Reboilers energy cost

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

Unfortunately, high reflux rates are expensive. They represent an energy cost, in that reflux is actually generated from the reboiler. The... 

Reducing reflux saves reboiler duty. Also, the lower pressure will reduce the tower-bottom temperature, and this also cuts the reboiler energy requirement. For most distillation towers, the energy cost of the reboiler duty is the main component of the total operating cost to run the tower. 

Of course, there is additional capital investment in the distillation column and its associated heat exchangers (condenser and reboiler) and most importantly, there will be an energy cost to provide heat to the reboiler. We neglect any energy cost in the... 

Operating the column at the minimum pressure minimizes the energy cost of separation. Towering this pressure increases the relative volatility of distillation components and thereby increases the capacity of the reboiler by reducing operating temperature, which also results in reduced fouling. Reducing pressure also affects other parameters, such as tray efficiencies and latent heats of vaporization. 

Low investments are required for these external modifications to adjust the piping network to remove the inefficiencies and to install a new reboiler at the bottom of the LD side stripper. External modifications were considered first. This is because of the low investment that is required by these modifications. The throughput through the modified column could be increased by 39%. There are two options concerning the use of a reboiler or a combination of live steam and a reboiler at the KE side stripper. A trade-off exists between these two options where the use of a reboiler only is more beneficial with respect to the savings in the energy costs while the use of the combination of reboiler and steam save 16% in the furnace duty. 

In most distillation columns, the major operating cost is reboiler energy consumption. Of course, if refrigeration was used in the condenser, this heat removal expense would also be... 

Equations to calculate the capital cost of all the equipment and the energy cost of the heat added to the reboiler are needed to perform economic optimization calculations. The major pieces of equipment in a distillation column are the column vessel (of length L and diameter D, both with units of meters) and the two heat exchangers (reboiler and... 

The other columns in Table 4.3 give results for columns with other total stages. If the number of stages is reduced to 24, which gives a shorter column, reboiler heat input increases. This increases column diameter and heat-exchanger areas. This results in an increase in both capital and energy costs. 

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 economic parameters and sizing relationships given in Chapter 4 are used to determine heat-exchanger areas (reboiler and condenser), capital investment, and energy cost. Total annual cost (TAC) takes the total capital investment, divides by a payback period (3 years), and adds the annual cost of reboder energy. 

In the low-pressure amine absorption system, a low stripper pressure is used to keep stripper reboiler duty as small as possible. In the high-pressure TEG absorption system, there is a smaller dependence of reboiler energy on pressure. A higher pressure in the stripper reduces the compression costs to raise the recovered carbon dioxide gas up to the required pipeline pressure for sequestration. Therefore, there is an optimum economic stripper pressure (ISOpsia) that balances compression costs with stripper reboder energy cost. The stripper distillate is cooled to 110 °F to minimize the amount of water in carbon dioxide gas product from the stripper reflux drum. Diameters of the columns are very large due to the enormous throughput. 

The beer still distillate flow rate decreases slightly as distillate composition is increased but less organic reflux (R2) is required. This reduces reboiler duty in the azeotropic column (QR2). However, the reboiler duty in the beer still (QRl) increases as distillate composition is increased, as does the optimum number of stages in the beer still (NTl). So, beer still capital and energy costs increase while those costs in the azeotropic column decrease. 

In tower design, the reflux ratio is determined based on the trade-off between operating cost in the reboiler and capital cost for the tower. In other words, use of more separation stages requires less reflux rate and thus less reboiling energy but at the expense of additional capital cost. 

However in operation, optimal reflux ratio is determined based on the trade-off product value and energy cost. When the reflux ratio increases, the separation improves at the expense of increased reboiling duty (Figure 14.5). As a result, top product rate decreases while the bottom product rate increases. As shown in Figure 14.5, the cost of reboiling duty presents a linear relationship with reflux ratio but the product rate is nonlinear and presents a different trend as reboiling duty. 

For example, if in our case study LPG sphtter, we fix the reboiler at its hmit and vary pressure we obtain the result shown in Figure 12.132. With the energy balance scheme in place, increasing the pressure reduces distillate yield and increases the C3 content of bottoms. However, there is a maximum pressure above which the bottoms product will be off-grade. Since the energy cost is constant, changing pressure simply shifts yield between distillate and bottoms. If the bottoms product is more valuable then we should maximise the pressure within the composition target If the distillate product is more valuable then we should minimise pressure until some other constraint is reached, such as condenser duty. 

Reboiler Heat transfer coefficient Ur 0.568 kJ K Temperature difference ATr 34.8 K Condenser Heat transfer coefficient Uc 0.852 kJ K m Temperature difference ATc 13.9 K Energy cost 4.7 (lO kJ) Payback period Ppay 3.0 years ... 

Table 8.11 gives the basis for equipment sizing and the costs used for equipment capital investment and energy. Design parameters, capital costs, and energy costs for the separation sections of both processes are listed in Table 8.12. The reboiler heat inputs in the two columns of the extractive distillation process are about 30% of those in the pressureswing process. This reduces column diameters and heat exchanger areas, so the capital cost is also much smaller (about 40% lower).

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... 

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