Utilization of Temperature Difference

The coefficients a and b in Equation 23.10 can be correlated as functions of the saturation temperature difference across the turbine9. In fact, the coefficients in Equation 23.10 are related to the pressure drop across the turbine. However, in the model, the pressure drop is replaced by its equivalent saturation temperature difference. Use of temperature difference allows easier interface to utility calculations with process heating and cooling demands. Thus ... 

On the basis of this finding, a two-step (stepwise) development technique12 was applied to separations of possible three binary mixtures of polybutadienes with different chain microstructure, namely, those of cis-1,4 + tram-1,4, tram-1,4+1,2-1,2-vinyl, and 1,2-vinyl + cis-1,4. The principle consisted of a utilization of the different development characteristics exhibited by carbon tetrachloride and amyl chloride. An example of this procedure applied by these authors83 will be described below. A mixture cis-1,4 + tram-1,4 was developed primarily with amyl chloride until the solvent front reached a distance, e.g., 10 cm from the starting point by this development only the cis-1,4 polymer should have migrated up to the solvent front (cf. Table 4). In order to identify the immobile component, the chromatogram was dried in vacuum at room temperature and treated with carbon tetrachloride until the solvent front reached an intermediate distance, e.g., 5 cm. It is obvious that this procedure can be alternatively used for identification of any unknown binary mix-... 

Example Seebeck and Peltier Effect. The Seebeck effect utilizes a temperature difference to generate a potential gradient as illustrated in Fig. 7.4. Two pieces of distinct metals, A and B (i.e. Cu/Al or Fe/Ni), are joined at 1 and 2. The junctions are exposed to different temperatures and this in turn gives rise to a (small) voltage drop, A, at 3. The Seebeck coefficient... 

Increasing the chosen value of process energy consumption also increases all temperature differences available for heat recovery and hence decreases the necessary heat exchanger surface area (see Fig. 6.6). The network area can be distributed over the targeted number of units or shells to obtain a capital cost using Eq. (7.21). This capital cost can be annualized as detailed in App. A. The annualized capital cost can be traded off against the annual utility cost as shown in Fig. 6.6. The total cost shows a minimum at the optimal energy consumption. 

The philosophy in the pinch design method was to start the design where it was most constrained. If the design is pinched, the problem is most constrained at the pinch. If there is no pinch, where is the design most constrained Figure 16.9a shows a threshold problem that requires no hot utility, just cold utility. The most constrained part of this problem is the no-utility end. Tips is where temperature differences are smallest, and there may be constraints, as shown in Fig. 16.96, where the target temperatures on some of the cold... 

Figure 16.10 shows another threshold problem that requires only hot utility. This problem is different in characteristic from the one in Fig. 16.9. Now the minimum temperature difference is in the middle of the problem, causing a pseudopinch. The best strategy to deal with this type of threshold problem is to treat it as a pinched problem. For the problem in Fig. 16.10, the problem is divided into two parts at the pseudopinch, and the pinch design method is followed. The only complication in applying the pinch design method for such problems is that one-half of the problem (the cold end in Fig. 16.10) will not feature the flexibility offered by matching against utility.

Example 16.2 A problem table analysis of a petrochemicals process reveals that for a minimum temperature difference of 50°C the process requires 9.2 MW of hot utUity, 6.4 MW of cold utility, and the pinch is located at 550°C... 

Thus loops, utility paths, and stream splits offer the degrees of freedom for manipulating the network cost. The problem is one of multivariable nonlinear optimization. The constraints are only those of feasible heat transfer positive temperature difference and nonnegative heat duty for each exchanger. Furthermore, if stream splits exist, then positive bremch flow rates are additional constraints. 

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

Because of its low and regular thermal expansion, vitreous sHica is employed ia apparatus used to measure the thermal expansion of soHds. A detaHed account of the different methods used for this purpose has been pubHshed (227). The most common form of dHatometer utilizes a vitreous sHica tube closed at the bottom and containing the test sample. A movable rod of vitreous sHica, resting on the sample, actuates a dial iadicator resting on the top of the rod. The assembly containing the sample is placed ia a furnace, bath, or cooling chamber to attain the desired temperature. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Consider a process which has two process hot streams (Hj and H2), two process cold streams (Cj and C2), a heating utility (HUi which is a saturated vapor that loses its latent heat of condensation) and a cooling utility (CUi). The problem data are given in Table 9.8. The cost of the heating utility is 4/10 kJ added while the cost of the coolant is 7/10 kJ. The minimum allowable temperature difference is lO C. Employ graphical, algebraic and... 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Utility operators may choose from several technologies to generate electricity, although the most common approach is via the use of high-temperature, fossil fuel boiler plants. In this case, the boiler (steam generator) itself may be of several different design types. 

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