Defined feed-tray location

The design of a distillation column involves many parameters product compositions, product flow rates, operating pressure, total number of trays, feed-tray location, reflux ratio, reboiler heat input, condenser heat removal, column diameter, and column height. Not all of these variables are independent, so a degrees of freedom analysis is useful in pinning down exactly how many independent variables can (and must) be specified to completely define the system. 

The mathematical model described in Section 5.2.1 indicates that two variables must be specified in order to define the performance of a column with fixed pressure, number of stages, feed tray location, and feed rate, composition, and thermal conditions. Three specifications are... 

The basic approach then consists of defining a base case, usually the design case, to firm up feed-tray location (number of trays above and below the feed tray), reflux ratio, and boilup ratio. With these in hand, we may calculate the effect of changing reflux ratio, and so forth. 

A stream containing benzene, toluene, and biphenyl is to be separated in a distillation column to produce purified benzene in the distillate. The separation will take place in an existing column with a total condenser, a partial reboiler, and several optional feed locations. The feed stream is of fixed flow rate, composition, and thermal conditions. The entire feed may be introduced at any one of the available feed trays, but may not be split and introduced at more than one feed tray. The condenser pressure is controlled by an inert gas flowing in and out of the reflux drum. Using column modules representation, determine the degrees of freedom for this operation, and recommend a set of specifications to define the column performance. 

The model for which the solution methods are developed represents a fixed configuration column. Thus, parameters such as number of trays, feed and draw tray locations, and heater and cooler locations are all fixed. The column is completely specified by defining a number of additional performance specifications equal to the degrees of freedom of the column. The model is then solved to determine all the other variables. 

The number of independent variables required to define the operation of an absorber or stripper may also be determined by applying the description rule, stated in Section 5.2.1. The number of trays or the column height is set by construction and may, in the design phase, be used as design variables. Since, by definition, the feeds are introduced at the top and bottom of the column, the feed locations are not variable. The feed compositions and thermal conditions are set outside the column region and are therefore beyond the operator s control. The operator can, however, control the valves on the two feeds and the two products. One of these four valves, usually the bottoms product valve, cannot be controlled independently since it must be set at steady state such as to maintain the required liquid level in the bottom of the column. The overhead valve is usually used to control the column pressure. The two feed valves may be controlled independently one controls the main process stream rate and the other controls the solvent or stripping gas flow rate. 

A distillation column with an attached side stripper is used to separate a feed stream F into products A, B, and C, as shown in the diagram. The main column has a total condenser and a partial reboiler, and the side stripper has a partial reboiler. The columns are existing units with fixed number of trays and feed and draw locations. The external feed, a product of an upstream unit, is of fixed flow rate, composition, and thermal conditions. The pressure in the columns is determined independently and is not available as a variable. Using basic modules representation, determine the degrees of freedom of this system. What variables would you specify to define the performance of these columns ... 

When the feed is introduced at its optimum location, the rectifying and stripping sections are balanced with regard to the amount of fractionation taking place in each. Uniform fractionation is characterized by even variation from tray to tray of the separation parameter, defined as the logarithm of the ratio of the light to heavy key concentrations in the hquid on a given tray ... 

As in two-product columns, total reflux in multiple product columns is a limiting condition where the colnmn internal liqnid and vapor flows are very large compared to each of the products and feed(s). Multiproduct columns are considered to consist of column sections defined by the product locations. Each section is bonnded by two products, one at its top and the other at its bottom. Thus, a column with. y sections has h- 1 products. As in two-product columns, multiproduct columns operating at total reflux achieve the maximum separation possible with a given number of stages in each section and for a given set of product rates. Conversely, if the separation between the different products is specified, the minimum trays required in each section are evaluated by total reflux calculations. 

Calculate the reflux heat, at Tray Dl. Reflux heat is defined as the apparent heat imbalance between external heat quantities at the point in question in the tower. These external heat quantities are denoted as Q with appropriate subscripts to signify their location. External heat input quantities are defined as the heat contained in the feed plus all heat to the system at product strippers either directly as steam or indirectly throu reboilers. External heat output quantities at a given tray are defined as the heat contained in liquid products leaving the system from points lower in the towier, the heat contained in the internal vapors of products plus steam and the heat contained by a product liquid flowing to the sidestream stripper. If the tray is nbt a sidestream draw tray, this latter quantity does not enter into the heat balance. 

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