Catalyst dual beds

Some OCs were of the monolithic honeycomb type, but all General Motors cars used peUetted OCs. For a period in the late 1970s and throughout the 1980s, both TWC and OC were used in a dual-bed catalyst. Oxygen needed for OC performance was provided by an engine driven air pump or a reed... 

The synergism of a dual-catalyst system comprising of Pt/ZSM-12 and H-Beta aiming to improve the benzene product purity during transalkylation of aromatics has been studied. Catalyst compositions of the dual-catalyst system were optimized at various reaction temperatures in terms of benzene product purity and premium product yields. Accordingly, a notable improvement in benzene purity at 683 K that meets the industrial specification was achieved using the cascade dual-bed catalyst. 

Previously, we have developed several techniques for platinum supported zeolite catalysts to improve the benzene product purity, including on-line sulfiding [3], precoking [6], and dual-bed catalyst system [7]. We report herein an in-depth investigation on the synergism of proton zeolite and platinum supported ZSM-12 catalyst (Pt/Z12) in a cascade dual-catalyst system. 

Whereas over the dual-bed catalyst system, namely Pt/Z12(80) HB(20), a significant improvement in benzene purity up to 94.60% was observed. This is ascribed due to selective cracking of naphthenes over acidic zeolite H-Beta at the bottom bed. 

The effect of the H-Beta ratio (y in wt%) in the dual-bed Pt/Z12(x) HB(y) catalyst system on the benzene purity at a reaction temperature (Tr) of 623 K is shown in Fig. 1. It is evident that the benzene purity gradually increased with increasing H-Beta ratio (Fig. la), eventually reaching a plateau value which meets the industrial specification of 99.85% at y 40 wt%. The effects of catalyst bed ratio on product yields are shown in Fig. lb. Comparing to the single-bed catalyst Pt/Z 12 (i.e., y = 0), the overall premium product yields of benzene and xylene (A68 yield) over the dual-bed catalyst Pt/Z12(x) HB(y) system reached an maximum at y 10 wt%. That the A68 yield dwindled and tetramethylbenzene (TEMB) increased with further increase in the H-Beta ratio may be attributed to the larger pore opening possessed by the bottom (H-beta) catalyst, which may provoke disproportionation of TMB to form tetramethylbenzene (TEMB) [8],... 

Table 1. Product yields of transalkylation reaction of toluene and 1,2,4-trimethylbenzene (at 623 K) over Pt-supported single- and dual-bed catalysts.

Figure 1. Effects of H-Beta ratio (y) in the dual-bed catalyst Pt/Z12(x) HB(y) on (a) benzene purity and (b) product yields during transalkylation reaction (see text) at 623 K.

The effects of Tr on benzene product purity and product yields over various dual-bed catalyst systems with different bottom bed catalyst ratios are shown in Fig. 2. As shown in Fig. 2a, over the single-bed Pt/Z12 catalyst alone (i.e., y = 0), a drastic increase in benzene purity with increasing Tr was observed, for example, the benzene purity value increased from 10.87% to 98.36% as Tr increased from 553 K to 683 K. However, upon... 

A dual-bed catalyst system has been developed to tackle the key problems in benzene product impurity during heavy aromatics transalkylation processing over metal-supported zeolite catalysts. It was found that by introducing zeolite H-Beta as a complementary component to the conventional single-bed Pt/ZSM-12 catalyst, the cascaded dual-bed catalyst shows synergistic effect not only in catalytic stability but also in adjustments of benzene product purity and product yields and hence should represent a versatile catalyst system for heavy aromatics transalkylation. 

Exhaust emission standards since the 1981 model year vehicles have required the use of three-way catalysts, either alone or in combination with an oxidation catalyst. Three-way catalysts are designed to operate in a very narrow range about the stoichiometric air/fuel ratio. In this range the HC and CO are subject to oxidation and the NO, compounds undergo reduction. The downstream oxidation catalyst in a dual bed system is generally used as a "clean-up catalyst lo further control HC and CO emissions. The most common catalytic combination in three-way uses is platinum/rhodium. Current production applications use these elements in a relatively rich proportion of 5 1 lo 10 1. whereas the respective mine ratio is about 19 1. 

Fig. 7. Axial lead profiles on dual-bed catalysts. [From Gandhi et al. (JO).] (Reprinted with permission of the Society of Automotive Engineers.)...

Dispersions at micron scale are usually made by merging gas and liquid streams in a mixing element and subsequent decay of the gas stream to a dispersion [251-262]. Mixing elements often have simple shapes such as a mixing tee (dual-feed gas-liquid) or triple-feed (liquid-gas-liquid) arrangements. The dispersion is passed either in a microchannel (or many of these) or in a larger environment such as a chamber, which, for example, provides volume to fill in porous materials such as catalyst particle beds, foams or artificial structures (microcolumn array). The mechanisms for bubble formation have not been investigated for all of the devices... 

Fixed-bed reactor (dual-bed catalyst in Mobil s process)... 

The third concept is the dual-bed emission control catalyst. In this, the catalytic converter is made of two different types of catalyst. The first is either a multifunctional catalyst or at least one capable of promoting NO.v reduction reactions. The engine is calibrated so as to guarantee a net reducing exhaust gas composition. Under these conditions, the first catalyst will lead to an elimination of the nitrogen oxides. The second catalyst is an oxidation catalyst. Extra air is injected in front of the second catalyst to assist the removal of carbon monoxide and hydrocarbons. The secondary air can be added either by mechanically or by electrically driven air pumps. 

The current US emission limits for light duty vehicles are achieved for gasoline fueled cars by engines equipped with a controlled threeway catalyst system including an oxygen sensor and fuel injection. For the older engine types dual bed reduction/oxidation catalyst systems are often applied (Ref. 3). 

Fig. 5. Efficiency scan for a dual-bed catalyst and a three-way catalyst.

Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S. three-way converters and dualbed converters. Both converters contain three-way catalysts, but with the dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. As for the earlier oxidation catalysts two forms of catalyst support are used pellets (thermally stable transitional alumina) and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. 

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