Practical Process Engineering

Lets' move forward to next step and talk about reforming catalyst deactivation. Different mechanisms are already introduced. 81- Catalyst poisons: Three poisons are introduced in literatures widely: Sulfur, Arsenic and Chlorine. Sulfur is the severest poison for theses catalysts. All the sulfur compounds in the feed will be converted into H2S at reforming conditions, and H2S will chemisorb on nickel surface: H2S+ Ni (surface) <——> Ni(surface)-S + H2 Severe sulfur poisioning in the top of the reformer results in an almost total loss of activity below 700 C and ,therefore, an increase in tube temperature. 82- Under normal conditions, the feedstock purification system will remove sulphur from the feed such that the sulphur slip to the reforming catalyst has minimal impact on the life of the catalyst. Occasionally, due to changes in feedstock or mal-operation of the purification system, etc, the reforming catalyst may be severely poisoned by sulphur. It is possible to remove most of the sulphur by steaming the catalyst. Temperatures should be kept as high as possible and the progress of sulphur stripping can monitored by measurement of hydrogen sulphide (H2S) in the reformer effluent gas. As with measurement of carbon oxides, an addition of a small constant flow of nitrogen into the steam assists the measurement of H2S when taking dry gas samples. 83- The effect of arsenic to be much less than that of sulfur. In contrast, it was demonstrated that 50-100 ppm of As2O3 on the catalyst affected the activity of an alkali-promoted catalyst. The reformer tube also picked up arsenic, which was transfered to subsequent charge of new catalysts and poisoned them too. Nielsen found evedience that the poisioning effect of arsenic is due to alloying with nickel. 84- Chlorine has no impact on a Ni/Mgo catalyst. However, for very large contents of chlorine (i.e. 1 wt% on the catalyst or 1000 ppm in the feed) scientists were able to detect deactivation of an alkali-promoted catalyst. 85- Other components (lead from the hydrocarbon feed, silica and other solids from steam, etc.) should be absent in the feed stream because they may eventually result in blockage of the catalyst pore system. Phosphorus was reported to be a poson for reforming catalysts, whereas Cadmium and Zink, which may escape from the sulfur removal system, have no detectable effect on the catalyst activity.

80- Typical physical properties of the Johnson Mattey catalysts

79- This graph compares 4 main type of catalyst manufactured by Johnson Mattey

CATASEL SSR structured catalyst manufactured by Johnson Mattey

KATALCO quadrolobe catalyst manufactured by Johnson Mattey

Wagon Wheel catalyst

73- Two main factors influence catalyst activity: chemical composition and surface area. But of no less importance for the performance are the heat transfer characteristics, which are governed by the size and shape of the particles. 74- Support materials are a-alumina, calcium aluminate, and magnesia - alumina spinel and alkalized calcium aluminate. 75- Alumina is now the predominant support for natural gas feedstocks, but is increasingly used in other parts of the world, too. Culcium Aluminate is still used worldwide on account of its natural alkalinity, which helps to suppress carbon deposition (more details will be posted later). It is therefore preferred for the manufacture of naphtha steam reforming catalysts. Catalysts intended for dealing with higher hydrocarbons are alkalized with potash. 76- Alkalization suppresses acidic spots on the surface of the catalyst and also promotes the reactions which remove deposited carbon from the catalyst surface. It is also advisable when processing natural gas which contains C4+ hydrocarbons to fill the first third of the tube, where the highest heat flux occurs, with alkalized reforming catalyst. 77- Effect of the size and shape of the catalysts on heat transfer and consequently performance is very important. However, the catalyst material itself is a very poor conductor and does not transfer heat to any significant extent. Therefore, the main mechanism of heat transfer from the inner tube wall to the gas is convection, and its efficiency will depend on how well the gas flow is distributed in the catalyst bed. It is thus evident that the geometry of the catalyst particles is important. 78- Various catalyst shapes have been developed by the individual catalyst manufacturers. Topso, Johnson Mattey and Clariant are the main catalyst manufacturers. Here are some photos of them.

Catalyst Activation/ Reduction Insitu activation VS. factory activation 69- In the industry, the catalyst is activated by exposure to natural gas, Hydrogen gas or hydrogen- rich gas at high S/C ratios , low space velocities and moderate outlet temperatures. In case of natural gas, ignition of the catalyst take place when a small part of the nickle is reduced, producing so much hydrogen that more nickle can be reduced leading to a fast increase in the activity. 70- Often, the catalyst first activates at the bottom of the bed where high temperatures are encountered. The activation proceeds faster in a feed containing higher hydrocarbons. The upper part of the bed is activated by back diffusion of hydrogen from one active catalystpellet to the one above. 71-Process gas to activate the catalyst must be free of catalyst poisions (mainly sulfur). 72- Factory reduction of the catalyst in pure hydrogen at the inlet of the reformer also results in smaller nickel crystalls, increased nickel surface area, and higher activity and tolerance toward sulfur at the reformer enterance.

66- In the tubular furnace of the steam reforming section, two elements are extremely important for performance; the catalyst activity and heat transfer through the walls of the reformer tubes, which strongly influence each other. 67- High temperatures are sometimes applied to compensate the catalyst malfunction due to deactivation. This excess heat leads to hot tube skin. On the other hand, heat plays role as a reactant for the reaction. Thus, a tube wall with good heat transfer characteristics transfers heat well and support the reaction. 68- Most reforming catalysts are delivered in the oxidised state and nickle is normally bound to the oxygen as NiO, which must be reduced by hydrogen to Ni by the reaction below before the catalyst is active for reforming. The nickel oxide content of unreduced catalyst is between 15 and 25 %. NiO+ H2 <——> Ni + H2O (-delta H @ 298 K= 1.2 KJ/ mol)

65- One of the proposed reaction mechanism. * is the catalyst. From an interpretation of the kinetic measurements, it was assumed that the surface reaction should be the rate-determining step (Step4)

Steam Reforming Catalyst 63- All steam reforming catalysts in the activated form contain metallic nickel as active component, but the composition and structure of the support and the nickel content differ considerably in the various commercial brands. 64- There is a general agreement that the steam reforming reaction is first order with respect to methane, but for the Qther kinetic parameters the results from experimental investigations differ considerabiy for various catalysts and reaction conditions studied by a number of researchers.

62- How to calculate S/C ratio? S/C ratio is molar flow rate of steam divided by molar flow rate of carbon. Here is an example.

61- Methane steam reforming conversion in terms of T, P and S/C ratio

60- Outlet CO2 for Naphta steam reforming

59- Outlet H2 for Naphta steam reforming

58- Methane slip for Naphta steam reforming