Topic 4: further analysis of distillation in plate columns Flashcards
typically, in mccabe thiele analysis construction is there an integer number of steps?
no
how would you estimate the fractional number of total steps?
N+1 = 9 + (x9-xB) / (x9 - x10) = 9.4
example problem
how do we identify the feed stage from the mccabe thiele analysis graph
the feed stage is identified by the triangle intersected by the feed line
can you construct stages from the bottom up?
yes
if you construct stages from the bottom up how do you identify the feed stage
the feed stage is identified as the same way as before, where the q line intersects a triangle
if you construct from the bottom up, what does stage 1 represent
the partial reboiler
if you construct from the bottom up how do you calculate the fractional total number of stages
N+1 = 9+ (yD-y9)/ ( y10- y9)
previously, the feed state parameter q was introduced as the liquid fraction in the feed, is this consistent for any feed state?
no ,it is only satisfactory for saturated feeds
in the mccabe thiele analysis, how is the difference between the liquid flow rates below and above the feed calculated?
the difference between the liquid flow rates above and below the feed is calculated from the feed state parameter as follows:
/L - L = qF
how would you make the feed state parameter q consistent for any feed state?
from /L - L =qF
q= (/L-L) / F
noting that:
for subcooled liquid feeds, all the feed descends and also some rising vapour condenses
for superheated vapour feeds, all the feed rises and also some descending liquid is vaporised
how do you calculate q from the feed stream enthalpy?
F + L + /V = /L +V material balance around the feed stage
Fhf + Lhl + /Vhv =/Ll + Vhv energy balannce around the feed stage
q= /L + L / F = 1 + (/V-V)/F from new definition of q
hf= qhl +(1-q)hv
therefore,
q = hv-hf / delta( h vap) where hvap= hv-hl
for saturated feeds, q remains the liquid fraction in the feed
what does q= …. represent?
q=1.5 subcooled liquid q=1 saturated liquid q= 0.5 partially vaporised q=0 dew point vapour q= -0.5 superheated vapour
analyse the distillation column that has two column feeds
A distillation column with two columns feeds has;
an upper operating line with slope L/V = R/R+1 passing through xD,xD
a lower operating line with slope /L/ /v passing through xB,xB
an intermediate operating line with slope L’/V’
two q lines for each feed
the operating line intersect on the q lines
how do you calculate V’, L’, / V, /L?
V’ = V-(1-q1)F1
L’=L +q1F1
/V= V’ -(1-q2)F2
/L= l’ +q2F2
how do you construct the mccabe thiele diagram for a case of two column feeds?
plot the q lines
plot the upper operating line based on specified xD and R (slope R/R+1)
calcuate V’ and L’ and hence find L’/V’ and plot the intermediate operating line
knowing xB, plot the lower operating line
what is a side stream?
a side stream is treated like a feed but with negative flow rate. side streams may either top off either saturated liquid or saturated vapour with molar flowrate Fs
how do you use the mccabe thiele analysis for feed plus side stream
plot the q lines
plot the upper operating line based on the specified xD and R
calculate V’ and L’ hence find L’/V’ and plot the intermediate operating line
where V’=V-(1-q)F1 and L’=L+q1F1 and F1=-Ls
knowing xb, plot the lower operating line
when would direct steam injection be used
for cases where water is the heavy component in a binary mixture, saturated steam may be injected directly into the base of the column
e.q distillation of CH3OH +H2O
why is direct steam injection useful
it eliminates the need for a reboiler which saves on CapEx
does direct steam injection affect the columns performance?
the columns performnce is similar to the reboiled one
