Processing considerations

Many polymers have their own process characteristics. NR and SBR process very easily on a two-roll mill. Other polymers process differently. Nitrile rubbers are well known for bagging (non-adherence to the rolls), while other polymers, especially those that are chlorinated, tend to stick firmly to the mill roll. Consequently, specific solutions have been found for each polymer s processing idiosyncrasies. 

The requirement for ease of processing demands that the rubber behave as a plastic material. While the effect of heating will aid in converting the tough elastic polymer into a plastic, the process is far from complete. Many grades of polymer are available in a pre-crosslinked form to assist in this respect. These polymers have usually been crosslinked prior to coagulation of the emulsion from which they were derived. The three-dimensional network helps to reduce the elastic component of the compound but at the expense, usually, of mechanical strength. Reclaimed rubbers form a supply of polymer with similar properties because of the residual crosslink structure from the parent material. 

The strength of rubbers is influenced strongly by the effect of filler type and loading. This is discussed further in Section 7.6.1. The addition of plasticisers is known to reduce 

Those elastomers that exhibit low modulus plus high strength in the unfilled gum state because they undergo stress-induced crystallisation. 

Those with a high modulus and high strength. 

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Sterile Filtration of Gases. Primary appHcations for sterile gas filtration are the sterilization of fermentor inlet air, fermentor vent gas, vents on water for injection tanks, and vacuum break filters during lyophilization. Operational and process considerations apply. Typically, the membrane in gas... 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Process considerations must not only take into account characteristics of the particular alcohol or phenol to be esterified, but also the self-propagating by-product reaction, which results in polymer formation. 

Conditions of high pressure and low temperature favor the formation of the complex, whereas low pressure and high temperature tend to release the complexed carbon monoxide from solution. These conditions typify the operation of the absorber-stripper shown in Figure 2. Specific design conditions for the process are given in references 86—88, and an excellent summary of processing considerations is presented in reference 85. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

Economic and process considerations usually dictate that agitated thin-film evaporators be operated in single-effect mode. Veiy high temperature differences can then be used many are heated with Dowtherm or other high-temperature media. This permits achieving reasonable capacities in spite of the relatively low heat-transfer coefficients and the small surface that can be provided in a single tube [to about 20 m" (200 ft")]. The structural need for wall thicknesses of 6 to 13 mm (V4 to V2. in) is a major reason for the relatively low heat-transfer coefficients when evaporating water-like materials. 

If there is water to be settled and withdrawn from hydrocarbon, the water s settling time requirement needs to be checked. The water settling requirement, rather than other process considerations, might set the liquid surge capacity. Therefore, the liquid surge capacity we have previously estimated from tables might have to be increased. 

Compressors are normally one part of an overall process train. When a compressor is selected for use in a process, consideration must be given to the required operating range. It will become quickly apparent when a compressor with a fixed capacity is installed in a process with a variable capacity requirement. Modifying capacity of the individual compressors was discussed in the earlier chapters. 

Th chapter presents an overview of the types of compresL . ,. eratic i for selecting a type of compressor, a procedure for estimating hors iwer and number of stages, and some process considerations for both ciprocating and centrifugal compressors. Chapter 11 discusses... 

It is essential that the ASME code requirements be understood by the designer and individual rating and specifying the installation details of the safety device. It is not sufficient to merely establish an orifice diameter, since process considerations which might cause overpressure must be thoroughly explored in order to establish the maximum relieving conditions. 

Spenlehauer (59) reported that in the case of cisplatin-loaded microspheres, irradiation only changes the processing considerations and does not influence drug release. This observation is in conflict with other literature reports showing increases in drug release rates (60,61). 

Though the total degrees of freedom is seen to be (C + 4) some of the variables will normally be fixed by general process considerations, and will not be free for the designer to select as design variables . The flash distillation unit will normally be one unit in a process system and the feed composition and feed conditions will be fixed by the upstream processes the feed will arise as an outlet stream from some other unit. Defining the feed fixes (C + 2) variables, so the designer is left with ... 

Waste-heat boilers are often used to recover heat from furnace flue gases and the process gas streams from high-temperature reactors. The pressure, and superheat temperature, of the stream generated will depend on the temperature of the hot stream and the approach temperature permissible at the boiler exit (see Chapter 12). As with any heat-transfer equipment, the area required will increase as the mean temperature driving force (log mean AT) is reduced. The permissible exit temperature may also be limited by process considerations. If the gas stream contains water vapour and soluble corrosive gases, such as HC1 or S02, the exit gases temperature must be kept above the dew point. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

The single-stage, horizontal, overhung, centrifugal pump is by far the most commonly used type in the chemical process industry. Other types are used where a high head or other special process considerations are specified. 

Pump selection is made on the flow rate and head required, together with other process considerations, such as corrosion or the presence of solids in the fluid. 

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