Practical Process Engineering

Whisker Carbon

102- Why we call it self-poisoning? Deactivation mechanisms could be classified in another way as below: Physical causes ——> Sintering Poisioning by impurites in the feed ——> Poisoning Deactivation by products of the reactions on the catalyst surface In the last one, catalyst itself produces coke via the aforementioned reactions. So, it is called self-poisoning or self-deactivation. 103- Whisker carbon: It is the most destructive form of carbon formed in steam reforming over nickel catalyst. The reaction of hydrocarbons as well as carbon monoxide over transition metals can lead to the formation of filamentous carbons . The carbon grows typically in a whiskerlike structure with a nickel particle at the top as shown in following picture. 104- Carbon is formed from equilibrated gas if the overall steam to carbon ratio is too low. Carbon whiskers grow by the reaction of hydrocarbons at one side of nickel particle and the nucleation of carbon as a whisker on the other side of nickel particle. This type of carbon does not deactivate the catalyst , but due to high mechanical strength of the carbon structure, it will weaken and eventually destroy the pellet. This can be observed as an increased pressure drop over the reformer tubes. 105- The whisker diameter is very close to that of the nickel crystal and the nickel crystal is very often pear shaped which may indicate a reconstruction during carbon formation of the nearly ideally shaped nickel crystals. 106- The growth rate of the whisker is independent of time meaning that large amounts of carbon can accumulate. This is in contrast to the carbon formation in catalytic cracking where carbon deactivates the active site forming the carbon.In some situations, the whisker growth may cease because the nickel crystal becomes encapsulated in carbonaceous deposits. The whisker growth rate in the absence of steam is not significantly influenced by the support nor the presence of alkali.

96- It is already mentioned that S/C ratio is maintained higher than stoichiometric level to alleviate high temperature and pressure effects in the reformer. Higher S/C ratio prevents carbon deposition on the catalyst, which may not only increase the pressure drop but also reduce the catalyst activity. As the rate of the endothermic reforming reaction is lowerded this way, it can result in local overheating of the reformer tubes (hot bands) and premature failure of the tube walls. 97- In principle, carbon formation may occure via the following reactions: A) Boudouard reaction: 2 CO <—-> C + CO2 delta H @298= -172.5 KJ/mol B) Methane Cracking: CH4 <—--> C + 2H2 delta H @298= +74.9 KJ/mol C) CO reduction: CO + H2 <——> C +H2O delta H @298= -131.4 KJ/mol 98- These reactions are reversible, and there is a dynamic equilibrium between carbon formation and removal. Under typical steam reforming conditions, reactions (A) and (C) are carbon -removing, whilst reaction (B) leads to carbon formation in the upper part of the tube. 99- This means that for a fixed gas composition of H2 , H20, CO, CO2 , and CH4 there is a temperature, T1, below which there is a thermodynamic potential (affinity) for the exothermic Boudouard reaction and CO reductin, and a temperature, T2 , above which there is an affinity for carbon formation by the endothermic decomposition of methane, reaction. 100- When a catalyst is present it is necessary to consider also the reforming and water gas shift equilibria. The risk of carbon is then normally evaluated by means of the so-called principle of "Equilibrated Gas" which states: Carbon formation is to be expected if the gas shows affinity for carbon after the establishment of the methane reforming and the shift equilibria. Since the gas is at equilibrium it is sufficient to consider one of the two carbon-forming reactions: exothermic or endothermic. The principle is no law of nature as illustrated below. It is merely a rule of thumb, indicating process conditions which are critical for carbon formation. 101- The various support materials have different effects on potential carbon formation. This seems to go in parallel with the Lewis/Bronsted acidity. The main commercially used catalyst supports can be ranked as follows in decreasing order of carbon forming tendency (and thus in decreasing order of the important minimum practical steam/ carbon ratio): a-alumina > magnesium aluminate (spinel) > calcium aluminate > alkalized calcium aluminate.

94- Coking/ Fouling/ Self-Poisioning Carbon formation is the most probable and dangeous deactivation mechanism to steam reforming catalyst. It is important to know how to avaoid it. 95- Carbon may be formed via different routes, each influencing the morphology of the carbon. The most common types are: - whisker-like carbon - encapsulating carbon - pyrolytic carbon following table summarizes charactristics of these three routes👇🏼👇🏼👇🏼

Hi, I have used many books, papers, power points, etc. to collect data about catalysts. Pleass let me know if you need them to study more. Thanks

93- Effect of hydrogen and steam. K is an quilibrium constant.

92- These variations with temperature may be interpreted as a change in the sintering mechanism. A reasonable hypothesis is therefore that the additional loss of nickel surface area above 600 C is due to a change in the sintering mechanism from particle migration and coalescence to Ostwald ripening.

Effect of H2 and temperature on surface reduction. Average nickel particle grows considerably faster with temperature above than 600 C.

a series ——> Particle migration b series ——-> Ostwaled ripening

86- Sintering/ Aging Sintering of the nickel crystals results in loss of surface area, and in principle recrystallization may change the nickel ensembles available, and also cause a decrease of the specific gravity. Sintering is expected above 864 K for nickel. 87- Although the catalyst carrier is rugged and somewhat temperature resistant, small nickel crystallites essential for high activity can grow in size via surface diffusion at temperatures above 700-800°C. It is known that stable micropores of the carrier can impede crystal growth if the nickel crystallite is of the same order of magnitude as the pore diameter. 88- The most important parameters are the sintering temperature and the atmosphere over the catalyst. Increasing the temperature and the presence of steam accelerate the sintering process. The increased rate of sintering in the presence of steam is attributed to formation of Ni2–OH species at the surface of nickel particles. Sintering at the same pressure of steam but at elevated hydrogen pressure decreases the rate of sintering. 89- High surface areas of the carrier increases the stability toward sintering 90- Sintering influences other catalytic challenges. The coking limits are affected by the nickel particle size, the nickel surface area determines the sulfur capacity, and the activity is related to nickel particle size. 91- Three mechanisms for the metal particle growth have been proposed: (i) particle migration, where entire crystallites migrate over the support followed by coalescence (ii) Ostwald ripening (atom migration), where metal atoms emitted from one crystallite migrate over the support and are captured by another crystallite; and (iii) vapor transport between particles (at high temperatures). Following pictures show particle migration and Ostwald rippening using electron microscopy sequence👇🏼👇🏼👇🏼

ZnO is absorbent used to remove sulfur.