Feed Preheat Exchangers

Added a feed preheat exchanger so that feed was not subcooled. This unloaded the critical bottom tower section. 

Varying the enthalpy, or heat content, of the feed is an additional independent variable that an operator, or process design engineer, can use to optimize fractionation efficiency. An additional benefit of feed preheat is that a lower-level temperature heat source can be used. If valuable 100-psig steam is required for the reboiler, then low-value 20-psig steam might be adequate for the feed preheat exchanger. 

For the crude preheat train in the refinery plant, three beneficial modifications are identified. Modifications 1 and 2 add a new shell to each of the feed preheat exchangers serviced by the diesel pump-around (PA). Modification 3 adds surface area to the diesel product run down heat exchanger. Before implementing these changes, it is crucial to make sure that the existing pump can handle the increased diesel PA flow and the increase can be tolerated in product cut points. These modifications can save 47 million Btu/h of fuel in the crude heaters, equating to cost savings of 2.3 million per year. With only a 2.2 million installed cost, this project provides a fast payback. 

From the vapor-liquid equilibria, the bubble point of the residue is 99 C. Heat capacity of the residue is 4179, of the feed 3852 J/kg K. Enthalpy balance of the feed preheat exchanger ... 

This method is a modification of the Erdco jet-fuel testing procedure (ASTM D-1660). In this unit, the test fuel is pumped at a controlled rate over a heated surface 204 °C (400 °F), whieh is designed to simulate feed preheat exchanger conditions. Decomposition of materials in the process stream on the hot surface causes deposition of polymers and coke, some of which adhere to the surface. 

NO analyzers at the preheater inlet and catalyst vessel outlet monitor NO concentrations and control the ammonia feed rate. The effluent gives up much of its heat to the incoming gas in the feed/effluent exchanger. The vent gas is discharged at about 350° F. 

Identify cases where feed or intermediate heat exchangers would be helpful, when only one of the operating lines is too far from the equilibrium curve. A large gap for the bottom section could indicate potential for feed preheat or an interreboiler. A large gap for the top section could portend precooler or intercondenser need. 

Distilling a crude oil starts by preheating the feed by exchange with the hot product streams. The feed is further heated to about 320°C as it passes through the heater pipe (pipe still heater). 

This chapter has two alternative structures for feed preheating. Both use a feed effluent heat exchanger, but one also uses a furnace. Steady-state economics favor use of only a heat exchanger. Dynamic controllability favors the use of both a heat exchanger and a furnace. 

The crossover temperature must satisfy an overall heat balance around the furnace system, including transfer line heat exchangers and feed preheaters, and must not exceed certain limits to avoid incipient cracking. 

Figure 2. Simplified schematic drawing of the extraction plant (1 regenerative pump, 2 fluid cyclone, 3 storage tank, 4 gear pump, 5 circulation gas condensator, 6 diaphragm pump, 7 heat recovery, 8 preheater, 9 heat exchanger, 10 cyclone separator, 11 feed pump, 12 feed preheater, 13 thermostatic chamber)...

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