Columns pressure and

Figure 8-12. Algorithm for establishing distillation column pressure and type condenser. Used by permission, Heniey, E. J. and Seader, J. D., Equilibrium Stage Separation Operations in Chemical Engineering, John Wiiey, (1981), p. 43, aii rights reserved.

The composition and quantity of rich oil, and percent recovery of a specified key component are known, also column pressure and temperature. 

The thesis of Steward indicates that the overall liquid film and mass transfer coefficients were functions of the gas flow rate and the column pressure and are independent of the liquid flow rate and inlet air temperature. The gas film heat transfer coefficient was found to be a function only of the air flow rate. 

A valve position controller is used to minimize operating pressure in a distillation column. Assume that the openloop process transfer function between column pressure and cooling-water flow Gju = P/F is known. 

Service-Oriented Rules of Thumb Strigle (Packed Tower Design and Applications, 2d ed., Gulf Publishing, Houston, Tex., 1994) proposed a multitude of rules of thumb as a function of the service, column pressure, and physical properties. These rules are based on the extensive experience of Strigle and the Norton Company (now merged with Koch-Glitsch LP). 

Instances of a task are replicas of the task operating under different conditions. The concept is used to optimize the operating conditions, such as the column pressure, and assumes the development of an operating range and a discretization scheme. Feasible ranges of pressure are identified by the physical properties (e.g., critical pressure) of the key components (upper limit) and the available utility levels (lower limit). The discretization scheme may be either uniform or based on the available utilities. The modeler can use a small or large number of discrete levels to capture associated trade-offs. 

Controlled variables include product compositions (x,y), column temperatures, column pressure, and the levels in the tower and accumulator. Manipulated variables include reflux flow (L), coolant flow (QT), heating medium flow (Qb or V), and product flows (D,B) and the ratios L/D or V/B. Load and disturbance variables include feed flow rate (F), feed composition (2), steam header pressure, feed enthalpy, environmental conditions (e.g., rain, barometric pressure, and ambient temperature), and coolant temperature. These five single loops can theoretically be configured in 120 different combinations, and selecting the right one is a prerequisite to stability and efficiency. 

Of these, the feed mixture may or may not vary, but is generally taken as given. The column pressure and the degree of subcooling are normally fairly constant. The main operational variables are the reflux ratio R and the heat input to the reboiler QR and once these are set, the amount of product withdrawal at the bottom or at the top will also be given by the product specifications. An optimum exists for the reflux ratio in terms of operating costs, and normally a number of ratios are tested, and the economics of each scenario is investigated, before a decision is reached. 

The next component to be optimized is the reboiler. The reboiler area is fixed once the values for steam temperature, Tst, and process stream temperature, Ij, are fixed (assuming a constant heat transfer conductance). The process stream temperature is fixed by the column pressure and the product purities. Thus only Tst remains as a variable. The unit cost of the process stream,... 

His paper10 should be consulted for recommendations on selecting the best experimental conditions for small particle columns. He considers decreasing the column pressure (and flow rate), increasing the column length, and combinations of these. His scheme is based on a reduced form of Eq. (12), which is presented in Chapter 5. 

FIG URE 8.6 F factor as a function of column pressure and tray spacing. 

The temperature profile in a CD column is controlled by column pressure and is not isothermal. The kinetics of the reaction should be known prior to the CD experiments so that the CD packing could be located in the section of the column where the temperature is optimal for production of a specified product. 

The problem definition is such that the feed is of fixed rate, composition, and thermal conditions. Also, the column pressure and the number of stages are known. The condenser and reboiler duties are not presently of concern calculating them would require energy balance equations for the condenser and reboiler. Aside from these, the system of equations includes the following ... 

With the product rates fixed, the sharpness of separation is determined by the reflux ratio. (The column pressure and number of stages are fixed.) The reflux ratio mostly affects the product compositions of components with boiling points in the vicinity of the key components. Components that are either much lighter or much heavier than the key components are not significantly affected by variations in the reflux ratio. This may be seen by comparing the behavior of the distillate compositions /(Cg) and V (NC4) at a distillate rate of 50 kmol/h (Figure 7.1). 

An absorber column is to be designed to lower the concentration of ethyl alcohol in a CO2 stream using water as the absorbent. The ethyl alcohol is assumed to be the only component transferring between the vapor and liquid phases, so that the flow rates of ethyl alcohol and water are constant on a solute-free basis. The column pressure and temperature are held constant at 1 kPa and 30°C, at which conditions the K-value of (X), is 0.60. The inlet ( (),-ethyl alcohol flow rate is 200 kmol/h at 2 niol% ethyl alcohol. The inlet absorbent rate is 160 kmol/h pure water. Find the smallest number of absorber equilibrium stages that will bring the ethyl alcohol concentration in the effluent gas stream down to 0.1 mol%. 

Fenske equation. Use Raoult s law for the /f-values at average column pressure and an average column temperature assumed equal to the feed temperature. Select pentane as the reference component. 

The relative volatility of propylene/propane at column pressure and average column temperature is a,2 = 1.11 (0.22 = 1.00). The feed stream enters the column as saturated liquid. In order to optimize the column design, a number of options will be considered, each using a different number of stages. Determine the reflux ratio required to meet the above specifications for N = 100, N = 125, and N = 150 stages. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

To avert high-pressure blow-outs of plumbing and equipment, pressure gauges and pop-off valves should be installed in the following places (1) in the condenser to sense product pressure, (2) near the base of the stripping and rectifying column sections to sense internal column pressure, and (3) in the reboiler steam chamber or the steam injector nozzle to sense steam pressure being applied. 

For a partial condenser, D is withdrawn from the accumulator as a vapor at its dew-point temperature 7 at the column pressure, and yu = XDi. Thus... 

Consider the case of an existing column such as the one shown in Fig. 2-3. Suppose that the specifications are taken to be (1) the total number of stages and the location of the feed plate, (2) the complete definition of the feed (the total-flow rate, composition, and thermal condition), (3) the reflux rate, (4) the distillate rate, (5) the column pressure, and (6) the type of condenser (total or partial). On the basis of this set of specifications, it is desired to find the resulting compositions of the distillate and bottom products. 

To initiate the calculational procedure for the determination of the product distribution for specified reflux and distillate rates, a number of plates between the two pinches is selected. (As discussed in a subsequent section, too few plates but not too many plates may be selected.) Next L/V and temperature profiles for the plates between and including the two pinches as well as the distillate and bottoms temperatures are selected. Next the components of the feed are classified according to the above criteria. Since it is supposed that the complete definition of the feed, the reflux and distillate rates, as well as the column pressure and type of condenser are specified, the component-material balances can be solved for the component-flow rates throughout the column. The component-material balances may be simplified by taking advantage of the unique characteristics of the three classes of components, the distributed components, the separated lights, and the separated heavies. 

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