Once-through operation

In the once-through operation low sulfur fuels are produced and the fractionator bottoms are not recycled. In the total conversion mode the fractionator bottoms are recycled to the inlet of the reactor to obtain more middle distillates. 

MK2 peristaltic tube pumps were used to feed the CCS. The CCS was coimected to a Julabo MV basis temperature controlled water bath (accuracy 0.01°C). The water bath was set at 70°C. The CCS was operated in a once-through operation. The CCS was fed with 12.6 ml/min of pure sunflower oil. Subsequently, the centrifuge was started (40 Hz, which corresponds to 2400 rpm). As soon as the oil started flowing out of the heavy phase outlet, the reaction was started by feeding the methanolic NaOMe solution (containing 1% w/w NaOMe with regard to snnflower oil) at 3.15 mL/min. 

Cook(63) has presented data on the performance of a partial-recycle units based on a direct-fired burner using natural gas. These are compared with the once-through operation in Table 16.8. 

The basic elements of each process are similar (Figure 9-9) and consist of a once-through operation of the feedstock coming into contact with hydrogen and the catalyst in a downflow reactor which is designed to maintain activity... 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

Cracking conditions Conversion, % of fresh feed Once-through operation 40-50 45-60 45-60 45-60 45-60... 

Evaporators may be operated either as once-through units, or the liquid may be recirculated through the heating elements. In once-through operation, all the evaporation is accomplished in a single pass. The ratio of... 

The process permits recovery of oil from tar sands with high energy efficiency in a once-through operation with respect to sand. 

When recirculation is allowable without damaging the liquid, distribution of liquid to the tubes is facilitated by a moderate recycling of liquid to the tops of the tubes. This provides a larger volume of flow through the tubes than is possible in once-through operation. 

Temperature, F Max yield in once-through operation Time, sec, at max yield, approx. Phase Stock Refer- ence... 

Viscosity breaking, usually a once-through operation on straight-run residuum, although admission, of black stock to the cracking coil is practical in many plants. 

Reforming of naphtha, usually as a once-through operation but also as a recycling operation—and sometimes employing a catalyst. 

In the isomerization of naphtha" using aluminum chloride, the Research octane number in a once-through operation is increased from 65 to 80. Further increase in octane number occurs with recycling (91 at 2 1 recycle ratio), but the economy of recycling is doubtful because of difficulties with fractionation. When processing mixed hexanes" operating conditions are... 

After this firing is initiated, and the temperature of the steam leaving the primary superheater reaches about 300°C, the steam from the flash tank is led to the condenser. When the flash tank pressure becomes high, the saturated steam from the flash tank can be used to roll the turbine. After this, the turbine is ramped to about 7% load. At about 20% unit load, the startup bypass operation is shifted to once-through operation. When the temperatures become stabilized, the feedwater flow and the turbine load are increased slowly to 100% of full load value. 

The condensate demineralizer and condensate filter demineralizer clean the condensate system and feedwater system (called the main coolant system in safety design). The reactor is also cleaned with the reactor clean-up system. It should be mentioned that the reactor cleanup system is used for only the startup, and the coolant is purified in the condensate system during the once-through operation as done in FPPs. Also, as in BWRs, the condenser is vacuumized in order to prepare it for start up. 

Flow pattern in once-through operation closely resembles plug flow so conversion of batch to continuous processes is easy, and virtually any desired time/ temperature pattern is possible. 

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