Cold-feed

However, factors such as this should not he allowed to dictate design options at the early stages of flowsheet design because preheating the cold feed hy heat integration with the rest of the process might be possible. 

The gas leaving the heat recovery equipment contains soot and ash some ash is deposited in the bottom of the reactor for removal during periodic inspection shutdowns. The gas passes to a quench vessel containing multiple water-sprays which scmb most of the soot from the gas. Additional heat recovery can be accompHshed downstream of the quench vessel by heat exchange of the gas with cold feed water. Product gas contains less than 5 ppm soot. 

Figure 6-4 shows the cold feed distillation tower of Figure 6-3. The inlet stream enters the top of the tower. It is heated by the hot gases bubbling up through it as it falls from tray to tray through the downcomers, A flash occurs on each tray so that the liquid is in near-equilibrium with the gas above it at the tower pressure and the temperature of that particular tray.

Figure 6-5 shows a stabilizer with reflux. The well fluid is heated with the bottoms product and injected into the tower, below the top, where the temperature in the tower is equal to the temperature of the feed. This minimizes the amount of flashing. In the tower, the action is the same as in a cold-feed stabilizer or any other distillation tower. As the liquid falls... 

At the lop of the tower any intermediate components going out with the gas are condensed, separated, pumped back to the tower, and sprayed down on the top tray. This liquid is called reflux. and the two-phase separator that separates it from the gas is called a reflux tank" or reflux drum. The reflux performs the same function as the cold feed in a cold-feed stabilizer. Cold liquids strip out the intermediate components from the gas as the gas rises. 

A condensate stabilizer with reflux will recover more intermediate components from the gas than a cold-feed stabilizer. However, it requires more equipment to purchase, install, and operate. This additional cost must be justified by the net benefit of the incremental liquid recovery, less the cost of natural gas shrinkage and loss of heating value, over that obtained from a cold-feed stabilizer. 

It can be seen from the previous description that the design of both a cold-feed stabilizer and a stabilizer with reflux is a rather complex and involved procedure. Distillation computer simulations are available that can be used to optimize the design of any stabilizer if the properties of the feed stream and desired vapor pressure of the bottoms product are known. Cases should be run of both a cold-feed stabilizer and one with reflux before a selection is made. Because of the large number of calculations required, it is not advisable to use hand calculation techniques to design a distillation process. There is too much opportunity for computational eiToi. 

A gas-processing plant, as described in Chapter 9, is designed to recover ethane, propane, butane, and other natural gas liquids from the gas stream. A condensate stabilizer also recovers some portion of these liquids. The colder the temperature of the gas leaving the overhead condenser in a reflux stabilizer, or the colder the feed stream in a cold-feed stabilizer, and the higher the pressure in the tower, the greater the recovery of these components as liquids. Indeed, any stabilization process that leads to recovery of more molecules in the final liquid product is removing those molecules from the gas stream. In this sense, a stabilizer may be considered as a simple form of a gas-processing plant. 

It should be clear from the description of LTX units in Chapter 5 that the lower pressure separator in an LTX unit is a simple form of cold-feed condensate stabilizer. In the cold, upper portion of the separator some of the intermediate hydrocarbon components condense. In the hot, lower portion some of the lighter components flash. 

The ROD is similar to a cold feed stabilizing tower for the rich oil. Heat is added at the bottom to drive off almost all the methane (and most likely ethane) from the bottoms product by exchanging heat with the hot lean oil coming from the still. A reflux is provided by a small stream of cold lean oil injected at the top of the ROD. Gas off the tower overhead is used as plant fuel and/or is compressed. The amount of intermediate components flashed with this gas can be controlled by adjusting the cold loan oil retlux rate. 

The de-methanizer is analogous to a cold feed condensate stabilizer As the liquid falls and is heated, the methane is boiled off and the liquid becomes leaner and leaner in methane. Heat is added to the bottom of the tower using the hot discharge residue gas from the compressors to assure that the bottom liquids have an acceptable RVP or methane content. 

Understand the critical properties of the mixed compound that affect processability. It should be understood that the properties and acceptable operating values of those properties, which control the compound processability, are dependent on the processes under consideration, e.g., hot or cold feed extrusion, compression or injection molding. 

In the two typical shaft seal manufacturing processes shown in Fig. 2, the extrusion process is only required if the compression molding process is being used. There are basically two types of extruders in general use, the cold feed and the hot feed. In both cases their role is to produce the preforms for use with compression molding. The critical issues with the preform are the shape, the dimensions, and the weight. 

Cold feed extruders use compounds in a strip form. The strip is generally made during the take-off process in mixing a series of cutters are used on the blending mill... 

There are two types of cold feed extruders, the standard screw design type (Fig. 15) and one with a screw feed using a ram action to feed the extruder. In both... 

There are two types of hot feed extruders, one similar to the screw type cold feed extruder, except that the strip feeding the extruder is taken directly off the two-roll mill. The mill is used to further blend and heat the compound. It is then taken off the mill in strip form and fed directly into the extruder. The second type of hot feed extruder is based on the action of a ram being used to feed the compound into the extruder die (Fig. 16). The compound is placed on a two-roll mill to further blend and heat rubber. It is then taken off the mill in pig form to fill the extruder chamber. 

In both cases, the preforms are produced using the rotating cutter system as described for the cold feed extruders. The preforms are also cooled the same way. 

With watertube boilers economizers are normally used, therefore the situation of cold feed to the boiler would not apply. However, the feed to the economizer should be treated similarly to that to a boiler to reduce thermal shock. 

Boiler or water-heating (kW) Cistern size (1) Ball-valve size (mm) Cold-feed size (mm) Open-vent size (mm) Overflow size (mm)... 

Use temperature-control techniques that inherently scale as S e.g., cold feed to a CSTR, or autorefrigeration. 

Use Scalable Heat Transfer. The feed flow rate scales as S and a cold feed stream removes heat from the reaction in direct proportion to the flow rate. If the energy needed to heat the feed from to Tout can absorb the reaction exotherm, the heat balance for the reactor can be scaled indefinitely. Cooling costs may be an issue, but there are large-volume industrial processes that have Tin —40°C and Tout 200°C. Obviously, cold feed to a PFR will not work since the reaction will not start at low temperatures. Injection of cold reactants at intermediate points along the reactor is a possibility. In the limiting case of many injections, this will degrade reactor performance toward that of a CSTR. See Section 3.3 on transpired-wall reactors. 

The equipment requirements that we have determined are well within the realm of technical feasibility and practicality. The heat transfer requirements are easily attained in equipment of this size. The fact that some of the heat transfer requirements are positive and others negative indicates that one should probably consider the possibility of at least partial heat exchange between incoming cold feed and the effluent from the second or third reactors. The heat transfer calculations show that the sensible heat necessary to raise the cold feed to a temperature where the reaction rate is appreciable represents a substantial fraction of the energy released by reaction. These calculations also indicate that it would be advisable to investigate... 

In a semibatch reactor, a cold feed may be heated by mixing with the reactor contents. This technique is discussed in Illustration 10.7 later in this section. 

ILLUSTRATION 10.7 QUASIAUTOTHERMAL OPERATION OF A SEMIBATCH REACTOR USING ADDITION OF COLD FEED... 

If one were to operate this semibatch reactor under a filling schedule, which for the first 11 hr is identical to that considered previously, and then proceed to feed A at the maximum rate of 400 lb/hr for an additional 2.875 hr, the same total amount of A would have been introduced to the reactor. However, in this case, the heat transfer requirements would change drastically. There would be a strong exotherm beginning at the moment the cold feed is stopped. The results for this case are presented in Table 10.1.3. 

---