Overall heat transfer coefficient increases

Contrary to dense-phase fluidized beds, the radial and axial distributions of voidage, particle velocity, and gas velocity in a circulating fluidized bed are considerably nonuniform, resulting in a nonuniform heat transfer coefficient profile. Since the particle concentration decreases in the axial direction, the heat transfer also decreases. In the radial direction the heat transfer coefficient exhibits a steep profile near the wall, but is almost constant in the center region. The overall heat transfer coefficient increases with suspension density and particle circulation rate. 

Some of our own data are shown in Figure 4. They indicate that the overall heat transfer coefficient increases linearly with absolute temperature in the range 220-420°C, at a rate that seems to increase with mass velocity. Only a small part of this increase can be explained by variation in fluid physical properties (dotted curve), as determined by the correlations in Section 4. The majority of the increase is ascribed to radiation and can be explained quantitatively (solid curves), as in reference 4. 

Consider a single-zone jacket where there is an increase in the jacket flow, and a corresponding increase in the outside film coefficient because hj =f(Njjg, G). Therefore, a two-fold increase in the jacket flow results in an increase in hjj by 2 h . The overall heat transfer coefficient U = l/[FpoL + 1/hj], and a larger outside coefficient subsequently increases the overall heat transfer coefficient. The overall heat flux will increase due to the combined effects of the increased flow and lower jacket outlet temperature. The net result is an increase in the pressure drop. 

To reduce the pressure drop, a batch reactor with a half-pipe jacket of length L and flowrate W can be partitioned into a two-zone jacket, each with a length L/2 and each supplied with W jacket flowrate. This doubles the jacket flow at a lower pressure drop in each zone. The flow in each zone can then be increased to increase the outside and overall heat transfer coefficients, which is similar to those of the single-zone jacket. 

The effect of the fouling on the shell-side flow is to increase the cross-flow and increase the overall heat transfer coefficient for a fixed pressure drop (assuming the same fouling coefficients in both cases). 

The correlation should be used with caution outside the range 0.6 < Tr < 0.8 and should not be used below a pressure of 0.3 bar. When dealing with a clean, nondegrading material, the process fouling coefficient should be increased to around 11,000 W m 2 K 1, but should be reduced to 1400 to 1900 W m 2 K 1 for material that has a tendency to polymerize17. If a shell-side coefficient of process fouling coefficient different from 5700 W m 2 K 1 is required, the corrected overall heat transfer coefficient can be calculated from17 ... 

Instead of using a 1-1 design in Example 7, a 1-2 design is to be used subject to Xp = 0.9. Assume that the overall heat transfer coefficient is unchanged. (In practice, it would be expected to increase). Calculate... 

Estimate the change in the overall heat transfer coefficient and the new outlet temperatures for a 20% increase in the tube-side flow. 

The radiant heat transfer coefficient becomes important above about 600°C, but is difficult to predict. Baskakov et al. (1973) report that depending on particle size, hr increases from approximately 8% to 12% of the overall heat transfer coefficient at 600°C, to 20 to 33% of h at 800°C. 

Botterill et al. (1982) measured the overall heat transfer coefficient as a function of particle size for sand at three different conditions 20°C and ambient pressure, 20°C and 6 atmospheres, and 600°C and ambient pressure. They found that there was a significant increase in h with pressure for Group D particles, but the pressure effect decreased as particle size decreased. At the boundary between Groups A and B, the increase of h with pressure was very small. 

Increasing system temperature causes hgc to decrease slightly because increasing temperature causes gas density to decrease. The thermal conductivity of the gas also increases with temperature. This causes h to increase because the solids are more effective in transferring heat to a surface. Because hgc dominates for large particles, the overall heat transfer coefficient decreases with increasing temperature. For small particles where dominates, h increases with increasing temperature. 

With a stirrer, the overall heat transfer coefficient could be increased to 200Wm 2K 1. Since the heat transfer area for the filled tank is 2.26 m2, the thermal time constant is... 

Assuming the same aspect ratio (L/D = 2), the diameter is 10 times larger (2.29 m), which gives a heat transfer area that is only 100 times larger (32.95 m2). If the overall heat transfer coefficient is the same as in the pilot plant (we will come back to this issue in Chapter 2), the required temperature differential between the reactor and jacket increases by a factor of 10 (jacket temperature is 304 K instead of 330 K). The flowrate of makeup cooling water (19.54 kg/s) increases by a factor of 4000. 

The combined stream is preheated to 122°C in a FEHE. A heater (HX3) is installed after the FEHE so that inlet temperature of the coolant stream in REACT2 can be adjusted to satisfy the energy balance when the exit temperature of the coolant stream is specified in this countercurrent tubular reactor. This temperature is 150°C, and the heat load in HX3 is 9.34 x 106 kcal/h. The stream is further preheated to 265°C in the tube side of reactor REACT2 by the heat transfer from the reactions that are occurring in the hot shell side of this vessel. There is no catalyst on the cold tube side, so the feed stream does not react but its temperature is increased. The stream is then fed to reactor REACT 1, which contains 48,000 kg of catalyst. This reactor is cooled by generating steam. The coolant temperature is 265°C (51 bar steam). This vessel contains 3750 tubes, 0.0375 m in diameter, and 12.2 m in length. The overall heat transfer coefficient between the process gas and the steam is 244 kcal h-1 m-2 °C 1. The heat transfer rate is 42 x 106 kcal/h. 

When processing is controlled by heat transfer variables, a log mean temperature difference (ATlmtd) and heat transfer surface area will predominate over the agitation variables. Provided it is sufficient to give a homogeneous process fluid temperature, increased agitation can only reduce the inside film resistance, which is one of a number of resistances that determines the overall heat transfer coefficient. 

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