This will run a special program that should cause them to be formatted for viewing by your browser. Distillation calculations can be performed graphically on an enthalpy-concentration Hx diagram. This approach is sometimes called the Ponchon-Savarit Method. Working on the Hx diagram is more general than a McCabe-Thiele construction, because it takes direct account of the thermal effects and does not require an assumption of equimolal overflow. The very shape of the Hx diagram provides a clue as to the importance of the energy balances. If the dew point and bubble point lines are more or less straight and roughly parallel, it indicates that the latent heat of vaporization is basically constant with respect to composition.

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This will run a special program that should cause them to be formatted for viewing by your browser. Distillation calculations can be performed graphically on an enthalpy-concentration Hx diagram. This approach is sometimes called the Ponchon-Savarit Method. Working on the Hx diagram is more general than a McCabe-Thiele construction, because it takes direct account of the thermal effects and does not require an assumption of equimolal overflow.

The very shape of the Hx diagram provides a clue as to the importance of the energy balances. If the dew point and bubble point lines are more or less straight and roughly parallel, it indicates that the latent heat of vaporization is basically constant with respect to composition. This is the prerequisite for assuming equimolar overflow, and so the energy balances may be neglected. If the saturation curves show significant changes in curvature or separation, it suggests that to assume equimolar overflow will introduce error.

The coordinates are their composition and enthalpy. If the products are saturated liquid, as is the case for total condensers without subcooling and for partial reboilers, these points will lie on the bubble point curve on the Hx diagram.

Overall Enthalpy Balance The steady-state enthalpy balance for a distillation column is: One of the tools of graphical solution is the notion of colinearity.

This has been used before if you have used lever arm principles. For an adiabatic process, the feed and products will be colinear on an Hx diagram. Thus, it is useful to redefine our distillation system to be adiabatic, by bringing the condenser and reboiler inside the system boundary.

Rearranging the enthalpy balance gives: If we then define: The enthalpy balance becomes: Substituting from the overall material balance: After this modification, the system is adiabatic, so a line can be drawn through the feed point, F, and the points xD, hDp and xB, hBp. This line represents the system enthalpy balance, and so is called the overall enthalpy line. Remember: for a given separation only one of the reboiler and condenser duties is independent.

So, you will probably want to pick one duty and then construct the line through the feed point to determine the other duty. Notice that this represents the ratio of distances on the Hx diagram: the numerator is the vertical distance between the hDp point and the dew point saturation curve, while the denominator is the distance between the saturation curves. A calculation often begins by using the overhead product composition and temperature to obtain hDand Hy1. These in turn are used with the reflux ratio to get hDp.

Then the overall enthalpy line is drawn from hDp through the feed point, and the intersection with xB gives hBp. Stepping Off Stages In the McCabe-Thiele procedure, operating curves were constructed to represent the component balances for the column and to relate the liquid composition on a stage to the composition of the entering vapor.

These were then paired with the equilibrium curve, which relates the composition of liquid on a stage to the vapor leaving the stage. A similar procedure can be followed on an Hx diagram, except instead of using component balances, enthalpy balances are used. The equilibrium data is represented by the equilibrium tie lines on the Hx diagram. The operating lines are developed from enthalpy balances on the rectifying and stripping sections just as in the McCabe-Thiele approach operating equations were developed from the equivalent component balances.

The operating lines will connect the point representing the liquid on a stage with coordinates xn, hn to the point representing the adjusted enthalpy at the appropriate end of the column. Summary of Procedure Obtain enthalpy-composition diagram Fix the feed point F, and product points D and B using stream compositions and enthalpies Use the overhead product enthalpy and the reflux ratio to find the adjusted enthalpy of the overhead. Plot point V1. For a total condenser, the composition entering the condenser is the same as the overhead product, so this point will be vertically above point D on the saturated vapor curve.

Follow the tie line from point V1 to the saturated liquid curve. This intersection will be point L1. The intersection of the operating line with the saturated vapor curve will be point V2.

Repeat the two preceding steps until one of the V or L points is to the left of the overall enthalpy line. When xi is less than xB, construction is finished.

The number of stages can be read as the number of complete triangles. Limiting Conditions At total reflux, operating lines are vertical infinite slope. This can be used to determine the minimum number of stages. Not that operating curves are not required to do this -- only the endpoint compositions. While doing constructions on the yx diagram, a "pinch" was defined as the intersection point between the equilibrium curve and the operating curve.

The "pinch point" also expands, resulting in a single line where the operating and tie lines overlap. Minimum reflux still corresponds to a pinch at the feed conditions, so to determine the minimum reflux a line must be constructed so that the overall enthalpy line coincides with the tie line that runs through the feed point.

Example A feed containing 40 mole percent n-hexane and 60 percent n-octane is fed to a distillation column. A reflux ratio of 1. The overhead product is 95 percent hexane and the bottoms 10 percent hexane. Find the number of theoretical stages and the optimum feed stage. Assume that a total condenser is used. The column is to operate at 1 atm. Step 1: Equilibrium data is collected.

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## Ponchon-Savarit Method

Gur Ponchon-Savarit Graphical Equilibrium Stage Method Working on the Hx diagram is more general than a McCabe-Thiele construction, because it takes direct account of the thermal effects and does not require meghod assumption of equimolal overflow. Apr 20, 4. Related Topics Chemical Engineering Chemistry. Apr 19, 2. So, you will probably want to pick one duty and then construct the line through the feed point to determine the other duty. My understanding is that the McCabe Thiele method assumes that the liquid and vapor molar flow rates are constant in each section of the column.

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## Ponchon-Savarit Graphical Equilibrium Stage Method

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