Practical Process Engineering

125- Regeneration of Sulfur-poisoned Catalysts The sulfur poisoning is reversible as shown in the next figure. The catalyst regains its initial activity after removal of the sulfur compounds in the feed. For industrial reformers, the deactivation from sulfur poisoning is most dominating at the inlet of the reformer. At high sulfur coverages this may result in increased tube wall temperatures and eventually in unsatisfactory approach to equilibrium in the exit gas. Sulfur poisoning can easily result in carbon formation indirectly since the catalyst activity as well as the tube wall temperature are important parameters in the development of hot tubes. If higher hydrocarbons are present in the feed gas they may pass over a deactivated catalyst to hotter parts of the reformer tube where conditions for carbon formation are more critical on the nickel surface as well as in the gas phase via thermal cracking to olefins. A) Regeneration in reducing atmosphere: 126- In principle it should be possible to remove sulfur from a poisoned catalyst simply by decreasing the sulfur content of the feed because the chemisorption of hydrogen sulfide on the nickel surface is reversible. This may well be achieved in experiments using a high flow rate and showing no diffusion restrictions. However, on an industrial scale this method will normally result in a slow regeneration because the rate of diffusion controlled elution decreases exponentially with time. 127- Therefore, the rate of sulfur removal by means of a desorption process decreases with time. The use of high temperatures during regeneration in a reducing atmosphere will have an effect because of the high heat of chemisorption. However, this may often be difficult at the inlet of the reactor (where sulfur is normally accumulated), because of temperature restrictions in preheating of the feed gas. The partial pressure ratio of P(H2O)/ P(H2) has no influence on chemisorption equilibrium . Therefore no regeneration effect is to be expected when increasing the ratio PH20/PH2 provided the catalyst is still in a reduced state.

Feed: light naphtha, S/C = 1.6, P = 1.7 MPa

123- Encapsulated carbon (gum) It may be formed in the reforming of heavy feeds with a high content of aromatic compounds. Low temperatures and high final boilinmg point of the hydrocarbon mixture enhance the rate of gum formation. Encapsulating carbon is thin CHx film, which covers the nickel particles and leads to the deactivation of the catalyst bed. 124- In adiabatic reformers (Prereformers) the resulting deactivation causes a continuous movement of the temperature profile [40] in the flow direction as illustrated in the following figure. adsorbed hydrocarbon species may be gradually dehydrogenated into non-reactive residues at a rate being slow compared to that of the reaction with the gas phase.

Analyses of dry exit gas. About 0.4 ton of steam per ton of catalyst per h. Catalyst Temp.= 723-873 K. & Pressure = ca. 0.6 MPa.

119- Regeneration of coked catalyst Rapid formation of whisker carbon can result in spalling of the catalyst and accumulation of carbon. The spalling can result in break-down of the outer part of the particle into powder. The result is an increasing pressure drop and the development of hot tubes. However, if whisker carbon is formed at a very slow rate, significant amounts may build up in the catalyst without harming the performance of the reformer. The formation of pyrolytic carbon can result in coke deposits at the tube wall and consequently a reduced heat transfer coefficient, which may cause the development of a "hot band". The catalyst particles can be encapsulated, which results in increased pressure drop. However, the encapsulation does not harm the catalyst particles. 120- it is possible in many situations to regenerate the catalyst and to re-establish satisfactory performance of the reformer. The whisker carbon is highly reactive and may be gasified by means of hydrogen, carbon dioxide, and steam with nickel as catalyst. Steam is the most effective gasifying agent: C + H2O——-> CO + 2 H2 (delta H @298 = -132 kJ/mol) Therefore, non-aged whisker carbon may be removed simply by increasing the steam-to-carbon ratio, at the same time maintaining the catalyst in a reduced state. However, the whisker structure will collapse with time and be converted into a more dense layer of carbon, which may be difficult to remove under reducing conditions. 121- Regeneration in steam can be carried out at temperatures around 870-970 K or higher depending on the ageing of the deposits. 122- with the addition of a small percentage of air the burn off of carbon is easily performed at a temperature above 720 K. The addition of air should be well controlled to minimize the local overheating of the catalyst caused by the heat produced from oxidation of the nickel and the carbon. In an industrial plant the progress of the regeneration can easily be followed by analysis of the exit gas from the reformer. An example is shown in following picture. The addition of air is increased as the production of CO2 decreases. In this way overheating can be controlled.

118- Effect of pressure on carbon formation

Summary: How sulfur affect coking?

Sulfur as inhibitor

116- It is well known that the presence of more than 50 ppm of sulfur in the feed stream to steam crackers significantly reduces the coke formation on the tube walls. Moreover, the sulfur may inhibit the carburization of the construction materials. For nickel catalysts a similar effect is observed. At complete coverage the sulfur blocks the nickel surface which means that adsorbed carbon atoms cannot be dissolved in the nickel crystal and that the whisker growth mechanism is blocked. 117- For the steam reforming of methane a complete coverage of the catalyst with sulfur results in total deactivation. However, it was observed that carbon-free steam reforming operation could be obtained with partly poisoned catalysts at conditions which would otherwise result in carbon formation. A series of experiments were performed with different sulfur contents in the feed, and at conditions for which the principle of equilibrated gas predicts carbon formation at temperatures below about 1100 K. The results shown in the following table indicate a certain activity for methane reforming and the existence of a threshold content of sulfur below which rapid carbon formation occurs.

115- Pyrolytic carbon may be formed when unconverted hydrocarbons may pass to the hotter part of the reformer tube (i.e., T> 920 K). This is unlikely with an active catalyst , but it can be provoked by sulfur poisoning of the catalyst. Therefore, when carbon formation problems occur in a tubular reformer, the carbon is normally of the whisker type. It is evident that (apart from short upsets) conditions where whisker carbon is formed cannot be tolerated.

Whisker carbon vs. Pyrolytic carbon

Types of Carbon

Pyrolytic Carbon

111- Pyrolytic carbon/ Thermal carbon: Pyrolytic carbon will result from the exposure of higher hydrocarbons to high temperatures. Often hot tubes or "hot bands" are the result of pyrolytic carbon formed by the cracking of unconverted higher hydrocarbons in the upper part of the reformer, and therefore, reaching the high temperatures in the lower parts. Deactivation of the top part of the catalyst bed is normally the result of severe sulfur poisoning. The temperature of the tubes increases because pyrolytic carbon may thrmally isolate the tubes and encapsulate the catalyst pellets, resulting in no activity and no consumption of the supplied heat. 112- The steam reforming reactions on the nickel surface may be accompanied by thermal cracking reactions (steams cracking), which may start at temperatures above ca. 920 K. In fact, a steam/naphtha reformer with a completely deactivated nickel catalyst will work as a steam cracker producing olefins. Therefore, the risk of carbon formation is to be analysed in the same way as for a steam cracker. It is generally agreed that the gas film at the tube wall is overheated and acts as a source of radicals and coke precursors. In steam crackers the tube skin temperature is the most important parameter determining the rate of coke formation. 113- the coking reactions are related to the so-called kinetic severity function (KSF): KSF = Integral (k(T) dt) which describes the residence time - temperature history of the reactants in a way that is consistent with kinetics. This means that for a given temperature profile the risk of carbon formation is increased with higher residence time. The catalyst filling has an influence on these parameters, first by changing the film volume and secondly by influencing the residence time distribution via the void fraction. Moreover, the nickel and the surface adicity of the catalyst will promote the formation of coke deposits from the tar-like intermediates. 114- The pyrolytic coke is normally found as dense shales on the tube wall as shown in following picture, or as deposits encapsulating the catalyst particles and eventually filling out the void between the particles.

110- As the nickel crystallites are not obscured or covered during this carbon-forming process, the rate of carbon formation can be enormous in a steam-free atmosphere. The whiskers formed within the catalyst pores are very strong. As a result, these can cause significant damage to the structure of the pellet. If serious enough, catalyst breakage can occur, and if an attempt is made to remove the carbon with steam, further breakage is likely. In part, this is because the carbon has caused the structure of the catalyst pellet to crack, but acts as a binder while still in place. Also, gasification of carbon inside the pore structure may occur faster than the gas can escape, causing the pellets to shatter.

109- formation of whisker carbon at low temperatures being replaced by carbon formation from thermal reaction at high temperatures.

108- The rate of carbon formation shows a complex dependency on temperature. Feed: n-butene, P (n-butene)= 13 Kpa, P (H2)= 3.2 Kpa

107- The extent of carbon formation depends strongly on the unsaturated character of the hydrocarbon.S/C=2, P= 0.1 MPa, T= 773 K, Catalyst: 0.7 g Ni/MgO