Dynamic resilience

Known as EPR. this material is of limited use hecausc it cannot be vulcanized in readily available systems. However, (he rubbers arc made from low -cost monomers, have good mechanical and elastic properties, and outstanding resistance to ozone, heal, and chemical attack. They remain flexible to very low temperatures (brittle point about -95 C), They are superior to butyl rubber in dynamic resilience. 

The scope of this chapter is limited to resilience of HENs in the steady state. Obviously, it is important that a HEN be controllable and that it be resilient to dynamic changes in temperature and flow rate (Morari et al., 1985). However, dynamic resilience will not be addressed. Also, many of the resilience concepts reviewed here were developed for general chemical processes (Grossmann and Morari, 1983 Swaney and Grossmann, 1985a Grossmann and Floudas, 1987 Linnhoff and Kotjabasakis, 1986). However, in this chapter they will be applied specifically to HENs. 

Process design modifications usually have a bigger impact on operability (dynamic resilience). Dynamic resilience depends on controller structure, choice of measurements, and manipulated variables. Multivariable frequency-response techniques have been used to determine resilience properties. A primary result is that closed-loop control quality is limited by system invertability (nonmin-imum phase elements). Additionally, it has been shown that steady-state optimal designs are not necessarily optimal in dynamic operation. 

Morari, M. Design of Resilient Processing Plants III, A General Framework for the Assessment of Dynamic Resilience, Chem. Eng. Sci., 38, 1881-1891 (1983). 

This framework formed the basis for quantitative measures of dynamic resilience proposed in [3-5], A key feature of these methods is that the dynamic resilience metrics derived are plant-inherent and independent of specific controller type and tuning (within the class of linear, constant parameter controllers). A limitation is that the performance-limiting factors are considered individually, making it difficult to rank plants that exhibit combinations of these characteristics to varying degrees. 

B. R. Holt, M. Morari, Design of resilient processing plants - VI. The effect of right-halfplane zeros on dynamic resilience, Chem. Eng. Sci. 40 (1) (1985) 59-74. 

Such a dependence on (7- 7g) is not restricted to properties so obviously related to tensile strength. The rate of cut growth of diene rubbers due to ozone attack appears to be similarly dependent on 7g (see Chapter 9) whilst a surprisingly unique relationship between dynamic resilience and (7-7g) appears to hold for a homologous series of poly(vinyl alkyl ether)s (Fig. 4.10) (Lai et aL, 1965). 

Fig. 4.10. Relationship between dynamic resilience and (T-Tg) for poly (vinyl alkyl ether) vulcanizates O, ethyl A, n-butyl , isobutyl O, n-pentyl n-hexyl V, n-octyl , 2-ethylhexyl T = test temperature, T.

Although quite good elastomers may be obtained from several of the polyvinyl alkyl ethers they do not appear to possess any particularly outstanding property that will found them a place in the spectrum of commercial elastomers. The dynamic properties are of some interest and data on the relationship of the dynamic resilience at... 

The thermoplastic polyamide elastomers may be considered as premium grade materials available in a wide range of hardness values with, in some instances, very good heat resistance. Particular properties of interest are the flexibility and impact resistance at low temperatures and the good dynamic properties and related resilience, hysteresis and alternating flexural properties. 

An instrument for measuring the mechanical properties of rubbers in relation to their use as materials for the absorption and isolation of vibration. These properties are resilience, modulus (static and dynamic), kinetic energy, creep and set. The introduction of an improved version has recently been announced. 

Prompt and delayed ionization is familiar for very energy rich molecules. The special feature of high Rydberg states is the initial state that is optically prepared, a state directly coupled to the continuum on the one hand and to a very dense bound manifold on the other. The dynamical theory necessary to describe such states has been reviewed, with special reference to the extremely long-time decay. It is suggested that this resilience to decay is due... 

Dynamic Effects. Particles often differ in their resilience, inertia, and other dynamic characteristics which can cause them to segregate, particulady when they are forming a pile such as when charged into a bin or discharged from a chute. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

The most straightforward way to measure the effect of low temperatures on recovery is by means of a compression set or tension set test. Tests in compression are favoured and a method has been standardised internationally. The procedure is essentially the same as set measurements at normal or elevated temperatures and has been discussed in Chapter 10, Section 3.1. As the recovery of the rubber becomes more sluggish with reduction of temperature the dynamic loss tangent becomes larger and the resilience lower (see Chapter 9), and these parameters are sensitive measures of the effects of low temperatures. Procedures have not been standardized, but rebound resilience tests are inherently simple and quite commonly carried out as a function of temperature. It is found that resilience becomes a minimum when the rubber is in its most leathery state and rises again as the rubber becomes hard and brittle. 

The current method of determining the energy properties of polyurethane is the Dynamic Thermal Mechanical Analyzer (DTMA). This instrument applies a cyclic stress/strain to a sample of polyurethane in a tension, compression, or twisting mode. The frequency of application can be adjusted. The sample is maintained in a temperature-controlled environment. The temperature is ramped up over the desired temperature range. The storage modulus of the polyurethane can be determined over the whole range of temperatures. Another important property closely related to the resilience, namely tan delta (8), can also be obtained. Tan (8) is defined in the simplest terms as the viscous modulus divided by the elastic modulus. 

Dynamic heat buildup in applications such as wheels is important. Polyurethanes with a high resilience or a low tan 8 in the operating range are important. Polyurethane elastomers used in wheel applications can be evaluated using a test rig where the urethane can be run under load for a fixed period or until failure. 

Perfluoroelastomers (FFKM), such as KALREZ, are particularly suited for extreme service conditions. They are resistant to more than 1,500 chemical substances, including ethers, ketones, esters, aromatic and chlorinated solvents, oxidizers, oils, fuels, acids, and alkali and are capable of service at temperatures up to 316°C (600°F).55 Because of the retention of resilience, low compression set, and good creep resistance, they perform extremely well as static or dynamic seals under conditions where other materials, such as metals, FKM, PIPE and other elastomers, fail. Parts from FFKM have very low outgassing characteristics and can be made from formulations, which comply with FDA regulations.56 Primary areas of application of perfluoroelastomers are paint and coating operations, oil and gas recovery,... 

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