Bed plugging

Staged Packed Beds (Plug Flow) with Intercooling. The reasoning in Chapter 9 shows that we would like the reacting conditions to follow the optimal tempera-... 

Removal of the metal contaminants is not usually economical, or efficient, during rapid regeneration. In fact, the deposited metals are believed to form sulfates during removal of carbon and sulfur compounds by combustion that produce a permanent poisoning effect. Thus, if fixed-bed reactors are to be used for residuum or heavy oil hydrodesulfurization (in place of the more usual distillate hydro-desulfurization) it may be necessary to first process the heavier feedstocks to remove the metals (especially vanadium and nickel) and so decrease the extent of catalyst bed plugging. Precautions should also be taken to ensure that plugging of the bed does not lead to the formation of channels within the catalyst bed which will also reduce the efficiency of the process and may even lead to pressure variances within the reactor because of the distorted flow patterns with eventual damage. 

Catalyst bed plugging can arise in a variety of ways, but the overall effect of bed plugging is always the same expensive shutdowns and possibly complete renewal of the expensive catalyst. Thus, the deposition of rust, coke, or metal salts (e.g., sodium chloride) from heavier and dirtier feedstock may all contribute to the plugging of a catalyst bed. Vanadium and nickel may also be deposited onto the surface of the catalyst as well as into the pore system. Asphaltene deposition from residua and heavy oils is also a potential means of bed plugging— coagulation of the asphaltenes becomes appreciable at temperatures above 420°C (790°F) with the formation of hard, coke-like materials on the catalyst. 

When processing coal derived liquids which contain fine solid particles, the possibility of bed plugging may be minimized. 

PACKED BED PLUG-FLOW CATALYTIC REACTOR 9.8.1 Laboratory Scale Reactor... 

FIGURE 9.15 Schematic representation of a packed bed plug-flow catalytic reactor. 

It is clear that a behavior such as the one shown in Fig. 6-4 is caused by plugging of the catalyst bed by solids deposition. The plugging causes the bed void fraction to decrease. Some speculations on the modes of deposition can be made from analysis of the plugging material. This material consists primarily of oxides of iron in the form of loose particles and is densely deposited in the inlet portion of the reactor. The material is probably carried by the fluid while it is flowing through rusted pipelines. As the reaction proceeds, bed plugging in an HDS reactor also occurs as a result of metal deposits, both nickel and vanadium, and coking of the catalyst. 

Series and series-parallel reactions in a packed-bed plug-flow membrane reactor have been analyzed by Lund and his co-investigators [Agarwalla and Lund, 1992 Lund, 1992 Bernstein and Lund, 1993]. First consider the following consecutive catalytic reactions ... 

The gas flow through tubular reactors is of particular importance because the composition at any point is influenced by the linear velocity of the gas, the size of the reactor and the size of the catalyst particles. When gas flows through a pipe at low linear velocity (low Reynolds number), the radial velocity is not uniform. As the linear velocity increases, turbulence increases and the velocity profile approaches what is called plug flow. However, in a packed bed, plug flow can never be completely attained because of the high voidage near the reactor wall. 

Catalyst bed plugging can arise in a variety of ways, but the overall effect of bed plugging is always the... 

Ter-butanol dehydration and n-hexane cracking were studied on samples in H form, isobutane dehydrogenation was studied on dried samples activated in situ. Reactions were carried out on st.steel or pyrex integral, fixed bed, plug flow reactors at atmospheric pressure. Catalyst (1-2 cc) was crushed to 20-40 mesh size. On-line chromatographic analyses were carried out. Experimental conditions are outlined in Table 1. Kinetic constants were evaluated by applying eq.(1). 

Non-uniform physical properties of the packing inside the bed - plugging, channelling etc. wilt create hot/cold spots and uneven distribution of thrust. 

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