**4. Energy Analysis**

In an attempt to quantify the energy savings from each of the design modifications proposed, we undertook calculations of the heat loads and associated pumping requirements for the molten salt heat transfer fluid systems for each reactor. For reactor R-101 (propylene oxidation reactor), the main design modification is the use of an inert preheating zone for preheating the feed. This is in lieu of a traditional fired preheater. The energy for preheating is obtained from the circulating thermal fluid and hence a higher flowrate of thermal fluid would be required when compared to the base case (traditional preheater and thermal fluid required only for cooling of the exothermic process fluid). The total heat duty required for pre-heating the propylene, air and steam mixture is 6190 kW, which translates into an increase of 1.64 kmol/s in the molten salt flowrate. Under the specified operating conditions of the unit, this equates to an additional 283 m3/h of molten salt. With a standard molten salt pump operating at 75% efficiency, this requires 3.5 kW additional pumping power. Because the inlet and outlet temperatures of the thermal fluid are approximately the same, there would be practically no need for a large heat rejection system, e.g. using a waste heat boiler, and hence a saving on capital and maintenance cost. The overall energy saving for R-101 would then be associated with the energy requirements for preheating the feed. At 90% efficiency for a traditional fired heater, this equates to 6878 kW, which is far greater than the associated increase in pumping power for the molten salt circuit.

In R-102 (the acrolein oxidation reactor), the main design modification is the cold injection of air (which is implemented without the introduction of inert solid material into the catalyst bed). We have compared the energy requirements of this process configuration to that of the base case (no injection of cold air nor the use of inert material), in terms of the total flowrate of molten salt and pumping requirements for the molten salt circuit. With reference to Figure 5a (base case) and Figure 6a (cold air injection), the base case is characterized by a rapid increase in process temperature due to the exothermic reaction, with practically all conversion occurring within the first quarter of the reactor. The total molten salt flowrate required for the base case is 10.13 kmol/s, or 1658 m3/h, which equates to a pumping power of 23 kW. The total molten salt flowrate required for the modified configuration (cold air injection) is 7.8 kmol/s, or 1278 m3/h, equating to a pumping power of 17.6 kW. The saving in terms of pumping power is therefore 5.4 kW, with the additional benefit of increased selectivity and overall yield of acrylic acid.
