Energy savings/recovery

Many units have waste heat recovery systems that generate low pressure steam from reaction heat. Such steam is often employed to drive adsorption refrigeration units to cool the reactor feed stream and to increase polymer conversion per pass, an energy-saving process that reduces the demand for electrical power. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

The CCE spreads the investment over the lifetime of the measure into equal annual payments with the familiar capital recovery factor. The annual payment is then divided by the annual energy savings to yield a cost of saving a unit of energy. It is calculated using the following formula ... 

There are several measure sets with immediate payback SE, SF, SH, SI. Measures SE and SF are radiant panel systems with displacement ventilation. These systems have a similar cost to the base case, but they offer energy savings. Furthermore, significant sizing reductions, mainly in the cooling tower and chiller sizes, offset the incremental cost of the envelope and heat recovery measures. Because the elevator efficiency measures offer a net savings in capital cost, the capital cost of the other measures is further offset. 

Mimura, T., Simayoshi, H., Suda, T., Iijima, M., and Mituoka, S. Development of energy saving technology for flue gas carbon dioxide recovery in power plants by chemical absorption method and steam system, Energ. Corners, and Manag., 38(Suppl.), S57-S62, 1997. 

Due to the greater number of moles of product produced, versus moles of reactants consumed i.e., 65 vs. 33 as in SR of n-Cig), the SR reaction is favored at low pressures, as shown in Figure 20. However, industrial SR is carried out at high pressures i.e., 15-35 atm) because much of the H2 produced is supplied in ammonia and methanol plants where higher pressures facilitate better heat recovery and result in compression energy savings." ... 

Heat recovery. Many process streams are hot. Ultrafiltration usually works better with hot feeds so hot feed solutions are not a problem. If the hot, clean permeate can be recycled without cooling, the energy savings can be considerable. If the water is 50 °C above ambient temperature, the energy savings amount to about US 4.00/1000 gal. 

In unit operations control, the individual column variables are treated only as constraints and so long as the values of these constraints are within acceptable limits, the column is controlled (optimized) to maximize production rate, profitability, etc. Economics of individual fractionators may continually change throughout the life of the plant, because energy savings can be important at one particular time, whereas product recovery can be more important at other times. 

---