Quench or quenching

All pipes and fittings are hardened in electric induction or gas annealing furnaces. After annealing, the pipes are water spray quenched or quenched in a water-filled tank outdoors, depending on their size. 

In order to use the stopped-flow technique, the reaction under study must have a convenient absorbance or fluorescence that can be measured spectrophotometri-cally. Another method, called rapid quench or quench-flow, operates for enzymatic systems having no component (reactant or product) that can be spectrally monitored in real time. The quench-flow is a very finely tuned, computer-controlled machine that is designed to mix enzyme and reactants very rapidly to start the enzymatic reaction, and then quench it after a defined time. The time course of the reaction can then be analyzed by electrophoretic methods. The reaction time currently ranges from about 5 ms to several seconds. 

Some fraction of encounters between organic compounds and O, lead only to quenching to reform 302 without chemical transformation (Wilkinson et al., 1995). Some measured rate constants combine reaction and quenching or quenching alone and overestimate reactivity of the chemical toward 1C>2 as well as distort the SAR parameters. The Wilkinson et al. compilation (1995) usually indicates which values refer to reaction (kr) or quenching (kq) and which are combined values (ky). 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600°C. Once-tbrougb cbnversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or ghtly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fiactionation allows the recycling of the unconverted part of the feedstock. 

Chilling liquid sulfur [112] (a somewhat slower process than quenching) or quenching without stretching the product... 

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

Many of the adsorbents used have rough surfaces they may consist of clusters of very small particles, for example. It appears that the concept of self-similarity or fractal geometry (see Section VII-4C) may be applicable [210,211]. In the case of quenching of emission by a coadsorbed species, Q, some fraction of Q may be hidden from the emitter if Q is a small molecule that can fit into surface regions not accessible to the emitter [211]. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

If there are no competing processes the experimental lifetime x should equal Tq. Most connnonly, other processes such as non-radiative decay to lower electronic states, quenching, photochemical reactions or... 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Figure C2.5.2. Scaling of the number of MBS C(MES) (squares) is shown for the hydrophobic parameter = -0.1 and A = 0.6. Data were obtained for the cubic lattice. The pairs of squares for each represent the quenched averages for different samples of 30 sequences. The number of compact stmctures C(CS) and self-avoiding confonnations C(SAW) are also displayed to underscore the dramatic difference of scaling behaviour of C(MES) and C(CS) (or C(SAW)). It is clear that C(MES) remains practically flat, i.e. it grows no faster than In N.

E. Vedejs (1978) developed a general method for the sterically controlled electrophilic or-hydroxylation of enolates. This uses a bulky molybdenum(VI) peroxide complex, MoO(02)2(HMPTA)(Py), which is rather stable and can be stored below 0 °C. If this peroxide is added to the enolate in THF solution (base e.g. LDA) at low temperatures, oneO—O bond is broken, and a molybdyl ester is formed. Excess peroxide is quenched with sodium sulfite after the reaction has occurred, and the molybdyl ester is cleaved to give the a-hydroxy car-... 

Quenched dynamics can trap structures in local minima. To prevent this problem, you can cool the system slowly to room temperature or some appropriate lower temperature. Then run room temperature molecular dynamics simulations to search for conformations that have lower energies, closer to the starting structure. Cooling a structure slowly is called simulated annealing. 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Fig. 1. A typical process flowsheet for acrolein manufacture. A, Fixed-bed or fluid-bed reactor B, quench cooler C, absorber D, stripper E and F,...

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

The simplest form of melt extmsion is the use of a slot die to form the molten polymer into a thin flat profile which is then quenched immediately to a solid state (Fig. 1). This is usually done by contacting the hot web very quickly on a chilled roU or dmm. A liquid quenching bath may be used ia place of or contiguous to the chill roU. Depending on the polymer type or formulation, the quenched web is generally substantially amorphous. In some cases, the web may be drawn down ia thickness by overdriving the quenching roU relative to the extmsion velocity. 

When drawdown is high, the film may be uniaxially oriented and the properties of the final film isotropic. In the manufacture of strapping tape this effect is accentuated. If the cast or quenched film is to be used to feed an orientation tine, additional attention must be given to the amorphous—crystalline nature of the film ia the draw processes so that maximum strength can be achieved and uniform gauge and optical quality maintained. Slot casting is used for the orientation of these resins, polyesters, polyamides, and a variety of others. 

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