Overhead condenser heat-removal

The overhead condenser heat-removal duty is proportional to the reboiler heat duty. 

Heat removal is accomplished by transfer to the vessel jacket, use of internal cooling coils, circulation of the reaction fluid through an external cooling loop, or by use of an overhead condenser to remove heat from the monomer or diluent in the vapor phase. 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

Internal reflux is induced by means of externally cooled liquid pumparounds. A pumparound simply removes hot liquid from the tower, pumps it through a heat exchanger and then introduces this cooled liquid into the tower a few trays above. Use of pumparounds allows a better distribution of tower loadings than if all the heat were removed from the VPS using an overhead condenser. Four to six trays between sidestreams and two pumparounds are normally specified for a lube VPS. The three liquid sidestream products to be used as lube plant feed stocks are steam stripped to remove lighter boiling components which condense with tire sidestreams. 

Q. = heat load of overhead condenser (removed in condenser), Btu/hr... 

Figure 1 illustrates the process in more detail. The inert liquid is pumped upflow through the reactor at a velocity sufficient to fluidize the catalyst and to remove the reaction heat. The low Btu feed gas is passed simultaneously up the reactor where it is catalytically converted to a high concentration methane stream. The exothermic reaction heat is taken up by the liquid mainly as sensible heat and partly by vaporization (depending on the volatility of the liquid). The overhead product gases are condensed to remove the product water and to recover any vaporized liquid for recycle. The main liquid flow is circulated through a heat... 

Operating Temperature. The important temperatures for economic analysis are the overhead condenser temperatures at which heat is removed from the process and the reboiler temperatures at which heat is supplied to the process. Of similar importance is the temperature of the solvent-addition plate of the primary column since this temperature, together with that of the recovery column, establishes the temperature swing in the solvent circuit. 

For energy exchange equipment Supply sufficient excess of heat transfer area in reboilers, condensers, cooling jackets, and heat removal systems for reactors to be able to handle the anticipated upsets and dynamic changes. Sometimes extra area is needed in overhead condensers to subcool the condensate to prevent flashing in the downstream control valves. Too frequently, overzealous engineers size the optimum heat exchangers based on an economic minimum based on steady-state conditions and produce uncontrollable systems. 

For Distillation, Favor Sequences that Remove Components One-by-Oneas Overhead Product. This advice is based on the need to condense or remove heat from overhead streams and add heat to bottom streams. Adding nonkey components to the overhead stream increases the cooling load on the overhead condenser and the heating load on the reboiler for a simple column. Removing components one-by-one in Older of decreasing volatility also minimizes the vapor flow in the column supporting a direct sequence. 

Absorber and strippers may be classified as complex columns because they possess two feeds and because they possess neither an overhead condenser nor a reboiler. The sketch of the absorber in Fig. 4-1 depicts an historic application of absorbers in the natural gas industry. From a light gas stream such as natural gas that contains primarily methane plus small quantities of, say, ethane through n-pentane, the desired quantities of the components heavier than methane may be removed by contacting the natural gas stream with a heavy oil stream (say n-octane or heavier) in a countercurrent, multiple-stage column such as the one shown in Fig. 4-1. Since absorption is a heat-liberating process, the lean oil is customarily introduced at a temperature below the average temperature at which the column is expected to operate. The flow rate of the lean oil is denoted by L0, and the lean oil enters at the top of the column as implied by Fig. 4-1. The rich gas (which is sometimes called the wet gas) enters at the bottom of the... 

The boiling point-equilibrium data for the system acetone-methanol at 760 mm Hg are given in Table 18.7. A column is to be designed to separate a feed analyzing 25 mole percent acetone and 75 mole percent methanol into an overhead product containing 78 mole percent acetone and a bottom product containing 1.0 mole percent acetone. The feed enters as an equilibrium mixture of 30 percent liquid and 70 percent vapor. A reflux ratio equal to twice the minimum is to be used. An external reboiler is to be used. Bottom product is removed from the reboiler. The condensate (reflux and overhead product) leaves the condenser at 25°C, and the reflux enters the column at this temperature. The molal latent heats of both components are 7700 g cai/g mol. The Murphree plate efficiency is 70 percent. Calculate (a) the number of plates required above and below the feed (b) the heat required at the reboiler, in Btu per pound mole of overhead product (c) the heat removed in the condenser, in Btu per pound mole of overhead product. 

Bottom-product cooler Overhead-product cooler Heat to reboiier Heat removal in condenser Reflux ratio Rg LJD Rectifying section Stripping section... 

When the reaction is carried out in a low-boiling-point solvent or when one of the reactants is quite volatile, heat can be removed by allowing the solvent or reactant to vaporize. The vapors are condensed in an overhead condenser, and the liquid is returned to the reactor. The limiting factor in this design may be the allowable vapor velocity in the reactor. Too high a velocity will cause foaming or excessive entrainment of liquid, and the reaction rate per unit volume of reactor will decrease because of greater gas holdup. 

The heat-removal rate in the condenser is 6.894 x 10 Btu/h, which requires 96,390 Ib/h of 90 °F cooUng water. Notice that the temperature of the overhead vapor from the column is 171 °F, which is much higher than the reflux-drum temperature. This occurs because of the large difference between the boding points of DME (—12.7 F) and methanol (148.5 °F) and the 25 psia operating pressure. The condenser is designed for a... 

The distillate D has a methanol composition (28mol% methanol) that is near the azeotrope at 4 bar. It is fed at a rate of 1122kmoiyh to Stage 6 of a 12-stage extraction column. Water is fed on the top tray at a rate of 1050kmol/h and a temperature of 322 K, which is achieved by using a cooler (heat removal 1.24 MW). The column is a simple stripper with no reflux. The column operates at 2.5 atm so that cooling water can be used in the condenser (reflux drum temperature is 326 K). Reboiler heat input is 5.96 MW. The overhead vapor is condensed and is the C5 product stream. 

A very efficient alternative for heat removal is to use overhead condensers. This modification uses the latent heat of evaporation of the monomer to remove the heat of polymerization. Monomer is evaporated in the reactor, condensed in the overhead condenser, and the cooled liquid monomer is returned to the reactor. This design works well for propylene polymerization, but it is not a good option for ethylene because of its much lower boiling point. Overhead condensers are used in the El Paso bulk polypropylene process [72]. 

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