Heat boilers, reboilers

It is probably best to consume steam generated in small waste-heat boilers, locally, in steam reboilers and preheaters. Turbine drivers are... 

In these types of heat exchangers the two fluids are separated by the tube walls. As one fluid flows through the shell— the region outside the tubes — the other fluid flows through the tubes. Heat transfer occurs so as to cool, and perhaps even condense, the hotter fluid, and heat, and even vaporize, the cooler fluid. Heat exchangers are called by various names depending on their function such as chillers, condensers, coolers, heaters, reboilers, steam generators, vaporizes, waste heat boilers, and so on. 

The control performance of the heat-integrated reactor-column system shown in Fig.. 5.9 deteriorates as the auxiliary rehoiler provides less and less heat to the column. The reason is that uncontrolled variations in the steam pressure of the waste heat boiler affect the heat supplied to the column. When these variations are of the same order of magnitude as the total heat supplied by the auxiliary reboiler, the latter cannot compensate properly for the variations. Part of the prob-... 

One process modification is shown in Figure 8. In this modification, a heat recovery system uses the heat of the recovery column bottoms in the main column reboiler and in a bottoms preheater. If additional heat is needed in the main column reboiler, auxiliary heating is provided as shown by the (dotted) steam coil in Figure 8. However, a waste heat boiler is sometimes needed (shown dotted) to remove the excess heat in the recovery column bottoms. 

Fig. 5.14. Scot process combined with Claus unit, (a) Combustor (b) reactor (c) waste heat boiler (d) quench cooler, (e) blower, (/) absorber, (g) regenerator h) cooler, (0 heat exchanger (k) reboiler (0 condenser (m) separator...

The high-temperature process heat is normally recovered through steam generation however, to make the overall process efficient, lower temperature process heat must also be used. Therefore, processes normally pick up the heat after the high-pressure boiler and use it to reboil the distillation columns in the purification section, raise low-pressure steam, and heat boiler feedwater or... 

Reboiler Film-transfer Rates. Heat is transferred by a boiling film in such equipment as reboilers, refrigerator evaporators, and waste-heat boilers. As the rate of heat transfer is increased, vapor tends to blanket the surface and the rate is sharply retarded. This usually occurs at a temperature difference of 120 to 150 F, and the mimmum heat density or heat that can be transferred per square foot-hour is about as indicated in Table 17-5. 

Heat transfer to boiling liquids, e.g. in boilers, vaporizers, reboilers... 

The total condensation of a vapor to a liquid is best illustrated by the condensation of steam to water. Figure 13.1 is a rather accurate reproduction of the radiator that heated my apartment in Brooklyn. Steam flowed from the boiler in the basement. The steam condensed inside the radiator, and flowed back into the boiler, through the condensate drain line. This is a form of thermosyphon circulation. The driving force for the circulation is the differential density between the water in the condensate drain line and the steam supply line to the radiator (see Chap. 5, discussion of thermosyphon reboilers). 

The hardness deposits coat the inside of the boiler s tubes, interfere with heat transfer, and overheat the tubes. The carbon dioxide, which is also generated from the dissolved solids, creates more serious corrosion problems in downstream heat exchangers. When the steam condenses, the carbon dioxide may remain trapped in the reboiler or preheater as a noncondensable gas. Actually, there is no such thing as a noncondensable gas. Even C02 is somewhat soluble in water. As the C02 dissolves in the condensed steam, it forms carbonic acid, a relatively weak acid (pH typically between 5 and 6). Strong acids will have pH values of 1 to 2. Pure water has a pH of seven. Carbonic acid is particularly corrosive to carbon steel heat-exchanger tubes. 

The reformed gas leaves the furnace at a high temperature where high grade heat is recovered successively to a reformed gas boiler, steam superheater process feedstock heater and boiler, feedwater heater. The reformed gas then passes to the distillation area where low grade heat is efficiently recovered via column reboilers and a demineralized water heater. 

The flue gas heat content preheats reformer feed. Likewise, the heat content of the process gas is used to produce superheated high-pressure steam (5), boiler feedwater preheating, preheating process condensate going to the saturator and reboiling in the distillation section (6). 

Description Gas feedstock is compressed (if required), desulfurized (1) and sent to the optional saturator (2) where some process steam is generated. The saturator is used where maximum water recovery is important. Further process steam is added, and the mixture is preheated and sent to the pre-reformer (3), using the Catalytic-Rich-Gas process. Steam raised in the methanol converter is added, along with available C02, and the partially reformed mixture is preheated and sent to the reformer (4). High-grade heat in the reformed gas is recovered as high-pressure steam (5), boiler feedwater preheat, and for reboil heat in the distillation system (6). The high-pressure steam is used to drive the main compressors in the plant. 

In Design 4, the feed was sent to the low-pressure column, which produced a pure low-boiler distillate but a mixed underflow (again, a sloppy separation decreasing the temperature difference across the low-pressure column). The mixed bottoms was then completely separated in the high-pressure column. The condenser of the high-pressure column was the reboiler of the low-pressure column (heat integration in the opposite direction as flow). 

The heating medium in a reboiler will often be condensing steam, in which case the reboiler may be viewed as a pair of two-phase systems a singlecomponent condenser exchanging heat with a multicomponent boiler. See Figure 12.5. 

F (10.5 psia) to the tower top temperature of 111°F (1.3 psia). To transfer this heat through the reboilers, steam at 233°F (22 psia) was required. Because this heat is needed only at relatively low temperatures, it is very inefficient to obtain it by burning fuel under boilers, without making use of the heat at higher temperatures first. Two possible ways of providing low-temperature heat more efficiently are these ... 

TYPICAL DISTILLATION EQUIPMENT. A plant for continuous distillation is shown in Fig. 17.1. Reboiler A is fed continuously with the liquid mixture to be distilled. The liquid is converted partially into vapor by heat transferred from the heating surface B. The vapor formed in the reboiler richer in low boiler than the unvaporized liquid, but unless the two components differ greatly in volatility,... 

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