Heat exchangers crossover

The crossover temperature must satisfy an overall heat balance around the furnace system, including transfer line heat exchangers and feed preheaters, and must not exceed certain limits to avoid incipient cracking. 

Energy-recovery wheels should normally not be used in multiproduct facilities. When such wheels are used they should not become a source of possible contamination (see Fig. 27). Note. Alternatives to the energy-recovery wheels, such as crossover plate heat exchangers and water-coil heat exchangers, may be used in multiproduct facilities. 

Figure 6.15 Typical temperature crossover in a countercurrent reactor/heat exchanger. [After E. A. Grens and R. A. McKean, Chem. Eng. Sci., 18, 291, with permission of Pergamon Press, Inc., London, England, 1963.]...

As discussed in detail in Section 13.1, the minimum approach temperature in a countercurrent-flow heat exchanger may occur at an intermediate location rather than at one of the two ends when one of the two streams is both cooled and condensed. If the minimum temperature approach is assumed to occur at one of the two ends of the heat exchanger, a smaller approach ora temperature crossover that violates the second law of thermodynamics may occur at an intemie-diate location. To avoid this situation, the following heuristic should be applied ... 

Given the design decision to use < > = 0.25, based on the steady-state C R analysis, verification is performed by dynamic simulations with HYSYS.Plant. The hot stream of n-octane at 2,350 Ibmol/hr is cooled from 500 to 300°F using n-decane as the coolant, with F2 = 3,070 Ibmol/hr and F3 = 1,200 Ibmol/hr. Note that these species and flow rates are chosen to match the heat-capacity flow rates defined by McAvoy (1983), with F, slightly increased to avoid temperature crossovers in the heat exchangers due to temperature variations in the heat capacities. Additional details of the HYSYS.Plant simulation are... 

Now that all of the difficult parts have been set up and run successfully, it is time to finish adding the recycle streams. Start with the stripper bottoms product leaving the pump. Connect this to the heat exchanger that is currently heating the liquid product from the third flash drum, as shown in Figure 16b. Notice that, upon doing so, a recycle loop is created and an additional tear stream is selected (in this case, the pump outlet). Specify the hot-side pressure drop of 0.2 bar in the heat exchanger, run the simulation, and verify that there is no temperature crossover (that is, the hot outlet stream should still be hotter than the cold inlet). If there is temperature crossover. 

In Figure 2, the heatlng/cooling curves corre onding to each flow pattern are shown. If the cold stream outlet temperature is higher an the hot stre outlet temperature, then this heat exchanger has a temperature crossover. 

For a shell and tube heat exchanger, the rule of thumb is to keep F at about O.S or more for each shell. This design will result the cold stream outlet temperature equal or less than the hot stream outlet tempeiature. The purpose is to minimize required heat transfer area and inefficient heat transfer. If there is temperature crossover, two or more shells will be required for the service. 

Therefore, most of the nonoxidative generation methods that have evolved can be viewed as a crossover reaction of sorts whereby one o-QM product is exchanged for another by application of heat. The stereochemistry accruing in the products of these procedures is expectedly subject to thermodynamic control. For example, while exploring a synthetic approach for nomofungin (Fig. 4.2), Funk recently showed that... 

Abstract This review reports on the study of the interplay between magnetic coupling and spin transition in 2,2 -bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2(bpym), [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) (bpym = 2,2 -bipyrimidine, bt = 2,2 -bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2(bpym) at 0.63 GPa. LIESST experiments carried out on [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species... 

The exchange in the alkoxyamine-based polymer occurs in a radical process that is tolerant of many functional groups. The exchange process is therefore applicable to polymers with various functional groups. TEMPO-based polyester 43 and polyurethane 44 were synthesized for studies of the scrambling of disparate polymers imder thermodynamic control (Fig. 8.11) [37], Two kinds of TEMPO-based polymers were mixed and heated in a closed system. After 24 hours when the crossover reaction achieved equilibrium, GPC and NMR analyses revealed that they were totally scrambled through bond recombination on the TEMPO units. 

Aminoenone (87), when heated with phosphoryl chloride, cyclizes to 2-(trif uoro-methyl)quinoline, rather than the expected 4-isomer.126 A series of crossover experiments using different perfluoroalkyl and aniline moieties suggest an amine exchange process. A 1,3-diaminoallyl cation (88), i.e. a vinylogous formidinimn salt, is proposed to act as turntable in the process. 

Figure 6 shows the flow of material forward to the devolatilizer, and recycle to the side arm exchanger as required to maintain a heat balance. For the given conditions, the recycle rate of monomer-polymer goes through a maximum, and is quite low at both very low or very high temperatures. There is also a crossover at T2 = 115° C. where G and P + R are equal. 

Parkin and Bercaw reported that Cp 2W(Me)(H) eliminates methane to form Cp (ri5,ri1-C5Me4CH2)WH.26 For the mixed isotopomer, Cp 2W(CH3)(D), H/D scrambling to give Cp 2W(CH2D)(H) is competitive with the methane elimination process (Scheme 11.7). Although the authors point out that the H/D exchange process could occur by pathways other than formation of a methane-coordinated intermediate, the observation of an inverse kinetic isotope effect (KIE) for the methane reductive elimination (see bottom of Scheme 11.7) provides additional support for the reversible formation of coordinated alkane (see below for a more detailed discussion of KIEs for reductive elimination of C—H bonds). Furthermore, at relatively low concentrations, heating a mixture of Cp 2W(CH3)(H) and Cp 2W(CD3)(D) produces only CH4 and CD4 with no observation of H/D crossover, which is consistent with intramolecular C—H(D) processes. Similar results have been obtained for... 

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