Superheated feed

Superheated feeds from chemical reactors or petroleum cracking units are a special case, often requiring a baffle-tray desuperheating section in the tower. 

A feed that is superheated at Tf and Pf has a negative XHp and, therefore, a negative q. A superheated feed will increase the vapor rate above the feed tray (Vr > Vj) and decrease the liquid rate below the feed tray (Lj < L ). If the feed is sufficiently superheated, it may dry up the liquid in the lower column section, making Lj approach zero. 

Inappropriate feed condition can be visualised by a large enthalpy variation near the feed. Excessively superheated feed could develop a large amount of vapour because of a flashing effect, dangerous for the hydrodynamics of internals. The same consequence may have an excessive sub-cooled feed, this time by a quenching effect. These situations can be easily... 

The above discussion pertains to subcooling and superheating effects that tend to persist. Additional considerations apply to handhng subcooled or superheated feeds during ahnormtil operation, such as startup, shutdown, or utility failure. Some of these are ... 

Account for vaporization due to superheat in column loading calculation. When attempting to reduce column loads by quenching a superheated feed, use a sufficiently subcooled liquid. 

From steam tables, the outlet temperature is 251°C, which is superheated by 67°C. Although steam for process heating is preferred at saturated conditions, it is not desirable in this case to desuperheat by boiler feedwater injection to bring to saturated conditions. If saturated steam is fed to the main, then the heat losses from the main will cause a large amount of condensation in the main, which is undesirable. Hence it is better to feed steam to the main with some superheat to avoid condensation in the main. 

Subcooled-hquid feed 9 > 1 Saturated-liquid feed 9 = 1 Partially flashed feed 1 > 9 > 0 Saturated-vapor feed 9 = 6 Superheated-vapor feed 9 < 0 The 9 value for a particular feed can be estimated from... 

FIG. 13-32 AU five cases of q lines (1) superheated-vapor feed, (2) saturated-vapor feed, (3) partially vaporized feed, (4) saturated-liquid feed, and (5) siih-cooled-liqiiid feed. Slope of q line = q/(q — 1). 

FIG. 13-45 Effect of feed on stream rates just above feed stage M + 1. (a) Subcooled or bubble-point feed, (h) Superheated or dew-point feed, (c) Partially flashed feed. 

Superheated steam is used to bring the feed to reaction temperature. Reactor effluent is quenched, distilled to remove unreacted feed plus benzene and toluene made during the reaction, and the crude styrene finished by vacuum distillation. Inhibitors are added during the distillation steps to prevent polymerization of the styrene monomer. 

The condition of the feed as it enters the column has an effect on the number of trays, reflux requirements and heat duties for a given separation. Figure 8-15 illustrates the possible situations, i.e., sub-cooled liquid feed, feed at the boiling point of the column feed tray, part vapor and part liquid, all vapor but not superheated, and superheated vapor. The thermal condition is designated as q, and... 

This represents feed as all vapor (not superheated). Slope of q line ... 

This represents feed as a superheated vapor, and there is a decrease in liquid overflow from feed plate. 

The feed is first preheated by exchange with the products, charged to the vacuum unit heater, and then passed to the vacuum tower in an atmosphere of superheated steam. Using superheated steam is important it... 

Hot gas. A branch pipe from the compressor discharge feeds superheated gas to the coil. The compressor must still be working on another evaporator to make hot gas available. Heat storage capsules can be built into the circuit to provide a limited reserve of heat for a small installation. 

A two-step evaporation process saves energy (Fig. 10). The feed is a concentrated alkanesulfonate solution with 35-47 wt % solid and 8-15 wt % neutral oil. In the first step the water and part of the neutral oil are vaporized at 0.4-0.5 MPa. In the second step the main part of the neutral oil is swept out by superheated steam at standard or reduced pressure. A spiral coil vaporizer or such a vaporizer in combination with a rotary film evaporator serves as a suitable apparatus. The pressurized vapor of the first step can be used for producing low-pressure steam. 

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