Question

A flash drum is to flash 10000 lb moles/hr of a feed that is 65 mole % n-hexane and 35 moles % n-octane at 1 atm pressure. V/F = 0.4. Find Tdrum, liquid composition, vapor composition. Use the K values given in Table.

T/

Answer 1

First of all, we have to know the kind of system which we have, the variables, the available data. According to the these (we have a table with temperature and partition coefficient;we have absolute pressure of our system and also we have the initial concentrations. Finally, we know the vapour/liquid ratio (vaporized fraction)), we can say that our problem is a vapour-liquid equilibrium problem, which can be solved by using of the isothermal flash equations.

As we know, these kind of problems are solved by means of iteration procedures. Therefore, we need to know our freedom degrees (equations and unknowns) to develop the proper iterative process. Also, we have to decide how we will conduct our iteration process.

By analysis of our problem we have:

ZC6/(KC6-1)*+1)+ZC8/(KC8-1)*?+1)-1 (1) objective function (the sum of fractions will be iqual to 1.

f(x3)=f(X1)+((x3-x1)*((f(x2)-f(x1))/(x2-x1))) (2) secand method used to iterate

KC6=YC6/XC6 (3) KC8=YC8/XC8 (4) Final equations to calculate the liquid and vapor concentrations.

Zi=feed concentration of i

Ki= partition coefficient of i.

= vaporized fraction.

Yi=vapour concentration of i.

Xi= liquid concentration of i.

The algorithm will be:

1. Assume 2 Temperatures.

2. Read all Ki from table.

3. Replace all of data in (1).

4. If equation (1)=0, go to step 5. Else, use equation (2) to estimate the temperature. Remember that we maybe need to interpolate to get the Ki values.

5. Replace the K-values in (3) and (4) and solve the 2x2 system equations.

Using the procedure, the results are:

Tassum	KC6 read	K8 read	f(x) cal	Tcal
90	1.8	0.26	-0.01042
100	2.37	0.27	-0.08575	88.61731
90	1.8	0.26	-0.01042
88.61	1.744	0.269	-0.00445	87.57516
87.56	1.702	0.268	0.002405	87.92861
87.92	1.717	0.268	3.85E-05	87.92586

T= 87.93