Module 4 : Hydrogen gas. Lecture 29 : Hydrogen gas

Transcription

1 1 P age Module 4 : Hydrogen gas Lecture 29 : Hydrogen gas

2 2 P age Keywords: Electrolysis, steam reforming, partial oxidation, storage Hydrogen gas is obtained in a very trace amount in atmosphere. It is found in the gases from some volcanoes, oil wells and coal mines. It may be liberated as a product of decomposition of organic matter. It is an important gaseous raw material used in many chemical and petroleum industries. Many metals absorb hydrogen, among which Palladium is one of the good examples. Gaseous hydrogen is shipped by truck, tanks or barge. The storage of hydrogen in the form of metal hydrides is getting increased interest as these can store hydrogen at moderate pressure, 2 MPa, instead of storing in hydrogen cylinders at higher pressure, 14 MPa. The common hydrides are iron-titanium, magnesium-nickel, lanthanum-nickel etc. Hydrogen gas production A number of methods are available to prepare hydrogen gas. The choice of method is dependent on the factors like, quantity of hydrogen required, extent of purity, availability and cost of raw materials. Among the processes of hydrogen gas preparation, the following are the most used methods. 1) Electrolytic method 2) Steam reforming 3) Partial oxidation 4) Coal gasification 5) Dissociation of ammonia

3 3 P age Electrolytic method A dilute aqueous solution of acid or alkali is decomposed by passing direct current through them to high purity hydrogen (99.7%) and oxygen gas by the Electrolytic method. 2H 2 O (l) 2H 2 (g) + O 2 (g) H= +569 kj Electrolysis is done in a photovoltaic cell using iron cathode and nickel anode with an asbestos diaphragm separating the cells into two compartments. A 15% sodium hydroxide solution is used to electrolyse at temperature of about C. Two types of cells are mainly used, one is bipolar or filter-press type and another one is uni-polar or tank type. The former one is such that each plate acts as an individual cell where, in the second one, two anode compartments are separated by a cathode compartment. Except dilute alkali solution, dilute acid or brine solutions may also be used in electrolysis process for production of hydrogen. Steam reforming Steam reforming is the most economic and efficient technology among many other techniques of hydrogen preparation and it uses a wide range of hydrocarbon feed stocks. The product gas is a mixture of hydrogen and carbon monoxide which is collectively called synthesis gas. The feed materials include natural gas, liquid gas or naphtha which are endothermally converted into synthesis gas with steam in catalytic tube reactors. The heat in flue gas produced is used for the steam generation. The hydro carbon feed is preheated in coils in the waste heat section of the reformer, and sulfur is removed over a zinc oxide catalyst. The desulfurized feed is mixed with superheated process

4 4 P age steam and this mixture is further preheated before entering to the tubular reformer. The tubes are arranged in vertical manner, heated from outside and the gas mixture flows from top to bottom through tubes. While flowing through the tubes, the hydrocarbon and steam mixture reacts together, forming hydrogen and carbon monoxide according to the following reactions: C n H m + n H 2 O nco + H 2 (1) CH 4 + H 2 O CO + 3 H 2 (2) CO + H 2 O CO 2 + H 2 (3) A higher steam to hydrocarbon ratio which is more than the theoretical one, is maintained to minimize the methane content and simultaneously maximizing the H 2 yield in the synthesis gas. High steam to hydrocarbon ratio also prevents the formation of elemental carbon which may deposit on the catalyst. The reactions are endothermic, hence heat is supplied externally which is done by firing burners. The burners for the firing are arranged on the ceiling of the firing area between the tube rows and fire vertically downward. The flue gas is then cooled down in a convection zone, generating steam. Nickel is the typical steam reforming catalyst. Sintering is an important cause of deactivation of nickel-containing steam reforming catalysts. The most important parameters are the temperature and the atmosphere in contact with the catalyst. Partial oxidation Partial oxidation is the second-most important process for preparation of hydrogen after steam reforming. The feed stocks which can be used are natural gas, refinery gases or other hydrocarbon gases, but the chief advantage of this process is the utilization of liquid hydrocarbon

5 5 P age feed stocks such as gas oil, diesel and even heavy fuel oil. This is a non catalytic partial combustion of hydrocarbon feed with oxygen in presence of steam at C temperature range. The reaction is exothermic. There are several advantages of partial oxidation. It is a simple operation with very little maintenance. Coal gasification Synthesis gas is produced by reacting pulverised coal with oxygen and steam under high pressures and temperatures. Synthesis gas is a mixture of carbon monoxide, carbon dioxide, hydrogen as well as small amounts of some other gases and particles. Cooling and cleaning of synthesis gas remove the other gases and particles, leaving only carbon monoxide, carbon dioxide and hydrogen.. During syngas cleaning, mercury, sulfur, trace contaminants, and particulate matter are removed. The carbon monoxide in the gas mixture is reacted with steam via the water-gas shift reaction to produce additional hydrogen and carbon dioxide in a shift reactor. Hydrogen is removed by a separation system, and the highly concentrated CO 2 stream can subsequently be captured and sequestered. Ammonia dissociation Cracking or dissociation of ammonia at high temperature produces one volume nitrogen and three volumes hydrogen. As nitrogen is an inert gas, hence, this gas mixture can be used as such for hydrogenation in industry. Liquid ammonia is vaporized from the cylinder and heated at

6 6 P age about C, passing over a catalyst, mainly nickel on alumina. The product gases are cooled in a heat exchanger by heating the reactant gas, ammonia.

7 7 P age Reference 1. Mc-Graw-Hill encyclopedia of science and technology, Vol 8, 9 th ed., Mc-Graw-Hill, Shreve s chemical process industries, G. T. Austin, 5 th ed., Mc-Graw Hill International Editions, Hydrogen production by steam reforming of hydrocarbons, Niels R. Udengaard, Am. Chem. Soc., Div. Fuel Chem., 49(2), 906, 2004.