Reboiler area requirement

Raises reboiler temperature, thereby requiring an unavailable or a more expensive heating medium. For the same heating medium, it increases the reboiler area requirement. 

Example 15.4 A reboiler is required to supply 0.1 krnol-s 1 of vapor to a distillation column. The column bottom product is almost pure butane. The column operates with a pressure at the bottom of the column of 19.25 bar. At this pressure, the butane vaporizes at a temperature of 112°C. The vaporization can be assumed to be essentially isothermal and is to be carried out using steam with a condensing temperature of 140°C. The heat of vaporization for butane is 233,000 Jkg, its critical pressure 38 bar, critical temperature 425.2 K and molar mass 58 kg krnol Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The thermal conductivity of the tube wall can be taken to be 45 W-m 1-K 1. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 W m 2-K 1. Estimate the heat transfer area for... 

Outside tube area required for heating coils in reboiler (coils to contain saturated steam at 250 psia, and average heat of vaporization of hydrocarbons in reboiler may be taken as 5000 cal/g mol)... 

Running at 0.4 atm reduces the base temperature to 410 K, which permits the use of low-pressure steam (433 K at 7.78/GJ). In addition, the reboiler energy requirement drops to 0.9147 MW. However, the diameter of the column increases to 2.044 m because of the lower vapor density at the lower pressure. This increases the capital cost of the vessel. In addition, the required condenser area increases rapidly as pressure is reduced because of the smaller temperature differential driving force. This also increases capital costs. Table 4.4 gives results over a range of pressures for Column C2. Operation at 0.4 atm gives the smallest TAC. 

The diameter of the stripping column is determined on the basis of the largest cross-sectional area required to handle gas and liquid flows at any point in the column. This is normally at the bottom of the column immediately above the reboiler. The vapor load consists of the water and glycol vapors from the reboiler plus any hydrocarbons which may be desorbed from the glycol or added as stripping vapor. The liquid load consists of the glycol stream plus reflux water. 

Stressed, such as heat-affected zones near welds, in areas of high acid-gas concentration, or at a hot gas-liquid interface. Therefore, stress-relieving all equipment after manufacturing is necessary to reduce corrosion, and special metallurgy in specific areas such as the still overhead or the reboiler tubes may be required. 

The boiling film coefficient for a kettle reboiler can be estimated from the correlation for pool boiling. Equation 15.96 gives one such method due to Palen15. However, the correlation requires the heat flux to be known, and therefore the heat transfer area to be known. Hence the calculation will need to be iterative. An initial estimate of the overall heat transfer coefficient of 2000 W-m 2-K 1 gives ... 

The higher heat transfer coefficients experienced by Hickman led to the concept of placing a peripheral reboiler and core condenser on either side of a rotating packed bed (50). This concept would be useful for distillation applications that need reflux and boilup. The internal exchangers as part of the rotor would decrease the required heat transfer surface area but would involve additional design and fabrication complexity. 

There is a condition on this rule of thirds—it is to be limited to areas of latent heat and not to the tube area dedicated to sensible heat. Most reboilers of this type comply to the requirement for very little sensible heat because the large flywheel of circulating liquid is near or exactly equal to the fractionator column bottoms temperature. Therefore, only latent heat applies, and this rule of thirds also applies. 

SubcooUng heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturation temperature in the mean temperature difference. For horizontal and vertical thermosiphons, a separate calculation is required for the sensible heat transfer area, using appropriate sensible heat transfer coefficients and the liquid temperature- profile for the mean temperature difference. 

Estimate the heat transfer area, using the maximum allowable heat flux. Take as 39,700 W/m for vertical and 47,300 W/m for horizontal reboilers. Choose the tube diameters and length. Calculate the number of tubes required. Estimate the recirculation ratio, not less than 3. 

In large-diameter packed colxunns where I-beams support the packing support plate, a larger space between the reboiler return nozzle and the packing support plate may be required. The prime consideration here is allowing sufficient open area between the bottom of the beam and the high liquid level for adequate vapor distribution. Detailed discussion is in Sec. 8.2. 

Where a large surface area is required. Alternative reboiler types can generally offer more area per shell and more shells per column. 

Note that the second and third considerations make horizontal thermosiphon reboilers attractive in many superfractionators (e.g., propane-propylene splitters), where high heat transfer areas are required and where increasing column height to supply additional reboiler head is costly. 

The temperature of the cooling medium in the condenser or the temperature of the heating medium in the reboiler are set, and then Aspen calculates the required UA product (overall heat-transfer coefficient U and heat-transfer area A) from the known heat-transfer rate and temperature differential driving force. This temperature is manipulated in the dynamic simulations. No heat-exchanger dynamics are considered. 

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