Liquid-to-Vapor Ratios

Example 8.2 examines the variation in the LIV ratio, temperature, and key component composition in an absorber column and the effect of the relative feed rates on the performance of the column. 

Dodecane (NC ) is the solvent or lean oil used to recover propane and heavier components from a rich gas mixture by absorption. The initial conditions of both feed streams are fixed at 38°C and 2760 kPa. Under these conditions the lean oil is a subcooled liquid, and the rich gas is a superheated vapor. The column has ten theoretical stages and is maintained at 2760 kPa. 

Going up the column, the vapor is at equilibrium with leaner oil (dodecane with lower concentrations of propane and other dissolved gases). Hence, the propane concentration in the vapor is lower in the upper column stages. The /(-values of propane are higher in the lower column stages because the temperature is 

To investigate the effect of the L/V ratio on the separation, the lean oil rate is varied from 25 to 5000 kmol/h at a fixed rich gas feed rate of 1000 kmol/h. Table 8.4 lists the recoveries in the rich oil and concentrations in the lean gas of several 

Effect of Lean Oil Rate on Column Performance (Example 8.2) 

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The tubes are usually 1 V4 -in. O.D. but never smaller than 1-in. O.D. because the flow contains vapor as well as liquid. The recirculation ratio i.e., liquid-to-vapor ratio in the outlet, is seldom less than 5 and more often is 10-15, sometimes reaching 50. 

This criterion is similar to using a reflux ratio of 1.2 times the minimum reflux ratio in a full distillation column. It provides a reasonable compromise between the number of trays and the vapor boilup required. The slope of the resulting operating line is the liquid-to-vapor ratio F/D ... 

As an alternative to trays, especially at low volumetric liquid-to-vapor ratios, packing can be used to promote vapor-liquid contact. One approach is to dump specially shaped pieces of metal, glass, or ceramic material into the column, wherein they are supported on a grid. An example of dumped or random packing is shown in Fig. 7. 

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby "fight" each other. 

Low pressures favor high vapor velocities and low liquid flow rates and, therefore, spray regime dispersions. Flooding in vacuum columns and in columns operating at a low liquid-to-vapor ratio is usually caused by the spray entrainment mechanism. 

Entrainment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantify of entrained liquid, and a lower limit is set. 

Related Calculations. It is usual practice in distillation operations to keep the liquid-to-vapor ratio constant as the throughput is varied. When this is the case, the percent of flood is usually defined at constant L/V rather than at constant L. The procedures for solving for the tower diameter are similar to those in step 1, except that the liquid-capacity factor at flooding is instead given by CLF = CL/f. For the present case, this would introduce a factor of 0.8 in the denominator of the second term in the brackets of Eq. (11.5). 

A 5.5-ft-diameter tower is to be used to countercurrently contact a vapor stream and a liquid stream. The mass flow rates are 150,000 lb/h for both. The liquid density is 50 lb/ft3, and the gas density 1 lb/ft3. Determine the approach to flooding at constant liquid loading L and at constant liquid-to-vapor ratio (L/V) for Flexipac type 2Y (FP2Y) structured packing. 

For new tower design, packing is the favored choice in 1) low-pressure operation 2) low-pressure drop operation 3) high liquid-to-vapor ratios 4) low liquid-to-vapor ratios 5) ceramic and polymer... 

The operating line shown in Figure 8.5 is constructed such as to meet the absorption specifications with three equilibrium stages. This is accomplished by trial and error, with the lower end of the operating line fixed at X g = 0 and Y = 0.01, and its upper end moved along the Y = 0.03 line to a point where the number of stepped stages is three. The liquid to vapor ratio is calculated from the slope of the operating line ... 

Equation (6-106) cannot be integrated analytically because the relationship between xD and xw depends on the liquid-to-vapor ratio, the number of theoretical stages, and the equilibrium distribution curve. However, it can be integrated graphically with pairs of values for xD and xw obtained from the McCabe-Thiele diagram for a series of operating lines of the same slope. 

The limited number of comparisons shown for the ten-component system include some of the best as well as some of the poorest agreement observed between the predicted and experimental results. Generally, the predictions are in better agreement with the experimental data at the higher temperatures. As the system pressure approaches the saturation pressure, the predicted K values do not converge toward unity as rapidly as the experimental K values, particularly at the lower temperatures. This results in a predicted saturation pressure which is greater than the experimental value, and in erroneous liquid-to-vapor ratios at elevated pressures. This appears to be a problem which would be expected to occur in reservoir fluid systems. The predicted results correctly accounted for the effect of the addition of an aromatic component to an otherwise n-paraffln system. 

Plate efficiencies and HETP values are complex functions of measurable physical properties temperature, pressure, composition, density, viscosity, diflusivity, and surface tension measurable hydrodynamic factors pressure drop and liquid and vapor flow rates plus factors that cannot be predicted or measured accurately foaming tendency, liquid and gas turbulence, bubble and droplet sizes, flow oscillations, emulsification, contact time, froth formation, and others. Values for plate efficiency, HETP, or HTU, particularly those that purport to compare various devices, are usually taken over a limited range of concentration and liquid-to-vapor ratios. The crossovers in Fig. 2.5 and the rather strange behavior of the ethyl alcohol-water system, Fig. 2.6, demonstrate the critical need for test data under expected operating conditions. ... 

Equation (9-2) cannot be integrated directly if the column has more than one stage, because the relationship between yp and x , depends on the liquid-to-vapor ratio and number of stages, as well as the phase equilibrium relationship. Thus, as shown in the following example, (9-2) is integrated graphically with pairs of values for Xp and Xw obtained from the McCabe-Thiele diagram. 

Figure 3.1 Effect of poor distribution on HETP (at constant liquid-to-vapor ratio).

Increasing the liquid-to-vapor ratio in a section of a column increases the separation that occurs in that section. 

Natural circulation calandrias (or thermosiphon reboilers) depend upon density differences to produce required flow rates. Vaporization creates an aerated liquid with a density less than that of the liquid in the system. The hydraulic head resulting from this density difference causes the fluid in the system to circulate. Circulation rates are high with liquid-to-vapor ratios ranging from 1 to 50. 

The gases are cooled down to 30°C at constant pressure and are fed to an absorption-oxidation tower that operates isothermically. We feed an HNO3 solution 2.5% w/w with a liquid to vapor ratio of 0.25. The acid produced is expected to reach 55% concentration. A summary of the operating conditions of the tower is given in Table 3.3. Determine the flow rate of acid solution used, flows, and tanperatures at the different units. 

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