PROCESS DESIGN AND CONTROL Efﬁcient Conversion of Thermal Energy into Hydrogen: Comparing Two Methods to Reduce Exergy Losses in a Sulfuric Acid Decomposition Reactor Leen V. van der Ham, † Joachim Gross,* ,† Ad Verkooijen, † and Signe Kjelstrup †,‡ Department of Process & Energy, Delft UniVersity of Technology, Delft, The Netherlands, Department of Chemistry, Norwegian UniVersity of Science and Technology, Trondheim, Norway Two methods for increasing the exergy efﬁciency of thermochemical processes for the production of hydrogen from water and high temperature thermal energy are presented and compared. Increasing the exergy efﬁciency is equivalent to reducing the entropy production. Starting from a reference reactor for the decomposition of sulfuric acid, two new reactor designs are developed that both reduce the entropy production by 26%. The ﬁrst design uses optimal control theory to obtain a more uniform distribution of the entropy production. As a result of this functional optimization we obtain optimal temperature proﬁles over the reactor length. This optimized design is found to perform the best, but it requires signiﬁcant changes in the heating equipment in order to approximately realize the optimal temperature proﬁles. A second design is obtained by increasing the reactor length. This leads to a higher pressure drop and requires additional compressor duty. 1. Introduction Driven by the need for affordable, environmentally friendly, and reliable energy sources and carriers, the development of thermochemical processes for the production of hydrogen from water has received considerable attention. 1 High temperature thermal energy is used as energy source, originating from nuclear reactors or solar collectors. Naturally, a high efﬁciency is essential for the feasibility of such processes. 1.1. Energy and Exergy Efﬁciencies. The performance of energy conversion processes can be evaluated using several types of efﬁciencies. 2 Nowadays, the most commonly used efﬁciency is based on the ﬁrst law of thermodynamics. It is called the energy efﬁciency and is deﬁned as the useful energy output divided by the total energy input. In the case of hydrogen production from thermal energy, this reduces to the ratio between the chemical energy stored in the produced hydrogen and the amount of thermal energy that is added to the process. The energy efﬁciency is based on quantities of energy, but it says nothing about the quality of the energy that is used. The quality of energy is usually described by its potential to perform work, also known as exergy. The difference between the exergy inputs and outputs of a process is the amount of work that has been consumed or produced. For each work consuming process, there exists a minimum amount of required work, which is called the ideal work. In practice the amount of consumed work is always larger than the ideal work because of irreversibilities that are present in the process, as given by the second law of thermodynamics, and the difference between these two amounts is called the lost work. The exergy efﬁciency of a work consuming process is deﬁned as the ratio between the ideal work and the actual amount of consumed work. In the case of hydrogen production from thermal energy, the exergy efﬁciency is equal to the ratio between the exergy of the produced hydrogen and the exergy of the thermal energy that is added to the process. Nowadays, it is increasingly argued that it is more important to optimize the exergy efﬁciency than the energy efﬁciency. Contrary to the energy efﬁciency, the exergy efﬁciency always gives meaningful and useful values. In addition, it can be used to indicate possible improvements. More details and examples on optimizing exergy efﬁciency or exergy analysis are discussed for example by Kotas, 3 Leites et al., 4 Rosen and Scott, 5 and Bejan et al. 6 In the case of hydrogen production from thermal energy, optimization of the exergy efﬁciency can be translated into minimizing the average temperature at which the thermal energy is added or reducing heat emissions to the environment. In general, optimization of the exergy efﬁciency is equal to minimization of the lost work, which is equivalent to minimiza- tion of the entropy production according to the Gouy-Stodola theorem. 7 1.2. Minimizing Entropy Production in Practice. From a thermodynamical viewpoint it is always useful to minimize entropy production, but the actual industrial process and equipment design is driven by costs rather than by thermody- namic variables. By including economical evaluations in the optimization, however, the results become dependent on the current socio-political situation and on varying technical maturity in different ﬁelds. The entropy production is by itself unambigu- ous and independent of time. This is why we deliberately chose to perform a purely thermodynamic optimization in this study. It is of course interesting to perform an analysis that includes economics, for example using an exergo-economics approach. 6 The economic trade-off is between lower operational costs, caused by a higher efﬁciency and indicated by lower entropy production, and higher investment costs, caused by the need for more complex equipment. 1.3. Approaches to Reduce Entropy Production. When identifying approaches to reduce the entropy production of a process unit, it is necessary to consider its design. The ﬁrst step in the design of a process unit is typically to deﬁne its primary objective, for example the transfer of a certain amount of heat, the realization of a certain displacement, or the conversion of a certain amount of chemicals. The next step is to deﬁne the time * To whom correspondence should be addressed. E-mail: j.gross@ tudelft.nl. Tel.: +31-152786734. Fax: +31-152782460. † Delft University of Technology. ‡ Norwegian University of Science and Technology. Ind. Eng. Chem. Res. 2009, 48, 8500–8507 8500 10.1021/ie801585e CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009 Downloaded by TECHNICAL UNIV OF DELFT on September 13, 2009 | http://pubs.acs.org Publication Date (Web): August 6, 2009 | doi: 10.1021/ie801585e