Sensible heat conversion

Any gas, when compressed, rises in temperature. Conversely, if it is made to do work while expanding, the temperature will drop. Use is made of the sensible heat only (although it is, of course, the basis of the air liquefaction process). 

In a typical example (33) a fresh feed of 8% polybutadiene rubber in styrene is added with antioxidant, mineral oil, and recycled monomer to the first reactor at 145 lbs./hr. The reactor is a 100-gallon kettle at approximately 50% tillage with the anchor rotating at 65 rpm. The contents are held at 124°C and about 18% conversion. Cooling is effected via the sensible heat of the feed stream and heat transfer to the reactor jacket. In this reactor the rubber phase particles are formed, their average size determined and much of their morphology established. Particle size is controlled to a large measure by the anchor rpm. 

The first term represents the sensible heat of cooling down from ambient to boiling point temperature, the second term represents the latent heat of the phase change, and the third term represents the energy contained in the ortho-para (o-p) conversion. 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 22.1, the refrigerant is chilled from 100 to 40°F. Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

Conversely, condensation from a vapor to a liquid is a heating process, as the latent heat is converted to sensible heat in the liquid, and the liquid temperature rises (as observed in a cooling tower plume). 

As temperature increases for a given conversion, reactor size decreases, as expected. The heat transfer rate also decreases because there is a greater contribution of the sensible heat of the cooler feed as reactor temperature increases. 

The reactor in the reactor-stripper process is larger than the first reactor in the 3-CSTR process and therefore has more heat transfer area. However, all the conversion occurs in this one reactor, so we would expect the heat transfer rate to be high. In fact, it is not high but is even lower than the heat transfer rate in the 1-CSTR process (1.78 x 106 kJ/s, as shown in Fig. 2.21). The reason for this unexpected result is the large recycle stream in the reactor-stripper process. We assume that its temperature is 322 K. The total flow into the reactor is 0.1591 krnol/s (the sum of the recycle 0.1241 kmol/s and the fresh feed is 0.03506 kmol/s), while the flow into the first reactor of the 3-CSTR process is just the fresh feed. Thus the larger sensible heat of the larger stream reduces the heat than must be transferred in the reactor. Note that the total heat of reaction for a 98% conversion is 2.395 x 106 kJ/s. The sensible heat of the large feed stream to the reactor in the reactor-stripper process is 1.398 x 106 kJ/s. In the 3-CSTR process (and in all the multiple CSTR processes) the sensible-heat term in the first reactor is only 0.616 x 106 kJ/s because the flowrate is smaller. 

Note that for this adiabatic endothermic reaction, the reaction virtually dies out after 2.5 owing to the large drop in temperature, and very little conversion is achieved beyond this point. One way to increase the conveision would be to add a diluent such as, which could supply the sensible heat for this endothermic reaction, However, if too much diluent is added, the concentration and rate will be quite low. On the other hand, if too Uttle diluent is added, the temperature will drop and virtually extinguish the reaction. How much diluent to add is left as an exercise (see Problem PS-2). 

P8-23g The vapor-phase praddng of acetone is to be carried out adiabatically in a bank of 1000 1-in. schedule 40 tubes 10 m in length. The molar feed rate of acetone is 6000 kg/h at a pressure of 500 kPa. The maximum feed (emperature is 1050 K.. Nitrogen is to be fed togethw with the acetone to provide the sensible heat of reaction, jbetermine the conversion as a function of nitrogen feed rate (in terms of Oj. .,) for... 

Steps 8 and 9 (Continued) Enthalpy data have been taken from Table 4.3. The heat of reaction at 25°C (77°F) and 1 atm from Example 4.31 after conversion of units is — 121,672 Btu/lb mol of CO. We can assume that the slightly higher pressure than SC of 2 atm has no effect on the heat of reaction or the enthalpy values. Figure E4.40b shows the sensible heat (enthalpy) values for the entering and exiting materials. 

Some of the energy released by the reaction will appear as sensible heat in stream F2, and some concern exists as to whether the fixed flow rates will be sufficient to keep the fluids from boiling while still obtaining good conversion. Feed data is as follows. 

Figure 5.2 gives the response of a one-CSTR process for step changes in feed rate from 100 to 150 lb-mo 1/hr and from 100 to 50 Ib-mol/hr for the system with k = 0.5 and 95 percent conversion. When feed rate is increased, the temperature in the reactor initially decreases. This is due to the sensible heat effect of the colder feed (70°F versus 140°F). After about five minutes, the temperature starts to increase because the concentration of reactant has increased, which increases the rate of reaction. The maximum temperature deviation is only 0.06°F, but it takes over five hours to return to the setpoint because of the slow change in reactor concentration and the large reset time. 

Reduction and oxidation step in two separate reactors allow for a continuous hydrogen production. A major drawback is the small conversion rate of 60 % of the synthesis gas in the reduction step. The effluent from the steam-iron reactor contains 37 % H2 plus 61 % steam and 96 % H2, respectively, if the steam is condensed. The remaining heating value plus sensible heat at 825 °C, however, can be used to cogenerate electricity. With a plant capacity of 110,000 Nm /h of H2, the byproduct electric power is 158 MW [55]. 

Watt never claimed anything more than his opinion . Priestley said Watt had thought that if steam could be made red-hot, so that all its latent heat should be converted into sensible heat , there would be a possibility of the conversion of water, or steam, into permanent air . Watt had, in fact, arrived at the idea of a critical temperature of water, but he went astray in supposing that the water was converted into air. He wrote to Boulton on 10 December 1782 ... 

Latent heat-storage materials are also called phase change materials (PCM). PCM can absorb or release heat with a slight temperature change. PCM may be repeatedly converted between solid and liquid phases to utilize their latent heat of fusion to absorb, store and release heat or cold during such phase conversions. The latent heats of fusion are greater than the sensible heat capacities of the materials. 

If the secondary reformer is operated with an excess of process air (about 50% above the stoichiometric amount), or if the process air is enriched to an Oj-content of about 30%, then the sensible heat in the product gas from the secondary reformer is sufficient for the primary conversion of natural gas in a heat exchange reformer, meaning that the fired reformer can be eliminated. Such schemes require, however, extra installations, either for production of enriched air or for removal of excess N2 from the synthesis gas, and they suffer of course also from the reduced efficiency of the steam production described above. 

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