Impurity components product concentration

As was mentioned in Section 11.2, in multicomponent distillations it is not possible to obtain more than one pure component, one sharp separation, in a single column. If a multicomponent feed is to be split into two or more virtually pure products, several columns will be needed. Impure products can be taken off as side streams and the removal of a side stream from a stage where a minor component is concentrated will reduce the concentration of that component in the main product. 

The task of the lights column is to remove the light components from the recycled EDC, with chloroprene and tri-chloroethylene being the most important impurities. Therefore, a concentration-cascade scheme was implemented, with chloroprene concentration and reboiler duty as controlled and manipulated variables, respectively. The distillate to feed ratio was kept constant using feedforward control. This ratio can be used to adjust the level of tri-chloroethylene in the bottom product. The level in the condenser drum was controlled by the reflux. Note that fixing the reflux and controlling the level by distillate does not work, because the distillate rate is very small. 

The output solutions contain only enriched products without auxiliary ion impurities. At the same time that the ion separation takes place concentration of the mixture components occurs. The product concentration in column I equals the total concentration of the ion mbmire put into the column and the product concentration in column II, that is, the ion-displacer concentration. This is riiy, other conditions being equal, the use of more concentrated solutions is beneficial. However, it should be taken into account that increase in the total concentration of the mixture of ions separated may result in a decrease in the sorbability differences between the separated and auxiliary ions. Moreover, the resultant decrease in the single-stage separation coefficient reduces the efficiency of the column operation. 

Purification separation is meant here as the removal of one or more impurities from a stream to achieve very high concentration (purity) of the dominant component. The initial concentration of impurity in mixture should be lower than 2% or 2000 ppm. The final concentration of impurity in product should be less than 100 ppm. Separation methods suitable for purification are equilibrium adsorption, molar sieve adsorption, chemical absorption and catalytic conversion. 

Make sure that the overall component balances for all chemical components can be satisfied. Light, heavy, and intermediate inert components must have a way to exit the system. Reactant components must be consumed in the reaction section or leave the system as impurities in product streams. Therefore, either reaction rates (temperature, pressure, catalyst addition rate, etc.) must be changed or the flow rates of the fresh feed makeup streams must be manipulated somehow. Makeups can be used to control compositions in the reactor or in recycle streams, or to control inventories that reflect the amount of the specific components contained in the process. For example, bring in a gaseous fresh feed to hold the pressure somewhere in the system, or bring in a liquid fresh feed to hold the level in a reflux drum or column base where the component is in fairly high concentration (typically in a recycle stream). 

Low concentrations of chemicals may not be declared Raw materials may contain hidden impurities Final product may contain starting materials Decomposition of components Contamination of residues... 

Section trajectories at quasisharp and nonsharp separation and at reflux bigger than minimum are examined below. At quasisharp separation, each product of the column contains, besides the product components itself, small amounts of impurity components, mostly of the key nonproduct component. The purpose of separation is to obtain in each product a prescribed set of product components at a prescribed summary concentration of impurity components. 

At the design calculation, the following summary concentrations of impurity components in separation products are set (specified) ... 

Trial calculations of the top section are realized at different httle concentrations of non-key impurity components in the top productXx),A +2, a +3 XD,n, " trial calculations of the bottom section are realized at different little concentrations of non-key impurity components in the bottom product x. i, Xb,i XB,k-i-... 

The algorithm of design calculation includes a preliminary search for little concentrations of non-key impurity components in separation products and subsequently more precise definition of these concentrations. 

The little concentrations Xg j, Xg 2 b,a -i at which trajectory of bottom section comes to point xj, where the concentrations of the non-key impurity components of the bottom product are the same as in point (x f)f corresponding to point (xf lYi in material balance are determined in the same way that is, =... 

For the above-described conditions of validity of trial calculations, the concentrations of the non-key impurity components in the feed cross-section are monotonously increasing functions of the concentrations of these components in the separation products. Besides that, in the vicinity of points and the concentration of each non-key impurity component is a linear function of the concentrations of all non-key impurity components in the corresponding separation product. 

By means of solution of Eq. (7.7), we determine more precise values of little concentrations of non-key impurity components in the separation products >, +2 >.k+v suring smaller values of discrep-... 

Component 4 is the non-key impurity component in the top product, and component 1 is the non-key impurity component in the bottom product. Trial calculations of the top section are carried out until the summary concentration of components 3 and 4, which is equal to the concentration of these components in a chosen point (xof segment [x/ i]f, is achieved at some tray. Similarly, trial calculations of the bottom section are performed until the summary concentration of components 1 and 2, which is equal to the concentration of these components in a chosen point (xf)f , is achieved at some tray. 

The little concentrations of non-key impurity components in products Xda and xb, are determined during the search process with the help of the described algorithm, and the concentrations of the rest of the components in the products are... 

The system of the equations for the componentwise discrepancies of the material balance in the feed cross-section is solved for the set value of CT = R/Rmm. determining the more precise values of little concentrations of the non-key impurity components in the separation products and of tray numbers in the column sections. The difference of that from the corresponding step of the algorithm for intermediate splits consists of the fact that tray numbers in the sections are included into the number of independent variables besides the concentrations of the non-key impurity components in the separation products. In accordance with that, it is accepted that the concentration of each component in the feed cross-section is a Unear function not only of the little concentrations of the non-key impurity components in the corresponding product, but also of the tray numbers in the corresponding section ... 

Table 7.2. Product concentration of non-key impurity component and number of trays...

As far as the joining of trajectories of the top and the intermediate sections is concerned, the composition point at the first tray below the cross-section of the entrainer input Xe should be located quite close to the boundary element of the concentration simplex that contains components of the top product and of the entrainer (in Fig. 7.12 - to side 1-2). Allowable concentration of impurity components in point Xe is determined by the requirements to the purity of the top product. Therefore, the composition in point Xe is not an optimized parameter and the composition in point Xe i at the first tray above the cross-section of entrainer input is determined by the conditions of material balance in this cross-section. 

Special requirements to the refining property make necessary a preliminary step to the design procedure the determination of the set of pseudocomponents specified by their normal bubble temperatures (at 1 atmosphere) in each product, and the determination of admissible concentration of impurity components in each product. 

We examine the main steps of the algorithm. The given data at heteroazeotropic and heteroextractive distillation are concentration of impurity components in the bottom product. This information unambiguously determines the bottom product composition if one component is impurity one (see Section 6.9 Figs. 7.16e,f and 7.17a,b). If two or more components are impurity ones (Fig. 6.16a d), then the bottom product composition is set in initial approximation, taking into consideration the ratios of phase equilibrium coefficients of impurity components in the bottom product point. In this case, the bottom product composition is defined more exactly later at iterations. 

The method of dynamic programming synthesizes the optimal sequence starting from its end. Therefore, expenditures Su should be determined without calculation of the previous part of the flowsheet. For this purpose, it is necessary to determine the composition of feeding of colunm IJJi.. It can be done easily, if it is accepted that each product of separation sequence i contains as impurity components only adjacent components i -1) and i +1) (i.e., the set permissible concentrations of impurity components) ... 

Where and rjF are set permissible concentrations of light and heavy impurity components in product i correspondingly, and concentrations of components (i - 1) and (i + 1) in product i correspondingly. 

Tear-off point or at minimum refihix (5, 5 ) point where the section trajectory at minimum reflux tear-off from boundary element of concentration simplex containing aU product components and one impurity component which volatility is niar to product component volatilities. 

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