Distillation column process characteristics

Byproducts are materials that are not one of the intended products of a production process. An example is the sediment remaining at the bottom of a distillation column. Byproduct is a catch-all term and includes most wastes that are not spent materials or sludges. Listed byproducts are solid wastes when reclaimed used in a manner constituting disposal burned for energy recovery, used to produce a fuel, or contained in fuels or accumulated speculatively. On the other hand, characteristic byproducts are not solid wastes when reclaimed, unless they are used in a manner constituting disposal burned for energy recovery, used to produce a fuel, or contained in fuels or accumulated speculatively (Table 13.1). 

The next thing that is needed is a program that keeps track of all the process and utility streams, and determines the order in which the individual equipment calculations will be performed. This is sometimes referred to as the executive program. The user of this system has merely to put into computer language the flow diagram, which identifies the units (areas of heat exchangers, number of trays in a distillation column) and their interrelations, and to list the operating characteristics of each unit (the pressures, temperatures, exit compositions), the input variables to the plant... 

Figure 1 lists the major industrial processes that use thermodynamic data and the kind of data that are relevant. The design and use of distillation columns are the largest consumers of thermodynamic data. The feed stream to most industrial columns contains at least several components. The predication of the operating characteristics of such columns requires complicated calculations, and much computer software has been written... 

The process control scheme proposed for the column is based upon similar suggested layouts used for the control of distillation columns (Ref. A8). Modifications were made in accordance with the individual operating characteristics and requirements of this system. Final verification of the proposed scheme is achieved by consulting Refs. A7 and A9. 

Many consumer products are produced, at least in part, using chemical processes. A characteristic chemical process involves a chemical and/or physical transformation of raw materials into products or intermediates that are then further processed. Process flowsheets or process flow diagrams are used by process engineers to depict the flow of process streams through the basic unit operations involved in a chemical manufacturing process. A unit operation typically refers to a vessel where a chemical or physical transformation occurs. Examples include chemical reactors and distillation columns. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

Operating Line and "Equilibrium" Curve. Both terms are of importance for the graphical solution of a separation problem, i.e., for the graphical determination of the number of stages of a cascade. This method has been developed for the design of distillation columns by MacCabe and Thiele and should be well known. For all cases, the operating line represents the mass and material balances. In distillation, the equilibrium curve represents the thermodynamical va-por/liquid equilibrium. For an ideal binary system, the equilibrium curve can be calculated from Raoult s law and the saturation-pressure curves of the pure components of the mixture. In all other cases, however, for example, for all membrane processes, the equilibrium curve does not represent a thermodynamical equilibrium at all but will represent the separation characteristics of the module or that of the stage. 

The integration of the recycling reactor directly into the distillation column leads to the process configuration shown in Fig. 5.2, in which the reaction takes place within the column total reboiler. On top of the reboiler a fully non-RD section is installed. This process can be seen as a simple hybrid RD column with only one reactive tray. Comparing the curves in Fig. 5.4 and Fig. 5.6, the operational characteristics of the two processes, recycling system and distillation column with reactive reboiler, are identical at 93 = 00 and R = co. 

Understand Process Characteristics H2, H2S, and NH3 and light ends are removed from reaction effluents through a series of separation and flashes, resulting in the reaction products in a liquid form, which goes to the stripper, the feed heater, and then to the main product fractionator. The task of the product fractionation is to separate different products based on their product specihcations such as distillation endpoint, ASTM D-86 T90% or T95% point, and so on. Side draws from the column go to the product strippers where kerosene and diesel products are made. The net draw from the column bottom is called unconverted oil (UCO), which is recycled back to the reaction section for nearly complete conversion. There are two pump-arounds, namely, kerosene and diesel pumparounds, as a main feature of heat recovery from the main fractionation column. 

The application of rigorous simulation packages, such as HYSYS, provides a valuable basis for the design and overall evaluation of advanced process control applications to the simulated processes. Steady state and dynamic simulations help in the process development by analysing and validating the design and/or ideas before their implementation to avoid costly modifications and to ensure safe operation. The simulation of the VCM plant is developed in HYSYS.PLANT in both steady state and dynamic modes and could be used for further economical, environmental and operational evaluations. Table 2 shows the characteristics of the VCM plant model and the detailed data and specifications of the main processes. Fig. 10 shows the process flowsheet of the simulated VCM plant in HYSYS including the main reactors and distillation columns. 

Currently, virtually all industrial scale separations of hydrocarbons are performed by distillation (43,44). Distillation alone is inherently inefficient when the vapor/liquid equilibrium line is close to the operating lines in McCabe-Thiele diagrams. This occurs when components have similar volatilities, form azeotrope(s), or when high product purity is required. While it is unlikely that membrane separation processes will displace distillation, the use of membranes to eiihance the performance of distillation columns is a possibility, if membrane materials with the necessary performance characteristics can be developed. Membrane/distillation column hybrid processes offer significant advantages in a number of situations ... 

The pyrometallurgical methods were developed based on the differences between zirconium and hafnium in oxidation and reduction characteristics [11, 12] volatility [13-16] electrochemical properties [17-19] and molten metal-molten salt equilibrium [20, 21], The extractive distillation process, using carbochlori-nation of zircon [13], is in operation by CEZUS in France. Both chlorides are sublimated and run through a vertical distillation column containing molten aluminium chloride and potassium chloride. Both hafnium and zirconium tetrachloride chlorides dissolve, but hafnium tetrachloride has a higher vapour pressure and is therefore condensed from the top of the column in a hafnium-enriched mixture. The zirconium tetrachloride is partitioned to a liquid phase and recovered from a salt, typically containing less than 50 ppm hafnium. 

For processes whose uncompensated load-response curves cross the set point, lead-lag may be inadequate. For these applications, an additional lag is useful in canceling the first part of the curve, while the lead-lag function compensates the balance. Distillation columns typically exhibit this characteristic. Further discussion of the problem will be found in Chap. 11 and in reference (3). 

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