Catalyst residence time

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

Space velocity is related to the CjO ratio through the catalyst residence time as ... 

Equation 6 relates the catalytic coke yield (as a fraction of the feed) to the delta coke, to the conversion, and to the catalyst residence time. 

Continuous polymerization in a staged series of reactors is a variation of this process (82). In one example, a mixture of chloroprene, 2,3-dichloro-l,3-butadiene, dodecyl mercaptan, and phenothiazine (15 ppm) is fed to the first of a cascade of 7 reactors together with a water solution containing disproportionated potassium abietate, potassium hydroxide, and formamidine sulfinic acid catalyst. Residence time in each reactor is 25 min at 45°C for a total conversion of 66%. Potassium ion is used in place of sodium to minimize coagulum formation. In other examples, it was judged best to feed catalyst to each reactor in the cascade (83). 

Catalytic coke is a byproduct of the cracking of FCC feed to lighter products. Its yield is a function of conversion, catalyst type, and hydrocarbon/catalyst residence time in the reactor. 

Catalyst flux Stripping steam rate Stripping steam superficial velocity Catalyst residence time Steam quality 500-700 Ib/min/ft" (40 to 55 kg/sec/m ) 2-5 lb/1,000 lb of circulating catalyst 0.5-0.75 ft/sec (.15-.25 m/sec) 1-2 minutes Superheated 100°F (55°C)... 

Catalyst residence time in the stripper is determined by catalyst circulation rate and the amount of catalyst in the stripper. This amount usually corresponds to the quantity of the catalyst from the centerline of a normal bed level to the centerline of the lower steam distributor. A higher catalyst residence time, though it increases hydrothermal deactivation of the catalyst, will improve stripping efficiency. 

Slip factor is defined as the ratio of catalyst residence time in the riser to the hydrocarbon vapor residence time. Some of the factors affecting the slip factor are circulation rate, riser diameter/geometry, and riser velocity. 

Slip Factor is the ratio of catalyst residence time to hydrocarbon vapors residence time in the riser. 

The catalyst residence time, T depends both on the ZSM-5 and base catalyst makeup rates... 

Habib (4) has emphasized the importance of the sulfur-release step in the mechanism for SOx reduction. If a catalyst captures SOx but cannot release it, it soon becomes saturated and ineffective. For example, if CaO captured SOx until it was transformed to CaSO, it would capture 57% sulfur, based on the weight of the CaO. For the FCCU under consideration, 50 tons of CaO added to the 500-ton unit (10% additive) would capture 28.6 tons of sulfur. At a sulfur capture rate of 10 tons a day, the CaO would be effective for only 2.9 days. Since the average catalyst residence time in the unit is 100 days, use of such a material would not be practical. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

In a typical fluid catalytic cracker, catalyst particles are continuously circulated from one portion of the operation to another. Figure 9 shows a schematic flow diagram of a typical unit W. Hot gas oil feed (500 -700°F) is mixed with 1250 F catalyst at the base of the riser in which the oil and catalyst residence times (from a few seconds to 1 min.) and the ratio of catalyst to the amount of oil is controlled to obtain the desired level of conversion for the product slate demand. The products are then removed from the separator while the catalyst drops back into the stripper. In the stripper adsorbed liquid hydrocarbons are steam stripped from the catalyst particles before the catalyst particles are transferred to the regenerator. 

The entire catalyst inventory is continually circulated through the three parts of the unit. The catalyst residence time in the riser reactor section is typically 1 to 3 seconds (with current trends to even shorter residence times), and the entire reactor-stripper-regenerator cycle is less than 10 minutes. To achieve cycle times... 

Similar dependence of catalyst deactivation on coke or catalyst residence time is suggested by Corella et al. (5-7). The authors give details on possible mechanisms of catalyst deactivation by coke, and also suggest, based on their data, that the deactivation order n may not be a constant. For our analysis, however, we will assume that n is constant and a function of catalyst type. Further theoretical treatment of catalyst decay is given by Wojdechowski (8.9). 

A fixed bed reactor described by ASTM Method No. D3907 was employed for catalytic testing. A sour, imported heavy gas oil with properties described in Table II was used as the feedstock. Experiments were carried out at a reactor temperature of 800°K and catalyst residence time (9) of 30 seconds. Liquid and gaseous products were analyzed with gas chromatographs. Carbonaceous deposit on the catalyst was analyzed by Carbon Determinator WR-12 (Leco Corp., St. Joseph, MI). The Weight Hourly Space Velocity (WHSV) was varied at constant catalyst contact time to generate selectivity data of various products as a function of conversion. For certain experiments, conversion was also varied by varying the catalyst pretreatment conditions. 

In order to achieve this, equipment to strip the spent catalyst must be carefully designed. Stripping nitrogen and catalyst residence time must then be optimized in order to achieve good stripping. If more nitrogen is used than necessary, the product gas is diluted, making accurate gas analysis for yield determination more difficult. 

For many kinetic studies the selectivities of different catalysts are compared at the same level of conversion. However, the main drawback is that sometimes very different reaction conditions (mass of catalyst, residence time, reactor temperature, feed gas composition) are required to achieve this. Another option is the comparison of conversion and selectivity under a given set of reaction conditions. The main drawback of this approach appears if catalysts show full or very different conversions, then the comparison of the selectivities is not appropriate. Despite this, it is still possible to conclude if a catalyst heavily favors one of several reaction pathways, for example, if partial or total oxidation is prevailing. 

More recently, Yavorsky, et al., (5) have reported on the development of a liquid phase (solvent) hydrogenation of coal In a highly turbulent tubular reactor In the presence of a packed solid catalyst. Residence times In the order of several minutes were reported and an oil product Is formed. This system Is a considerable Improvement over the Berglus Process since reduced pressure. 

Features - particles of growing polymer form as suspension in hydrocarbon diluent - catalyst residence time 1 hour for Phillips loop slurry process - morphology and psd of catalyst are important - wide range of comonomers may be used... 

Conversion Temperature Pressure Space Velocity Catalyst Residence Time (or Process Period)... 

Catalyst residence time or process period). Catalyst residence time in a moving-bed or fluid unit, or process period in a fixed-bed unit, is the length of time a catalyst particle is used to crack oil in each cycle before it is regenerated. Catalyst residence time is equal to the ratio of the amount of catalyst in the reactor to the catalyst-circulation rate. In moving-bed units, it is essentially the same for all catalyst particles, whereas in fluid units there is a variation, as already discussed. 

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