Heat recovery effectiveness

In Chap. 10, modification of the process for reducing process waste was considered in detail. It also was concluded that to minimize utility waste, the single most effective measure would be improved heat recovery. The energy-targeting methods presented in Chaps. 6 and 7 maximize heat recovery for a given set of process conditions. However, the process conditions can be changed to improve the heat recovery further. 

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

Frequently, the difference ia exchanger type does not influence the desired topology to any significant extent. For iadustrial problems, however, it is necessary to consider iadividual heat-exchanger shells rather than just the match that is called the heat exchanger. If a high level of heat recovery is desired, the effect of the F factor can be important. This problem has been solved but is beyond the scope of this article. 

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Hea.t Pumps. The use of heat pumps adds a compressor to boost the temperature level of rejected heat. It can be very effective in small plants having few opportunities for heat interchange. However, in large faciHties a closer look usually shows an alternative for use of waste heat. The fuel/steam focus of energy use has led to appHcation of heat pumps in appHcations where a broader examination might suggest a simpler system of heat recovery. 

As a general rule, the optimum number of effects increases with an increase in steam cost or plant size. Larger plants favor more effects, partly because they make it easier to install heat-recovery systems that increase the steam economy attainable with a given number of effects. Such recoveiy systems usually do not increase the total surface needed but do require that the heating surface be distributed between a greater number of pieces of equipment. 

Fig. 6-11. Schematic diagram of the kraft pulping process (6). 1, digester 2, blow tank 3, blow heat recovery 4, washers 5, screens 6, dryers 7, oxidation tower 8, foam tank 9, multiple effect evaporator 10, direct evaporator 11, recovery furnace 12, electrostatic precipitator 13, dissolver, 14, causticizer 15, mud filter 16, lime khn 17, slaker 18, sewer.

El-Halwagi, M. M. (1993). A process synthesis approach to the dilemma of simultaneous heat recovery, waste reduction and cost effectiveness. In Proceedings of the Third Cairo International Conference on Renewable Energy Sources ( A. I. El-Sharkawy and R. H. Kummler, eds.), Vol. 2, pp. 579-594. ... 

The basic idea of using TCR in a gas turbine is usually to extract more heat from the turbine exhaust gases rather than to reduce substantially the irreversibility of combustion through chemical recuperation of the fuel. One method of TCR involves an overall reaction between the fuel, say methane (CH4), and water vapour, usually produced in a heat recovery steam generator. The heat absorbed in the total process effectively increases... 

For any network there will be a best value for the minimum temperature difference that will give the lowest total annual costs. The effect of changes in the specified ATIllin need to be investigated when optimising a heat recovery system. 

A microturbine with heat recovery was one of the measures modeled. It has not been included in any of the measure sets. A more comprehensive analysis was done that took account of the different treatments of electricity and thermal energy and the effects of varying electricity and natural gas prices. This analysis is available in a separate report [6],... 

The effect of the heat recovery is to decrease the energy requirements. Hence, the energy cost of the process ... 

As excess air is reduced, theoretical flame temperature increases. This has the effect of reducing the stack loss and increasing the thermal efficiency of the furnace for a given process heating duty. Alternatively, if the combustion air is preheated (e.g. by heat recovery), then again the theoretical flame temperature increases, reducing the stack loss. 

Figure 23.38 The effect of steam levels on heat recovery.

The advantage of catalytic thermal oxidation is that the lower temperature of operation can lead to fuel savings (although effective heat recovery without a catalyst can offset this advantage). The major disadvantages of catalytic thermal oxidation are that the catalyst needs to be replaced every two to four years and the capital cost tends to be higher than thermal oxidation without a catalyst. Catalytic thermal oxidation also tends to increase the pressure drop through the system. 

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