Reboiler operation

When MEA is used in the presence of COS and CS2, they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper. The heat-stable salts remain in the reclaimer until the reclaimer is full. Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. 

The design of a distillation column requires a reboiler operating at 2.23 psia (vapor space above bottom liquid). The heat duty is 1,528,600 Btu/hr. The properties of the acrylonitrile mixture have been calculated to be... 

The manufacture of fatty acids from triglycerides via their reaction with alkali followed by acidification is not used commercially. However, the recovery of fatty acids from alkali refining of fats/oils (to produce acid soaps) and soap reboiling operations (the recovery of fatty acids from the acidification of soap waste streams) are practiced still in special situations, for example, in edible oils refining where soap-stock that is generated from chemical (alkali) refining requires acidulation to produce acid oils with commercial value. 

Saponification/Acidification Process. The recovery of fatty acids from soapstocks and soap reboil operations is of commercial value ... 

This has implications in two different cases. For columns with auxiliary reboilers, we should obtain better control by using heat input to the base reboiler (operating at the highest temperature) compared with heat input to the other reboilers. We should also expect better control for columns with large temperature differences between the top and bottom than for columns with small temperature differences. 

This process differs from others in several engineering details. The severe duty placed on the reboiler of the aluminum chloride recovery tower due to fouling was a problem which was solved by a vertical thermosyphon reboiler operating ivith sufficient head to provide a high rate of liquid circulation. Another problem was suitable pumps to recycle aluminum chloride slurry to the top of the reactor. Centrifugal pumps with mechanical seals using butane flush were most satisfactory. 

A distillation column separates 10,000 Ib/hr of a 40% benzene-60% chlorobenzene liquid solution which is at 70°F. The liquid product from the top of the column is 99.5% benzene, while the bottoms (stream from the reboiler) contains 1% benezene. The total condenser uses water that enters at 60°F and leaves at 140°F, while the reboiler uses saturated steam at 280°F. The reflux ratio (the ratio of the liquid overhead returned to the column to the liquid overhead product removed) is 6 to 1. Assume that both the condenser and reboiler operate at 1 atm pressure, that the temperature calculated for the condenser is 178°F and for the reboiier 268°F, and that the calculated fraction benzene in the vapor from the reboiler is 3.9 wt% (5.5 mole %). Calculate the following ... 

Vertical Thermosiphon Reboilers Vertical thermosiphon reboilers operate by natural circulation of the hquid from the still through the downcomer to the reboiler and of the two-phase mixture... 

The liquid from the expander is fed to the top of the demethanizer column as external reflux, and the valve outlet stream is fed to an intermediate tray. In this example, the column has 12 theoretical trays and a reboiler (a total of 13 theoretical stages). As shown in Figure 9.1, one stream goes to tray 1 and the other to tray 5, counting trays from the top of the column. Streams 3 and 4 are the overhead and bottoms products, respectively. The column and the reboiler operate at a constant pressure of 2900 kPa. The compositions, flow rates, and thermal conditions of the streams are given in Table 9.1. The process requirement is that the bottoms product should have a maximum of 0.001 mole fraction methane. 

If the distillation column is equipped with a partial reboiler, this reboiler is equivalent to a single theoretical contact since, in effect, the reboiler accepts a liquid feed stream and discharges liquid and vapor streams in equilibrium B and Vi in Fig. 8.10. However, not all reboilers operate this way. Some columns are equipped with total reboilers in which either all of the entering liquid from the bottom stage is vaporized and the bottoms are discharged as a vapor, or the liquid from the bottom stage is divided into a bottoms product and reboiler feed that is totally or partially vaporized. 

Increasing column mechanical design pressure This is one of the most effective techniques in cases where credit can be taken for a temperature pinch (see Sec. 9.4). When a close-boiling mixture is distilled, and the reboiler operates at a relatively small AT, a modest increase in column pressure can drive the reboiler AT to zero... 

Flow regimes. Five main flow regimes are important in reboiler operation (Fig. 15.2). In order of increased vaporization these are... 

Effect of Liquid Levei on Verticai Thermosiphon Reboiler Operation... 

Problems described for vertical thermosiphon reboilers (see previous section) are shared by their horizontal counterparts. Some may even be more potent in horizontal thermosiphon reboilers. Operating problems experienced with horizontal thermosiphon reboilers and not experienced in vertical thermosiphon reboilers are ... 

Compared to a forced-circulation reboiler, a fired heater often operates at a higher pressure drop, higher velocities, and a larger fractional vaporization. It is common to have fired reboilers operate at 30 to 50 percent vaporization. 

The condensate outlet scheme can destabilize thermosiphon reboiler operation. The condensate-side level fluctuations inherent in this scheme are just as detrimental to thermosiphon reboiler stability as process-side level fluctuations (Section 15.3). This is most troublesome in vacuum services. 

To overcome this problem, a submerged condensate pot is often installed instead of the steam trap (Fig. 17.le) as described earlier (item 5 above). An alternative remedy is replacing the steam trap by a level condensate pot (Fig. 17.1/). By varying the level control set point, the surface in the reboiler can be adjusted so that the reboiler operates at a pressure high enough to ensure condensate removal at all times without a pump. Note that the bottom of this drum is located below the bottom of the condensing side of the reboiler (189) otherwise, "dry reboiler operation at high rates will not be possible, and reboiler capacity will be reduced. 

For conceptual design it is reasonable to represent the process operation in a classical McCabe-Thiele diagram (Fig. 5,7). A relative volatility of a = 10 was assumed for the sake of better visualization. The specified distillate composition (assume = 0.95) fixes the operating point of the condenser. The reboiler oper-... 

Fig. 5.7 shows how the reboiler operating point is shifted by changing the Damkohler number. All reboiler operating lines for the different Da numbers meet in the pole point Tt eh whose coordinates are given by... 

Minimize q at points of heat use where Tp is high. If heat can be put into a distillation column by heating the feed rather than the residue, the presence of the volatile components of the feed will keep the boiling temperature low, and since the feed is injected some way up the column the pressure at which the feed boils will be less than the pressure at which the reboiler operates. 

F2. A distillation column with a total condenser and a partial reboiler operates at 1.0 atm... 

D6. We are testing a new packing for separation of benzene and toluene. The column is packed with 3.5 meters of packing and has a total condenser and a partial reboiler. Operation is at 760 mm Hg, where a varies from 2.61 for pure benzene to 2.315 for pure toluene fPerryetal.. 1963. p. 13-3). At total reflux we measure a benzene mole fraction of 0.987 in the condenser and 0.008 in the reboiler liquid. Find HETP ... 

For a common steam system with three steam headers, the balancing procedure usually starts from the HP header. HP steam is mainly generated from steam boilers as well as from convection sections of process furnaces. While HP steam is mainly used for steam turbines as process drivers for pumps and compressors, it can also be used for column reboiling operating at relatively high temperatures. 

At the high reflux/reboil operation virtually all the C4 is recovered from the distillate. As a result the bottoms yield increases as shown in Figure 12.97. 

Example 19.4 shows that the reboiler operation requires that the pressure of the boil-up stream be increased. This is not the only possible alternative. All that is necessary is that the temperature difference be 21.7°C. This could be accomplished by reducing the temperature of the steam to 163.4°C. In most chemical plants, steam is available at discrete pressures, and the steam used here is typical of medium-pressure steam. Low-pressure steam would be at too low a tenperature to work in the scaled-down column. However, the pressure and temperature of medium-pressure steam could be reduced if there were a throtding valve in the steam feed line to reduce the steam pressure. The resulting steam would be superheated, so desuperheating would also be necessary. This could be acconplished by spraying water into the superheated steam. A change in a utility stream is almost always preferred to a change in a process stream. The condenser is now analyzed in Examle 19.5. 

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