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Multicomponent Flash Drum Calculation
Reference: Excel workbook file FLASH.XLS. Cell addresses shown
below in []
A ml!icom"onen! mix!re is flashed !hro#ha $al$e in!o a drm. %he
li&id and $a"or
"rodc!s are remo$ed !hro#h $al$ed lines.%he s"readshee! carries
o! a s!ead'(s!a!edesi#n calcla!ion) incldin# !he si*in# of!he !hree
con!rol $al$es.
%he feed is s"ecified b' i!s com"osi!ion)!em"era!re) "ressre and
flow ra!e.
%he s's!em chosen here is:ace!one) +("ro"anol) wa!er
and !he feed s"ecifica!ions are:
com"osi!ion) iz ,i 1,2,3= ) mole frac!ion: ,.+-) ,./) ,.+++
[0:01]
!em"era!re: /,C [02+]"ressre: +, "sia [022]flow ra!e: 3 k#4s
[0/]
%he densi!' of a li&id mix!re of !hese com"onen!s can be
"redic!ed from !he formla
3
i i
i 1
3
i i
i 1
mw z
a z
=
=
=
[025] where imw are !he moleclar wei#h!s) 3/.,/) 1,.,-)
2/.,21)
res"ec!i$el') and ia are cons!an! "arame!ers) ,.,6-6) ,.,//)
,.,26-3) res"ec!i$el'.
%his e&a!ion is fi! b' nonlinear re#ression on !he 7ensi!'
7a!a workshee! and !he"erformance of !he fi! is "resen!ed on !he
7ensi!' Char! shee!. %he densi!'8ex"erimen!al8 $ales were ob!ained
from !he H9S9S "ro"er!' "acka#e.
%his allows !he feed $olme!ric flow ra!e !o be calcla!ed.
[0-:02,]
%he feed mass frac!ions are ob!ained from !he mole frac!ions and
!he moleclar wei#h!s:
i ii 3
j
j 1
mw z
zmw
=
= [7:71]
%he hea! ca"aci!' of !he feed mix!re is com"!ed b' a wei#h!ed b'
mole frac!ion; smof !he com"onen! hea! ca"aci!ies a! !he feed
!em"era!re.
Feed
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3
P Pi i
i 1
C C z=
= [026]
%he com"onen! hea! ca"aci!ies are
ace!one: [ ]P1 kJC 123 0.186T C kgmol K
= + g
[02]
+("ro"anol: [ ]P2kJ
C 178.1 0.3066T C kgmol K
= +
g[023]
%his e&a!ion is de$elo"ed b' linear re#ression on !he Hea!
Ca"aci!' shee!.
wa!er: P3kJ
C 75.4kgmol K
=
g[021]
And !he feed en!hal"' is es!ima!ed b'
f pf fh C T= 2 [0+/]
%he feed $al$e is desi#ned from !he nominal desi#n
e&a!ion
[ ]
[ ]V
q gpmC
P pi
!p"#
= [0+,] wi!h a desi#n P of 3.5 "si [02-]
%he abo$e resl! wold be sed !o s"ecif' !he $al$e !o a
$endor.
A dimensional si*in# of !he $al$e is done wi!h !he formla
[ ]
3
2
V
3
mq
C m
P Pa
kg
m
=
[0+2]
%he $al$e is desi#ned !o be +3= o"en f; a! !he nominal desi#n
feed flow ra!e.
[0++:0+5]
>i!h an ?= !rim and ran#eabili!' $ ; of 3,:2 [0+]) !his
re&ires a s!em "osi!ion l; of,.11) accordin# !o !he formla
2@o!e: !his mi#h! be be!!er done b' in!e#ra!in# !he hea!
ca"aci!ies from ,C !o !he feed!em"era!re.
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( ) l 1f l $ = [0+3]
%he flash $essel is desi#ned on !he basis of a 3(min!e hold" of
feed li&id [E2,:E22].%his #i$es a li&id $olme of s! o$er +
m5[E2+]. An inside diame!er of 2.3 m is chosen
[E25] #i$in# a nominal li&id de"!h of 2.23 m [E23]. %he
ac!al $essel is si*ed !o ahei#h! of +.3 m [E21] !o allow sfficien!
$a"or s"ace and freeboard.
%he o"era!in# "ressre of !he $essel is selec!ed as 2 a!m
[E2/:E+,]. %he ideal #as lawis sed !o com"!e !he nmber of moles
[E+3] and !he mass [E+1] of !he $a"or in !he$essel. %he la!!er
makes se of !he a$era#e moleclar wei#h! of !he $a"or [B5])
whichcomes from ha$in# calcla!ed !he $a"or(li&id e&ilibrim.
%ha! is discssed below.
%he flash calcla!ion block makes se of $a"or(li&id
e&ilibrim and !he en!hal"' andcom"onen! balances on !he $essel
!o de!ermine !he com"osi!ions and flow ra!es of !heli&id and
$a"or s!reams lea$in# !he $essel. %his is an i!era!i$e calcla!ion
and"roceeds from !he es!ima!ion of !he li&id flow ra!e [H3] and
!wo li&id com"osi!ions
[H1:H6]. %he $a"or flow ra!e can !hen be calcla!ed from !he
o$erall mass balance [H-].
%he com"osi!ion of !he li&id is known) now !ha! i! has been
es!ima!ed. n !he
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[ ]f
28&" "
T K= #as s"ecific #ra$i!' a! flowin# !em"era!re [@+5]
f
1.63 P
% C P
= cri!ical4sbcri!ical flow fac!or [@+2]
For !he $a"or $al$e) nominal flow ra!e is chosen for +3= o"en
and an ?= !rim isselec!ed.
