"Thermodynamic Analysis of Processes for Hydrogen Generation by Decomposition of Water"

Transcription

1 "Thermodynamc Analyss of Processes for Hydrogen Generaton by Decomposton of Water" by John P. O'Connell Department of Chemcal Engneerng Unversty of Vrgna Charlottesvlle, VA A Set of Energy Educaton Modules for Chemcal Engneerng Sponsored by The Center for Energy Intatves of The Amercan Insttute of Chemcal Engneers Insttute for Sustanablty Module 1: Fundamentals Introducton Hydrogen s beng proposed as an mportant factor n meetng the energy demands of the future global communty. Whle hydrogen s not a fuel, t can serve as an energy carrer and storage for fuel cells that provde statonary and moble electrcty, as well as have chemcal value n producng ammona fertlzer and upgradng heavy ols, coal and bomass [1, 2]. Snce, under normal condtons, Nature prefers to combne hydrogen wth other atoms, especally to oxdze t to water, energy must be put n to obtan pure hydrogen gas, as descrbed by the Second Law of Thermodynamcs. As wth all real processes that go aganst natural tendences, rreversbltes cause the nput energy to always be greater than the energy that mght be recovered by oxdzng the hydrogen. The prncpal engneerng ssues are how to mnmze the extra energy nput, and to defne the scale of chemcal processes and equpment to be constructed for a specfc hydrogen technology. There are three prncpal methods for obtanng hydrogen nvolvng water: chemcal reformng or gasfcaton from water and fossl fuels or bomass, electrolyss of water, and photo-nduced or thermochemcal decomposton of water [1-3]. Fossl fuels contan carbon along wth varable amounts of hydrogen, so carbon doxde s always produced along wth the hydrogen. At present, as much as 95% of the 9 MM tons of hydrogen annually produced n the US s from natural gas, prncpally methane, va the "water gas" and "water-gas shft" reactons [1]. Ths process yelds four moles of hydrogen and one mole of carbon doxde from each methane and two water molecules. However, any carbon source can be used n ths approach, wth the rato of H 2 to CO 2 reduced as the H content n the fuel decreases. A rough ndcator of the H/C rato s the phase of the fuel; gas has the most H and sold the least. For lquds, the greater the fludty, the hgher the H/C. The declne n fossl fuel resources and the producton of carbon doxde make ths approach less attractve over the long run. In electrolyss, the energy s put n va electron removal and addton at electrodes under electrochemcal potentals, bascally the reverse operaton of a fuel cell. Unfortunately, there are sgnfcant losses of useful energy due to rreversble processes n the electrode reactons and to speces transport. Photo-nduced decomposton uses sunlght energy for ether bactera or -1-

2 semconductors to drectly produce hydrogen. But the mechansms are poorly understood and the technology s undeveloped. Fnally, thermochemcal decomposton of water uses hgh temperature heat and closed-cycle processes wth added substances to separate the hydrogen and oxygen nto the pure components. There are several feasble approaches [2, 3], but most nvolve costly chemcals and materals of constructon, and none have been bult and operated at ndustral scale. The ntenton of the present modules s to gve experence n usng thermodynamcs for process analyss of energy effects and process constrants. Thermochemcal decomposton of water to manufacture hydrogen wll be the prncpal llustraton, though related unts and processes wll be examned. The set of modules s comprsed of the followng components: Module 1. Fundamentals Module 2. Analyss of Sngle-Unt Processes Module 3. The Whole-Process Perspectve for Thermochemcal Hydrogen Module 4. A Smplfed Multsecton Process for Thermochemcal Hydrogen Module 5. The 3-Secton Sulfur-Idodne Process for Thermochemcal Hydrogen Module 1 formulates the thermodynamc relatons that wll be used n all Modules. The later Modules start wth a revew of ths materal and then proceed to example exercses wth and wthout solutons. Module 5 has more extensve problems that to allow ndvdual and group nvestgaton. Thermodynamcs for Process Analyss Thermodynamc evaluatons, usng fundamentals and modelng, can provde lmted, but vtal, nformaton for analyss and desgn of chemcal processes, such as hydrogen producton. In partcular, best-case energy nputs and outputs, as well as a bass for estmatng the nevtable addtonal energy from rreversble processes, can be obtaned. The Laws gve always-true relatons that constran processes va mass and energy conservaton, entropy generaton, and chemcal potentals for phase and reacton equlbra. Models then provde values for these nonmeasurable, or conceptual, propertes n terms of measurables, such as temperature, pressure, and composton. The prncpal approach of open-system thermodynamcs uses balance equatons about "black-box" unts wth only flows of materal, and ther accompanyng energes and entropes, to account for the effects of nteractons of the system wth ts surroundngs. In addton, the streams can be constraned to phase and reacton equlbrum condtons whch can be solved for. Thus, no detals of the process, such as equpment or nternal stream flows, need to be ncluded; only streams and energes crossng the boundary of the defned system are used. The balance equatons force the heat, work, and materal nteractons of the process unt to obey the Laws, and quanttatve results are relable to the extent that the propertes are accurate. A man goal s to determne the process confguratons for mnmum energy requrements for specfed sets of constrants by mposng reversblty. In addton, the mpacts of rreversbltes characterzed by rates of entropy generaton, or other measures [4], can be determned. Besdes ths desgn actvty, the relatons allow analyss of entropy generaton for processes developed wth process smulaton software, determne consstency among soluton propertes, and compare results from dfferent property models [5]. -2-

3 Fundamental Thermodynamc Analyss Thermodynamcs offers many ways, both rgorous and approxmate, to evaluate the utlzaton of energy flows for open systems, as descrbed n chemcal engneerng texts [6, 7]. The basc analyss here s descrbed n [6]. Fgure 1.1 llustrates the concept for a steady-flow system, wth nlet and outlet streams at specfed absolute temperatures, T, and pressures, P, as well as sets of molar or mass amounts for the components, {N}, and energy that crosses the boundares as "shaft work", W s, and heat, Q. Note that f a stream has both vapor and lqud, ts specfcaton must nclude the amounts of components n both the phases. For pure components, ths means ether T or P, the total flow, N, and the qualty or fracton of the system that s vapor, x, would specfed. For mxtures, defnng the state s more elaborate. Fgure 1.1. Steady Flow System for Applyng Materal, Energy, and Entropy Relatons, Eqs. (1.1) and (1.2). The balance equatons for steady flow processes are: N h (T, P, x ) N o h o (T o, P o, x o ) s W s Q b Q e 0 b Q N s (T, P, x ) N o s o (T o, P o, x o ) b b T b Q e T e S gen 0 (1.1) (1.2) Here h s the molar enthalpy, s s the molar entropy, and {x} s the set of component mole fractons found from the set of numbers of moles of components, {N}, n a stream. Knetc and potental energy dfferences n the flowng streams have been gnored n Eq. (1.1). The summaton s over all nput streams,, and the summaton s over all output streams, o. Consequently, all molar flow numbers, {N} and {N} o, are postve. The summatons and are for the work and heat effects, respectvely, assocated wth external utltes. The speces amounts, {N} and {N} o are related by mass conservaton; moles are conserved only n nonreactng systems. These two relatons express the conservaton of energy among the forms generally treated n chemcal processes, Eq. (1.1), and the balance of entropy, Eq. (1.2), whch has entropy -3- o s b

4 conservaton for reversble cases (s gen rev = 0) and postve entropy generaton (s gen > 0) n real systems. The heat effects, {Q b} and Q e, are defned to be postve when heat s put n; they cross the outsde of the system boundary (surroundngs) at temperatures T b and T e. We dstngush Q e as the total heat dscharged or rejected (< 0) by the system to the process envronment at T e. If an amount of heat s transferred over a range of temperature such as n a heat exchanger wth a sngle phase flud, the log mean temperature should be used. A reversble process gves the absolute upper lmt, the best case, of the effcency of energy usage. That s, when s gen = 0, the soluton to Eqs. (1.1) and (1.2) wll gve the mnmum nput shaft work, hgh-temperature heat, or energy-carryng materal, to accomplsh a process that does not occur spontaneously. The shaft work s usually n the form of electrcty for pumps, compressors, and turbnes and s postve for work put n across the system boundary. Here, t s not the tradtonal Pv work assocated wth expandng or contractng the system boundary, snce that work cannot occur steadly. The choce of boundary locatons s made on the bass of convenence, known devce effcences, and avalable property estmates. Thus the boundary of a system may be drawn so that all work devces are nsde the system and so are unseen n the dagrams. Such devces may be drven by a stream that enters and leaves the system at dfferent condtons, such as by steam or hgh-pressure helum from a nuclear reactor, or by heats that cross the boundary at dfferent temperatures to run a heat engne. As long as the specfcatons of the work devces are consstent wth the boundary-crossng flows, the process results wll also be consstent. In partcular, as shown below, t can be qute convenent to do analyses that avod detals of partcular peces of equpment, whle connectng to real process flows. In any case, the locaton of the system boundares wll determne f W s s explct n the energy balance, Eq. (1.1), though t never appears n the entropy equaton, Eq. (1.2). These equatons for a steady-flow system must be computed n relaton to a bass chosen to represent the system. Here we wll use a defned amount of the desred product, e.g., per kmol of H 2 product, though producton rate per unt of tme, e.g., kmol per hour, can also be chosen. Note that ths model-free analyss does not expose locatons wthn a process where heat would need to flow up a temperature gradent. Ths must be found from a "pnch analyss" [8], though "exergy" analyses also can be used [9]. However, ths nvolves treatng the specfc process unts, whch we wsh to avod. In any case, snce complex processes normally have the possblty of temperature crossover, real energy requrements wll be dfferent, leadng to lower effcences than those found here, and process desgn must fnd where pnches appear. Ths may requre ultmate adjustment of the process flows, but once the present analyss s set up, that aspect can be done easly. The two energy/entropy relatons force two unknown quanttes to be found from the known varable values, whle gvng consstency among molar flows for all chemcal reactons occurrng. The result s that many dfferent cases can be set up; Table 1 llustrates a few of these. Table 2 lsts the common practcal applcatons of the cases. Some generalzatons about effects of changng specfed varables can be made for closed and for sngle-unt open systems. For example, we can state the consequences of s gen > 0,.e., of puttng n rreversbltes whle keepng the same flow condtons. For work-absorbng devces, such as heat pumps, Eq. (1.2) shows that entropy generaton means more heat must be removed (Q e real < Q e rev < 0), so Eq. (1.1) gves more work nput (W s real > W s rev > 0). For devces that produce work, such as heat engnes, real systems yeld less work (W s rev < W s real < 0) whle less heat s put n (0 < Q b real < Q b rev). -4-

5 Table 1.1. Some Optons for Specfcatons and Soluton Varables for Eqs. (1.1) and (1.2). Case Specfcatons Soluton Varables A T, P, N, T o, P o, N o, W s, Q b, T b, T e, s gen Q e, Q bn B T, P, N, T o, P o, N o, W s, Q b, T b, T e, s gen Q e, W sn C T, P, N,, T o, P o, N o, W s, Q b, T b, T e, Q e W sn, s gen D T, P, N, T o, P o, N o, W s, Q b, T b, T e Q e, s gen E T, P, N,, P o, N o, W s, Q b, T b, T e, Q e T o, s gen F T, P, N,, P o, N o, W s, Q b, T b, T e, Q e, s gen T o, N n G T, P, N,, P o, N o, W s, Q b, T b, T e, s gen Q e, T o H T, P, N,, P o, N o, W s, Q b, T b, T e, Q e, s gen T o, W sn J T, P, N,, T o, P o, N o, W s, Q b, T b, T e, s gen Q e, N n * Includes all elements of set except n whch s solved for Note agan that 2-phase systems requre a specfcaton of the relatve amounts of the phases, such as by the qualty, x V, n a pure component system. Table 1.2. Practcal Applcatons of the Cases n Table 1. Case Common Practcal Applcaton A Heat effects on a dstllaton column (Desgn or Analyss) B Work and heat rejecton of a turbne, compressor, or pump (Analyss) C Work and rreversblty of a turbne, compressor, or pump (Analyss) D E F Outlet state and flow nto a turbne, compressor, or pump (Desgn) G H J Problem 1.1 Complete Table 1.2 for Cases E - J. For complex open systems, the nterplay among the work, heat, and stream flows and condtons can prevent smple generalzatons. Yet n cases where energy or hgh-energy substances are produced, entropy generaton handles all of the effects of rreversbltes. These can be cataloged as energy costs: (A) puttng n more energy by ncreasng nput heat (more postve Q b), hgher h (usually by hgher T ), and/or greater {N }, and (B) takng out more energy, ether by more heat output (more negatve Q e), hgher h o (usually by hgher T o), and/or greater {N o}, whch must be consstent wth {N }. Thus, to obtan the same power from a real steam power cycle, rreversblty (postve entropy generaton) requres more nput heat to produce more steam or the nput heat must be at a hgher temperature (and/or pressure). In addton, ether the temperature of the outlet heat s hgher, or more heat must be removed. For a process producng a gven amount of hydrogen from nput energy carred by helum at a gven pressure and temperature from a nuclear plant, f s gen s ncreased, the soluton to Eqs. (1.1) and (1.2) means more He flow or hgher energy condtons and more heat rejecton. Problem 1.2 Extend Tables 1.1 and 2.1 to other cases. Comment on ther expected usefulness n real engneerng work. -5-

6 References [1] U.S. Department of Energy Hydrogen Program (2011). Retreved August 31, 2011, from [2] Rand, D.A.J., Dell, R.M., Hunt, J.C.R., "Hydrogen Energy: Challenges and Prospects" RSC Publshng, Cambrdge, 2007, ISBN-10: X. [3] Hydrogen Producton (2006). Retreved August 31, 2011, from [4] Sankaranarayanan, K., de Swaan Arons, J., van der Koo, H.J. "Effcency and Sustanablty n the Energy and Chemcal Industres: Scentfc Prncples and Case Studes, 2nd Edton", CRC Press, 2010, ISBN-10: [5] O'Connell, J.P., Narkprasert, P., Gorensek, M.B., Process model-free analyss for thermodynamc effcences of sulfur processes for thermochemcal water decomposton, Int. J. Hydrogen Energy, 2009, 34, [6] Smth, J.M., Van Ness, H.C., Abbott, M.M., "Introducton to Chemcal Engneerng Thermodynamcs, 7th Edton", McGraw-Hll; 2005, ISBN-10: Sandler, S.I., "Chemcal, Bochemcal, and Engneerng Thermodynamcs, 4th Edton", Wley, 2006; ISBN-10: Koretsky, M.D., "Engneerng and Chemcal Thermodynamcs", Wley; 2003, ISBN-10: Ellott, J.R., Lra, C.T. "Introductory Chemcal Engneerng Thermodynamcs", Prentce Hall; 1999, ISBN-10: [7] O'Connell, J.P., Hale, J.M. "Thermodynamcs: Fundamentals for Applcatons: Cambrdge Press, Cambrdge, 2005, ISBN-10: [8] Kemp, I.C., "Pnch Analyss and Process Integraton, 2nd Edton: A User Gude on Process Integraton for the Effcent Use of Energy", Butterworth-Henemann; 2007, ISBN-10: [9] Sato, N. "Chemcal Energy and Exergy. An Introducton to Chemcal Thermodynamcs for Engneers", Elsever, 2004, ISBN 10: X. -6-