Column Vapor and Liquid Flows

Side coolers or condensers are commonly used in multi-product columns to prevent excessive variations in the vapor and liquid flow profiles along the column. The multi-product column described in Section 9.2 had, for simplicity, only one condenser and a reboiler. If the feed in this column is introduced at its lower part, the vapor and liquid rates would continuously increase above the feed to the top of the column. A similar situation would exist in crude columns if no side coolers were used. 

The variations in the column flow profiles are caused by the liquid side draws. The liquid flow at the top of the column is the reflux rate. Each liquid draw removes a portion of the column liquid flow. With a reduced liquid flow, the vapor flow also decreases since the LIV ratio remains unchanged, assuming other factors are constant. The column flow profiles can be manipulated by redistributing the condenser duty among side coolers along the column. 

If a condenser or side cooler is placed on a draw tray or right below it, a portion of the rising vapor condenses. This causes the liquid flow, and consequently the vapor flow, below the side draw to rise, which tends to equalize the profiles above and below the draw tray. The side cooler duty may be adjusted to obtain the desired profiles. In this manner, maintaining the desired liquid and vapor flows in the lower sections of the column does not require unnecessarily high flows in the upper sections. 

Each side cooler or heater placed on the column adds one degree of freedom to the column. By varying the side cooler or heater duties, it is possible to meet certain performance specifications such as liquid or vapor profiles or side product purities. The side condensers can enhance the side product purities because better fractionation may be achieved by manipulating the LIV ratio below the side draw. 

Example 9.8 examines the effect of side coolers on the column performance. 

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In a distillation column, vapor and liquid flow in countercurrent directions to each other. Liquid is vaporized at the bottom, and vapor is condensed from the top product and withdrawn from the column. A number of trays are placed in the column, or the column is packed with open material, so that the vapor phase contacts the liquid phase, and components are transferred from one phase to the other. As you proceed up the column the temperature decreases, and the net effect is an increase in the more volatile component(s) in the vapor and a decrease in the less volatile components in the liquid. Vapor is withdrawn from the top of the column and liquid from the bottom. Feed to the column usually enters part way up the column. 

The column vapor and liquid flows at this reflux ratio are estimated from the... 

Notice that the column pressure drop (upper right graph in Figure 18.15) increases to over 0.5 bar at the high feed flow rate because of the increase in column vapor and liquid flow rates. In the following section, we assume that there is a 0.5 bar constraint on the allowable pressure drop, so a high pressure-drop override controller is required. 

The column vapor and liquid flows at this reflux ratio are estimated from the relationships V, = L + D and R = L, D, where and L, are the vapor and liquid molar flows in the column rectifying section (assuming constant molar flows in the section). The distillate rate, D, is 60 Ibmol/hr (Example 12.1). 

Feed composition and flow rate normally are known. Feed condition (temperature and pressure) must be determined carefully, because a liquid feed may flash on entering a vacuum column. Feed enthalpy changes significantly with feed condition, and this in turn, affects the reboiler duty and the internal column vapor and liquid flow rates. 

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

The column diameter is sized to suit the maximum anticipated rates of vapor and liquid flow through the column. Usually, the diameter is determined primarily by the vapor flow rate, and a rough estimate can be obtained from ... 

Columns operated at vapor and liquid flow rates greater than those for which they were designed become flooded. Unexpected foaming can also cause flooding. In a flooded column, liquid cannot properly descend against the upflowing vapor. Poor separation performance results, the overhead condensation circuit fills with process liquid, the reboiler is starved of process liquid, and the column quickly becomes inoperable. 

Conceptually, product quality is determined by the heat balance of the column. The heat removal determines the internal reflux flow rate, whereas the heat addition determines the internal vapor rate. These internal vapor and liquid flow rates determine the degree of separation between two key components. 

Solving the equations simultaneously, D = 71, B = 129. step 2 Set vapor and liquid flow in the column. 

The initial total flow rate and temperature profiles can make the difference between success and failure of a rigorous method. Usually for distillation columns, the condenser and reboiler temperatures are estimated and a calculation that assumes constant molal overflow Sec. 2.2.2) is used to initialize tbe internal vapor and liquid flow... 

Example 3 Simple Distillation Column Compute stage temperatures, interstage vapor and liquid flow rates and compositions, and reboiler and condenser duties for the butane-pentane splitter studied in Example 1. The specifications for this problem are summarized below and in Fig. 13-37. 

Example 4 Calculation of the BP Method Use the BP method with the SRK equation-of-state for K values and enthalpy departures to compute stage temperatures, interstage vapor and liquid flow rates and compositions, and reboiler and condenser duties for the light-hydrocarbon distillation-column specifications shown in Fig. 13-51 with feed at 260 psia. The specifications are selected to obtain three products a vapor distillate rich in Cg and C3, a vapor side-stream rich in TI-C4, and a bottoms rich in n-Cj and n-Ce. 

Example 5 Calculation of the SR Method Use the SR method with the PR equation of state for K values and enthalpy departures. The oil was taken as n-dodecane. To compute stage temperatures and interstage vapor and liquid flow rates and compositions for absorber-column specifications shown in Fig. 13-52. Note that a secondary absorber oil is used in addition to the main absorber oil and that heat is withdrawn from the seventh theoretical stage. 

Vapor and liquid flows in a packed column truly flow in opposite directions. Contrast this with the flow in a tray column where the vapor-liquid contact is between the vapor as it rises up through a liquid that is flowing laterally across the column prior to flowing down to the tray below. 

If we count the equations listed, we will find that there are 2n + 4 equations per stage. However, only 2 n + 3 of these equations are independent. These independent equations are generally taken to be the n component mass balance equations, the n equilibrium relations, the enthalpy balance, and two more equations. These two equations can be the two summation equations or the total mass balance and one of the summation equations (or an equivalent form). The 2n + 3 unknown variables determined by the equations are the n vapor mole fractions the n liquid mole fractions, the stage temperature 7 and the vapor and liquid flow rates LJ and Ly. Thus, for a column of 5 stages, we must solve s 2n + 3) equations. 

The bottom product rates were specified in molar units. For column 1 the side stream flow rate also was specified in molar units. Specification of the molar flow rates makes the simulations converge more easily. Nonstandard specifications can be harder for a nonequilibrium model to converge than for a corresponding equilibrium model because nonstandard specifications are more likely to lead to large variations in vapor and liquid flows from iteration to iteration. Since mass transfer coefficient and pressure drop calculations may... 

The separation column is assumed to have one feed and two products overhead (or distillate) and bottoms. The feed has a fixed flow rate and composition. The temperatures and pressures of the feed and products and of streams within the column are of no concern in the present context. Moreover, the column configuration, number of stages, and internal vapor and liquid flows are immaterial since the focus is on the net performance of a black box column. 

It will be shown that if certain assumptions hold, the vapor and liquid flows Vj and Lj may be considered constant throughout the column section. If the sensible heat of the vapor and liquid is neglected when compared to the latent heat, that is, if hj i hj and f/y+i Hj, Equation 5.5 becomes... 

The purposes of side heaters and coolers may be categorized as follows utilization of heat sources or sinks at different temperature levels, removal of heat of absorption, and control of vapor and liquid flows in the column. 

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