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Azeotropic distillation with an internal decanter - …

Computers and Chemical Engineering 24 (2000) 2435 2446 Azeotropic distillation with an internal decanterAmy R. Cirica, Hassan S. Mumtazb, Grafton Corbettb, Matthew Reaganb,Warren D. Seiderb,*, Leonard A. Fabianoc,1, David M. Kolesarc,2,Soemantri WidagdodaDepartment of Chemical Engineering,Uni6ersity of Cincinnati,Cincinnati,OH45202-0171,USAbD epartment of Chemical Engineering,Uni6ersity of Pennsyl6ania,Towne Building D3, Street,Philadelphia,PA19104-6393,USAcWay ne,PA,USAd3M,St Paul,MN55144,USAR eceived 14 June 1999; received in revised form 5 June 2000; accepted 7 June 2000 AbstractA distillation column with an internal decanter is used to separate a mixture containing five some oxygenated and hydrocarbonsand having at least three carbon atoms, and water. One of the species is partially miscible with water, as well as another organicspecies. Both binary pairs exhibit azeotropes above the minimum bubble-point temperature.

Computers and Chemical Engineering 24 (2000) 2435–2446 Azeotropic distillation with an internal decanter Amy R. Ciric a, Hassan …

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Transcription of Azeotropic distillation with an internal decanter - …

1 Computers and Chemical Engineering 24 (2000) 2435 2446 Azeotropic distillation with an internal decanterAmy R. Cirica, Hassan S. Mumtazb, Grafton Corbettb, Matthew Reaganb,Warren D. Seiderb,*, Leonard A. Fabianoc,1, David M. Kolesarc,2,Soemantri WidagdodaDepartment of Chemical Engineering,Uni6ersity of Cincinnati,Cincinnati,OH45202-0171,USAbD epartment of Chemical Engineering,Uni6ersity of Pennsyl6ania,Towne Building D3, Street,Philadelphia,PA19104-6393,USAcWay ne,PA,USAd3M,St Paul,MN55144,USAR eceived 14 June 1999; received in revised form 5 June 2000; accepted 7 June 2000 AbstractA distillation column with an internal decanter is used to separate a mixture containing five some oxygenated and hydrocarbonsand having at least three carbon atoms, and water. One of the species is partially miscible with water, as well as another organicspecies. Both binary pairs exhibit azeotropes above the minimum bubble-point temperature.

2 The column is very sensitive to smalldisturbances which can lead to flooding, poor product quality, and migration of an embedded two-liquid phase region within thecolumn. These disturbances can cause the column to move from one steady state to another for the same specifications. Multipleoperating regimes are exhibited, with unusual transitions between them. Two regions involving multiple steady states are observed,one of which involves the partial-miscibility of two organic phases. The dynamics of moving interfaces between trays having one-and two-liquid phases, as well as controllers to insure that unwanted transitions do not occur, are examined. 2000 ElsevierScience Ltd. All rights : Azeotropic distillation ; internal decanter ; Oxygenated :locate:compchemeng1. IntroductionThe sensitivity of Azeotropic distillation columns withinternal decanters to small disturbances can lead toflooding and the deterioration of distillate and sidedrawpurities, from which it is difficult to return to normaloperation.

3 Thus far, this sensitivity and the distur-bances that trigger shifts in the operating regimes havenot been well understood. Furthermore, while there isan extensive literature on heterogeneous Azeotropic dis-tillation, that is, Azeotropic distillation towers with twoliquid phases on at least one of the trays or decanter , toour knowledge, Azeotropic distillation columns withinternal decanters have not been studied in the litera-ture (Widagdo & Seider, 1996).This paper examines the behavior of columns withinternal decanters, their operating regimes, and theexistence of multiple steady states. In normal operation,these columns have a sidedraw composed of a singleliquid phase, usually an aqueous phase, and two liquidphases on trays near the decanter tray. When operationin an anomalous regime occurs, there is only one liquidphase on the decanter tray and no sidedraw, two liquidphases appear on trays throughout the stripping sec-tion, the bottoms product contains two liquid phases,and the internal vapor flow rates are increased.

4 Fur-thermore, small disturbances, such as an increase in thefeed rate of the light species, can move these columnsfrom normal to anomalous operation. It will be shownthat the vapor flow is increased, which results in flood-ing when insufficient capacity is provided. For thesereasons, it is an objective of this paper to show thesensitive responses to small disturbances, to show how* Corresponding author. Tel.: 1-215-8988351; fax: ( Seider).1 Present address: CDI Corporation, Philadelphia, PA 19103-2768, address: Rohm & Haas Corporation, Bristol, PA 19007, :00:$ - see front matter 2000 Elsevier Science Ltd. All rights :S0098-1354(00) et al.:Computers and Chemical Engineering24 (2000) 2435 24462436multiple steady states arise, and to explore the dynamicresponses of columns equipped with PID paper also examines a related behavior thatoccurs as the bottoms flow rate is reduced, resulting inan increased recirculation rate.

5 Over a narrow range ofthe bottoms flow rate, two steady-state solutions arecomputed, one which involves only one liquid phase onthe decanter tray and no sidedraw, and the other inwhich two partially-miscible organic phases form on thedecanter tray, providing an organic A typical ternary systemColumns with internal decanters are used commonlyto remove one species, typically water, in a sidedrawthat concentrates in one of two partially misciblephases. When designing and analyzing these columns, itis usually beneficial to focus attention on three principalspecies, one of which concentrates in the sidedraw, withthe others concentrating in the distillate and bottomsproduct. Typical ternary systems have the residue-curvemaps shown in Fig. 1. (Note that all possible residue-curve maps associated with internal decanters are notshown). Species A, the lowest boiler, is recovered in thedistillate.

6 It may form maximum-boiling azeotropeswith water and species C, and, when two liquid phasesare present, it often strongly prefers the liquid phaserich in species C. Water may be either the heavy or theintermediate boiler. It forms a heterogeneous mini-mum-boiling azeotrope with species C. While water isconcentrated in a side stream from an internal decantertray, species C is an intermediate or heavy boiler recov-ered in the bottoms stream. Often, species A is a lightorganic chemical and species C is a heavy equilibria and residue cur6esIn this section, physical and thermodynamic proper-ties are presented for a specific, proprietary A water C system. This system is the subject of a theoretical andexperimental study of a distillation tower introduced inthe next section. Note that care was taken to obtaininteraction coefficients for the NRTL equation thatrepresent both experimental VLE and LLE data withacceptable understand the interactions within the system, thebinary pairs are considered first.

7 Fig. 2 shows thebubble- and dew-point curves for the three binary pairsat near-atmospheric pressure, computed usingASPENPLUS . The water C system exhibits a minimum-boil-ing azeotrope at 75 mol% water and C and a largemiscibility gap, while the A water system has a largermiscibility gap, but exhibits no azeotrope, and the A Csystem exhibits neither an azeotrope nor an miscibilitygap. Species A is the most volatile compound (normalboiling point of 40 C) and species C is the heaviestcompound (normal boiling point of 144 C).Next, the ternary system is considered. Fig. 3 showsthe residue curves and binodal curve at C. Notethat the residue curves are computed assuming vapor liquid equilibrium. The binodal curve is computed C to approximate the binodal curve at the highertemperatures at which vapor liquid liquid equilibriumexists.

8 There are four singular points, three at thevertices and one at the water C azeotrope. Because theminimum-boiling azeotrope is at a higher temperaturethan the boiling point of species A, a distillationboundary connects vertex A with the C waterazeotrope. Consequently, the operating line, or residuecurve, for a simple distillation process cannot cross thisdistillation towerIn this section, the proprietary distillation tower isdescribed. Results of steady-state simulations are com-pared with experimental temperature profiles from anindustrial tower in the next tower consists of 34 trays with a liquid feed totray 16 and a sidedraw from tray 12. It is modeled,assuming an overall efficiency of 70%, with 23 theoreti-cal trays, a liquid feed to tray 12, as shown in Table 1,Fig. 1. Residue-curve map for the A H2O C et al.

9 :Computers and Chemical Engineering24 (2000) 2435 24462437 Fig. 2. Bubble- and dew-point temperatures for binary pairs atnear-atmospheric pressure. (a) Water C binary. (b) A water binary.(c) A C a sidedraw from tray 9. Depending on the operat-ing conditions, some of the trays contain both aqueousand organic phases. The sidedraw removes some of theaqueous phase flowing from tray 9, while the organicphase flows to the tray below. This is accomplishedusing a built-in donut decanter tray that collects theliquid flowing from the tray above and permits phaseseparation, as illustrated in Fig. 4. The cavity in thecenter of the tray carries both the vapor, rising to thetray above, and the overflowing organic phase. Aninterfacial level controller maintains the position of theinterface between the aqueous and organic phases andprevents the organic phase from escaping in the side-draw and the aqueous phase from overflowing to thetray below.

10 It does not prevent entrainment and doesnot insure perfect phase simulationTheRADFRAC subroutine inASPEN PLUSwas usedto simulate the distillation asso-ciate a decanter with any tray in the tower. Phasestability is checked on every stage to determine whetheror not a second liquid phase exists. The decanter tray ismodeled by specifying the fraction of each liquid phaseto be returned to the tower, L1 and L2, for the organicand aqueous phases, respectively. When L1 is specifiedas unity and L2 is zero, the decanter is assumed to beperfectly efficient with all of the organic phase returnedto the tower, and all of the aqueous phase removed asa sidedraw. An imperfect decanter is modeled by speci-fying L1 less than unity and:or L2 greater than simulation was tuned to fit an experimentaltemperature profile by varying L2, while keeping L1 atunity. As shown in Fig.


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