Feed preheat

Merging more than one operation into a single piece of equipment (e.g., feed preheating and reaction in the same vessel), providing these operations are not limiting the cycle time. 

Fig. 38. Caustic purification system a, 50% caustic feed tank b, 50% caustic feed pumps c, caustic feed preheater d, amonia feed pumps e, ammonia feed preheater f, extractor g, trim heater h, ammonia subcooler i, stripper condenser j, anhydrous ammonia storage tank k, primary flash tank 1, evaporator reboiler m, evaporator n, caustic product transfer pumps o, purified caustic product cooler p, purified caustic storage tank q, ammonia stripper r, purified caustic transfer pumps t, overheads condenser u, evaporator v, evaporator vacuum pump w, aqueous storage ammonia tank x, ammonia scmbber y, scmbber condenser 2, ammonia recirculating pump aa, ammonia recycle pump. CW stands for chilled water.

Heat Recovery and Feed Preheating. The objective is to bring the reactants to and from reaction temperature at the least utihty cost, and to recover maximum waste heat at maximum temperature. The impact of feed preheating merits a more careful look. In an exothermic reaction, preheated feed permits the reactor to act as a heat pump, ie, to buy low and sell high. The most common example is combustion-air preheating for a furnace. 

Fractionator-feed preheaters partially vaporize charge stock from an upstream unfired preheater en route to a fractionating column. A typical refinery application a crude feed to an atmospheric column enters the fired heater as a liquid at 505 K (450°F) and leaves at 644 K (700°F), having become 60 percent vaporized. 

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. 

Excessive feed preheat. Flooding of rectification section packed bed. Operational error. 

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

Liebert [218] studied feed preheat conditions and the effects on the energy requirements of a column. This topic is essential to the efficient design of a distillation system. 

Example 10-12. Steam Heated Feed Preheater— Steam in Shell... 

Most FCC units use fired heaters for FCC feed final preheat. The feed preheater provides control over the catalyst-to-oil ratio, a key-variable in the process. In units where the air blower is constrained. 

Antimony-based passivation was introduced by Phillips Petroleum in 1976 to passivate nickel compounds in the FCC feed. Antimony is injected into the fresh feed, usually with the help of a carrier such as light cycle oil. If there are feed preheaters in the unit, antimony should be injected downstream of the preheater to avoid thermal decomposition of the antimony solution in the heater tubes. 

Increasing catalyst-to-oil ratio by decreasing the feed preheat temperature... 

Compare the cyclone loading with the design. If the vapor velocity into the reactor cyclones is low, consider adding supplemental steam to the riser. If the mass flow rate is high, consider increasing the feed preheat temperature to reduce catalyst circulation. 

Decrease in the feed preheat temperature and subsequent increase in the catalyst-to-oil ratio... 

Figure 1-5 shows a typical feed preheat configuration. A hydraulic limitation usually manifests itself when increasing fresh feed rate and/ or installing high efficiency feed injection nozzles. 

The hydraulic pinch points in the feed preheat system are identified with a single-gauge pressure survey. The bottlenecks are often related to ... 

There is a middle steady state, but it is metastable. The reaction will tend toward either the upper or lower steady states, and a control system is needed to maintain operation around the metastable point. For the styrene polymerization, a common industrial practice is to operate at the metastable point, with temperature control through autorefrigeration (cooling by boiling). A combination of feed preheating and jacket heating ensures that the uncontrolled reaction would tend toward the upper, runaway condition. However,... 

Feed temperature is not normally controlled, unless a feed preheater is used. 

Liebert T.C (1993) Distillation Feed Preheat - Is It Energy Efficient Hydrocarbon Process, Oct 37. 

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