Catalyst replacement

Earlier processes (e.g. hydroforming) used MoO,i —AI2O3 catalysis but platinum-based catalysts are now extensively used, enabling longer on-stream times before catalyst replacement. 

In some liquid-phase processes, catalyst components are slowly leached from the catalyst bed and eventually the catalyst must be replaced. The feasibility of this type of process involves economics, ie, the costs of catalyst maintenance and keeping a unit out of service for catalyst replacement, and product quality and safety, ie, the effects of having catalyst components in the product and their ease of removal. 

The advantages of the ease of catalyst replacement or regeneration are offset by the high cost of the reactor and catalyst regeneration equipment. 

The ageing and decay characteristics of catalysts are of immense importance in defining the economics of processes. The simplest criterion that can be applied is that of total productivity during the life of the catalyst and also loss of productivity during the shut down required for catalyst replacement. Figure 2 illustrates notional performances for two catalysts A and B in hypothetical processes in which productivity is simply a measure of quantity of product produced. Catalyst A has a lower initial productivity but is more stable in use and dies off at a much lower rate than catalyst B, which has a high initial productivity which falls relatively... 

Temperature control is also reasonably simple. An important advantage in the case of a rapidly deactivating catalyst is the possibility of continuous catalyst replacement. There are, however, a number of problems associated with handling fine catalyst particles. They have to be separated from the products, which is usually troublesome, plugging of lines and valves can occur, and pyrophoric catalysts may also require special procedures. This is less important if the product can be removed from the reaction mixture (e.g. products are volatile and are stripped during the operation). In case of excessive gas flow rates, however, small catalyst particles can be entrapped and deposited in downstream equipment. The catalyst load is limited to what can be kept in. suspension with a reasonable power input. 

When a catalyst has sufficiently deactivated to justify taking some action is determined by economics. Both Gas and Liquid Recycle hydroformylation plants may be operated to give essentially constant production rates as the catalyst deactivates. Hydroformylation is approximately first order in both rhodium and alkene concentration. As the rhodium catalyst deactivates, the alkene concentration may be allowed to increase to compensate for the declining catalyst activity. Action is taken when the alkene efficiency declines to the point where it approximates or exceeds the cost of catalyst replacement or reactivation. 

Another PHOX analogue has the aryl ring of the PHOX catalyst replaced by a thiophene unit 16 (Fig. 29.4) [15]. The synthesis is similar to that of the PHOX catalysts, starting with oriho-nu lallaliori of the thiophene. The catalysts showed similar selectivity to PHOX, and were used to hydrogenate substrates 1 and 2 with maximum enantioselectivities of 99% and 94%, respectively. 

Using higher-quality catalysts will lead in increased process efficiency while the required frequency of catalyst replacement can be reduced. Similarly, the replacement of ceramic catalyst support with activated alumina supports presents the opportunity for recycling the activated alumina supports with the spent alumina catalyst. 

Sulfur and carbon monoxide can be killers (literally) with hydrogenation catalysts. It will poison them, making them completely ineffective. Some sulfur often shows up in the benzene feed, carbon monoxide in the hydrogen feed. The alternatives to protect the catalyst are either to pretreat the feed and/or the hydrogen or to use a sulfur resistant catalyst metal like tin, titanium, or molybdenum. The economic trade-offs are additional processing facilities and operating costs vs. catalyst expense, activity, and replacement frequency. The downtime consequences of catalyst replacement usually warrarit the more expensive treatment facilities. 

In the initial screening of various Cinchona alkaloids, the addition of diethyl phosphate 41 to IV-Boc imine 40 in toluene revealed the key role of the free hydroxyl group of the catalyst. Replacing the C(9)-OH group with esters or amides only results in poor selectivity. Quinine (Q) was identified as an ideal catalyst. A mechanistic proposal for the role of quinine is presented. Hydrogen-bonding by the free C(9)-hydroxyl group and quinuclidine base activation of the phosphonate into a nucleophilic phosphite species are key to the reactivity of this transformation (Scheme 9). 

Although at first sight, the Citrate Process may not appear to be in any way related to the traditional Claus, it is in fact an H2S/SO2 redox reaction in solution with the activating bauxite, carbon, or metal salt type catalyst replaced by a citrate complex with SO2. The chemistry of the process is clearly interesting and of some importance but for the purposes of this review it is sufficient to draw the analogy indicated above. The Citrate Process is yet another reduction process that may require the ancillary generation of H2S from natural gas and product sulphur if the effluent gas stream is solely SO2 as far as sulphur content is concerned. 

Catalyst replacement costs and yield loss costs can rise rapidly if poisoning occurs. Obviously, a high metals level must be dealt with appropriately. We accelerated our effort to discover a way to deal with vanadium poisoning. 

Catalyst Cost. Catalyst replacement cost represents a large operating expenditure, in addition to the effect that catalyst performance (good or bad) can have on yield and associated profit. Therefore, in addition to all the other objectives, we continually evaluated catalyst composition and method of manufacture as they impacted on catalyst cost. Catalyst manufacturing modifications as they impacted cost were always carefully reviewed and such review was a key part of the catalyst development program. 

The decomposition of the catalyst beads can cause a secondary air pollution emission consisting of the particulate dust generated by abrasion of the surface of the catalyst. Operating cost for catalyst replacement varies directly with catalyst attrition rate. The system can process waste streams with VOC concentrations of up to 25% of the lower explosive limit (LEL). The proprietary catalyst contains up to 10% chromium, including 4% hexavalent chromium. This could lead to the emission of hexavalent chromium in some applications of the technology. 

A liquid/solid separation step is usually required before the catalytic reaction with nondisposable catalysts. This is an expensive step. To remove it, the catalyst must be compatible with the contaminating solids. Ebulating beds can partially satisfy this condition, but the rate of catalyst replacement becomes large if the deactivation is severe. 

The simplest model of a bubbling fluidized bed, with uniform bubbles exchanging matter with a dense phase of catalytic particles which promote a continuum of parallel first order reactions is considered. It is shown that the system behaves like a stirred tank with two feeds the one, direct at the inlet the other, distributed from the bubble train. The basic results can be extended to cases of catalyst replacement for a single reactant and to Astarita s uniform kinetics for the continuous mixture. 

In these processes, metal complexes find a number of uses as light-sensitive latent-image-catalyst formers, catalyst replacements for image silver in low-silver systems, and oxidants for various developers in image amplification baths. 

Expanded-bed reactors operate in such a way that the catalyst remains loosely packed and is less susceptible to plugging and they are therefore more suitable for the heavier feedstocks as well as for feedstocks that may contain considerable amounts of suspended solid material. Because of the nature of the catalyst bed, such suspended material will pass through the bed without causing frequent plugging problems. Furthermore, the expanded state of motion of the catalyst allows frequent withdrawal from, or addition to, the catalyst bed during operation of the reactor without the necessity of shutdown of the unit for catalyst replacement. This property alone makes the ebullated reactor ideally suited for the high-metal feedstocks (i.e., residua and heavy oils) which rapidly poison a catalyst with the ever-present catalyst replacement issues (Figure 5-8). 

Catalyst consumption is a major aspect of the hydrodesulfurization process and costs of the process increase markedly with the high-metal feedstocks. The ease with which the catalyst can be replaced depends, to a large extent, on the bed type, and with the high-metal feedstocks it is inevitable that frequent catalyst replacement will occur. From the data available (Table 5-7) (Nelson, 1976), attempts have been made to produce a correlation (Figure 5-10) (Nelson, 1976)... 

The process centers on a fixed-bed downflow reactor that allows catalyst replacement without causing any interruption in the operation of the unit (Figure 9-28). Feedstock is introduced to the process via a filter (backwash, automatic) after which hydrogen and recycle gas are added to the feedstock stream which is then heated to reactor temperature by means of feed-effluent heat exchangers whereupon the feed stream passes down through the reactor in trickle flow. Sulfur removal is excellent (Table 9-18), and substantial reductions in the vanadium content and asphaltenes content are also noted. In addition, a marked increase occurs in the API gravity, and the viscosity is reduced considerably. 

Note In the above analysis, the annual costs for insurance, local taxes, and maintenance were not changed with varying investment costs. Royalties and catalyst replacement costs were not changed with operating volumes. 

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