Reboiler duty

In this mode heat input to the reboiler is held constant throughout. In practice it is set to its maximum limit, the value of which depends on the heat exchange system to the reboiler. Domenech and Enjalbert (1974), Greaves et al. (2001), Greaves (2003) used this mode of operation in their laboratory conventional batch column. Cuille and Reklaitis (1986) and Mujtaba (1989) also used this mode of operation in their simulation studies. 

This is characterized by two modes of operation, called transient total reflux and stripping. During the total reflux portion of the cycle, liquid reflux is returned to the column, but no product is withdrawn and during the stripping portion of the cycle, the product is withdrawn but no reflux is returned to the column (Barb and Holland, 1967). The extreme difficulty of accurate measurement and control of small flow rates in laboratory columns strongly favour cyclic operation (Holland and Liapis, 1983 Sorensen and Skogestad, 1994 Sorensen and Prenzler, 1997 Greaves, 2003). 

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Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Convergence was achieved in 3 iterations. Converged values of temperatures, total flows, and component flow rates are tabulated in Table 13-14. Computed reboiler duty is 1,295,000 W (4,421,000 Btu/h). Computed temperature, total vapor flow, and component flow profiles, shown in Fig. 13-54, are not of the shapes that might be expected. Vapor and liquid flow rates for nC4 change dramatically from stage to stage. 

For checking designs one can roughly relate tower diameter to reboiler duty as follows ... 

A small change in reboiler duty can be approximated by adding the increased traffic to S, and S . If the top temperature is automatically controlled, extra reflux would also result. 

Scale blocking pinched downcomer. Very poor separation with limited ability to change reflux. At low reboiler duty, feed rates greater than design could be sent through the column. Any increase in reboiler duty resulted in surges of liquid overhead. This was an installation error. 

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 heat duty of amine reboilers varies with the system design. The higher the reboiler duty, the higher the overhead condenser duty, the higher the reflux ratio, and thus the lower the number of trays required. The lower the reboiler duty, the lower the reflux ratio will be and the more trays the tower must have. 

Typically for a stripper with 20 trays, the reboiler duties will be as follows ... 

Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. 

Calculate approximate reboiler duty with 250°F reboiler temperature. 

Table 8-2 can be used for an initial approximation of reboiler duties, If the reboiler is heated with a fire tube, the fire tube should be sized for a maximum flux rate of 8,000 Btu/hr-ft . 

Determine glycol circulation rate and estimate reboiler duty. 

Determine Glycol Circulation Rate and Reboiler Duty... 

The tower operates in the same manner as a condensate stabilizer with reflux. The inlet liquid stream is heated by exchange with the gas to approximately 30 F and is injected in the tower at about the point in the tower where the temperature is 30 F. By adjusting the pressure, number of trays, and the amount of reboiler duty, the composition of the bottoms liquid can be determined. 

Net heat in through reboiler, reboiler duty, Btu/hr or heat added in still or bottoms Qc = Net heat out of overhead condenser, Btu/hr, =... 

L/D = reflux ratio Pf = packing factor Qc = condenser duty, Btu/hr Or = reboiler duty, Btu/hr Vr = reboiler vapor rate, Ib/hr Vs = superficial vapor velocity, ft/sec Pi = liquid density, Ib/ft ... 

The thermosiphon reboiler has inherent instabilities. A valve or other flow restriction in the inlet line helps overcome these instabilities. Adjustment possibilities of a valve also compensate for variations in reboiler duty as imposed by changes in operation of the fractionator. 

Installation of a valve in the liquid circulation line as shown on the illustration can aid in overcoming instability and variations in reboiler duty. 

The distillation columns shown in Figure 21.3 both fit. Figure 21.3a shows a case in which the reboiler duty can be supplied by hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Figure 21.3b. This distillation column also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty in Figure 21.3b must also be integrated, while the remainder of the condenser duty can be rejected to cold utility. 

A direct sequence of two distillation columns produces three products A, B and C. The feed condition and operating pressures are to be chosen to maximize heat recovery opportunities. To simplify the calculations, assume that condenser duties do not change when changing from saturated liquid to saturated vapor feed. This will not be true in practice, but simplifies the exercise. Assume also that the reboiler duty for saturated liquid feed is the sum of the reboiler duty for saturated vapor feed plus the heat duty to vaporize the feed. Data for the two columns are given in Tables 21.7 and 21.8. 

Prior to 1974, when fuel costs were low, distillation column trains used a strategy involving the substantial consumption of utilities such as steam and cooling water in order to maximize separation (i.e., product purity) for a given tower. However, the operation of any one tower involves certain limitations or constraints on the process, such as the condenser duty, tower tray flooding, or reboiler duty. 

Table 9 presents a summary of the variables in a typical real-time run. The raw measurements are initially used to run the simulation with PROCESS (therefore, only the simulation switch is activated). The first column of the table shows the raw measurements, and the second indicates the results from PROCESS. It is clear that the results from the simulation are not in agreement with the measurements.1 It can be seen from Table 9 that the measurements of the condenser and reboiler duties are quite different from the simulation results. This suggests that there are gross errors in those measurements. The gross error detection and data reconciliation modules are then activated. The third, fourth, and fifth columns show the rectified and reconciled data. 

If an error probability of 0.1 is considered, then 12.03 > 9.24 (from statistical tables) and one may say that inconsistency is important at this error probability level. After sequential processing of the measurements, as is shown in Table 10, feed temperature and reboiler and condenser duties are suspected to contain gross errors. Since feed temperature and reboiler duty do not appear in separate equations, it is difficult to isolate the gross error when it happens in one or both of duties. In this case, we need... 

In the first experiment, Amberlyst-15, a strongly acidic cation exchange resin, was used as a catalyst to synthesize mesityl oxide, the precursor of MIBK, from acetone without hydrogenation. The effects of acetone feed rate, reboiler duty and reaction temperature on the mesityl oxide productivity and product distribution were investigated. Preliminary results of this experiment are outlined in Table 1. 

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