Experimental Studies of Feed Plate Manipulation for Distillation Column Feedforward Control Emmett J. Speicher and William 1. Luyben’ Department of Chemical Engineering, Lehigh University, Bethlehem, Pa. 18015
An experimental investigation of the feasibility of changing feed plate location to control a distillation column was undertaken to confirm earlier theoretical studies. Top product composition was successfully held constant a t 0.94 mole fraction methanol in a 24-tray, 8-inch4.d. methanol-water column b y gradually changing the feed tray from tray 14 to tray 8 when the feed composition dropped from 0.40 to 0.31 mole fraction methanol.
T l i e iclca of iisitig tltc fee({ tray 1oc:itioii as :i iit:iiiil)ul:ttivc v:irinlilc t o coinpciis:itc for ii feed coiniiositioii (1istiirl):iiicc \vas es~ilorccli i i :in cw1ic.r 1):iper by digital siiiinlatioii (LuyIrii> 1968). This lisper ])re,-(vifs the rewlts of nil cslirriiiietital study to vrrify soiti(~of tlic tlieort4c:il result.;. .in :icrur:itc stc:itly-st:ltc~ rllodcl of colulllll \v:i> develo]jctl and i i s ( ~ 1to predirt the ste:idy-st:ito cffccts of (~Iiniigiiigfeed tr:iy 1oc:itioii. The dyii:ittiic~:idjustiiieiit of the feed 1)latc W L I S dcterniiiictl cs~~ci~itiieiit:ill\-, ubiitg trial :iiitl error 1)r~ic~Iiircs. l k i i this crridc ciiii)iricnl tccliiiique r c 4 t c d iti cffcctive stwdy-state and (lyti:iiiiic fctdforwircl coiitrol. Description of Equipment
‘Ylie (1istill:itioii uitit, \vas a 24-tr:i~~, ~-ii!cIi-~li:iiiieter,liul~lile c~oluiiiii.l’lie tii:itcri:il tliitilled NYIS the biitiiry metlt:iiiol\vuter mist w(’a t :itriiosplieric i)re>wre. Tlic colurnn \viis equil)lml wit,li a vertical, tlter~iio~iphoii relioiler, a feed 1)i.eheater, two coiideiiiers, aiid a reflus drum from \vliieli reflux returned t o the coluniii hy gravity flow. Flow rates were me:isruttl 11y Fosboro I)/l’ cells. Steam f l o ~t o tlie reboiler and reflux flow were flo\v-coiitrolletl by l l o o r e 1’1 feedback controllers. ‘Yliermocouples tliroughout t’lie sybteni were ITc ~ ~ r d c011 t l :I Lecd.; & Sort’liruli8peedoni:is. Feed mid hottoms roiiipo4tioiis w i ’ e deteimiiied by taking s:implcs and u i n g liytlrorneters t o find denhities aiitl thus rompositiotis of the mctliaiiol-n.nter niistut.e. A coiitiiiuous on-line inensureinelit of overheat’ Iirotliict coniiiosition \vas obtained froni a l’rince I)ensitid. Specific details of the equipment are giveii by Speiclter (1969). cnli
Dynamic Tests
Steady-State Model
l‘lie first step \vas to see if feed plnte loc~tioiicould be used our exiieriiiiciit:il sy,+m to control diqtillate coiiil)o~itioii from :I 1)iireIy steady-state standpoiiit. .I digital coiiiiiutcr program was nrittcii that iiiclutled tlic tionidealities of the tern : iioiiequimolal overflow, inefficient trays, 1ie:it locqe5, and varialilc relative wlatility. Steady-btate espeririieiitnl data were obtaiiied :ind the tray t%ciciiry parameters in the computer program were adjusted to makc tlic steady-state model rtiatcli tlie data. l‘:iblc I cornpares coniliiitcd n i i J esi)erinieiitnl coluniti Iirofilcs :ind give> Fte:itly-stntr v:iliies of varialile.: and ~ ~ u r a r i i c t e r , ~ . S e s t , tlicl steady-state ~irogriun \vas ~ i s c d to coiistruct curves of distillate coriilioiitioii, SD, vs. bottoms coinpositioii, S g , for various fwd tray locntioiis, -ITp, and feed c~)iril)ositions, ATP (Figure 1). ‘I’lie iihual maiiipulative v:iri:i1hj reflus :md v:il)or l)oilu~),\ v ( w licltl coitst:iiit, iti
To whom correspoiideiice hhould be sent, Ind. Eng. Chem. Fundam., Vol. 10, No. 1 , 1971
147
Table I. Steady-State Results (Experimental VS. model) Feed Tray Compositions (m.C Methanol)
XI3
X D
8
8
Exptl.
Model
0,052 0.334 0,348 0.645 0,959
0.050 0,281 0.376 0.615 0.959
14
14 Exptl.
Model
0,021
0,034
...
...
... ... 0.931
0.934
Temp., OC
81 76 75 72 70 68 67 66
T2
T4 T8 TI" 7'14
y
17
y '20 T24
82 77 76 75 75 72 70 66
80.2 76.9 75.5 73.3 70.6 68.4 66.5 65.1
81.8 77.3 75.4 75.2 75.1 71.3 68.5 65.5
Efficiencies (Murphree Vapor)
Feed coinpohition Feed temperature, "C Reflux temperature, "C Steam preshure in reboiler (psig)
Reboiler 90% Stripping section 5574 Rectifying section 40P-j,
Rates
Feed Diytillate Ijott(Jlllb Heflu\
stcnrn
GPM
0 0 0 0
MoleslHr
70 14 0 38 4 8 36 9 5 39 50 (175 lb hr)
P
X 0.69
z
0 k
fn 0 n 2 0.92 0 V W
t-
a 10.95 _1 tv,
-
n 0.9E
0.03
0.06
0.09
BOTTOMS COMPOSITION
Figure 1 .
148
Ind. Eng. Chem. Fundom., Vol. 10, No. 1 , 1971
0.12
Xe
Open-loop steady state
0.15
0.355 58 60 18
TIME v
gQ
o
(MINS.)
-0.01
(MINS.)
1
I
-0.01
Figure 3.
-0.02
o.011 P
x a
Responses of controlled variable
GRADUAL
o
C H A N G E IN
NF-,
20
5
-o"lt
DIRECT
A
CHANGE
IN
>
TIME c (MINS.)
NF
I
Figure 4.
Responses of controlled variable
tioii iy set Iiy the reflux flow controller; the reflux overflow liiie i; li:ilf-way ul) the reflux drutn). If tlic p r e - ~ r eiii the top of the column v, too liipli, liquid 1i:id t o I)uild ill) all tlic way into the reflux drum. 'Tlic f l o ~ of reflux did iiot stay coiiqtaiit, c ~ c i as i the f l o ~ controller decrtasetl the tli+till:ite f l o y 1iecxu.c of t h e liydrirulic eff cct+. The tlrol) iii reflux made overhead coriil)ositioii dccrc:ihe for :i wliile uiitil the 1iydr:iuIics stabilized aiitl t h e t reflux rate r v : ~acliieved. Tlicii overlientl roiiil)ositioii huik u1i to it- exl)ectctl value. T h i b iiiiexi)ectetl iioi~liiieu1)eli:ivior i i :I good cxiiiililc of :I Iihciionieiioii that would probably never lie thought of it1 a tlieorctical study of tlie coluriiii dyialiiici xitliout the exi)erimeiitnl ~ v o r k . Nomenclature
.YF T, XB
feed tray iiuinher teni1)erature on nth tray>"C: hott,onis composition, niole fraction metlianol X D distillatmecompositioii, mole fraction rnetliniiol X F = feed coniposition, mole fractioii methanol S,,= liquid composition on ntli t,ray, mole fraction methxnol = = = =
literature Cited
Luvbeii, IV. L., IND.EKG.CHEM.FL-NDAM. 7, 502 (1968). Speicher, E. J., X1.S. thesiu, Lehigh Universitl-, Bethlehem, Pa., 1969. RECEIVED for review llarch 24, 1970 ACCEPTED November 30, 1970
Ind. Eng. Chem. Fundam., Vol. 10, NO. 1, 1971
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