Heat balance, distillation

This type of evaluation of a distillation system involves a material and heat balance around each tray. It is extremely tedious to do by conventional means, and is now handled with computers. But even with this untiring worker, the volume of calculations is large and requires a relatively long time. Only those special systems that defy a reasonable and apparently economical solution by other approaches are even considered for this type of solution. 

Distillation calculations result in a reflux ratio L/D = 0.8, with 4 theoretical trays for rectification and 4 theoretical trays for stripping, or a total of 8 trays. The design heat balance (neglecting heat losses) is as follows ... 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Because the reboiler is usually used in conjunction with distillation columns, the terminology and symbols used here will relate to that application. Assume a column with an overhead total condenser and a bottoms reboiler (see Figures 10-96D and 10-96E). Assuming all liquid feed, the heat balance is ... 

In order to get heat balance from the condenser with control of distillate temperature, a recirculating pump was Included, rather than only an intermittently regulated coolant supply. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

The solution of the mass balances and the temperature correction has been heavily studied, and is now quite rapid. In distillation columns, the correction of flows through solution of the stage heat-balances is also simple and well developed. Even for complicated columns, distillation processes are readily solved by the present iteration methods, convergence often being reached after only a few iterations. 

Figure 13.9. Combined McCabe-Thiele and Merkel enthalpy-concentration diagrams for binary distillation with heat balances, (a) Showing key lines and location of representative points on the operating lines, (b) Completed construction showing determination of the number of trays by stepping off between the equilibrium and operating lines.

Controlling Quality of Two Products Where the two products have similar values, or where heating and cooling costs are comparable to product losses, the compositions of both products should be controlled. This introduces the possibility of strong interaction between the two composition loops, as they tend to have similar speeds of response. Interaction in most columns can be minimized by controlling distillate composition with reflux ratio and bottom composition with boil-up, or preferably boil-up/bottom flow ratio. These loops are insensitive to variations in feed rate, eliminating the need for feedforward control, and they also reject heat balance upsets quite effectively. 

Multiproduct fractionator controls, where, after dynamic correction, the boil-up, side-draw and distillate flows are ratioed to the feed flow (left). On the right, the true boiling points are controlled by throttling the product flows, while heat balance is controlled by manipulating the reflux flows. 

The converged mass and heat balances and the exergy loss profiles produced by the Aspen Plus simulator can help in assessing the thermodynamic performance of distillation columns. The exergy values are estimated from the enthalpy and entropy of the streams generated by the simulator. In the following examples, the assessment studies illustrate the use of exergy in the separation sections of a methanol production plant, a 15-component two-column... 

The earliest employ s supported silver crystals. It leads to a heat balance equalized between the % apon2er and the exchanger/reaaor stage. It avoids methanol distillation and recyclings and achieves sufficient once-through conversion to leave the residual alcohol in the fonnaldehyde solunon. The total yield is 87.5 molar per cent based on the methanol introduced, and 91 per cent in relation to the metlianol cooverted The formaldehyde concentration of the final product ranges from 40 to 44 per cent weight... 

Calculations for the extractive distillation of aqueous ethanol mixtures containing 85.64% m ethanol have been carried out with the aid of a UNIVAC 1108 computer. The computer program calculates all phase equilibria and tray-to-tray material and heat balances for each component... 

As mentioned above, the only wastes discharged from the plant are hydrochloric acid, pretreatment residue, and moisture contained in the waste plastics. Therefore, the recycling ratio is calculated as 99.0 wt% in the pyrolysis and distillation sections, and 91.1 wt% in the whole plant as shown in Table 26.13. The heat balance shown in the Table 26.15 indicates that the total heat recovery ratio is 72.6%. 

Although the thermal demands of crystallization processes are small compared with those of possibly competitive separation processes such as distillation or adsorption nevertheless, they must be known. For some important systems, enthalpy-composition diagrams have been prepared, like those of Figure 16.4, for instance. Calculations also may be performed with the more widely available data of heat capacities and heats of solution. The latter are most often recorded for infinite dilution, so that their utilization will result in a conservative heat balance. For the case of Example 16.3, calculations with the enthalpy-concentration diagram and with heat of solution and heat capacity data are not far apart. 

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