Heat overall

Miscellaneous Heating Overall Heat Transfer Coefficient, U... 

The reforming reactions require the input of water and heat. Overall, the reformer thermal efficiency is calculated as the latent heat of vaporization (LHV) of the product hydrogen divided by the LHV of the total input fuel. This thermal efficiency depends on the efficiencies of the individual processes, the effectiveness to which heat can be transferred from one process to another, and the amount of energy that can be recovered through means such as turbochargers. In the end, high-temperature reformer efficiencies are approximately 65% and low-temperature methanol reformers can achieve 70%-75%. 

Fouling resistance Clean overall heat Overall heat Area required for... 

It must be emphasized that it is not worth expending any effort optimizing pressure, feed condition, or reflux ratio until the overall heat-integration picture has been established. These parameters very often change later in the design. 

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

Figure 5.9 The temperature-heat profiles of a sequence with low overall flow rate of nonkey components is favorable. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195. 1988 reproduced by permission of the Institution of Chemical Engineers.)...

It is thus recommended that in a first pass through a design, thermal coupling should not be considered. Rather, simple columns should be used until a first overall design has been established. Only when the full heat-integration context has been understood should thermal coupling be considered. 

In Fig. 6.33a. heat Qftjel is taken into the boiler from fuel. An overall energy balance gives... 

As with the steam turbine, if there was no stack loss to the atmosphere (i.e., if Qloss was zero), then W heat would he turned into W shaftwork. The stack losses in Fig. 6.34 reduce the efficiency of conversion of heat to work. The overall efficiency of conversion of heat to power depends on the turbine exhaust profile, the pinch temperature, and the shape of the process grand composite. 

The problem with Eq. (7.5) is that the overall heat transfer coefficient is not constant throughout the process. Is there some way to extend this model to deal with the individual heat transfer coefficients ... 

By constrast, Fig. 7.46 shows a diflFerent arrangement. Hot stream A with a low coefficient is matched with cold stream D, which also has a low coefficient but uses temperature diflferences greater than vertical separation. Hot stream B is matched with cold stream C, both with high heat transfer coefficients but with temperature differences less than vertical. This arrangement requires 1250 m of area overall, less than the vertical arrangement. 

Thus, for a given exchanger duty and overall heat transfer coefficient, the 1-2 design needs a larger area than the 1-1 design. However, the 1-2 design offers many practical advantages. These... 

Hgura 7.10 A large overall temperature cross requires shells in series to reduce the cross in individual exchangers. (From Ahmad, Linnhoff, and Smith, Trans. ASME, J. Heat Transfer, 110 304, 1988 reproduced by permission of the American Society of Mechanical Engineers.)... 

Once a design is known for the first two layers of the onion (i.e., reactors and separators only), the overall total cost of this design for all four layers of the onion (i.e., reactors, separators, heat exchanger network, and utilities) is simply the total cost of all reactors and separators (evaluated explicitly) plus the total cost target for heat exchanger network and utilities. 

The overall inventory. In the preceding chapter, the optimization of reactor conversion was considered. As the conversion increased, the size (and cost) of the reactor increased, but that of separation, recycle, and heat exchanger network systems decreased. The same also tends to occur with the inventory of material in these systems. The inventory in the reactor increases with increasing conversion, but the inventory in the other systems decreases. Thus, in some processes, it is possible to optimize for minimum overall inventory. In the same way as reactor conversion can be varied to minimize the overall inventory, the recycle inert concentration also can be varied. 

It must be emphasi2ed that any energy costs for the separation in the tradeoffs shown in Fig. 10.7 must be taken within the context of the overall heat integration problem. The separation might after all be driven by heat recovery. 

Energy efficiency of the process. If the process requires a furnace or steam boiler to provide a hot utility, then any excessive use of the hot utility will produce excessive utility waste through excessive generation of CO2, NO, SO, particulates, etc. Improved heat recovery will reduce the overall demand for utilities and hence reduce utility waste. 

Now consider the placement of the reactor in terms of the overall heat integration problem. 

By contrast. Fig. 13.46 shows an endothermic reactor integrated below the pinch. The reactor imports Qreact from part of the process that needs to reject heat. Thus integration of the reactor serves to reduce the cold utility consumption by Qreact- There is an overall reduction in hot utility because, without integration, the process and reactor would require (Qumin + Qreact) from the utility. 

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

Unfortunately, the overall design problem is even more complex in practice. Spare driving forces in the process could be exploited equally well to allow the use of moderate utilities or the integration of heat engines, heat pumps, etc. in preference to distillation integration. 

LinnhoflF, B., Dunford, H., and Smith, R., Heat Integration of Distillation Columns into Overall Processes, Chem. Eng. Sci., 38 1175, 1983. 

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