1.1.5 Flash distillation

Points: 7 No answer

This task is supposed to make you familiar working with the enthalpy/composition (hxy)-diagram. We considered this kind of diagrams for the determination of the temperature of binary mixtures in the flash distillation kettle.
The figure below shows the respective slide from the lecture.

In this OPAL exercise we consider a

  • binary mixture of ethanol (1) and water (2).
  • The mixture features a composition of $$w_1=0.5$$.
  • The isenthalpic throttle depressurizes the binary mixture from $$4MPa$$ to $$0.1MPa$$.
  • The temperature of the binary mixture before throttling is $$150°C$$.

The respective hxy-diagram is shown below. This kind of diagram is also referred to as PONCHON SAVARIT diagram. It is from a rather old book "Technische Thermodynamik, Franjo Bošnjaković, T Steinkopff (1935", which is also available in the TUBAF library (https://katalog.ub.tu-freiberg.de/Record/0-114246749X/Holdings). As the book is old, the energies are given in kcal instead of J. You can transfer the energy units like this:

  • 1 kcal = 4.1868 kJ
  • 1 kJ = 0.2388 kcal

Attention! The compositions are given in mass fractions! Therefore, the ordinate provides specific instead of molar enthalpies!

 

TASK 1:

What is the specific enthalpy of the binary mixture before it has passed through the throttle?

  • The mixture is supposed to be ideal (no excess enthalpies)!
  • The reference states, where the pure compound enthalpies (for both compounds) are zero are "LIQUID", "0°C" and 1 bar.
  • The influence of pressure onto the enthalpy can be neglected.
  • The boiling point temperatures of the two pure compounds at $$4MPa$$ are above $$150°C$$. Therefore, the binary mixture is liquid, before it passes through the nozzle.
  • The specific isobaric heat capacities are $$c_{p,1}=2.5 kJ (kg K)^{-1}$$ and $$c_{p,2}=4.2 kJ (kg K)^{-1}$$ for the two liquid pure compounds.

Before the mixture has passed through the isenthalpic nozzle, its specific enthalpy (check the unit!) is $$h^F=$$ $$kcal (kg)^{-1}$$.

TASK 2:

What is the specific enthalpy of the binary mixture after the mixture has passed through the isenthalpic throttle.

$$h^F=$$ $$kcal (kg)^{-1}$$

TASK 3:

You can read the temperature (in °C) of the binary mixture after the throttling from the Ponchon Savarit diagram. You also can read the compositions of the coexisting phases.

The temperature after isenthalpic throttling is $$T=$$ $$°C$$

The composition (as weight fraction) of the liquid phase is $$w^L_1=$$ .

The composition of the vapour phase is $$w^V_1=$$ .

The mass flow ratio $$L/G$$ is . (you can use the lever rule for the determination of the mass flow ratio; simply put your ruler on the screen)

TASK 4:

Assume that the mass flow ratio is $$G/F=0.15$$ and that the vapour phase composition is $$w^V_1=0.7$$.
The condenser condenses the vapour stream "G" to a boiling liquid. What is the feed specific cooling energy of the condenser (unit $$k J/kg$$)?
Read from the Ponchon Savarit diagram, how much cooling energy is required for the condensation of $$1kg$$ saturated vapour and then additionally consider the ratio $$G/F=0.15$$.

The feed specific cooling energy is $$Q ̇_C/F ̇ =$$ $$k J/kg$$.