Reboilers, cost

Kettle-type-reboiler costs are 15 to 25 percent greater than for equivalent internal-floatiug-head or U-tube exchangers. The higher extra is applicable with relatively large kettle-to-port-diameter ratios... 

Solution. The variable costs involved are cost of column, cost of reboiler, cost of condenser, cost of steam, and cost of cooling water. Each of these costs is a function of the reflux ratio, and the optimum reflux ratio occurs at the point where the sum of the annual variable costs is a minimum. The total variable cost will be determined at various reflux ratios, and the optimum reflux ratio will be found by the graphical method. 

If we use average values for U and for the water tenperature in the condenser, we can estimate the condenser area. With the steam pressure known, the steam tenperature can be found from the steam tables then, with an average U, the area of the reboiler can be found. Condenser and reboiler costs can then be determined. Area is directly proportional to Q, and thus is directly proportional to F. is also directly proportional to (1 + L/D) and Qr depends linearly on (1 + L/D). 

Another variable that needs to be set for distillation is refiux ratio. For a stand-alone distillation column, there is a capital-energy tradeoff, as illustrated in Fig. 3.7. As the refiux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required (see Fig. 3.7). If the capital... 

Example A.4.1 The purchased cost of a distillation column is 1 million, and the reboiler and condenser are 100,000. Calculate the annual cost of installed capital if the capital is to be annualized over a 5-year period at a fixed rate of interest of 5 percent. 

Reboiler. The case shown in Figure 8 is common for reboilers and condensers on distillation towers. Typically, this AThas a greater impact on excess energy use in distillation than does reflux beyond the minimum. The capital cost of the reboiler and condenser is often equivalent to the cost of the column they serve. 

The concept of an optimum reboiler or condenser AT relates to the fact that the value of energy changes with temperature. As the gap between supply and rejection widens, the real work in a distillation increases. The optimum AT is found by balancing this work penalty against the capital cost of bigger heat exchangers. 

The reconcentration of dilute (50—60%) sulfuric acid is one of the more costly operations in the manufacture of ethanol by this process. An acid reboiler, followed by a two-stage vacuum evaporation system, raises acid concentration to about 90%. The 90% acid is then brought to 96—98% strength by fortification with 103% oleum (fuming sulfuric acid). 

The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty). 

The pump gas can be used to fuel the reboiler. The amount of pump gas is normally close to balancing the reboiler fuel gas requirements. The pump gas can also be routed to the facility fuel gas system or to a low-pressure system for compression and sales. If it is not recovered in one of these ways and is just vented locally, the cost of using this type of pump can be very high. 

Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosyphon effect. This will increase the cost of the column supporting-structure. Horizontal reboilers require less headroom than vertical, but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

Selling price of MEK Steam raising cost Cost of tower shell Cost of plates Cost of reboiler... 

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