Overheating method

Another technique used is the overheating method where highly oriented, crystalline polymer fibers are constrained and heated. As the polymer chains must relax to reach the isotropic liquid state, preventing this relaxation can mean increases in of around 10 °C [89], therefore widening the processing window for SPC manufacturing. Unfortunately, it is not always possible to constrain the polymer chains effectively if the molecular chains are folded, or the chain mobility is high. 

Co-extrusion is another technique, utilizing once again the similar physics discussed in Sections 19.2.3 and 19.4.1. Here a single polymer composite is created by extruding and combining tapes of the same polymer, but with differing melting temperatures, and then 

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Primary alkanolamine solutions require a relatively high heat of regeneration. Also excessive temperatures or localized overheating in reboilers cause the MEA to decompose and form corrosive compounds. An inhibitor system, such as the Amine Guard system developed by Union Carbide, is an effective method of corrosion control (52). Inhibitors permit the use of higher (25—35%) concentration MEA solutions, thus allowing lower circulation rates and subsequendy lower regeneration duty. 

Heaters. The preferred methods for heating are a double-jacket heated tank, nonmetaUic heat exchangers, quartz heaters, or Teflon-coated low watt density stainless steel heaters. Localized overheating must be avoided. 

The proper method to remove the catalyst involves stabilization. The method for this is usually recommended by the catalyst manufacturer. With the reactor still closed, cold and flushed with nitrogen, admit nitrogen with less than 1 % oxygen in it, while the impeller is running. This oxidizes the organics and the metallic surface of the catalyst under well-controlled conditions after which the catalyst can be exposed to air without danger of overheating. 

Protection from Fire Exposure and Pressure Relief Considerations - Pressure relief valves cannot protect a vessel that becomes locally overheated on an unwetted surface, although they do prevent the pressure from rising beyond the accumulation pressure of the valve. However, in such a case the vessel may be effectively protected against failure by either one of two methods for mitigating the effect of fire ... 

Two methods of preventing overheating are available either general ventilation to provide fairly even conditions over the whole space at working level or local ventilation to give spot cooling for localized hot spots. 

With the hot plate method, the bearing is simply laid on the plate until it reaches the approved temperature, using a pyrometer or Tempilstik to make certain it is not overheated. Difficulty in controlling the temperature is the major disadvantage of this method. 

Another problem studied and solved with PCM products is the overheating of laptop computers. One of the possible alternative methods to the used the noisy traditional fans is to add a PCM pad under the computer chassis (Figure 168). When the temperature of the computer chassis rises to the melting point of the PCM, the temperature of the computer will be kept close to this temperature. 

The causes of variations in yield by the use of the okler methods can now be explained. When benzaldehyde is added to the alcoholate, and especially when the latter is still warm, local overheating results in fact, the temperature may rise far above xoo° with the result that benzyl ether is formed. Simultaneously, the sodium benzylate is converted into sodium benzoate, which is of no value for inducing the desired reaction, and consequently very little benzyl benzoate is obtained. The same side reactions explain the failure of this experiment when the benzyl alcohol used in preparing the catalyst (sodium benzylate) is contaminated with benzaldehyde. 

Drying, usually by physical methods, is one of the most common unit operations in both laboratory or industrial scale process chemistry, and since heating is usually employed to remove volatiles, thermally unstable materials may decompose if overheated. As a light-hearted example, when a faulty oven thermostat led to overheating of mercuric thiocyanate, a monstrous Pharaoh s serpent resulted. Drying moist cadmium propionate in an electric oven led to explosive ignition of the diethyl ketone vapour produced as an unforeseen by-product. Drying 3,5-dinitro-2-toluamide had more serious consequences. 

A PTFE tube (1), for example, with an outer diameter of 3 mm and an inner diameter of 2 mm, is cut some centimeters longer than finally needed. Tubes from 0.5 mm up to 6 mm outer diameter have been successfully applied with this method. The tube is heated at the end (ca. 2-3 cm) with a hot air blower until it becomes clearly transparent (2, ca. 250 °C, using a gas flame includes the risk of overheating and decomposition of the polymer). Then the tube is quickly stretched so that the diameter becomes smaller than the borehole (3). The squeezed end is cut off (4). 

There are many questionable points in this description. To begin with, the dilution of the batch by plunging into water is uneconomic. This can also easily cause denitration of the product owing to local overheating. A stabilization method that omits boiling does not, in general, yield a product of satisfactory stability. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

The continuous method of nitration is particularly useful when the rate of nitration is rapid (such as in case of manufg NG, NGc, or PETN) and an overheating and expln might take place with the batch process. Nitration of cellulose, on the other hand, does not proceed very rapidly (unless mixed acid contains phosphoric or glacial... 

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