Practical Process Engineering

CO2 Capture by different solvents (Rate based simulations)

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Pumps and Systems Magazine- September 2018 https://bit.ly/2wHsHrQ

Chemical Engineering Magazine, September 2018 https://bit.ly/2Q3NFdt

Professor Ehsan Yarshater (1920-2018) R.I.P.

Catalyst Settling

I am done on steam reforming catalyst deactivation topic. Let's move forward to next topic and study a little bit about reformer tubes. 131- Reformer tube appearance One of the key performance indicator of any reformer is its appearance. Visual inspection of steam reformer tubes and temperature measurements with pyrometers provide a good guide to the performance of the catalyst inside the tubes. During normal operation the tube wall temperature profile of all the tubes should be similar. Tubes will look hot when the steam reforming reaction is inhibited and they cannot absorb the heat input from the furnace. Some guidance in assessing the cause of problems can be obtained from the tube appearance. Several typical forms of overheated tubes are: 132- Tiger tailing Clear, well defined, hot rings alternating with cooler rings which develop at random on individual tubes are known as ‘Tiger Tailing’. These indicate voids in the catalyst packing, and are often associated with bridging in the catalyst as a result of improper charging. If vibration or careful hammering does not remove the voids, the tube should be discharged and refilled at the earliest opportunity. Hammering with a rubber mallet is possible during plant operation but does involve some risk which could make the problem worse. Care is essential. 133- Hot bands These are not so well defined as the hot zones of ‘Tiger Tailing’ and are generally the result of catalyst deactivation. The deactivation may be caused by a general decrease in catalyst activity either because the catalyst is past its useful life, has been poisoned or if the catalyst surface becomes coated by a thin layer of carbon or some other deposit. Hot bands may often be removed by steaming, but if this has no effect the catalyst in the affected tubes should be discharged and replaced with new catalyst when a suitable opportunity is available. 134- Giraffe necking These are random hot zones or patches which can occur for three main reasons: • An extreme form of the hot band problem which is caused by extensive deactivation from catalyst poisoning or a surface deposit. • Channelling through the catalyst. This cools the tube where the gas is still reacting and overheats the tube where the flow of gas is impeded. Channelling can be caused by pockets of broken catalyst and dust, or by accumulation of carbon. • Catalyst may be too old for further use, so that part may be falling below the minimum required activity level. It should therefore be replaced. 135- Hot tubes Where extensive carbon deposition or catalyst breakdown restricts the flow of gas a whole tube can become overheated. It may be possible to remove carbon in such tubes by steaming but this is usually ineffective because of preferential flow through the unaffected tubes. Catalyst in hot tubes should be discharged at the next opportunity and replaced. When all tubes are hotter than expected with no obvious flow restriction, it is possible that the charge has not been properly reduced or that temperature indications are inaccurate. If so the proper corrective actions should be taken. 136- Settling Care should be taken to allow for the effect of tube expansion. Sufficient catalyst must be charged into the reformer tube when cold to make sure that when operating, and therefore hot, the catalyst does not settle down so far as to expose empty space at the top of the reformer tube.

Effect of catalyst support on oxidizing regeneration. Catalysts with sodium and potassium supports show the lowest regeneration level.

Reaction sequence for regeneration in oxidizing atmosphere. Sulfur to hydrogen sulfide and then to sulfur dioxide

Influence of H2O/H2 on regeneration. S/S1:sulfur ratio, S1 is the sulfur content before the regeneration. Catalyst: 0.5 g Ni/MgAl2O4 (S1= 963 wt. ppm), T= 973 K

128- Regeneration in oxidizing atmosphere When the ratio P(H20)/P(H2) is increased, no improvement is observed as expected from the chemisorption experiments. However, at P(H20)/P(H2) above 150-250 a significant change of the regeneration degree of the catalyst is obtained as shown in below figure. Increased sulfur removal is achieved at a P(H20)/P(H2) which may be close to the equilibrium constant for the oxidation of the catalyst. 129- Analysis of the exit gas shows the presence of sulfur dioxide as well as hydrogen sulfide. A mall amount of hydrogen will inhibit the conversion of hydrogen sulfide to sulfur oxides. Therefore, sulfur removal following this reaction pattern requires a total oxidation of the catalyst. If some part of the nickel surface is still exposed to the gas, hydrogen formed by reforming reactions will cause hydrogen sulfide to be retained at the surface by the chemisorption reaction. 130- The regeneration in steam is very sensitive to the composition of the support because the sulfur dioxide may react with the support. The following figure shows that the sulfur content of catalysts promoted with sodium and potassium is nearly unaffected by the treatment with steam. Sulfur dioxide reacts with the alkali forming alkali sulfate.

The sulfur poisoning is reversible. Catalyst: Ni/Sio2, S/C= 2.7, sulfur addition: 120-140 mg/s/(m^3 methane)

Axial profiles of sulfur uptake on catalyst for Naphta feed (wt. ppm)