Tower pressure optimum

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

Figure 3.5 Point A represents the optimum tower pressure.

Figure 3.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as that operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure, to obtain the best efficiency point for the trays Answer—look at the temperature profile across the column. 

Air-stripping tower diameter is selected as a function of the liquid loading rates necessitated by the required design flow capability. The optimum tower diameter may be determined with the use of pressure-drop curves developed by Eckert (11) as shown in Fig. 3. The volumetric air-to-water ratio, calculated by Eq. (9), is converted to a weight-to-weight ratio and plotted on the abscissa in the form ... 

The best inemals and the optimum values of pressure, vapor velocity, and reboil vapor ratio are those that permit production of heavy water at minimum cost. The initial cost of the plant depends on a number of factors including the total number of towers, the total amount of reboiler and condenser surface, and the total volume of tower internals. The principal operating cost is for power, which is proportional to total loss in availability of steam as it flows through the towers. A complete minimum-cost analysis requires knowledge of the unit cost of all the important cost components and is beyond the scope of this book. Design for minimum volume of tower internals or minimum loss in availability due to tower pressure drop and for minimum cost of these two important contributors to total cost can be carried out without complete unit-cost data and will be discussed. Because the same choice of reboil vapor ratio minimizes the number of towers, their volume, and the loss of availability within them, this reboil vapor ratio is close to that which leads to minimum production cost. An equation for this optimum reboil vapor ratio will now be derived, and expressions will be developed for the total volume of towers and the total loss in availability in towers designed for the optimum ratio. 

Second, the optimum target for the DIP overhead mCs was implemented into the APC system. The APC controller on the DIP DCS system was reconfigured to operate at this mCs target, while maximizing the rcboiler duty as limited by the high limit on tower pressure. This APC system allowed the DIP operation to be maintained at an economic optimum, accounting for the isomerization unit capacity and the economics of the day. 

Low flash-zone temperature. Have the instrument mechanic check the furnace outlet thermocouple. The optimum tower top temperature for a vacuum tower equipped with a precondenser is usually not the minimum temjjerature. As the tower top temperature is raised, heavy naphtha boiling-range materials are flashed overhead into the precondenser. Acting as an absorption oil, they absorb a portion of the light hydrocarbons that would otherwise overload the jets. However, getting the vacuum tower top too hot can overload the precondensers. By field trials, find the tower top temperature (usually 230°F to 280°F), that minimizes flash-zone pressure. 

In the design of any vacuum tower, the first question to be settled is the selection of the optimum operating pressure of the system. In order to simplify this discussion, let us consider certain facts, assuming that a maximum allowable flash zone temperature has been set. 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

Optimum Reflux Ratio. The reflux ratio affects the cost of the tower, both in the number of trays and the diameter, as well as the cost of operation which consists of costs of heat and cooling supply and power for the reflux pump. Accordingly, the proper basis for choice of an optimum reflux ratio is an economic balance. The sizing and economic factors are considered in a later section, but reference may be made now to the results of such balances summarized in Table 13.3. The general conclusion may be drawn that the optimum reflux ratio is about 1.2 times the minimum, and also that the number of trays is about 2.0 times the minimum. Although these conclusions are based on studies of systems with nearly ideal vapor-liquid equilibria near atmospheric pressure, they often are applied more generally, sometimes as a starting basis for more detailed analysis of reflux and tray requirements. 

An important thermodynamic parameter in cooling tower calculations is the ratio of the thermal capacity of the water stream to that of the sir stream. This parameter is referred to as, the tower capacity factor. It is shown that when air or water efficiency, are plotted against the capacity factor test points for a given tower are found to lie on a single smooth curve. The correlation is obtained, irrespective of whether the equipment is used as a water cooler or air cooler, and irrespective of the temperature levels, temperature ranges and barometric pressures. The paper also shows that when a specified amount of heat has to be rejected into a specified air stream, optimum performance giving the lowest average water temperature is obtained when the water flow rate is chosen so that its thermal capacity is equal to the potential thermal capacity of the air stream. 13 refs, cited. 

Example 6 Determination of optimum reflux ratio. A sieve-plate distillation column is being designed to handle 700 lb mol (318 kg mol) of feed per hour. The unit is to operate continuously at a total pressure of 1 atm. The feed contains 45 mol% benzene and 55 mol% toluene, and the feed enters at its boiling temperature. The overhead product from the distillation tower must contain 92 mol% benzene, and the bottoms must contain 95 mol% toluene. Determine the following ... 

Thns, for a valne of G/L of 4.0, the valnes of Z/HTU for XJX = 0.1 are 1.49 for 85°F and 1.95 for 75°F. The height of the tower at 85°F is 1.48 x 23.5 = 34.8 ft, whereas at 75°F it becomes 1.95 x 23.5 = 45.8 ft. This shows that setting the GIL ratio to 4.0 instead of 2.0 wonld resnlt in a shorter tower. The amount of air requirecf, however, is not doubled 34.8 times 2 divided by 49.5 = 1.40. Thus, at this higher air loading rate, only 40% more air is reqnired. To find the optimum GJL ratio, the entire design mnst be priced and the minimnm cost tower selected. This requires repetitive calculations using a computer and incorporating reasonably accurate cost data as well as mass transfer, enthalpy transfer, and pressure drop characteristics on the detailed analysis. 

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