Catalyst continued deactivation

The molecular weight of the polymers is controlled by temperature (for the homopolymer), or by the addition of organic acid anhydrides and acid hahdes (37). Although most of the product is made in the first reactor, the background monomer continues to react in a second reactor which is placed in series with the first. When the reaction is complete, a hindered phenoHc or metal antioxidant is added to improve shelf life and processibiUty. The catalyst is deactivated during steam coagulation, which also removes solvent and unreacted monomer. The cmmbs of water-swoUen product are dried and pressed into bale form. This is the only form in which the mbber is commercially available. The mbber may be converted into a latex form, but this has not found commercial appHcation (38). 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

In Test 1 (3 ppm sulfur in the feed gas), the catalyst showed continuous deactivation it did not maintain some intermediate level of... 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

Agrawal et al.33 performed studies of Co/A1203 catalysts using sulfur-free feed synthesis gas and reported a slow continual deactivation of Co/A1203 methanation catalysts at 300°C due to carbon deposition. They postulate that the deactivation could occur by carburization of bulk cobalt and formation of graphite deposits on the Co surface, which they observed by Auger spectroscopy. 

DuPont and Dow use solution polymerization technology to produce LLDPE resins. The process is based on continuous polymerization of ethylene with 1-octene in cyclohexane at about 250°C and 1200 psi. The catalyst is again Ziegler type. Residence time is of the order of several minutes. The catalyst is deactivated by treatment with an alcohol or complexing agent such as acetyl-acetone, and adsorbed on a silaceous adsorbent before stripping the solvent. The Stamicarbon (Dutch State mines) process is similar to the DuPont process, and it uses a short-residence-time solution process for HDPE production. 

Polyterpene resins are related to the oldest reported polymerization, as they were first observed in 1789 by Bishop Watson by treatment of turpentine with sulfuric acid [92]. Commercial polyterpene resins are synthesized by cationic polymerization of /3- and a-pinenes extracted from turpentine, of rf,/-limonene (dipentene) derived from kraft-paper manufacture, and of d-limonene extracted from citrus peels as a by-product of juice industry [1,80,82,93]. The batch or continuous processes are similar for the three monomers. The solution polymerization is generally performed in mixed xylenes or high boiling aromatic solvent, at 30-55° C, with AlCl3-adventi-tious water initiation. The purified feedstream (72-95% purity, depending on monomer) is mixed in the reactor with solvent and powdered A1C13 (2—4 wt% with respect to monomer), and then stirred for 30-60 min. After completion of the reaction, the catalyst is deactivated by hydrolysis, and evolved HC1 is eliminated by alkaline aqueous washes. The organic solution is then dried, and the solvent is separated from the resin by distillation. 

Distributed inflow to a moving catalyst bed of continuously deactivated catalyst accompanied by continuous regeneration... 

The reaction will continue until the monomer is exhausted or until the catalyst is deactivated this can happen under any circumstance when the potassium is destroyed. 

The reaction occurs over a relatively small zone in the catalyst bed. As the reaction moves down the catalyst bed, coke deposits deactivate the front part of the bed. The reaction continues down the bed until a substantial part of the catalyst is deactivated and unconverted methanol "breakthrough" is detected in the reactor effluent stream. Use of sufficient catalyst permits reactor onstream periods, or cycles, sufficiently long to avoid excessive regenerations. To enable this to be done onstream, multiple reactors are provided and operated in parallel on a cyclic mode. The New Zealand plant is designed to operate with four reactors onstream, with a fifth reactor in regeneration. 

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