Process/catalyst development deactivation

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

It has been ten years since Amoco announced the UltraCat process O) for SOx control in FCC units. In those ten years, as well as in the years previous to the announcement, much work was done to develop catalysts that would control SOx emissions. The evidence is the 80 or more U.S. patents that have issued in that time to Amoco and others. One of the first patents issued was to Amoco in 1974 ( ) for the addition of magnesia and other group IIA oxides to cracking catalyst. This paper reviews the SOx catalyst developments and emphasizes the work done at Amoco to identify the active materials, explain the deactivation mechanism and, finally, to make a side-by-side comparison of various catalytic systems that are being pursued commercially today. 

At slightly higher temperatures (100°-200°C) catalysts consisting of chlorided alumina in combination with a noble metal, such as platinum, are used. As a cocatalyst HC1 or an organic chloride is supplied with the feedstock. The high reactivity of these catalyst systems requires careful feed pretreatment for removal of deactivating materials. Several plants (1, 2) using this type of catalyst, and one version of this process especially developed to convert C5/C6 feed, have recently been built. 

Current Processes. The development of superactive third-generation supported catalysts enabled the introduction of simplified processes, without sections for catalyst deactivation or removal of atactic polymer. By eliminating the waste streams associated with the neutralization of catalyst residues and purification of the recycled diluent and alcohol, these processes minimize any potential environmental impact. Investment costs arc reduced by approximately one-third over slurry process plants. Energy consumption is minimized by elimination of the distillation of recycled diluent and alcohol. The total plant cost for the production of polymer is less than 130% of the monomer price, when a modem process is used, compared to 175% for a slurry process. 

The feasibility of hydrotreating whole shale oil is demonstrated by means of several long pilot plant tests using proprietary commercial catalysts developed by Chevron. One such test was on stream for over 3500 hr. The rate of catalyst deactivation was very low at processing conditions of 0.6 LHSV and 2000 psia hydrogen pressure. The run was shut down when the feed supply was exhausted although the catalyst was still active. 

Catalyst deactivation is of major concern in catalyst development and design of packed bed reactors. Decay of catalytic activity with time can be caused by several mechanisms such as fouling, sintering and poisoning. Although much fundamental experimental work has been done on deactivation,very little attention has been focused on modelling and systematic analysis of nonadiabatic fixed bed reactors where a deactivation process occurs. 

Catalysts currently employed in process development units for coal liquefaction are hydroprocessing catalysts developed for petroleum refining (5l6). They are composed of combinations of Mo or W with Co, Ni or other promoters dispersed on alumina or silica-alumina supports. When used in liquefaction, these catalysts deactivate rapidly f6-9i causing decreases in product yield and quality and problems with process operability. Thus the... 

These examples show the importance of performing bench-scale tests as part of the process and catalyst development, but it also illustrates the importance of applying the pseudo-adiabatic reactor principle to observe the deactivation phenomena and to ensure reliable, adiabatic bench-scale tests when providing data for industrial design. 

Obviously, both communities, those involved in catalyst development and those active in process development, have to meet and cooperate at an early stage of the project. This is true for catalytic processes in general Why not for catalyst deactivation ... 

Catalyst developers experience woe in their quest to find a practical catalyst owing to the very general taidency of catalysts to deactivate during use onstream. To live with this phenomenon, a developer must know the time scale of the deactivation process in order to know what kind of reactor to use. If development schedules permit, the developer may even have the luxury of making a detailed study of the deactivation process in the laboratory or in a pilot plant to find conditions to increase the life of the catalyst on stream. 

It is necessary to determine rj(e) under reaction conditions, and a life test should be included in any catalyst development effort. The data from this test will allow r] to be fitted as a function of time on stream, 6. Equations 10.35 and 10.36 can obviously be used to model deactivation processes other than site sintering, and ko can be regarded as an empirical constant with units of reciprocal time. 

Platinum (metal)- and acid (oxide)-catalyzed processes were developed to convert petroleum to high-octane fuels. Hydrodesulfurization catalysis removed sulfur from the crude to prevent catalyst deactivation. The discovery of microporous crystalline alumina silicates (zeolites) provided more selective and active catalysts for many reactions, including cracking, hydrocracking, alkylation, isomerization, and oligomerization. Catalysts that polymerize ethylene, propylene, and other molecules were discovered. A new generation of bimetallic catalysts that were dispersed on high-surface-area (100-400 m /g) oxides was synthesized. 

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