorbent. As can be expected, the H2S slip level is more sensitive to the

328

operating temperature, increasing from 5 ppm at 375 °C to 22 ppm at 550 °C. The black

329

symbols are thermodynamic calculations for the H2S slip for a Cu2S formation at the reactive

330

gas partial pressure conditions of the study. Although the temperature dependency is stronger

331

for the thermodynamic prediction, the experiments show the same order of magnitude for the

332

H2S slip. The results are in very good agreement considering the short bed used in this study.

333 334

Figure 11. H2S capacity and H2S slip as a function of the cycle temperature. Open symbols

335

represent points measured during the stability testing, with a larger L/D for the bed than the

336

solid points.

337

Figure 12 shows the mass spectrometry results of the packed bed reactor for a series of

338

sequential cycles performed at 375 °C. For simplicity, only the SO2 and H2S mass

339

scpectrometry results are shown in this figure. Large SO2 peaks in this figure correspond to

340

the sorbent regeneration, when a large amount of SO2 was released by H2 introduction to the 19 ACS Paragon Plus Environment


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Page 20 of 27

341

reactor to regenerate the sorbent according to reaction (3). This figure also illustrate that a

342

small amount of SO2 is released at the starting point of the oxidation mode. This small SO2

343

release is similar to the conventional high temperature (550 °C and above) technique for

344

regeneration of copper sulfide to copper oxide using air. At lower temperatures, which are

345

also the main purpose of this study, CuSO4 is the stable phase after oxidation, thus reduction

346

with H2 is required to regenerate the sorbent, as also explained in the thermodynamics

347

analysis section. Quantification of the 50 cycles at 375 °C in Figure 13, confirms that the

348

developed sorbent and desulfurization methodology is highly reproducible, and no

349

deterioration in terms of H2S slip or sorption capacity after 50 cycles was observed. Post-

350

characterization SEM analysis, shown in Figure 14, confirm that after 50 cycles at 375 °C

351

only some minor Cu agglomeration and minor cracks in the outer layers are observed. The

352

sorbent shows good stability after 50 cycles, reflected by nearly unchanged microstructure.

353 354

Figure 12. H2S and SO2 mass spectrometry results for repetitive cycles at 375 °C performed in

355

the packed bed reactor.


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356 357

Figure13. Long term packed bed rector testing of sorbent for 50 cycles at 375 °C.

358 359

Figure14. SEM analysis of cross section after 50 cycles.

360

Conclusion

361

An effiecient Cu based sorbent material has been produced for H2S removal from syngas at

362

intermediate temperatures through chemical swing adsorption. This sorbent shows a slip of

363

less than 20 ppm below 500 oC and all the way down to 5 ppm at 375 oC. This regenerative

364

copper–based sulfur sorbents is a good alternative for hot syngas desulfurization applications 21 ACS Paragon Plus Environment


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in the temperature range of 300–500 oC avoiding penalties introduced by scrubbing . The

366

capacity for the developed sorbent is around 2 wt% when the thermodynamic limit is reached.

367

The sorption reaction rate is sufficiently high for the desulfurization process operation even at

368

these low temperatures. The sorbent showed good stability throughout 50 cycles in packed

369

bed testing. The regeneration process using an oxidation followed by reduction is very

370

efficient, led to full recovery of capacity and results in a highly concentrated stream of

371

SO2.The optimal temperature, based on sufficient kinetics combined with low H2S slip and

372

low SO2 release during the oxidation mode, is 375 °C. This temperature suits well to the

373

operation temperature for palladium membrane feed stream desulfurization for hydrogen

374

purification applications.

375

Acknowledgements

376

The research leading to these results has received funding from the European Union through

377

the EU-7FP CACHET-II project under Grant Agreement Number 241342.

378

References

379

(1) Hildenbrand, N.; Readman, J.; Dahl, I. M.; Blom, R. Sorbent Enhanced Steam Reforming

380

(SESR) of Methane Using Dolomite as Internal Carbon Dioxide Absorbent: Limitations due

381

to Ca(OH)2 Formation. Appl. Catal., A 2006, 303, 131.

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(2) Liu, Y.; Li, Z. S.; Xu, L.; Cai, N. S. Effect of Sorbent Type on the Sorption Enhanced

383

Water Gas Shift Process in a Fluidized Bed Reactor. Ind. Eng. Chem. Res. 2012, 51, 11989.

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(3) Uemiya, S.; Sato, N.; Ando, H.; Kikuchi, E. The Water Gas Shift Reaction Assisted by a

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Palladium Membrane Reactor. Ind. Eng. Chem. Res. 1991, 30, 585.

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(4) Uemiya, S.; Sato, N.; Ando, H.; Matsuda, T.; Kikuchi, E. Steam Reforming of Methane in

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a Hydrogen-Permeable Membrane Reactor. Appl. Catal. 1990, 67, 223. 22 ACS Paragon Plus Environment

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(5) Peters, T. A.; Kaleta, T.; Stange, M.; Bredesen, R. Development of Thin Binary and

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Ternary Pd-based Alloy Membranes for Use in Hydrogen Production. J. Membr. Sci. 2011,

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383, 124.

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(6) Peters, T. A.; Kaleta, T.; Stange, M.; Bredesen, R. Development of Ternary Pd–Ag–TM

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Alloy Membranes with Improved Sulphur Tolerance. J. Membr. Sci. 2013, 429, 448.

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(7) Ockwig, N. W.; Nenoff, T. M. Membranes for Hydrogen Separation. Chem. Rev. 2007,

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107, 4078.

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(8) Gazzani, M.; Manzolini, G. Using Palladium Membranes for Carbon Capture in Integrated

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Gasification Combined Cycle (IGCC) Power Plants. In Palladium Membrane Technology for

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Hydrogen Production, Carbon Capture and Other applications; Doukelis A., Panopoulos K.,

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Koumanakos A., Kakaras E., Eds.; Woodhead Publishing: Amsterdam, 2015; pp 221–246.

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(9) Gazzani, M.; Turi, D. M.; Ghoniem, A. F.; Macchi, E.; Manzolini, G. Techno-Economic

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Assessment of Two Novel Feeding Systems for a Dry-Feed Gasifier in an IGCC Plant with

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Pd-Membranes for CO2 Capture. Int. J. Greenhouse Gas Control 2014, 25, 62.

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(10) Gazzani, M.; Turi, D. M.; Manzolini, G. Techno-Economic Assessment of Hydrogen

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Selective Membranes for CO2 Capture in Integrated Gasification Combined Cycle. Int. J.

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Greenhouse Gas Control 2014, 20, 293.

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(11) Løvvik, O. M.; Peters, T. A.; Bredesen, R. First-Principles Calculations on Sulfur

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Interacting with Ternary Pd–Ag-Transition Metal Alloy Membrane Alloys. J. Membr. Sci.

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2014, 453, 525.

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(12) Cheah, S.; Carpenter, D. L.; Magrini-Bair, K. A. Review of Mid- to High-Temperature

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Sulfur Sorbents for Desulfurization of Biomass- and Coal-derived Syngas. Energy Fuels

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2009, 23, 5291. 23 ACS Paragon Plus Environment


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(13) Zeng, B.; Yue, H.; Liu, C.; Huang, T.; Li, J.; Zhao, B.; Zhang, M.; Liang, B.

412

Desulfurization Behavior of Fe–Mn-Based Regenerable Sorbents for High-Temperature H2S

413

Removal. Energy Fuels 2015, 29, 1860.

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(14) Ren, X.; Chang, L.; Li, F.; Xie, K. Study of Intrinsic Sulfidation Behavior of Fe2O3 for

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High Temperature H2S Removal. Fuel 2010, 89, 883.

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(15) Wang, J.; Liang, B.; Parnas, R. Manganese-Based Regenerable Sorbents for High

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Temperature H2S Removal. Fuel 2013, 107, 539.

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(16) Mureddu, M.; Ferino, I.; Rombi, E.; Cutrufello, M. G.; Deiana, P.; Ardu, A.; Musinu, A.;

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Piccaluga, G.; Cannas, C. ZnO/SBA-15 Composites for Mid-Temperature Removal of H2S:

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Synthesis, Performance and Regeneration Studies. Fuel 2012, 102, 691.

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(17) Fenouil, L. A.; Lynn, S. Study of Calcium-Based Sorbents for High-Temperature H2S

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Removal. 3. Comparison of Calcium-Based Sorbents for Coal Gas Desulfurization. Ind. Eng.

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Chem. Res. 1995, 34, 2343.

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(18) Rezaei, S.; Tavana, A.; Sawada, J. A.; Wu, L.; Junaid, A. S. M.; Kuznicki, S. M. Novel

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Copper-Exchanged Titanosilicate Adsorbent for Low Temperature H2S Removal. Ind. Eng.

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Chem. Res. 2012, 51, 12430.

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(19) Woods, M.; Gangwal, S.; Jothimurugesan, K.; Harrison, D. P. Reaction Between

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Hydrogen Sulfide and Zinc Oxide-Titanium Oxide Sorbents. 1. Single-Pellet Kinetic Studies.

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Ind. Eng. Chem. Res. 1990, 29, 1160.

430

(20) Slimane, R. B.; Abbasian, J. Copper-Based Sorbents for Coal Gas Desulfurization at

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Moderate Temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338.


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(21) Abbasian, J.; Slimane, R. B. A Regenerable Copper-Based Sorbent for H2S Removal

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from Coal Gases. Ind. Eng. Chem. Res. 1998, 37, 2775.

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(22) Selim, H.; Gupta, A. K.; Al Shoaibi, A. Effect of Reaction Parameters on the Quality of

435

Captured Sulfur in Claus Process. Appl. Energy 2013, 104, 772.

436

(23) Stirling, D. The Sulphur Problem: Cleaning Up Industrial Feedstocks. (Chapter 1:

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Introduction: The Sulphur Problem); RSC Clean Technology Monographs, Clark, J. H.,

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Series Ed., The Royal Society of Chemistry: Cambridge, 2000; pp 1–9.

439

(24) Ashar, N. G.; Golwalkar, K. R. A Practical Guide to the Manufacture of Sulfuric Acid,

440

Oleums, and Sulfonating Agents. (Chapter 2: Processes of Manufacture of Sulfuric Acid);

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Springer: Cham, 2013; pp 9–30.

442

(25) Yagi, S.; Kunii, D. Studies on Combustion of Carbon Particles in Flames and Fluidized

443

Beds. Symp. Int. Combust. 1955, 5, 231.

444

Figure Captions

445

Figure 1. The gas mixing system and apparatuses used in this study. (a) magnetic suspension

446

balance; (b) Packed bed reactor.

447

Figure 2. Illustration of the regenerative sulfide-sulfate-oxide desulfurization process. Sorbent

448

sulfurization mode: Desulfurization of H2S from syngas; Oxidation mode: Sorbent oxidized

449

from sulfide to sulfate; Regeneration mode; SOx removal from sorbent.

450

Figure 3. Predominant phase diagram for the Cu system at 375 °C for P(H2S) and P(O2).

451

Figure 4. Predominant phase diagram for the Cu system at 375 °C for P(SO2) and P(O2).

452

Figure 5. Cross-section SEM image of impregnated and sintered copper based sorbent used in

453

this study. 25 ACS Paragon Plus Environment


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454

Figure 6. Thermogravimetric isothermal cycle of sorption–desorption.

455

Figure 7. Effect of temperature on the sorbents capacity during sulfidation.

456

Figure 8. Effect of temperature on the sorbents conversion during sulfidation. Constant total

457

flow of 600 ml.min-1 and H2S concentration of 600 ppm was used.

458

Figure 9. Effect of H2S partial pressure on the sorbents conversion during sulfidation. Inner

459

box: Effect of gas speed on the sorbents conversion during sulfidation, while constant H2S

460

partial pressure.

461

Figure 10. Mass spectrometry results for a typical cycle at 375 °C of the packed bed reactor

462

experiments.

463

Figure 11. H2S capacity and H2S slip as a function of the cycle temperature. Open symbols

464

represent points measured during the stability testing, with a larger L/D for the bed than the

465

solid points.

466

Figure 12. H2S and SO2 mass spectrometry results for repetitive cycles at 375 °C performed in

467

the packed bed reactor.

468

Figure 13. Long term packed bed rector testing of sorbent for 50 cycles at 375 °C.

469

Figure 14. SEM analysis of cross section after 50 cycles.

470


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