Diffusion distillation — a new separation process for azeotropic mixtures part II: Dehydration of isopropanol by diffusion distillation

Diffusion distillation — a new separation process for azeotropic mixtures part II: Dehydration of isopropanol by diffusion distillation

265 DiEusion Distillation - A New Separation Process for Azeotropic Mixtures Part II: Dehydration of Isopropanol by Dilksion Distillation Diffusionsd...

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DiEusion Distillation - A New Separation Process for Azeotropic Mixtures Part II: Dehydration of Isopropanol by Dilksion Distillation Diffusionsdestillation - ein neues Trennverfahren ftir azeotrope Gemische Teil II : Isopropanolentw%sserung mit Hilfe der Diffusionsdestillation D. FULLARTON Bayer AG, IN/AN, 5090 Leverkusen

(F.R.G.)

E. U. SCHLijNDER Institut fir I%ermische Karlsruhe 1 (F.R.G.) (Received

January

Verfahrenstechnik,

Universitri’t Karlsruhe

(TH), Postfach

6980, 7500

30,1986)

Abstract A design is submitted for a diffusion distillation apparatus in the form of a tube bundle, which represents a new means for separating azeotropic mixtures. It is combined with two rectification columns for separating binary mixtures. The results thus obtained in separating isopropanol-water mixtures are compared with those for an established extractive distillation process. It is shown how the energy consumption in the entire plant can be minimized.

Kurzfassung Es wird ein Entwurf eines technischen Diffusionstrennapparates in Form eines Rohrbiindeis vorgestellt. Fiir die Auftrennung eines binlren Gemisches in die reinen Komponenten ist es zweckmtissig, diese Diffusionstrennapparatur mit zwei Rektifiiationskolonnen zu verschalten. Es wird gezeigt, wie der Energieverbrauch der Gesamtanlage minimiert werden kann. Fiir die Isopropanolentw&erung wird die Diffusionsdestillation mit einem in der Praxis eingesetzten Schleppmitteldestillations-Verfahren verglichen. Der giinstige Energieverbrauch der Schleppmitteldestillation kann mit Hilfe der Diffusionsdestillation nur bei relativ hohen SelektivitPten, d.h. aber grossen Austauschflachen bzw. hohen Apparatekosten erzielt werden. Zie Diffusionsdestillation diirfte daher vor allem fiir die Trennung von azeotropen Gemischen, fbr die sich nicht giinstige Schlepp- oder Extraktionsmittel finden lassen, von Interesse sein. S ynopse In Teil I fl] wurde der Einmss verschiedener Parameter auf die Selektivitdt und iibertragungsleistung der Diffisionsdestillation diskutiert. In diesem Teil sol1 der Entwurf eines technischen Diffsionstrennapparates und dessen Verschaltung mit zwei Rektifikntionskolonnen vorgestellt werden. I. Technische Ausj?ihtungeines

Diffusionstrennapparates

Eine Trennzelle besteht aus einem Doppelrohrapparat, in dem das zu trennende Gemisch als Rieselfilrn aufgegeben wird (s. Teil I, Bild 3). Diese aus Doppelrohren bestehenden nennzellen fasst man zweckmtissig zu 0255-2701/86/$3.50

Chem. Eng. Process,

einem Rohrbiindelapparat (s. Bild 1) zusammen. Im Mantelraum kann mit Dampf geheizt und das Kiihlwasser im Gleich- oder Gegenstrom durch die Innenrohre geji2hrt werden. Der ZulauA die fioduktabfiufe und der Heizdampffaum mit Kondensatabzug werden durch RohrbGden voneinander abgeteilt. Obwohl dieser Apparat aus konventionellen Bauelementen zusammengesetzt ist, stellt er aufgrund der erfbrderlichen kleinen Ringspaltweite doch recht hohe fertigungstechnische Anspriiche. Die Auftrennunn des Gemisches kann entweder kontinuierlich mit ausreichend langen Rohren in einem Durchlauf in einem sogenannten Integralapparat oder absatzweise mit Hilfe eines mit entsprechend kiirzeren Rohren bestickten Differentialapparates erfolgen.

20 (1986) 265-270

0 Elsevier Sequoia/Printed

in The Netherlands

266

2. Verschahung des Diffusionstrennapparates mit zwei Rektifikationskolonnen-Energieverbrauch der Gesamtanlage Da mit der Diffusionstrennapparatur nur eine Trennstufe realisiert wird, ist es zwecknuissig, mit ihr nur den azeotropen finkt zu iiberwinden und die Auftrennung in die annahernd reinen Komponenten mit zwei Rektifikationskolonnen durchzufiihren (s. Bild 2). Das zulaufende IsopropanolWasser Gemisch F wird praktisch vollkommen in den Wasserstrom W (Kolonne I) und den Isopropanolstrom P (Kolonne II/ zerlegt. Beide Kolonnen arbeiten bei atmosph&‘schem Druck. Die Kopfproduktstrome HI und HII haben anrrahernd azeotrope Zusammensetzung. Der nicht als Riickfluss R, und RI1 zuriickgefihrte Anteil der Kopfstrome wird im Diffisionstrennapparat D in eine wasserreiche Fraktion N, und eine isopropanolreiche Fraktion NV zerlegt. Der Diffsionstrennapparat kann mit einem Teil der dampffiirmigen [email protected] beheizt werden (s. Bild I ). Mit Hilfe der Energie- (Gl. (1)) und der Stoffiilanzen (Gl. (4)-(U)) tisst sich die Summe der Kopfstriime (Cl. (I 0)) und damit der Energieverbrauch der Gesamta&age (Gl. (3)) berechnen. Der Energieverbrauch ist in erster Naherung proportional zu den Riicklaufverhaltntisen der Kolonnen und umgekehrt proportional zu der . . integralen Selekttvttat S,, = xv - xK der Diffisionsdestillation. Der Energieverbrauch der Isopropanolentwtisserung durch Diffisionsdestillation wird dann mit einem in der Praxis eingesetzten Verfahren verglichen (s. Bild 3). Bei diesem Verfahren wird durch die Zugabe von Diisopropylather reines Isopropanol im Sumpf der ersten Kolonnegewonnen. Das iiber Kopfgehende IsoproparwWasser-A ther-Gemisch zerfallt in z wei Phasen. Die leichtere isopropanolreiche Phase wird als Riicklauf in die erste Kolonne gefiihrt, die schwere wasserreiche Phase in einer kleinen Nebenkolonne aufgetrennt. Eine vereinfachte Rechnung zeigt (s. Gl. (13)), dass der Enelgieverbrauch der Schleppmitteldestillation (3 k W h (kg Iso.)-‘) mit Hilfe der Diffisionsdestillation nur bei einer integralen Selektivitat von S,, =0.25 enielt wird. Dieser Wert entspricht einer differentiellen Selektivitat von S = x I’ - x1” = 0.15 bei einer Verdunstungstemperatur von ca. 50 “C. Aufgrund der relativ geringen iibertragungsleistung bei dieser Verdunstungstemperatur wird eine recht grosse Austauschfiche eflorderlich sein. Der grosse Vorteil bei der Schleppmitteldestilkztion liegt in der vergleichsweise niedngen Verdampfungsenthalpie des Athers. Die Diffusionsdestillation durfte daher vor allem fiir die Trennung von Gemischen, fur die sich nicht derart gmtstige Schleppoder Extraktionsmittel &den lassen, von Interesse sein.

3. Minimierung

des Energieverbrauchs

Fur eine gegebene Verdunstungstemperatur sol1 durch Wahl eines optimalen Dennschnittes, d.h. den Grad der Eindampfing in der Dif~sionstrennapparatur, der Energieverbrauch minimiert werden. Ausgehend von Gl. (1.5) ergibt sich mit Hilfe einer Mengenbilanz urn die

Dtffisionstrennapparatur (Cl. (18)) und der Differentialgleichung der Eindampfkurve (Gl. (22)) die Bedingung fir den optimalen Trennschnitt (Cl. (24)). Wenn die lokale Zusammensetzung .x1” an der Phasengrenze des Kondensatfilms bzw. der relative iibergehende Staff Strom r glcich der azeotropen Zusammensetzung xA= ist, liegt der optimale l’rennschnitt vor (s. Bild4). Ausgehend von Messergebnissen oder den theoretischen Grundlagen (s. Teil I) ergib t sich mit der Bedmgung x In = x&._ = 0.68 die optimale Zusammensetzung auf der Verdunstungsseite xlopt = xv. opt und durch numerische Integration der Eindampjkurve ein optimaler Schnitt (NvIN,),,,. In der Tabelle 1 sind fir die Spaltweite s0 = 2.5 mm die aus den Messungen abgeleiteten, optimalen Werte _f%r verschiedene Verdunstungstemperaturen und zum Vergleich die gemessenen Selektivitaten und @rertragungsleistungen angegeben. Die optimale integrale Selektivitat tit urn einen Faktor von 1.5-2 grosser als die differentielle Selektivitat. 1. Introduction A new separation process for azeotropic mixturesdiffusion distillation-is proposed. A liquid mixture is evaporated below the boiling temperature, diffuses through an inert gas gap and is recondensed. Hence the separation effect is not only based on the relative volatility of the components concerned but also on their diffusivity in the inert gas. Part I of this article [l] presented the experimental layout and the effects of composition, evaporation and condensation temperature, different inert gases and gap widths on the selectivity and transfer efficiency of the diffusion distillation process. It was shown that the experimental results can be adequately described by the vapour-liquid equilibrium and the Stefan-Maxwell equations, that is, by steady-state molecular diffusion. In this part of the paper, the design of a technicalscale diffusion distillation apparatus is presented together with a flow sheet showing the connections to two rectification columns. The energy consumption in the entire plant can be minimized by selecting an optimum cut. 2. Design

of a diffusion

distillation

apparatus

A separation cell consists of two concentric tubes (see Part I, Fig. 3). The feed is introduced as a falling film on the inside of the outer tube. The mixture is then partially evaporated, diffuses through the inert gas in the annular gap, and recondenses on the outside of the inner tube. Several separation cells can be combined to form a tube bundle apparatus (Fig. 1). For the dehydration of isopropanol--water mixtures, the feed is introduced at azeotropic composition and split up into a water-rich and an isopropanol-rich fraction. The water, which diffuses more rapidly, is enriched in the falling films on the inner tubes, and isopropanol on the outer tubes. The outer tubes are heated with condensing vapours, and the inner tubes are cooled by mains water. The feed, the product outlets and the vapour space are separated from one another by the tube support plates.

267

fi

Detail

..Z”

Mains

water

PIXp

WlX,

water

P‘Opa”Ol

Fig. 2. Flow sheet for the dehydration of isopropanol by diffusion distillation.

thins water t Fig. 1. Diffusion distillation apparatus.

In conventional falling-film equipment, the film is introduced through a simple weir. In the diffusion distillation apparatus, the falling film can be evenly distributed by inserting a short corrugated sleeve between the inner and outer tubes, thus forming several small channels with the inner wall of the outer tube (cf. Part I, Fig. 3(b)). A sleeve of this nature can be produced from standard tubes with longitudinal corrugations and ensures that the inner and outer tubes are centred. A distinction must be made between two fundamentally different designs and operating conditions. If the azeotropic mixture is to be separated in a single passage, the tubes must be sufficiently long. In this case, the term ‘integral design’ is used. The ‘differential design’ with correspondingly short tubes must be operated batch-wise. Furthermore, the transfer efficiency and thus the evaporation temperature effect the operating conditions. At low evaporation temperatures, that is, high selectivities, the equipment must be operated batchwise, because the low transfer efficiency does not lead to fractionation within tubes of reasonable length. The advantage of the batch equipment is that the short twin concentric tubes can be correctly centred at low gap width, thus enabling high transfer efficiencies to be achieved.

3. Flow sheet for the dehydration of isopropanol -energy consumption of the process Since the diffusion distillation equipment requires only the one theoretical stage, a practical expedient would be to use it solely to overcome the azeotropic point. The mixture could then be separated into its

components in two rectification columns. Figure 2 depicts the flow sheet for an isopropanol-water separation plant with diffusion distillation. Similar flow sheets apply for azeotropic and extractive distillation. The isopropanol-water feed F is almost completely separated into the water stream W leaving rectification column I and the isopropanol stream P from rectifrcation column II. Both columns operate at atmospheric pressure. The streams Hr and Hrr withdrawn at the heads of the two columns have approximately azeotropic composition. Portions of them are returned as reflux Rr and Rrr and the remaining streams are separated in the diffusion separation apparatus D into a water-rich fraction NK and an isopropanol-rich fraction Nv. The diffusion separation apparatus can be heated by part of the gaseous head streams (Fig. 1). Thus the energy consumption in the entire plant is governed solely by the amounts of heat Qr and Qrr supplied to the reboilers at the foot of the columns. The energy balance for the entire plant is QI+QII+Fh,=Wh,+Ph,+Q, If the change in enthalpy streams is neglected, FhF-PhpThe energy given by

(1) due to the feed and product

Whw=O consumption

(2) per unit

weight

of product

is

Thus, minimum energy consumption in the entire plant corresponds to the minimum of the head streams HI and HII, which are obtained from the mass balances for the entire plant, F= W+P

(4)

FxF=

(5)

Wxw+Pxp

268

column

I,

F+R,+N,=H,+W

(6)

FxF + RIxAZ + NKxK = HIxAz + Wx,

(7)

and column

II,

R,,+N,=H,,+P RIIXAZ

(81

+&xv

= HIIXAZ

If complete assumed,

separation

+ PXP

into

xp=

components

(9) is

XAz = x,,

is

I--Xx =(I

+ur)

l-x, + (1 + 4

XAZ ~ XK

XK = &v/2

The energy consumption per unit weight of isopropanol is then obtained by inserting the numerical values for the azeotropic composition, that is, x,, = 0.68, and the latent heat of evaporation, Ah, AZ = 42 kJ mol-‘:

1

the sum of the head streams

P

pure

xv

xw=o,

HI +-HI,

the

water is removed. The isopropanol and ether passing overhead are recycled. The following simplifying assumptions can be made to compare the energy consumption in the two processes. The reflux ratios in the two columns of the diffusion distillation process are related by the expression vr = uII = 2 and the azeotropic feed of the diffusion distillation apparatus is separated ‘symmetrically’, that is,

(10) xV

~

xAZ

E/P = 0.75/S,,

kW h (kg iso.)-’

(13)

where the reflux ratios are VI = R,I(Hr

~ R,)

(11)

VII = R,,/(HII - RII) (12) Hence it can be seen that the head streams and thus the energy consumption in the entire plant can be restricted to a minimum if the reflux ratios are as small as possible and the overall selectivity S,, = xv - xx is as high as possible. The energy consumption in the isopropanol-water separation by diffusion distillation will now be compared with that in an extractive distillation process that has been adopted in practice (Fig. 3). By adding di-isopropyl ether at the head, pure isopropanol is obtained at the foot of the fist col~~mn. The overhead isopropanol-water-ether mixture is cooled and separates into two layers. The lighter isopropanol-rich phase, consisting mainly of ether, is fed as reflux to the head of the first column. The heavier water-rich phase--only 3% of the amount in the settling tank-is fed into the smaller second column, where the

Thus, as a first approximation, the energy consumption in diffusion distillation is inversely proportional to the overall selectivity. If it is to be comparable with the energy consumption in the practical extractive distillation, that is, 3 kW h (kg iso.))’ the overall setectivity must be S,, = 0.25. As will be shown later, a differential selectivity S = xi’ - xi’ = 0.15 (Table l), which can be realized at an evaporation temperature of about 50 “c, is required to obtain this value of overall selectivity. However, the low transfer efficiency at 50 “C necessitates a large area for mass transfer. Hence there would be no point in replacing the well-established extractive distillation by diffusion distillation for dehydrating isopropanol. In the former case, an advantage is to be gained by adding ether as a separating agent because of its low latent heat of evaporation, Thus, although there is much more internal circulation, the energy consumption is generally less. Diffusion distillation will attract interest in separating mixtures for which no suitable separating agents or extractants can be found.

B3OOx12000

Fig. 3. Flow sheet for the dehydration

of isopropanol

by extractive

distillation.

269

4. Optimization

of the energy consumption

where the relative

Minimization of energy consumption in the entire plant corresponds to a minimum of the head streams Hr and Hr, in the columns. Hence, if the influence of the reflux ratios on the energy consumption is not considered, the energy consumption is governed solely by the stream NC entering the diffusion distillation apparatus. Inserting N,,=H,-RI+HI,-RI1

(14)

in eqn. (10) gives No _= P

(XV ~~&(l

- XAZ)

(15) @AZ - xK)(%’ - xAZ) Thus a high overall selectivity So, = xv ~ xK yields a low energy consumption, and it would be an advantage to choose the lowest possible evaporation temperature to attain high selectivities. But the transfer efficiency declines rapidly with decrease in evaporation temperature, with the result that the required exchanger area and thus the capital investment costs would be increased. The aim of this paper is not to optimize the total costs of the plant by selecting an optimum evaporation temperature, but to minimize the energy consumption at a given evaporation temperature. The overall selectivity and thus the energy consumption are also affected by the degree of fractionation, that is, the breakdown of the azeotropic mixture No into isopropanol-rich and water-rich fractions NV and NK, The mass balances for the diffusion apparatus No =Nv+NK

(16)

NOVA, = NV + NKXK yield

(17)

NV xAZ - xK _= NO Xv--% Equation (15) can be rearranged 1

No _=_ P

1

(18) to give

-xAZ

(19) NV/NO xV - xAZ With increasing degree of fractionation, the remaining stream NV decreases, but the difference xv ~ xAZ becomes greater. It can be shown that there exists an optimum fraction NV/No that allows minimum energy consumption. In order to allow the optimum to be determined the relationship between the differential and the overall selectivity must be known. The composition of the falling films changes continuously with increase in the degree of fractionation. The differential selectivity describes the local separation effect. The differential mass balance of a given point on the falling film is as follows: [email protected]

= -(N,

d(Nvxv)/d/l

+ N2)/A

It gives rise to the differential curve cw, -=_ NV

T = Nr l(Nr + NT) On differentiating (19) becomes

r=x”=xAz

or

Nr/Na

= xl”/xp”

= 0.6810.32

(24)

In Fig. 4 the change in composition of the falling films on the evaporation side xv and condensation side xK is plotted against the degree of fractionation NV/N,,. The optimum fraction is attained if the local composition x” of the condensate film is equal to the azeotropic composition xAZ. To determine the optimum cut in the diffusion distillation process, the relationship between the local compositions x’ and x” must be known. It was shown in Part I that this relationship can be obtained with the aid of the vapour-liquid equilibrium and the Stefan-Maxwell equations. Of course this relationship can also be derived from the experimental results. Thus, the condition xl” = xAZ = 0.68 gives rise to the optimum composition on the evaporation side ~6,~ = xv, ,,nt, and numerical integration of the boiling curve (eqn. (22)) yields the optimum cut (NV/No),,,. The optimum values at various evaporation temperatures are listed in Table 1. They were obtained from the experimental results with a gap width so = 2.5 mm. The measured differential selectivity S and the dimensionless transfer efficiency 4 are also quoted for comparison purposes. It can be seen that about two-thirds of the mixture must be evaporated to obtain the optimum cut. The optimum overall selectivity is 1.5-2 times higher than the differential selectivity. With rise in evaporation temperature the energy consumption E/P increases due to the decreasing overall selectivity, while the required area A/P and thus the cost

of the boiling 0

(22)

cut, eqn.

l--AZ r-xAZ (23) XV ~ xAZ dNV xV - xAZ Repeated differentiation proves that a minimum exists, if the local relative molar flux ir is equal to the azeotropic Composition xAZ. If it is assumed that no mixing occurs in the falling film on the condensation side, the relative molar flux corresponds to the local composition x” on the condensation side, as determined in the differential test section (see Part I). Thus the condition for the optimum cut is given by

&V r-xv

the optimum

= x

(21) equation

(22a) to determine

No

d(NoIP)

(20)

= -N,/A

flux is

Fig.

0

4. Optimum

cut.

210 TABLE

T'

1. Optimum

values at various evaporation

S

0

xv, opt

xK, opt

S 0”

0.158 0.145 0.115 0.076

0.190 0.357 0.687 1.256

0.837 0.838 0.815 0.777

0.582 0.588 0.607 0.632

(0 40 50 60 70

temperatures

bwNO)o*t

E/P (kW h (kg iso.)-‘)

A I?’ (m2 h (kg iso.)-‘)

0.25.5

0.386 0.369 0.352 0.332

2.9 3.0 3.6 5.2

0.34

0.250 0.208 0.145

of equipment for the diffusion distillation apparatus decrease. An estimation of the total costs shows that the optimum evaporation temperature is between 50 and 60 “c. As stated above, the energy consumption is then about the same as in the extractive distillation process, but of course the diffusion distillation apparatus is more expensive than a simple settling tank.

Acknowledgement The financial support of the AIF (Arbeitsgemeinschaft Industrieller Forschungsvereinigungen e.V.), Cologne, and DECHEMA, Frankfurt-on-Main, is gratefully acknowledged.

X

annular gap width, m differential selectivity overall selectivity temperature, “c reflux ratio waste stream, mol s-l molar concentration

Q,

dimensionless

: S 0” T

U W

A F

h Ahv H

P

Q L

area, m2 feed, mol s-r enthalpy, J mol-’ latent heat of evaporation, head stream, mol s-l product stream, mol s-l heat flow, W relative flux reflux, mol s-l

transfer

efficiency

Indices 0 1 2

,

I,

Nomenclature

0.19 0.12 0.10

AZ K opt V

inlet of diffusion apparatus isopropanol water evaporation side (local) condensation side (local) azeotropic point condensation side (outlet of diffusion apparatus) optimum value evaporation side (outlet of diffusion apparatus)

J mol-’

Reference 1 D. Fullarton and E. U. Schliinder, Diffusion distillationa new separation process for azeotropic mixtures. Part I, Chem. Erg. Process., 20 (1986) 255 -263.