Lecture 2 Flashcards
(13 cards)
What are Flash calculations? (basically the whole topic of this lecture)
For multi-component systems of more than two components, graphical methods are not convenient and often calculations are made using (1) component material balances coupled with (2) K-value expressions, such as Raoult’s laws, and
(3) an energy balance if heat transfer occurs. These calculations are often known as flash calculations.
Note that Flash A flash is a single-equilibrium-stage distillation in which a liquid feed is partially vaporized to give a vapor richer than the feed in the more volatile components.
Break moment
Okay so this whole lecture basically talks about how we can find the vapour and gas compositions of different multicomponent systems, we will first add an extension for the Gibbs phase rule for a flash operation to determine how many and which variables can be specified, then we will define different systems (thermal, adiabatic, etc) and present ways to solve them using flash calculations.
What is the extension of Gibbs’ Phase Rule?
Gibbs’ phase rule as discussed in lecture 1 does not deal with extensive variables in feed, product, and energy streams, for
either a batch or continuous process. However, the rule can be extended for process applications by adding extensive
variables for flow rates or amounts in material and energy streams, together with corresponding additional independent
equations. To illustrate, consider the extension of a two-phase (vapour and liquid), multicomponent equilibrium condition, to a continuous, single-stage partial condensation or flash vaporization process as shown in written notes.
Final notes about the derivation?
- If we restrict NP to two phases (vapor and liquid), NV from becomes (3C + 10) and NE becomes (2C + 6)
- Note that we could write C instead of (C − 1) component material balances (EQ1) and eliminate the total material balance (EQ2). However, a procedure for solving the equations favours the former. F
- For process calculations, it is common to completely specify the feed variables: F, TF, PF, but only (C-1) feed mole fractions, because the missing feed mole fraction must satisfy the sum of feed mole fractions [Seen in EQ 3 and shortly explained in written notes]
- When it comes to the exam double check the system you have, you might need to change the number of variables or (not as likely) the number of independent equations, so don’t only memorize this but also understand it
Defining our new systems…(imp)
As we saw from the derivation, we have a total of C + 2 additional variables, this leaves us with 2 variables two more variables to specify. Process simulators permit the specification of the following combinations of two variables: (Found in digital notes).
With some process simulators, it is also possible to specify one or even two product mole fractions. However, when attempting this, one must be careful to avoid irrational values that can lead to infeasible results.
What is the procedure used for cases of an isothermal flash?
The procedure used is the of Rachford and Rice (RR). They recognized that the (2C + 6) equations constituted a nonlinear
system of algebraic equations that could not be solved directly. They developed a procedure that reduces the number of nonlinear equations that need to be solved to just one.
What is the procedure of Rachford and Rice (RR)?
That is shown in written notes
Before we carry out the RR procedure what must we first do?
Before applying the RR procedure, a check should be made to determine whether the mixture is between the bubble and dew points at the specified conditions. These checks can be seen in the written notes.
What is some important intuition that we gain from the results of the RR?
Once we have all the L and V compositions, we will see how well a flash operation (1 Equilibrium stage) will separate the components. If the results are not desirable then we will opt to include more equilibrium stages (distillation)
What is the proccedure for Adiabatic, Nonadiabatic, and Percent Vaporization (𝚿) Flash Calculations?
When the pressure of a liquid stream is reduced adiabatically across a valve as in Figure 4.8b, an adiabatic-flash (Q = 0) calculation determines the resulting phases, temperature, compositions, and flow rates for a specified downstream pressure. The calculation can be made by applying the isothermal-flash calculation procedure in an iterative manner. First, a guess is made of the flash temperature, TV. Then Ψ, V, x, y, and L are determined by the RR procedure. The guessed value of
TV (equal to TL) is then checked by an energy balance to determine Q. If Q is not zero to an acceptable degree of accuracy, a new value of TV is assumed and the RR procedure is repeated.
The nonadiabatic flash (Q ≠ 0) calculation is identical to the adiabatic-flash calculation except for a non-zero specification of Q. The percent vaporization flash can also utilize the RR procedure in the following manne
A smart thing we can do to make the guessing easier (nice to know)?
After the first two guesses, a plot of the calculated Q versus the assumed TV can be made with interpolation or extrapolation used to provide the next guess of TV. T
Bubble- and Dew-Point Calculations?
This is found in digital notes
Final comment about Bubble- and Dew-Point Calculations?
The bubble- and dew-point equations are nonlinear in temperature, but only moderately nonlinear in pressure, except at
high pressures. The latter is especially the case in the region of the convergence pressure, where K-values of very light
or very heavy components change drastically with pressure, as discussed in §2.5.2. Therefore, iterative procedures are
required to solve for bubble- and dew-point conditions unless Raoult’s law is applicable. In that case, a direct calculation of bubble-point pressure for a given temperature can be made with (4-30). I think that we only deal with Raoult’s law being applicable!!
