Tandem reactors

Fig. 7.5. Tandem reactor concept for single-stage, high-conversion Claus process with integrated elutive regeneration.

Looking again at Figure 2.7, it will be remembered that resins such as LLDPE made with heterogeneous Ziegler-Natta catalysts and tandem reactors have a rather complex relation between MWD and average comonomer composition. Because this relation has such an important impact on the mechanical properties of polyolefins, an FTIR detector is often added to the GPC (GPC/RI-FTIR) to measure comonomer fraction as a function of molecular weight. 

Examination of the GPC data in Figure 4.28 shows the bimodal MWD of the polyethylene produced with the bimetallic (Ti/Zr) catalyst in a single gas-phase reactor and illustrates that the relatively LMW polymer component is produced by the Zr catalyst component, while the relatively HMW polymer component is provided by the Ti-based catalyst component. This GPC data is very. similar to a similar product produced with tandem reactors. As discussed previously, the Conoco scientists developed this polymer composition to improve the mechanical properties of the film fabricated from this type of polyethylene. The relatively high MW polymer... 

The LLDPE with a broad MWD can be produced in a single reactor with a Cr-based catalyst or in tandem reactors such as two gas-phase reactors... 

Several polymerization processes use only one reactor, but two or more reactors can also be operated in series (tandem reactor technology) to produce polyolefins with more complex microstructures [5]. Each reactor in the series is maintained under different operating conditions to produce products that are sometimes called reactor blends . Although, in principle, the post-reactor blending of different resins could lead to the same product, in reactor blends the chains are mixed on the molecular scale, permitting better contact between the polymer chains made in different reactors at a lower energy cost. 

For heterogeneous catalysts, tandem reactor technology also relies on the fact that each polymer particle is in fact a microreactor operated in semibatch mode, into which monomers and chain-transfer agents are fed continually, while the polymer formed never leaves the microreactor. In this way, polymer populations with different average properties are produced in each reactor and accumulate in the polymer particle microreactor, as illustrated in Figure 8.37. In theory, an optimal balance does exist between the fractions of these different populations to meet certain performance criteria. This creates a truly fascinating reactor and product design problem because the fractions of the different polymer populations per particle will be a function of the residence time distribution in the individual reactors in the reactor train. 

Since the same will happen in Reactor 2, in the end the ratio of polypropylene to ethylene-propylene copolymer per particle exiting Reactor 2 will also vary widely, which may be undesirable in some applications. Some of the reactor configurations shown in Figure 8.35 can reduce this phenomenon, particularly the configuration adopted for the gas-phase horizontal reactor, because the residence time distribution of this reactor is the equivalent to about three to four CSTRs in series. (Remember that the residence time of an infinite series of ideal CSTRs is that of a plug-flow reactor.) A more recent solution for this problem, in fact a completely new alternative to tandem reactor technology, is the multizone reactor that will be described in more detail below (see Section 8.6.4). 

In our previous indole oxidation experiments, H202 has been added continuously with a flow rate of 10 pmol min 1 to a buffered indole solution in a batch reactor. In this case a constant maximum conversion at pH values between 3.0 and 8.0 was observed, whereas the indole conversion of the tandem system is limited by the H202 formation rate. At pH... 

Chemat et al. have reported several microwave reactors, including systems that can be used in tandem with other techniques such as sonication [68], and ultraviolet radiation [69]. With the microwave-ultrasound reactor, the esterification of acetic acid with n-propanol was studied along with the pyrolysis of urea. Improved results were claimed compared with those from conventional and microwave heating [68]. The efficacy of the microwave-UV reactor was demonstrated through the rearrangement of 2-benzoyloxyacetophenone to l-(2-hydroxyphenyl)-3-phenylpropan-l,3-dione [69]. 

Another example of the retention of volatile DA reagents is that of cyclopentadiene in a tandem retro-DA/DA prime reaction [15, 16, 38], This reaction type is the thermal decomposition of a DA adduct (A) and the generation of a diene (generally the initial diene) which is trapped in situ by a dienophile leading to a new adduct (B) [39]. Cyclopentadiene (22) (b.p. 42 °C) is generated by thermolysis of its dimer at approximately 160 °C [40]. An equimolar mixture of commercial crude dicyclopenta-diene (21) and dimethyl maleate was irradiated in accordance with the GS/MW process, in an open reactor, under 60 W incident power, for 4 min (8 x 30 s). The expected adduct 23 was isolated in 40% yield (Scheme 7.1). The isomeric composition of 23 (endo-endoIexo-exo = 65/35) was identical with that obtained under classical conditions from 22 and methyl maleate [41]. The overall yield of this tandem reaction can be increased from pure dimer 21 (61%) and the same tandem reaction has also been reported using ethyl maleate as dienophile [31]. 

The use of two or more different catalysts in the same reactor, sometimes known as in situ reactor blending or tandem catalysis, has been widely employed industrially as means of controlling the properties of a polyolefin (e.g. molecular weight and the molecular weight distribution). Recent years have seen a variety of reports emerge on the use of bis(imino)pyridine iron/cobalt systems as one component of the process [169, 170, 171, 172, 173, 174, 175, 176, 177,178, 179],... 

We also thank Dr. J. A. Kuehner and his operations staff at the McMaster Tandem for their support and assistance for this project. We are grateful to the staff at the McMaster Reactor and the Toronto Slowpoke reactor for irradiating our Be test samples. 

Fig. 9. Reversed-phase separations of cytochrome c digests obtained with trypsin-modified beads (left) and trypsin-modified monolithic reactor (right) in a tandem with a chromatographic column (Reprinted with permission from [90]. Copyright 1996 Wiley-VCH). Conditions digestion (left curve) trypsin-modified beads reactor, 50 mm x 8 mm i.d., 0.2 mg of cytochrome c, digestion buffer, flow rate 0.2 ml/min, 25 °C, residence time, 15 min (right curve) trypsin immobilized onto molded monolith other conditions the same as with trypsin-modified beads. Reversed-phase chromatography column, Nova-Pak C18,150 mm x 3.9 mm i.d., mobile phase gradient 0-70% acetonitrile in 0.1% aqueous trifluoroacetic acid in 15 min, flow rate, 1 ml/min, injection volume 20 pi, UV detection at 254 nm...

In terms of coupling flame generation and detection methods, several combinations are common. Generally, shock tubes are coupled with IR and UV absorption and gas chromatography (GC) detectors, while flow reactors are used in tandem with GC, electron spin resonance, and resonance fluorescence detection. 

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