Hydrogen production in microchannel reactors by process intensification

Hydrogen production in microchannel reactors by process intensification

Junjie Chen

Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China

Contributor: Junjie Chen, ORCID: 0000-0001-5055-4309, E-mail address: [e-mail]

In reactions under normal laboratory conditions, matter is neither created nor destroyed, and elements are not transformed into other elements. Therefore, equations depicting reactions must be balanced; that is, the same number of atoms of each kind must appear on opposite sides of the equation. The balanced equation for the iron-sulfur reaction shows that one iron atom can react with one sulfur atom to give one formula unit of iron sulfide. Chemists ordinarily work with weighable quantities of elements and compounds. For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams of iron sulfide. Because matter is not created or destroyed in a chemical reaction, the total mass of reactants is the same as the total mass of products. If some other amount of iron is used, say, one-tenth as much (5.585 grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of sulfur were initially present with 5.585 grams of iron, then 28.859 grams of sulfur would be left over when the reaction was complete. The ratio of reactants and products in a chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that matter is conserved in chemical processes, and calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved. A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels. Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide and hydrazine, require energy input when they are formed from the elements. The decomposition of limestone to make lime is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur. The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction. Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favourability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work. A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning methane. Methane has a low entropy. When methane burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

Streamwise distance (meter), Heterogeneous reaction rate along the length of the reactor (mole per square meter per second)

0                                 14.1419

0.00025                             14.1567

0.0005                               14.1167

0.00075                             13.9733

0.001                                 13.7817

0.00125                             13.5773

0.0015                               13.3834

0.00175                             13.2078

0.002                                 13.0467

0.00225                             12.8884

0.0025                               12.7233

0.00275                             12.549

0.003                                 12.3695

0.00325                             12.193

0.0035                               12.0273

0.00375                             11.8751

0.004                                 11.7343

0.00425                             11.5992

0.0045                               11.4645

0.00475                             11.3284

0.005                                 11.1919

0.00525                             11.0595

0.0055                               10.9352

0.00575                             10.8199

0.006                                 10.7123

0.00625                             10.6092

0.0065                               10.5073

0.00675                             10.4052

0.007                                 10.3035

0.00725                             10.2049

0.0075                               10.1114

0.00775                             10.0239

0.008                                 9.94127

0.00825                             9.86107

0.0085                               9.78104

0.00875                             9.70044

0.009                                 9.61993

0.00925                             9.54112

0.0095                               9.46564

0.00975                             9.39386

0.01                                   9.32487

0.01025                             9.25686

0.0105                               9.1882

0.01075                             9.11849

0.011                                 9.04832

0.01125                             8.97901

0.0115                               8.91153

0.01175                             8.84618

0.012                                 8.78242

0.01225                             8.71871

0.0125                               8.65378

0.01275                             8.58747

0.013                                 8.5204

0.01325                             8.45357

0.0135                               8.38782

0.01375                             8.3233

0.014                                 8.25958

0.01425                             8.19529

0.0145                               8.12949

0.01475                             8.06216

0.015                                 7.99387

0.01525                             7.92554

0.0155                               7.8578

0.01575                             7.79065

0.016                                 7.72352

0.01625                             7.65551

0.0165                               7.58549

0.01675                             7.51373

0.017                                 7.44118

0.01725                             7.36834

0.0175                               7.29539

0.01775                             7.22242

0.018                                 7.1493

0.01825                             7.07505

0.0185                               6.999

0.01875                             6.92134

0.019                                 6.84253

0.01925                             6.76289

0.0195                               6.683

0.01975                             6.60311

0.02                                   6.5227

0.02025                             6.44094

0.0205                               6.35734

0.02075                             6.27217

0.021                                 6.18588

0.02125                             6.09878

0.0215                               6.01121

0.02175                             5.92324

0.022                                 5.8344

0.02225                             5.74426

0.0225                               5.65277

0.02275                             5.56008

0.023                                 5.4663

0.02325                             5.37179

0.0235                               5.27682

0.02375                             5.18138

0.024                                 5.08523

0.02425                             4.988

0.0245                               4.88969

0.02475                             4.79046

0.025                                 4.69054

0.02525                             4.59019

0.0255                               4.48973

0.02575                             4.38924

0.026                                 4.28848

0.02625                             4.18738

0.0265                               4.08615

0.02675                             3.98492

0.027                                 3.88396

0.02725                             3.78374

0.0275                               3.68429

0.02775                             3.58597

0.028                                 3.48836

0.02825                             3.38251

0.0285                               3.28598

0.02875                             3.18522

0.029                                 3.08588

0.02925                             2.98803

0.0295                               2.89361

0.02975                             2.794388

0.03                                   2.694288

Contributor: Junjie Chen, ORCID: 0000-0001-5055-4309, E-mail address: [e-mail], Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China