Lean chemical processes

From the reaction-kinetic modeling point of view, the NSRC, sometimes called lean NOx trap (LNT) or NOx adsorber, is the most complex of the currently used automobile exhaust converters. A variety of different physical and chemical processes and the number of gas and surface components participating in typical periodic lean/rich operation form a large and closely linked system. 

Fig. 3 looks very different from a plot of conversion versus A/F obtained during steady-state tests or cycled-A/F tests. First, note that NO conversion is high at lean A/F s, whereas steady-state and cycled-A/F tests would show low conversion. Transient chemical processes, such as reaction of NO with reduced "oxygen storage" components in the catalyst, serve to maintain high NO conversions during transient lean A/F excursions. 

We now use the mass balance discrepancy to subtract the contribution of any "accumulation-reaction" processes present from the measured responses. For example, the corrected response curve for CO, shown in Fig. 12, was obtained at each instant in time by adding to the measured outlet CO concentration the amount of CO that was converted to CO2 by reaction with oxygen atoms stored in the catalyst, as determined from the oxygen balance. The fact that the corrected CO response and the corrected CO2 response, shown in Fig. 13, do not match the instantaneous response demonstrates the action of an "activity change" type of transient chemical process. The process resulted in lower-than-expected CO conversion, since the activity change process included in the simulation was partial deactivation of the catalyst in lean exhaust (e.g., by oxidation of the precious metal). 

Two chemical processes have been selected by automotive makers lean NO e trap (LNT) [11] or NO c storage reduction (NSR) and selective catalytic reduction by ammonia (SCR-NH3) [12]. For LNT process, the cost is very expensive, because a large amount of noble metal is required. It is why the second possibility (SCR-NH3) has paid attention and has been developed first on trucks and bus. The result is positive in terms of efficiency and decrease of pollutants, but the required location remains a major problem especially when the technology must be applied to a particular vehicle. In order to improve the process, a new concept has been proposed and involves bringing together two technologies into one. In this chapter, we will describe the integration of the SCR-NH3 catalyst in the structure of the particles filters for application in diesel posttreatment. 

These techniques can be used throughout an organization or in smaller pieces such as a particular chemical process. Eor example Lean Six Sigma has been used to improve a selenium analysis [6] and showed the importance of improving the digestion of the sample. 

The chemical processes responsible for changes in the production rates of H, O, and OH as ( ) changes were identified. Three flames were chosen for the analysis a lean flame with =0.6, a stoichiometric flame with the maximum rate of radical production, and the richest flame with ( )=5. The rate of H atom production in separate reactions was calculated, and the key reactions for each of the chosen flames were determined. In addition, to evaluate the role of each reaction qualitatively the authors (Shvartsberg et al., 2010) calculated the contribution of each reaction to the total rate of H consumption, considering only the reactions involving ICS. It was found out that, regardless of the flame equivalence ratio, the key reactions responsible for H atom removal are as follows ... 

Process conditions that favor chemical crimp formation are similar to those used for improved tenacity staple (2inc/modifier route). However, spin bath temperature should be as high as possible (ca 60°C) and the spin-bath acid as low as possible (ca 7%). Attempts have been made to overcome some of the leanness of high strength rayons by increasing the crimp levels. ITT Rayonier developed the Prim a crimped HWM fiber (36) and made the process available to their customers. Avtex developed Avdl 111. Neither remain in production. 

As stated earlier, turboexpanders are normally used in cryogenic processes to produce isentropic expansion to cool down the process gas. Two common applications are natural gas processing plants and chemical plants. In natural gas processing plants, turboexpanders are installed to liquify heavier hydrocarbon components and produce lean natural gas with specified dew point limits to meet required standards. 

A process lean stream and an external MSA are considered for removing H2S. The process lean stream, S1, is a caustic soda solution which can be used as a solvent for the reactive separation of H2S. An added bonus for using the process MSA is the conversion of a portion of the absorbed H2S into Na2S, which is needed for white-liquor makeup. In other words, H2S pollutant is converted into a valuable chemical which is needed in the process. The external MSA, S2, is a polym ic adsorbent. The data for the candidate MSAs are given in Table 8.2. The equilibrium... 

With the current emphasis on streamlined management and lean staffing in the processing industries, chemical engineers will have fewer peers and fewer superiors their ability to make decisions will be tested early and often. 

A number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

Clearly, when modelling an LNT it is important to include the most important processes occurring in this relatively complex catalyst system. Kinetic and experimental studies of lean NOx trap catalysts, including those describing chemical principles, have been published previously (Brogan et al., 1995 Dou and Bailey, 1998 Fekete et al, 1997 Miyoshi et al., 1995 Takami et al., 1995). These processes can be summarised as follows ... 

Recovery of the solvent, occasionally by chemical means but more often by distillation, is almost always required and is considered an integral part of the absorption system process design. A more complete solvent-stripping operation normally will result in a less costly absorber because of a lower concentration of residual solute in the regenerated (lean) solvent, but this may increase the overall cost of the entire absorption system. A more detailed discussion of these and other economical considerations is presented later in this section. 

The example given in Figure 3 illustrates this combination of two processes. In an absorber, one or several gas components are absorbed by a lean solvent, either physically or chemically. A rich solvent, after preheating in heat exchangers 111 and H3, is transported to the top of a desorption unit, which usually operates... 

Figure 6 shows a simplified C02 capture process with two major process units absorber and stripper Zhang et al., 2009. A lean amine solvent (low C02 loading) is fed into the top of the absorber and is in counter-current contact with the gas containing C02. The C02 is chemically absorbed by the amine solvent and the treated gas exits the top of the absorber. The rich (high C02 loading) amine leaves the bottom of the absorber and is preheated by a cross heat exchanger before... 

Other industries indicate one possible path for chemical companies going forward. If correctly implemented, the principles of lean manufacturing, developed first in the automotive and assembly industries and then used in process industries (such as chemicals, metals, pulp paper, and power) by companies like Alcoa, offer a promising approach. Major impact has been achieved by this method in large scale transformations over the last few years. In this chapter, we will focus on how to implement such a transformation rather than on the elements of lean manufacturing, since there are many publications on that subject. 

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