Reflux drum temperature

Next calculates the condenser outlet, or reflux drum temperature, that would result from the above water or air temperature (as discussed in Chap. 13). 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the reflux drum. As the liquid in the reflux drum is at its bubble point, reducing the reflux drum temperature will reduce the reflux drum pressure. As the stripper tower pressure floats on the reflux drum pressure, the pressure in the tower will also decline. 

C. Vaporize 10,000 lb/h of reflux from the 150°F reflux drum temperature to the tower overhead temperature of 160°F. 

The 140°F reflux drum temperature, shown in Fig. 10.3, is the dew point of the vapors leaving the reflux drum. Almost always, we would like to minimize this particular temperature. The lower the reflux drum temperature, the smaller the amount of steam in the off-gas. If the off-gas is H2S and NH3, flowing to a sulfur recovery plant, the steam carried into the sulfur plant reduces the sulfur plant s capacity and efficiency. 

On the other hand, a low reflux drum temperature increases the solubility of H2S and NH3 in the reflux water. As the concentration of H2S and NH3 in the reflux increases, the stripper has to work harder, to keep these components out of the stripped water. 

Controlling reflux drum temperature by throttling cooling water to condenser caused boiling of cooling water when control valve cloeed. This resulted in atmospheric product release. 

The specification of the feed pressure takes a little thought. We will discuss the selection of column pressure in more detail later in this chapter. We know that the distillate product is propane. We will want to use cooling water in the condenser because it is an inexpensive heat sink compared with refrigeration. Cooling water is typically available at about 305 K. A reasonable temperature difference for heat transfer in the condenser is 20 K. Therefore, reflux drum temperature will be about 325 K. The vapor pressure of propane at 325 K is about 14 atm (206psia). Therefore, the column will have a pressure at the feed tray of something a little higher than 14 atm. 

The other important pieces of information in the window are the condenser heat removal (—22.29 MW) and the reflux drum temperature (317.06 K) at 14 atm pressure, which we specified. If you recall, we guessed that a pressure of 14 atm would give us a reflux drum temperature of about 325 K, so cooling water could be used in the condenser. To attain the desired 325 K, the pressure should be increased a little. If we rerun the simulation with a pressure of 16.8 atm, the reflux drum temperature is 325.06 K. 

Many separations are favored by lower temperatures, so conventional distillation wisdom recommends operating at the lowest pressure permitted by the use of cooling water in the condenser. Therefore, many columns are designed for 120 °F reflux-drum temperatures. If the components going overhead in the distillate are fairly high boiling, the column could operate under vacuum conditions. 

Energy would be saved and a lower-temperature less-expensive heat soiuce could be used as pressure is lowered. However, there are competing effects that must be considered. Vapor density decreases as pressure is lowered, so the diameter of the column increases, which increases capital cost. In addition, the lower pressure means lower reflux-drum temperature, which decreases the heat-transfer differential temperamre-driving forces in the condenser. This results in more heat-transfer area being required, which increases capital cost. Therefore, an economic analysis is required to find the best balance between these effects. 

A reasonable differential temperature-driving force is about 20 K. If the AT is too small, the heat-transfer area of the condenser/reboiler heat exchanger becomes quite large. The pressure in the second column is adjusted to give a reflux drum temperature of 367 - - 20 = 387 K. The pressure in C2 is 5 atm. The base temperature in C2 at this pressure is 428 K, which will determine the pressure of the steam used in this reboiler. 

A distillation column that is designed to produce both a liquid distillate stream and a vapor distillate stream from the reflux drum has an additional design degree of freedom. This is usually specified to be the reflux-drum temperature. Under these conditions, the split between the flow rates of the vapor and liquid distillates is fixed. The condenser heat duty is also fixed. Specifying a reasonable design minimum temperature differential temperature (at the either the cold or the hot end of the condenser), and a reasonable overall heat-transfer coefficient fixes the heat-transfer area. This also fixes the required flow rate of... 

Figure 8.19 shows the flowsheet. The column has 36 stages (Stage 1 is the reflux drum and Stage 36 is the base). Feed is introduced on Stage 17. The reflux-drum pressure is set at 25 psia and the reflux-drum temperature is 120 F. A tray pressure drop of 0.1 psi per stage is assumed. NRTL physical properties are used in the Aspen simulations. 

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... 

In the first case (constant QC), the condenser duty was held constant. In the second case (constant TC), the reflux-drum temperature was controlled by manipulating condenser heat duty. In the third case (constant CW), the flow rate of cooling water was held constant (using the LMTD model). Disturbances in the flow rate and composition of the feed to the column were made. 

In Figure 8.22, feed flow rate is increased to 10%. If condenser duty is fixed (fixed QC), no additional heat transfer occurs in the condenser, so there are large increases in the flow rate of the vapor distillate (DV) and in the reflux-drum temperature. These responses are muealistic because the increase in the flow rate of the overhead vapor into the condenser should change the condenser heat-transfer rate. 

Figure 8.23 gives results for a 10% decrease in feed flow rate. Results are the opposite of those shown for an increase. Unrealistic responses are shown for the fixed QC and the fixed TC cases. Notice that the fixed QC case requires a reflux-drum temperature (TC shown in the bottom right graph in Figure 8.23) that is lower than the temperature of the available cooling water. 

Decreasing the DME concentration in the feed has the opposite effects, as shown in Figure 8.25. The fixed QC model predicts less change in the flow rate of the vapor distillate and a high reflux-drum temperature. The most realistic predictions are those given by the fixed CW model. 

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. 

The specific numerical case used is a ternary mixture of dimethyl ether (DME), methanol (MeOH), and water. The feed composition is 5 mol% DME, 50 mol% MeOH, and 45 mol% water. The feed flow rate is lOOkmol/h, and the feed is fed on Stage 32 of a 52-stage column. The liquid sidestream is withdrawn from Stage 12. The column pressure is set at 11 atm so that cooling water can be used in the condenser (reflux-drum temperature is 323 K with a distillate composition of 98 mol% DME and 2 mol% MeOH). The NRTL physical property package is used. 

The second column has 28 stages and is fed on stage 14. The reflux drum operates at 0.13 atm, giving a reflux-drum temperature of 322 K. Base pressure is 0.31 atm, which gives a base temperature of 378 K. The reflux ratio is 1.53, and the reboiler heat input is 24.53 MW. The column diameter is fairly large (8.22 m) because of the vacuum operation. 

Stripper. Absorber bottoms at 322 K is preheated to 380 K in a heat exchanger using the hot stripper bottoms at 400 K and fed to the top of a stripping column with 10 stages and operating at 2 atm in the column and 1.5 atm in the reflux drum. Reboiler heat input is 54.12 MW to maintain a reflux-drum temperature of 363 K, as suggested by Desideri and Paolucci as a balance between stripper reboiler energy and water losses in the vapor from the reflux dmm. 

Reflux-drum temperature in the stripper is controlled by manipulating stripper reboiler duty (temperature controller TCS in Fig. 14.10). A 1 min deadtime is inserted in the loop, and relay-feedback testing and Tyreus-Luyben tuning give tc = 2.65 and Tj= 13.2 min. 

Notice that the reflux-drum temperature is disturbed much less for these composition disturbances because the feed flow rates to the stripper change much less rapidly. 

Regardless, I tried culling ihe steam back to 5,000 Ib/hr to see how much the vacuum distillate rate would increase. Surprisingly, the flow of vacuum distillate remained constant. The only notable result on the crude unit was a drop in flow of wet gas from the reflux drum. The 3,000 Ib/hr cut in the bottom s stripping steam had unloaded the overhead condenser. This caused the reflux drum temperature to drop and hence reduced the flow of wet gas to the off-gas compressor. 

Reducing the water vapor content of acid gas to a minimum also increased Claus capacity. During periods when one of the two sulfur trains was out of service, the amine regenerator reflux drum temperature was reduced from 135°F to 110 F. This was achieved by spraying treated water on the exterior of the amine regenerators overhead fin fan tube bundles. This reduced the water content of the acid gas from 10% to 5% and thus increased sulfur recovery capacity by 2%. 

On the same tower, a hot-vapor bypass around the condenser starts leaking. This puts a small amount of vapor into the reflux drum inlet. On mixing with the condenser outlet, the vapor condenses and increases the reflux drum temperature 3°F. For the contents of the reflux drum to remain at its bubble point, the condenser outlet temperature must drop an additional 3°F. Now, a total of 7.5°F subcooling is required. This is a lot of subcooling. 

Find the tower top temperature, the reflux drum temperature, and the tower pressure. 

The reflux drum temperature in the distillation column is fixed at 316 K (to back-... 

The flowsheet in Figure 5.3 shows independent reboilers and condensers on each column. However, the reflux-drum temperature of the high-pressure column (136°C at 7.9 bar) is higher than the base temperature of the low-pressure column (106°C at the pressure in the base of this column 1.1 bar), so heat integration could be attractive in terms of energy consumption. 

At this point, the heat duty in the condenser/reboiler is known. All the temperatures throughout both columns are also known. Therefore, the heat-transfer area of this heat exchanger can be determined by using the difference between the reflux drum temperature... 

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