Distillation columns hydraulics

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

Changes in the hydraulic hold-up of liquid on the column plates is known to have a significant effect on the separating efficiency of batch distillation columns, and may be relatively easily incorporated into the batch simulation model. The hydraulic condition of the plates is represented in Fig. 3.52. 

You may wonder why we would ever be satisfied with anything less than a very accurate integration. The ODEs that make up the mathematical models of most practical chemical engineering systems usually represent a mixture of fast dynamics and slow dynamics. For example, in a distillation column the liquid flow or hydraulic dynamic response occurs fairly rapidly, of the order of a few seconds per tray. The composition dynamics, the rate of change of hquid mole fractions on the trays, are usually much slower—minutes or even hours for columns with many trays. Systems with this mixture of fast and slow ODEs are called stiff systems. 

The digital simulation of a distillation column is fairly straightforward. The main complication is the large number of ODEs and algebraic equations that must be solved. We will illustrate the procedure first with the simplified binary distillation column for which we developed the equations in Chap. 3 (Sec. 3.11). Equimolal overflow, constant relative volatility, and theoretical plates have been assumed. There are two ODEs per tray (a total continuity equation and a light component continuity equation) and two algebraic equations per tray (a vapor-liquid phase equilibrium relationship and a liquid-hydraulic relationship). 

Olujic, Z. Development of a complete simulation model for predicting the hydraulic and separation performance of distillation columns equipped with structured packings. Chem. Biochem. Eng. Q. 11, 31-46, 1997. 

Rocha, J.A., Bravo, J.L., Fair, J.R. Distillation columns containing structured packings A comprehensive model for their performance. 1. Hydraulic models. Ind. Eng. Chem. Res., Vol. 32, 641-651, 1993. 

Tung, L.S. and Edgar, T. F., "Development and Reduction of a Multivariable Distillation Column Model with Tray Hydraulics,"... 

The last step regards the detailed design of the reactive-distillation column and of other operational units. The hydraulic design is consolidated taking into account the optimal traffic of liquid and vapor. Additional internals are provided to ensure uniform distribution of fluids and a sharp residence-time distribution. 

Hydraulic analysis of the Aspen Plus simulator produces thermodynamic ideal minimum flow and actual flow curves for rigorous distillation column simulations. These types of calculations are performed for RADFRAC columns. Using the input summary given in problem 4.48 construct the stage-flow curves. Assess the thermodynamic performance of the column. 

One of the challenging aspects of distillation column control is the many limitations imposed on the operation of the column. There are hydraulic constraints, separation constraints, heat-transfer constraints, pressure constraints, and temperature constraints. We recommend the excellent books by Kister (1992 and 1990) on distillation design and operation. 

Only one liquid phase should be present. A second liquid phase can introduce problems in the hydraulic design of the distillation column. In some cases, this problem can be avoided through the use of a separate cosolvent, which then must be separated for recycle. 

Site the Distillation Column. This includes the hydraulic aanlysis to establish operating mages, pressure drop, and mass transfer efficiency. The result is a set of dimensious, including columa diameter and height, number of actual trays (or height of packed bed), details of internal devices, and profiles of temperature and pressure. 

This chapter covers all important aspects of distillation column rating or desiga. It follows generally the requence of the steps noted immediately above. The common case for discussion is conventional distillation operated in the continuous mode, Exceptions to this mode, such as extractive distillation or betch distillation, will be handled separately. Much of the material in Sections 5.7-5.10, dealing with hydraulics and mass transfer in columns, can also be used in the design and analysis of aheorption columns. Perusal of Chapter 6 will show the reader bow the connection may be made. 

For example, the cost of a distillation column can be assembled from the cost of elements vertical cylindrical vessel, plus internals (trays or packing), reboiler, condenser, and reflux drum. The height of the shell can be determined from the number of trays and inter-stage height. The column diameter can be found by hydraulic calculations based on the flooding point. In this way, the volume of the cylindrical part can be easily evaluated. The volume of auxiliary vessels, as drum and reboiler, can be estimated from the residence time, typically of 10 minutes. 

A gravity-flow condenser uses the hydraulic head of the liquid in the line from the condenser to overcome the pressure drop over the control valve and the difference between the pressure at the top of the distillation column P and the pressure at the bottom of the condenser Pi. The pressure difference is due to the flow of vapor through the vapor line and condenser. When the flow rate of vapor from the top of the column is 14 1.6 Ibm/min, the pressure drop P - Pi is 2 psi. The pressure drop due to the liquid flowing through the liquid return line is negligible. Liquid density is 62.4 lbm/ft ... 

An important example of a physical process that shows inverse response is the base of a distillation column with the reaction of bottoms composition and base level to a change in vapor boilup. In a binary distillation column, we know that an increase in vapor boilup V must drive more low-boiling material up the column and therefore decrease the mole If action of light component in the bottoms xg. However, the tray hydraulics can produce some unexpected results. When the vapor rate through a tray is increased, it tends to (1) back up more liquid in the downcomer to overcome the increase in pressure drop through the tray and (2) reduce the density of the liquid and vapor Ifoth on the active part of the tray. The first effect momentarily reduces the liquid flow rates through the column while the liquid holdup in the downcomer is... 

Liu, Z.Y. and Jobson, M. (2004) Retrofit design for increasing the processing capacity of distillation columns 1. A Hydraulic performance indicator. Chemical Engineering Research and Design, 82,... 

The diameter of a distillation column is determined by the maximum vapor velocity. If this velocity is exceeded, the column liquid and vapor hydraulics will fail and the column will flood. Reliable correlations are available to determine this maximum vapor velocity. 

The hydraulic capacity of trayed distillation columns is always limited by either a high liquid entrainment, or by an overload of downcomers. The technology of high capacity trays deals with these two fundamental limits by reducing the entrainment or by releasing the loads of downcomers. Both are generally connected (HUls, 2001). 

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