Article pubs.acs.org/IECR
Techno-economic Analysis of Membrane-Based Argon Recovery in a Silicon Carbide Process Thomas Harlacher,* Thomas Melin, and Matthias Wessling Chemical Process Engineering, RWTH Aachen University, Turmstrasse 46, 52064 Aachen, Germany ABSTRACT: This paper investigates the performance of membrane gas permeation processes for simultaneous argon recovery and hydrogen concentration adjustment. A thermal plasma synthesis and subsequent water gas shift results in a feed gas mixture predominantly containing argon, hydrogen and carbon dioxide, whereby argon and a certain amount of the hydrogen have to be recycled as the plasma gas. The separation performance of two-stage processes was determined by a flow sheeting analysis using Aspen Plus. The application of different membrane materials (polyimide, PEO based Polyactive) and the adaptation of the membrane areas in the two process stages allow an independent adjustment of the hydrogen and carbon dioxide concentrations in the product stream. The influences of temperature, membrane selectivity, and recycle flow rate are discussed. For the process evaluation, an economic analysis was performed using the various membrane process designs. For the assumed costing framework, the most economic feed pressure in the membrane system is between 5 and 8 bar with an argon recovery up to 94%. The main cost per volume argon recovered of the membrane process is just 15% of today’s argon prize.
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monoxide is achieved.6,7 Since the water gas shift reaction is an equilibrium reaction, water is added excessively. To remove the condensed water after cooling and to avoid water condensation during compression, the water has to be separated. In the gas permeation unit, the carbon dioxide and hydrogen resulting from the water gas shift has to be separated. While the carbon dioxide fraction should be reduced as much as possible, the amount of hydrogen should be set to a certain value. After compensating the argon losses caused by the gas treatment process, the recycled gas stream shall comply with the stream composition required for the plasma gas. The details and the economics of the membrane unit are investigated in this paper. By means of a validated membrane model,5 the membrane process was implemented in the flow sheeting program Aspen Plus.
INTRODUCTION For the synthesis of silicon carbide, the most widely used nonoxide ceramic, the thermal plasma synthesis is currently under investigation. In contrast to the state-of-the-art Acheson process, the thermal plasma synthesis of silicon carbide allows smaller grain sizes, less oxygen contamination and a continuous process. The synthesis is described by the equation: SiO2 (s) + 3C(s) ↔ SiC(s) + 2CO(g)
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Silicon dioxide and carbon in the form of char react to silicon carbide and carbon monoxide as a byproduct. Argon is used as the plasma gas. To improve heat conductivity and the enthalpy of the plasma, the argon is mixed with hydrogen.1 Therefore, the exhaust gas of the ceramic synthesis process consists of argon, carbon monoxide, and hydrogen. Since argon is an expensive gas, the plasma gas needs to be recycled. For this purpose, the carbon monoxide has to be removed from the exhaust gas of the synthesis reactor. Membrane processes became an industrial alternative for gas separation next to the conventional technologies as physical adsorption, chemical or physical absorption, and cryogenic fractioning.2−4 Therefore, the applicability of a membranebased gas separation process for the argon recovery was investigated in a prior study.5 The study showed that a membrane gas separation unit, using commercial polyimide and PEO-based membranes combined with a prior water gas shift reaction turning CO into CO2 is the most promising membrane process. The water gas shift reaction is a common method to lower carbon monoxide contents by conversion to carbon dioxide and hydrogen with the use of water. CO(g) + H 2O(g) ↔ CO2 (g) + H 2(g)
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BOUNDARY CONDITIONS The important boundary conditions for the process simulation are flow rate, compositions, and conditions of the streams entering and leaving the separation process. The specified values in Table 1 relate to a plasma gas flow rate of 1 kmol/h producing about 13.5 kg/h SiC. After ceramic synthesis, water gas shift reactor and gas drying, the flow rate adds up to 2.143 kmol/h. A completely dried feed gas stream is assumed after gas drying. The gas separation unit should maximize the argon recovery and ensure the argon purity at the same time. Since only the amount of the lost argon should be amended to the recycling stream, a certain hydrogen stream has to be set in the gas separation unit. The argon/hydrogen ratio of the plasma gas is defined to 5:1. Therefore, the recycle stream has to Special Issue: Enrico Drioli Festschrift
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Received: Revised: Accepted: Published:
The reaction is performed in two stages: The high temperature shift has a high reaction rate for the bulk conversion. In a following low temperature shift the required low level of carbon © 2013 American Chemical Society
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December 4, 2012 February 1, 2013 February 1, 2013 February 6, 2013 dx.doi.org/10.1021/ie303330b | Ind. Eng. Chem. Res. 2013, 52, 10460−10466
Industrial & Engineering Chemistry Research
Article
To perform the techno-economic analysis, a permeation model was implemented in Aspen Custom Modeler adapted to the membrane modules. The model includes the following assumptions: • solution-diffusion mechanism for mass transfer through the membrane • pressure losses in feed and permeate • no concentration polarization effects • no Joule-Thomson effect • pressure independent permeances The model was validated by experimental data and the assumptions are reasonable for this particular separation and the process conditions.5 For the multistage process design, the Aspen Custom Modeler model has been interfaced with Aspen Plus. For feed gas compression, an isentropic two-stage compressor was calculated, whereby the same pressure ratio was applied for both stages. The gas inlet temperature and the feed temperature after intercooling were set to 50 °C. The Soave−Redlich−Kwong equation of state was used for the thermodynamic calculations.
Figure 1. Schematic flow sheet of the considered gas treatment process for carbon monoxide removal from argon plasma gas.
Table 1. Boundary Conditions for the Membrane Gas Separation Unit Related to 1 kmol/h Plasma Gas feed after water gas-shift Ar CO2 H2 CO total
retentate
flow [kmol/h]
mole fraction [−]
flow [kmol/h]
0.833 0.572 0.718 0.020 2.143
0.389 0.267 0.335 0.009 1
