Temperature heating medium

Process variables material balance and flow rates, operating pressure, operating temperature, heating medium temperature, and flow rate. 

Figure 5.19 SSP discontinuous batch process for PET, showing the variation of temperature, pressure and intrinsic viscosity with batch time (reactor volume of 44 m ). Curves 1, temperature-heating medium (amorphous material) 2, product temperature (amorphous material) 3, pressure (in mbar) 4, temperature-heating medium (crystalline chips) 5, product temperature (crystalline chips) 6, IV (amorphous material) 7, IV (crystalline chips) [34]. From manufacturer s literature published by OHL Apparatabau and Verfahrenstechnik, Limburg, Germany, and reproduced with permission...

Fig. 17-2. Common arrangeme of flow (a) countercurrent, two-pass shell, 2 tube passes (b),parallel flow, two-pass shell, two tube passes (c) ipixed flow, single-pass shell, two tube passes (d) constant-temperature ccioling medium (e) constant-temperature heating medium, and (/) combination condensing and subcooling.

The use of an unnecessarily hot utility or heating medium should be avoided. This may have been a major factor that led to the runaway reaction at Seveso in Italy in 1976, which released toxic material over a wide area. The reactor was liquid phase and operated in a stirred tank (Fig. 9.3). It was left containing an uncompleted batch at around 160 C, well below the temperature at which a runaway reaction could start. The temperature required for a runaway reaction was around 230 C. ... 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

For extended, noncyclic exposures, it can be assumed that the entire piece teaches the temperature of the heating medium and is, therefore, subject to permanent strength losses throughout the piece, regardless of size and mode of stress application. Because dry wood is a good insulator, it often does not teach the daily extremes in temperature of the air around it in ordinary constmction thus, estimates of long-term effects should be based on the actual wood temperatures experienced by critical stmctural parts. 

Low Temperature Properties. Medium hardness compounds of average methyl acrylate, ie, VAMAC G, without a plasticizer typically survive 180° flex tests at —40° C. Such performance is good for a heat-resistant polymer. Low temperature properties can be greatly enhanced by the use of ester plasticizers (10). Careful selection of the plasticizer is necessary to preserve the heat resistance performance of the polymer. Plasticized high methyl acrylate grades lose only a few °C in flexibiUty, compared to grades with average methyl acrylate levels. 

Steam-Heated Exchangers Steam, the most common heating medium, transfers its latent heat in condensing, causing heat flow to be proportional to steam flow. Thus, a measurement olsteam flow is essentially a measure of heat transfer. Consider raising a hquid from temperature Ti to To by condensing steam ... 

Control of trav and compartment equipment is usually maintained by control of the circulating-air temperature (and humidiy) and rarely by solids temperature. On vacuum units, control of the absolute pressure and heating-medium temperature is utihzed. In direct dryers, cycle controllers are frequently employed to vary the air temper-... 

Steam is the usual heating medium, and a standard heater arrangement consists of a main heater before the circulating fan. When steam is not available or the diying load is small, electricalheat can be used. For temperatures above 450 K, products of combustion can be used, or indirect-fired air heaters. 

Unless material characteristics hmit the gas temperature, the inlet temperature is usually fixed by the heating medium employed i.e., 400 to 450 K for steam or 800 to 1100 K for gas- and oil-fired burners. The proper exit-gas temperature is largely an economic function. Its value may be determined as follows ... 

At,n = log-mean-temperature driving force from heating medium to the sohds, K. 

Heat-transfer-fluid heaters maintain the temperature of a circulating liquid heating medium (e.g., a paraffinic hydrocarbon mixture, a Dowtherm, or a molten salt) at a level that may exceed 673 K (750°F). 

Maintain inlet temperature of heating medium sufficiently below the minimum ignition temperature... 

Limit temperature of heating medium Use split heating/cooling system to eliminate heat transfer to unwetted surface Heat with sparged steam/tempered water Avoid splashing of material onto unwetted heating surface... 

Ratings in columns 3 and 4 are based on a temperature of the heating medium of 240°-400 F. If the heating medium temperature is over 400°F. and the cooling medium is known to scale, these ratings should be modified accordingly. 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

---