close

Вход

Забыли?

вход по аккаунту

?

The Measurement of High Pressure Vapour-Liquid-Equilibria Part I Dynamic Methods.

код для вставкиСкачать
The Measurement of High Pressure
Vapour-Liquid-EquiIibria
Part I : Dynamic Methods
J.D. Raal* and A.L. Muhlbauer'
Deparfmenf of Chemical Engineering, Universdy of Natal,
King George V Avenue, Durban 4001, SOUTH AFRICA
Accurate measurement of high pressure vapour-liquid equilibria is a most demanding
task and has elicited much effort and ingenuity from researchers. The number and
variety of approaches adopted attest to the exacting nature of the task offinding the most
reliable, accurate and cost-effective approach. There is as yet no firm consensus of
opinion as to which type of equipment is superior. Each type has its advantages and
drawbacks and persuasive advocates. There are few areas of research in chemical
engineering or physical chemistry in which so great a variety of equipment and
techniques has been reported. The purpose of this publication is to summarize in some
detail the equipment and procedures used to measure high pressure VLE and to comment
on some of the difficulties common to several methods. In Part I we discuss dynamic
measurement methods and refer to a new high-precision volumetric device we have
developedfor gas chromatograph calibrationfor gas mixtures or for gas-liquid mixtures.
In Part 11 we examine static methods for high pressure VLE measurement and present
proposals for a new cell suitable for multiphase equilibrium and volumetric data
measurement.
A complete survey of the numerous types of equipment used and proposed is not
feasible within reasonable length. Good reviews by Tsiklis[l] (1968) and Young[2]
(1978) in particular are available, and our emphasis is on more recent developments.
This review will be useful to those entering the field or those experiencing problem with
a particular type of apparatu.
1. Classification of Experimental Equipment
In their review of high pressure procedures for VLE measurement, Deiters &
Schneider [3] distinguished between the so-called synthetic method and the
analytical method. In the former method, a mixture of known composition is
prepared and its behaviour observed as a function of temperature and pressure - the
problem of analyzing the equilibrium compositions is thereby avoided. In the
analytical method overall compositions are not at issue. Temperature or pressure is
adjusted to bring about phase separation and withdrawn samples are then analyzed.
A simpler classificationof equilibrium cells is adopted in this review and is shown in
Figure 1. Classification depends upon whether either the liquid or vapour, or both,
are circulated through the equilibrium chamber. If circulation takes place it is
~~
* Author for correspondence.
I
Sentrachem, Sanaton, South Afica.
69
JD.Raal anti A.L. Miihlbauer
classified as a dynamic or flow method, othmise it is a static method. Sub-division
of the static method depends on whether the phases are sampled or not. In a new
development in our laboratories we have designed a static cell with internal
circulation through the sampling valves, i.e. the design combines elements of the
static and flow methods. The design and associated results will be published at a later
date.
OPTIC
NON-OPTIC
STATIC
TI
.c
STATIC
STATIC
NON-ANALYTICAL
t
OMBINED
STATIC
ANALYTICAL
E
1
OPTIC
+
NON-OPTIC
Figure I . Classification of experimental high pressure vapour-liquid equilibrium
equipmen(.
Dynamic and Static Analytical Methods
Main features of the analytical method
A schematic diagram of an apparatus in this category is shown in Figure 2. It consists
of the following:
(i) An equilibrium cell in which the vapour and liquid phases of the mixture are in
equilibrium.
(ii) An environment that controls the temperature of the equilibrium cell, e.g. airbath, oil or water bath, or a copper or aluminium jacket (Konrad et d.[4],Ng and
Robinson [5]).
(iii) A procedure for agitating and mixing the cell contents. Static methods use an
internal stirrer whereas in dynamic methods the circulation of one or more
phases fulfils this role. However, some vapour recirculation methods do have an
additional internal stirrer. Rocking of the equilibrium cell assembly to attain
equilibrium has also been used (Ashcroft et al. [6],Huang et al. [71)but this
procedure is unnecessarily cumbersome.
70
T k Measurement of High Presswe Vapour-Liquid-Equilibria
Part I : D y ~ m i Metho&
c
(iv) A method for sampling the vapour and liquid phases. In the two-phase
recirculation and single-pass vapour and liquid methods, sampling presents few
problems since a representative portion of the flow is readily diverted for
sampling. The vapour recirculation method requires some form of liquid
sampling device.
(v) A means for accurate analysis of the withdrawn samples.
(vi) Pressure and temperature measuring devices.
PRESSURE AND TEMPERATURE
MEASURING DEVICE
SAMPLING
SYSTEM
SAMPLING SYSTEM
7
LIQUID SAMPLING A
do
AG lTATl0N
DEVICE
SYSTEM
I
CONTROLLED ENVIRONMENT
Figure 2. Features of a typical analytical method.
Difficulties encountered in Analytical Experimentation
Problems encountered in the accurate measurement of high pressure isothermal VLE
which are common to all analytical methods include:
(a) obtaining truly isothermal equilibrium conditions, and establishing that this
equilibrium has been reached,
(b) withdrawal of representative samples of the phases without disturbing the
equilibrium;
(c) partial condensation of the heavier component(s) in the sample lines, or flashing
of the lighter component(s) from the equilibrium chamber when the latter is
connected to a sample space at reduced pressure;
(d) cell temperature and pressure must be measured accurately and liquid
components must be thoroughly degassed before introduction into the cell.
(e) Accurate analysis of withdrawn samples. This is normally done by
chromatography, although other procedures are discussed below. The greatest
difficulty in GC analysis is in the calibration of detectors for gas mixtures or gasliquid mixtures, particularly when response-factor ratios are not constant. A highprecision device has been developed by Raal [8] and avoids the reliance on
commercial calibration mixtures which may be unavailable or of uncertain
accuracy.
Obtaining truly isothermal conditions
It has been our experience that even small vertical temperature gradients in the
equilibrium chamber of a static or dynamic apparatus can cause considerable error.
71
J.D. Raal and AL.MWbauer
It is recommended that several temperature sensors be installed in the walls of the
equilibrium cell to test temperature homogeneity, which should be within 0.2K.
There should be no conductive paths to or from the cell through fittings or
attachments, and there should be no direct radiative energy exchange between the
cell and the bath heaters. A copper lining on the interior of the constant temperature
bath (if used) is recommended. Many authors have reported measurements of bath
or cell temperature profiles, e.g. Rogers and Prausnitz [9], and Figuiere et al. [lo].
The attainment of equilibrium
Since equilibrium requires a balance of all potentials that may cause change, a true
state of equilibrium is probably never reached due to continuous small changes in the
surroundings and to retarding resistances. Mechanical stining of the liquid phase
produces fluid friction and its dissipation to the surroundings must produce some
temperature gradient. Temperature,pressure, vapour and liquid composition,and in
some cases stability of refractive index (Besserer and Robinson [ll]) are important
indicators in judging whether equilibrium has been reached. Fredenslund [ 121
proposes, as a criterion, a change in pressure of less than 0.05% in 30 minutes.
Repeated vapour and liquid sample composition analyses must give reproducible
results within the limits of the analytical procedure.
Disturbance of equilibrium during sampling
Sampling of either the liquid or vapour phase may entail a change in the volume of
the equilibrium cell with the size of the effect being inversely proportional to the cell
volume. Pressure fluctuations of 0.1 and 0.01 bar due to sampling have been
reported by Besserer and Robinson [ 113, and Wagner and Wichterle [ 131,
respectively. In addition to the volume change due to sample removal, the sampling
device itself (e.g. a sliding rod sampler) may produce interior cell-volume changes.
A large cell-volume dampens these volumetric disturbances but the penalty in
increased use of chemicals may be unattractive. Comparatively large cell volumes
were used by Sagara et al.[14], Klink et al. [15], Aschroft et al. [6], and Reiff et al.
[16], to counteract volume changes due to sampling. More ingenious procedures,
where the volume change is due only to the withdrawn samples, have been proposed.
Rogers and Prausnitz [9] and Nakayama et al.[17] used sampling rods traversing the
entire cell and sealed at both ends.
In variable-volume cells, the pressure change due to sampling can be
compensated by pressure adjustment (Nakayama et al. [17]). Phase compositions
can also be analyzed in-situ by optical methods (Konrad et al. [4]) without
disturbanceof the cell content.
'
Sample homogenization
In liquid-phase sampling there is a tendency for the more volatile components to
flash preferentially, and thus to produce concentration gradients in the resultant
vapour. Homogenizing this vapour mixture without recondensation is one of the
more difficult problems in high pressure VLE measurement. Several procedures
have been adopted in attempts to overcome this problem, for example:
(i) the use of a stirred homogenization vessel in the sample line (Wagner and
Wichterle [131);
n
The Measurement of High Pressure Vapour-Liquid-Equilibria
Part I : Dynamic Methocis
(ii) circulation of the vapourized liquid sample with a pump through an evaporating
chamber in a closed loop, before expelling the sample to a gas chromatograph
(Nakayama et al. [ 171);
(iii) expelling the liquid sample into an evacuated jet mixer (in which the swirling
recirculation homogenizes the vapour, Miihlbauer and Fbal [18]). This device
is remarkably simple but its size must be carefully calculated. A pressure
uansducer flush-mounted in the wall of the jet mixer gives a reliable indication
of the constancy of the liquid sample size.
Analysis of withdrawn samples
Gas chromatography and spectroscopy are the most commonly used methods of
analysis. Refractive index measurement in conjunction with GC analysis has also
been reported (Besserer and Robinson [ll], and Kalra et al. [19]). A disadvantage of
this procedure is that the high-pressure high-temperature equilibrium state differs
considerably from that of the input to the gas chromatograph. In-situ phase
composition analysis by spectroscopy (based on Beer's law) overcomes sample
preparation difficulties. Konrad et al. (4) and Swaid [203report the use of infrared
spectra (where the absorption bands are often well separated) to determine phase
concentrations. Infrared, visual and ultraviolet spectroscopy or Raman scattering
methods require more extensive calibration procedures than gas chromatography
analysis, and the latter is used most frequently in VLE measurements. Visual and
ultraviolet spectroscopy is largely restricted to aromatic or coloured compounds.
Accurate gas chromatograph detector calibration (e.g. for thermal conductivity
detectors) remains a considerable problem for gas mixtures or for gas-liquid
mixtures when reliable commercial standards are not available. For many systems
the response-factor ratios are not constant over the entire mole fraction range, and
their variation with concentration must be determined. The precision volumehic
calibration device developed in our laboratories (see Figure 3) fulfils this purpose
(Raal[8]). The instrument is being developed commercially under international
patent protection. The device is based on a novel principle in which a piston with
an imbedded solenoid valve operates in a highly uniform cylinder. There is a shaft
compensating mechanism so that the piston movement produces no change in the
total interior volume, and displacement of a gas from the lower chamber to the upper
chamber is exactly proportional to the piston Davel. The latter can be measured to
0.001 mm by counting steps of the stepper motor which drives the piston. For the
calculation of volume/mole fractions, all equipment dimensions cancel and there is a
single invariant equipment constant. In principle, no other volumetric or flow device
or procedure can equal or surpass the accuracy of this instrument. A robust
microdroplet dispenser with a volumetric discharge independent of fluid properties
has been developed as an accessory for preparing standard gas-liquid mixtures, and
can dispense volumes from 10 picolitres to 2 cm'.
Temperature and pressure measurement
Temperatures are easily measured with satisfactory accuracy using platinum
resistance thermometers, thermocouples or thermistors. Highly stable thermistors
with very steep temperature gradients are now available although such sensitivity is
73
ID. R d and AL.MliMbawr
not usually required. Bourdon-type pressure gauges and pressure transducers, some
of the latter with temperature compensation over wide ranges, are favoured devices
but may require calibration with a dead-weight piston gauge.
Degassing
Degassing the non-volatile (liquid) component is necessary since dissolved gases
will compete with the volatile component at low liquid-phase mole fractions.
Degassing is especially important in systems where the two components have very
small mutual solubilities, e.g. the propane-water system (Miihlbauer and Raal [18]).
n
PRESSURE
TEMPERATURE
PROBE (Pt-loo1
PISTON
MAGNETIC COILS
-
IMPELLERS
SOLENOID VALVE /
TEMPERATURE
PROBE (Pt-100)
-7
BEARtNG
STEPPER MOTOR
---a
Figure 3. Gas chromatograph calibration device (Raal[18]), manual version.
74
The Measurement of High Pressure Vapour-Liquid-Equilibrh
Part I : @ ~ m i cMethook
The importance of degassing is stressed by Figuiere et al. [lo], Legret et al. 1211.
Liquid degassing can be done in-situ in the cell or before sample introduction using
equipment such as that of Van Ness and Abbott [22] or Battino et al. [23].
2. Dynamic VLE Measurement Methods
There are three types of flow apparatus: the single-vapour pass method, the phase
recirculation method, and the single vapour-liquid pass methd. These variations are
reviewed below.
Single-vapour pass method
The features of a typical single-vapour pass method are shown in Figure 4. A stream
of pure gaseous component at a specific pressure is passed through a stationary
liquid phase in the equilibrium cell. More gaseous component progressively
dissolves in the liquid phase until equilibrium is reached. Young [2] claims an
equilibrium time of only 15 minutes but the equilibration time will depend on the
transport coefficients of mass and momentum transfer in both phases, and on the
equilibrium solubilities of the system. Equilibrium vapour samples are obtained by
diverting a portion of the effluent stream. Liquid-phase samples are withdrawn from
the equilibrium cell. The pressure of the gaseous component and the liquid-phase
temperature are regulated to generate isobaric or isothermal VLE data. This is the
original dynamic method, it is the simplest and easiest to operate but suffers from
several disadvantages. The method has largely been superseded by recirculation
methods.
The principal difficulties encountered in the single-vapour pass method are:
(i) large quantities of gas are used;
(ii) droplet entrainment must be avoided,
(iii) liquid components are confined to those which have low partial pressures (e.g.
below 0.01 MPa, Young [2]) and consequently the method is not suitable for
critical region studies;
(iv) accurate gas flowrate control is vital. High gas-flowrates will produce more
rapid liquid saturation, however, the correspondingly shorter contact times, e.g.
for gas bubbles, is unlikely to produce saturation of the vapour-phase. The
flowrate problem is exacerbated when the gas is highly soluble in the vapour
phase. The liquid-phase level must be constant, and some form of liquid
agitation is necessary if the gas is sparingly soluble in the liquid phase.
Phase recirculation methods
Two different variations of this type of equipment have been reported: single-phase
and two-phase recirculation. The features of a typical phase-recirculation method are
shown in Figure 5. A description of the method as used is described as follows.
The components are charged into the equilibrium cell. Experience is required to
determine the quantities that need to be added to give the desired liquid level
(Freitag and Robinson [24]). The temperature and pressure of the mixture are
maintained at the required values while either one or both phases are continuously
75
JD.Raal andAL.MIihlbaurr
d
-1
w
0
E
2
cT
-m
s
=!
I-
SAMPLE
LIQUID
PHASE
3
VAPOUR FEED
-
SAMPLE
Figure 4. Features of a single-vapour pass method.
Figure 5. Features of the phase recirculation method
withdrawn from the cell and recirculated. In wephase recirculation apparatus, both
phases are circulated countercurrently through the equilibrium cell and either phase
may be dispersed. Equilibrium between the phases in a well-designed equilibrium
cell should be achieved fairly rapidly by efficient contact between the two phases.
This method removes some of the problems associated with the single-vapour pass
method, i.e. ensuring that equilibrium is reached, that the liquid component is not
continuously removed from the system, and that large quantities of the gaseous
component are not wasted. Liquids possessing high partial pressures may be studied.
Summaries of selected single and two-phase recirculation methods reported in the
literature are given in Tables 1 and 2 respectively.
The single-vapour phase recirculation method, like the single-vapour pass
method, requires some form of liquid sampling device to remove a sample of liquid
76
Y
1973
1983
1984
1985
1985
1986
1990
1990
(36) 1989
(12)
(52)
(39)
(50)
(50)
(24)
(53)
(55)
Date
100
15
168
230
50
56
100
cma
Cell
Volume
423
83
3251500
137
373
11200
31180
0.1 I350
21400
11365
350
701100
2231300
701290
701470
2651405
-
316 Stainess steel
Sapphire
304 Stainless steel
Austenitic steel
Chromium-nickel steel
Stainless steel
Stainless steel
Hastelloy C-276
(2)
Temp
K
Press
bar
Cell
Operating Range
I
TC
TC
TC
RT
RT
RT
RT
RT
RT
r"
m
m
B
B
DWP
DWP
DWP
B
B
Measurement Device
(1I Due t o space limitations only first author mentioned.
( 2 ) Material of construction of equilibrium cell.
(3) TC = thermocouple; RT = Platinum Resistance Thermometer (Pt 100 0).
(4) B = Bourdon pressure gauge; PT = Ressure transducer; DWP = Dead weight piston Qauge.
I
Kim
Recirculation
Recirculation
Fredenslund
Dorau
Weber
Meister
Meister
Freitag
Chou
Shah
!a!Qu
Ref.
Author
(1)
Table 1
Shale Phase Reckculstlon Aooarutus Aenorted in the Utersture
-
10
30
60
5/10
1201180
min
Claimed
Equilibration
time
25
-
501200
3.5
SOQOO
d
VaP
0.5
3.5
fl
Liq
Sample Size
Ref.
Date
Volume
60
40
65
100
1 50
750
100
700
200
300
106
60
100
160
cm3
Two-Phase Rockc
(2)
bar
RSSS
1250 403 Stainless steel
500 Stainless steel
800 304 Stainless steel
350
146-109
90 304 Stainless steel
K
233-303
773
283-353
533
311-588
298
60
161
80
250
422
100
1901103
250
Hastelloy C
300 Stainless steel
Stainless steel
316 Stainless steal
316 Stainless steel
Stainless steel
Sapphire
4 130
353
313
430
345
335
3001670
453
Temp
Operating Ranus
in the Utararure
131
(4)
Press
B
B
8
QT
TC
RT
m
PT
PT
PT
B
B
PT
m
DWP
PT
TC
RT
FT
B
8
RT
m
DWP
RT
TC
TC
TS
Temp
Measurement Device
I
(251 1965
Muirbrook
(261 1983
Kin0
(27)1983
Kubota
128) 1984
Radosz
(291 1985
Moms
Yorizane
(30)1986
Takishima
(31)1986
(32)1990
lnomata
133134) 1988
D'Souza
Adams
(35)1988
Kim
(36)1989
(37)1989
Shibata
Jennings
(38)1989
Weber
(40)1989
(41)1990
(42) 1990
Fink
suzuw
I
-
I
t
Equilibration
time
Sample Size
134700
@
11 600
100
03
Liq
-
100
201250
lo00
fl
120
15
10115
3 001
1
VSP
60
100
min
10
30145
0.2
1
750
10000
1
20
-
480
(1)Due to space limitations only first author mentioned.
(2)Material of construction of equilibrium cell.
(3)TC = thermocouple; RT = Platinum Resistance Thermometer If? 100 n).TS = Thermistor, QT = Quartz Thermometer
(4)B = Bourdon pressure gauge; PT = Presswe transducer; D W = Dead weight piston gauge.
-
R
The Measurement of High Pressure Vapour-Liquid-Equilibria
Part I : D y ~ m i Met&
c
from the cell. In the liquid phase and two-phase recirculation methods the liquid
phase can be sampled by isolating a quantity of the circulating phase, e.g. by
diversion through the external loops of commercially available liquid or vapour
chromatography valves. The use of Rheodyne or Valco valves was reported by
DSouza and Teja [331, DSouza et al. [34], Adams et al. [351, Jennings and Teja
[38], Kim et al. [36], and Fink and Hershey [42]. This feature makes liquid
sampling relatively easy and sfmightforward when compared to the static method,
for example. This simplicity of sampling in the two-phase recirculation method is,
however, somewhat negated by the added complexity of a liquid recirculation loop.
Another reason for the popularity of these methods is the increasing availability
of commercially manufactured (usually magnetically driven) pumps. Muirbrook and
husnitz[25] and Fleck and hausnitz[49] had to design and construct their own
pumps which they describe in detail. One of the earliest examples of this type of
apparatus was that used by Muirbrook and Prausnitz[25] to measure the ternary
system nitrogen-oxygen-carbon dioxide. The circulation of the phases provided
sufficient agitation to ensure equilibration of the phases, and no additional liquid
phase agitation was required. Samples for analysis were obtained by blocking off
sections of the circulation line. Calibration of these sample spaces permitted the
determination of phase molar volumes. Liquid entrainment in the vapour stream was
prevented by a simple baffle. The authors reported using a liquid level indicator.
The vapour recirculation apparatus of Fredenslund et al. [12] has a unique
sampling device. Its principle subsequently formed the basis of the liquid sampling
devices of Meister [50], and those developed for the static equilibrium cells of Bae et
al. [51], Ng and Robinson [5], and Muhlbauer and Raal[18]. It had a 5 mm diameter
rod with a 3.5 pl hole drilled near its tip. The latter was totally immersed in the
liquid phase upon insertion of the rod into the equilibrium cell. On withdrawal of the
sampling rod the hole came into alignment with sample ports drilled into the cell
wall. Carrier gas then flushed the sample to a gas chromatograph for composition
analysis. Freitag and Robinson [24] extracted liquid samples directly from the
equilibrium cell through a 90 cm capillary tube, at the end of which was located the
liquid sampling valve. The arrangement provided for the acquisition of a lowpressure vapour sample of the same composition as the high-pressure liquid in the
cell. All of the sample was captured as it flashed while flowing through the capillary
tube and needle valve. The low pressure sample was circulated to ensure
homogeneity before being analyzed.
Dorau et al. [52] extracted both the vapour and liquid samples directly from the
equilibrium cell through capillary tubes. These capillaries led to evacuated flasks
attached by rapid connection couplings. In order to obtain representative liquid
samples the authors reported that the flask volume had to be greater than the
inventory of the capillary tubing. The experimental method of Chou et al. [53] was
somewhat similar. The equilibrium cell had separate sampling ports for the vapour
and liquid phases respectively. A specially designed detachable microcell, similar in
principle to the ones used by Legret et al. [21], was attached to each sampling port.
The microcell collected an equilibrium phase sample from the sample analysis
system (enclosed in a separate air bath) for composition analysis. In the sample
analysis system, the microcell was attached to a variable-volume bellows assembly
79
J.D. Radl and A.L. MWbauer
(1)Equilibrium Cell; (2) windows,' (3)sample charging pump; (4) liquid sample; (5)
pressure transducer; (6)sampler; (7) magnetic pwnp; (8)magnetic stirrer; (9)jlash
tank; (10) temperature controller: (11) vacuum pump: (12) NH, sample cylinder;
(13)N2gas cylinder: (14)NH3gas cylinder: (15)He gas cylinder; (16)air bath: (I 7 )
gas chromatograph.
(Reprinted with permission from J.Chem.Eng.(Japan); Copyright 1986, American
Chemical Society)
that acted both as a flash vessel and pump. The bellows assembly circulated the
flashed sample through the gas chromatographic sampling valve until a
homogeneous sample suitable for composition analysis was obtained.
The experimental apparatus of Kubota et al. [27]used a 6-portvalve which (with
the appropriate setting) allowed the high pressure pump to circulate either the vapour
or liquid phase. Representative samples of the vapour and liquid phases were trapped
in the 4-port valve and subsequently released into a low pressure line. The sample
was circulated until it was homogenized before analysis by gas chromatography. The
authors claimed that equilibration of the phases required approximately 2 hours. In
recent years the most popular method of sampling the circulatingphases is by using a
commercial sampling valve located in the circulation lines. Shibata and Sander [37]
used a more elaborate method of trapping vapour and liquid samples in sample
bombs and transfemng these to a gas chromatograph for analysis. The latter is very
similar to the vapour recirculation methods used by Dorou et al. [521 and Chou et al.
[53]. Yorizane et al. [30]describe an unusual method of cell content agitation. The
apparatus consisted of two equilibrium cells which were connected to each other at
The Measurement of High Pressure Vapour-Liquid-Equilibria
Part I : Dynamic Metho&
both the top and bottom ends by means of flexible stainless steel tubes. One cell was
fixed while the other was moved slowly up and down by means of a mechanical
device. This motion produced a pressure merit which induced liquid- and vapour
phase-flows in opposite directions to equalize the pressure.
For recirculation methods in general, vapourization and condensation of the
circulating liquid and vapour streams must be avoided. This has been overcome in a
number of ways, for example:
(a) Having three separately controlled temperature zones, one for the equilibrium
cell and another for the vapour recirculation loop in which the temperature was
slightly greater than the equilibrium temperature. The third zone for the liquid
recirculation loop was maintained at a temperature slightly lower than the
equilibrium temperature. A good example is the equipment of Takishima et al.
[31] and Inomata et al. [32]. Both have similar circulation loops and sampling
systems housed in three different temperature zones (see Figure 6).
(b) Sampling the phases directly from the equilibrium cell, which partially negates
the advantages of a recirculation method.
(c) Removing samples from the equilibrium cell by some device, and transferring
these samples to an analysis device (Dorou et al. [52]; Shibata and Sandler [37];
Chou et al. [53]).
It is interesting to note that Kubota et al. [27], King et al. [26], Takishima et al.
[31], Inomata et al. [32] and Suzuki and Sue [41], all report the use of internal
stirrers to aid in the equilibration process.
Difficultiesencountered in the recirculation methods are:
(i) Maintaining an adequate liquid level in the equilibrium cell. The only authors
who mentioned the use of a liquid-level measuring device were Muirbrook and
Prausnitz [25] and Simnick et al. [43]. All other authors appear to rely on
visual observation to maintain the desired liquid level.
(ii) Prevention of droplet entrainment in the effluent vapour stream. The only
specific reference to a demisting device was by Muirbrook and Prausnitz [25].
a
‘8
PREHEATER
MIXER
LIQUID FEED
I
‘
,
8
,$LOURFEED
Figure 7 . Features of the single vapour-liquid pass method.
81
Simnick
Sebastian
tin
lnomata
Niesen
Roebers
Author
(1)
Ref.
(43)
(44)
(45)
(46)
(47)
(481
Date
1977
1980
1986
1986
1986
1990
.
TIM0 3
Cell
Press
bar
3 16 Stainless Steel
I
TC type K
TC type K
TC type K
TC type K
RT
NA
B
NA
I I
Measurement Device
Slnde Vmwr and liould Pass b r e t u r R e w r t d In the Utersturr
Operatiirig Range
K
Temp
250
(2)
Cm’
703
-
90
Cell
Volume
10
316 Stainless steel
316 Stainless steel
3 16 Stainless steel
450 Stainless steel
= 100 n
250
230
100
350
pt
710
710
623
673
30
250
10-50
(1 Due to space limitations only first author mentioned
(2) Material of construction of equilibrium cell
(3) TC = thermocouple; RT = Ratinurn Resistance Thermometer,
(41 B = Bourdon pressure gauge.
Hold up
time
sec
18-36
NA
Sample size
50012 000
1320/1 860
600/1 000
3001 1 140
The Measurement of Hizh Pressure Vapour-Liquid-Equilibria
P a n I : D y ~ m i Meihods
c
(iii) Ensuring that the pumps used do not contaminate the equilibrium mixture or
create stagnant spaces. The former problem has largely been overcome by the
use of magnetically coupled pumps.
(iv) Avoiding the possibility of partial condensation and vaporisation of the
circulating vapour and liquid streams respectively.
(v) Avoiding undesirable pressure gradients across the equilibrium cell caused by
the circulating pump. In principle this is a weakness of the circulation method
since flow cannot be produced without a pressure gradient. In practice data from
circulation methods compare well with those from static type cells. In a new
development in our laboratories (Raal [54]), the liquid stirrer itself provides a
small recirculating flow through a liquid sampling valve mounted in the cell
wall.
The Single-Vapour and Liquid Pass Method
This is a relatively recent development of the dynamic method and was developed
principally for high-temperature and high-pressure vapour-liquid equilibrium
measurements, where thermal degradation of a hydrocarbon could occur. The
features of a typical single vapour-liquid pass method are shown in Figure 7, and a
description of the method follows. Separate streams of vapour and liquid
components are contacted co-currently at a controlled temperature and pressure in a
mixing unit. The combined stream passes into an equilibrium cell where the mixture
separates into vapour and liquid phases. The two phases exit from the equilibrium
cell separately and sampling is achieved by the diversion of the effluent streams. A
summary of some of the apparatus described in the literature is given in Table 3.
Single Vapour-Liquid Pass Apparatus in the Literature
The apparatus used by Inomata et al. [46] for a variety of hydrocarbon systems
shows the essential features common to all flow apparatus of this kind. The gaseous
component at its critical pressure was supplied from a buffer tank. The gaseous and
liquid components were contacted co-currently before entering the preheater. The
preheater consisted of a tube which was initially heated by an electric line heater and
finally by an air bath. The air bath was used to adjust the mixture to the desired
equilibrium temperature. The static mixer produced homogenization and rapid
equilibration. The cell design incorporated features to minimise fluid entrainment, a
problem with this type of apparatus. The inlet nozzle was inclined so as to minimise
liquid entrainment in the vapour phase and as an additional measure a demister was
fitted at the vapour outlet. To avoid entrainment of the liquid withdrawn from the
cell and to maintain a steady liquid level, the cell was fitted with an overflow-type
level-control system and back-pressure regulator. The liquid level could be detected
by a capacitor sensor, and according to the authors proved a useful aid in achieving
steady-state operation.
Simnick et al. [43] combined the final heater and static mixer features into one
unit. They fitted a notched twisted ribbon inside the entire length of their
heater/mixer. This unit brought the final temperature of the mixture to within 1°C of
the equilibrium temperature. The apparatus of Sebastian et al. [443 was essentially
the same as that of Simnick et al. [43]. However, the equilibrium cell used by Lin et
al.[45] was different as they found the capacitor level-indicator of Simnick et al.[43]
83
J.D.Raal and A L . M W b a w r
to be unsuitable for the system being measured, and they designed an optical cell.
The sapphire window created a sealing problem as no organic elastomer could seal at
the temperatures of interest. The problem was solved by using a gold O-ring with a
copper shim. This seal was nevertheless considered to be the factor limiting the
operating pressures and temperatures. The auxiliary apparatus of Niesen et al. [47]
was essentially the same as that of Lin et al. [45] but their equilibrium cell differed.
Severe corrosion problems were experienced which ruled out the use of a liquid-level
measuring device and necessitated the use of a view cell. Simnick et al. [43],
Sebastian et al. [44], Lin et al. [45], and Inomata et a1 [46] indicated that the
measured compositions of the vapour and liquid phases were independent of the feed
flow rates tested, confirming that equilibrium had indeed been reached. Sebastian et
al. [44]and Inomata et al. [46] found thermal decompositions to be negligibly small.
Roebers and Theis [48] describe a well-designed equilibrium cell which
theoretically has superior temperature and pressure operating ranges to other
equilibrium cells. Experimental results using this cell were in press at the time of
writing and unavailable for comment.
Difficulties encountered by this method are :
(i) Ensuring that equilibrium has been reached in one pass.
(ii) Ensuring complete phase separation in the equilibrium cell.
(iii) Achieving a steady liquid level in the equilibrium cell.
(iv) Prevention of droplet entrainment in the effluent stream.
(v) Ensuring the pumps do not contaminate the equilibrium mixture.
(vi) Minimizingthe effect of undesirablepressure gradients across the equilibrium cell.
In addition, the designer faces materials selection problems due to the very reason
these apparatus were designed, namely high temperatures.
3. Conclusions
Several synthetic and analytical procedures and many different types of dynamic
equipment for high pressure VLE measurement have been surveyed. The problems
common to most methods were reviewed, including the attainment of equilibrium,
sampling from a high-pressure space, sample homogenization, and accurate sample
analysis. Calibration of gas chromatograph detectors for gas mixtures or gas-liquid
mixtures will be facilitated with a new high-precision volumetric device capable of
producing binary or multicomponent mixtures from ppm values up to 50 mole%.
In Part I1 static procedures used for high pressure VLE measurement are
reviewed.
References
1. Tsiklis, D.S.1968. Handbook of Techniques in High Pressure Research Engineering. New
York: Plenum Press.
2. Young,C.L. 1978. An Experimental Method for Studying Phase Behaviour of Mixtures at
High Temperatures and Pressures. In McGlaston, M.L. (senior reporter) A Specialist
Periodical Report, V01.2: Chemical Thermodynamics: 71-104. London. The Chemical
Society.
3. biters, U.K. & Schneider, G.M. 1986. High Pressure Phase Equilibria: Experimental
84
The Measuremenf of High Presswe Vapour-Liquid-Equilibria
Part I : Dynamic Methods
Methods. Fluid Phase Equilibria, 29,145-160.
4. Konrad R., Swaid 1. & Schneider. G.M., 1983. High-Pressure Phase Studies on Fluid
Mixtures of Low-Volatile Organic Substances with Supercritical Carbon Dioxide. Fluid
Phase Equilibria, 10,307-314.
5. Ng, H.J. & Robinson, DB. 1978. Equilibrium Phase Properties of the Toluene-Carbon
Dioxide System. J. Chem. Eng. Data, 23(4), 325-327.
6. Ashcroft. S.J.. Shean. R.B. & Williams C.J.J.. 1983. A Visual Equilibrium Cell for
Multiphase Systems at Pressures up to 690 Bar. Chem. Eng. Res. Des.. 61.51-55.7.
7. Huang, S.S.S., Leu. A.D., Ng. H.J. & Robinson, DB. 1985. The Phase Behaviour of Two
Mixtures of Methane. Carbon Dioxide, Hydrogen Sulphide and Water. Fluid Phase
Equilibria, 19,21-32.
8. Raal, J.D. 1992. A New Device for the Calibration of Gas Chromatographs for Gas
Mixtures. Pitcon 92, New Orleans, USA.
9. Rogers, B.L. and Prausnitz, J.M. 1970. Sample-Extraction Apparatus for High Pressure
Vapour-Liquid Equilibria. Ind. Eng. Chem. Fundam.. 9(1). 174-177.
10. Figuiere. P., Hom, J.F., Laugier, S., Renon, H., Richon, D. & Szwarc, H. 1980. Vapourliquid Equilibria up to 40 OOO kPa and 400OC: A New Static Method. AlChE J.. 26(5).
872-875.
11. Besserer, G.J. and Robinson, D.B. 1971. A High Pressure Autocollimating Refractometer
for Determining Coexisting Liquid and Vapour Phase Densities. Can. J. Chem. Eng., 49,
651-656.
12. Fredenslund, Aa., Mollerup, J. and Christensen, L.J. 1973. An Apparatus for Accurate
Determination of Vapour-Liquid Equilibrium Properties and Gas PVT Properties.
Cryogenics, 13,414419.
13. Wagner, 2. and Wichterle, I. 1987. High-Pressure Vapour-Liquid Equilibrium in Systems
Containing Carbon Dioxide, 1-hexene and n-hexane. Fluid Phase Equilibria, 33,109-123.
14. Sagara, H., Arai. Y. and Saito, S. 1972. Vapour-Liquid Equilibria of Binary and Ternary
Systems Containing Hydrogen and Light Hydrocarbons. J. Chem. Eng. (Japan), 54,339348.
15. Klink, A.E., Cheh, H.Y. and Amich (Jr), E.H. 1975. The Vapour-Liquid Equilibrium of
Hydrogen-n-Butane System at Elevated Pressures. AlChE J.. 21(6), 1142-1148.
16. Reiff, W.E., Peters-Gerth. P. and Lucas. K. 1987. A Static Equilibrium Apparatus for
(Vapour + Liquid) Equilibrium Measurements at High Temperatures and Pressures.
Results for (methane + n-pentane). I., Chem. Thermodynamics, 19,467477.
17. Nakayama, T., Sagara, H., Arai. K. and Saito, S. 1987. High Pressure Liquid-Liquid
Equilibria for the System of Water, Ethanol and 1.1-Difluoroethane at 323.2 K. Fluid
Phase Equilibria, 38, 109-127.
18. Miihlbauer, A.L. and Raal, J.D. 1991. Measurement and Thermodynamic Interpretation of
Vapour-Liquid Equilibria in the Toluene-C02 System. Fluid Phase Equilibria, 64.213236.
19. Kalra, H., Kubota, H., Robinson, D.B. and Ng,H.J. 1978. Equilibrium Phase Properties of
the Carbon Dioxide-n-HeptaneSystem. J. Chem. Eng. Data, 23(4). 317-321.
20. Swaid I. 1983. Dissertation, University of Bochum.
21. Legret, D. Richon, D. and Renon. H. 1980. Static Still for Measuring Vapour-Liquid
Equilibria up to 50 bar. Ind. Eng. Chem. Fundam.. 19,122-126.
22. Van Ness, H.C. and Abbott, MM. 1978. A Procedure for Rapid Degassing of Liquids.
Ind. Eng. Chem. Fundam.. 17,66-67.
85
J.D. Raal and A L . Miihlbawr
23. Battino. R., Banzhof, M., Bogan. M. and Wilhelm. E. 1971. Apparatus for Rapid
Degassing of Liquids, Part 1 1 1. Analytical Chemisay, 43(6), 806-807.
24. Freitag, N.P. and Robinson, D.B. 1986. Equilibrium Phase Properties of HydrogenMethane-Carbon Dioxide, HydrogenCarbon Dioxide-n-Pentane and Hydrogen-n-Pentane
Systems. Fluid Phase Equilibrii 31,183-201.
25. Muirbrook. N.K. and Prausnitz, JM. 1965. Multicomponent Vapour-Liquid Equilibria at
High Pressures: Part 1. Experimental Study of the Nitrogen-Oxygen-Carbon Dioxide
System at OOC. AlChE J.. 11(6), 1092-1096.
26. King, MB.. Alderson, D.A., Fallah, F.H.. Kassim, KM., Sheldon, J.R. andMahmud, R.S.
1983.Some Vapour-Liquid and Vapour-Solid Equilibrium Measurements of Relevance
for Supercritical Extraction Operations and their Correlation. In Paulaitis. M.E.,
Penninger. J.M.L., Gray, R.D. and Davidson. P. (eds). Chemical Engineering at
Supercritical Fluid Conditions. Ann Arbor: The Butterworth Group: pp.31-80.
27.Kubota, H.. Inotome. H.. Tanka, Y. and Makita, T. 1983.Vapour-Liquid Equilibria of the
Ethylene-PropyleneSystem under High Pressure. J. Chem.Eng. (Japan), 16(2), 99-103.
28.Radosz. M. 1984. Variable-Volume Circulation Apparatus for Measuring High-Pressure
Fluid Phase Equilibria. Ber. Bensenges. Phys. Chem., 88.859-862.
29.Morris, W.O. and Donohue. M.D. 1985. Vapour-Liquid Equilibrium in Mixtures
Containing Carbon Dioxide, Toluene, and I-methylnaphthalene. J. Chem. Eng. Data, 30,
259-263.
30.Yorizane, M., Yoshmura, S.. Masuoka, H.. Miyano. Y. and Kakimoto, Y. 1985.New
Procedure for Vapour-Liquid Equilibria. Nitrogen + Carbon Dioxide, Methane + Freon 22,
and Methane +Freon 12.J. Chem. Eng. Data., 30.174-176.
31.Takishima, S., Saiki, K., Arai., K. Saito. s. 1986.Phase Equilibria for CO, - C2HflH - H20
System. J. Chem. Eng. (Japan), 19(1), 48-56.
32.Inomata, H., Ihawa, N.. Arai, K. and Saito. S. 1988. Vapour-Liquid Equilibria for the
Ammonia-Methanol-Water System. J. Chem. Eng. Data, 33,2629.
33. DSouza, R. and Teja, AS. 1988.High Pressure Phase Equilibria in the System Glucose +
Fructose + Water + Ethanol + Carbon Dioxide. Fluid Phase Equilibria, 39,211-224.
34.DSouza, R., Patrick, J.R. and Teja, AS. 1988.High Pressure Phase Equilibrium in
the Carbon Dioxide-n-Hexadecane and Carbon Dioxide-Water System. Can. J. Chem.
Eng.. 66,319-323.
35. Adams, W.R., Zollweg. J.A.. Street, W.B. and Rizvi, S.S. 1988.New Apparatus for
Measurement of Supercritical Fluid-Liquid Phase Equilibria AlChE J.. 34(8), 1387-1391.
36. Kim,C.H., Clark, A.B., Vimalchand, P. and Donohue, M.D. 1989. High Pressure Binary
Phase Equilibria of Aromatic Hydrocarbons with CO, and C2H6.J. Chem. Eng. Data, 34,
391-395.
37.Shibata, S.K. and Sandler S.I. 1989. High Pressure Vapour-Liquid Equilibria Involving
Mixtures of Nimogen, Carbon-Dioxide and Cyclohexane. J. Chem. Eng. Data. 34,291298.
38.Jennings, D.W. and Teja, AS. 1989.Vapour-Liquid Equilibria in the Carbon-Dioxide-1Hexene and Carbon Dioxide-1-Hexyne Systems. J. Chem. Eng. Data, 34.305-309.
39.Weber. W., Zeck, S. and Knapp. H. 1984.Gas Solubilities in Liquid Solvents at High
Pressures. Apparatus and Results for Binary and Ternary Systems of N , CO, and CH50H.
Fluid Phase Equilibria, 18,253-278.
86
40.Weber. L.A. 1989. Simple Apparatus for Vapour-Liquid Equilibrium Measurements with
Data for the Binary Systems of Carbon Dioxide with n-Butane and n-hexane. Fluid Phase
Equilibria, 3(4), 171-175.
41. Suzuki, K. and Sue. H. 1990. Isothermal Vapour-Liquid Equilibrium Data for Binary
Systems at High Pressures. Carbon Dioxide-ethanol, Carbon Dioxide-1-1 -propanol,
Ethene-Ethanol,and Ethane-1-Propanol Systems. J. Chem. Eng. Data, 35.63-66.
42. Fink, S.D. and Hershey. H.C. 1990. Modelling the Vapour-Liquid Equilibria of 1.1.1Trichloroethane + Carbon Dioxide and Toluene + Carbon Dioxide at 308.323 and 353 K.
Ind. Eng. Chem. Res., 29.295-306.
43. Simnick, J.J., Lawson, C.C., Lm, H.M. and Chao. K.C. 1977. Vapour-Liquid Equilibrium
of Hydrogefletralin System at Elevated Temperatures and Pressures. AlChE J.. 23(4),
469476.
44. Sebastian. HM., Simmick. J.J., Lin, HM. and Chao, K.C. 1980. Gas-Liquid Equilibrium
in Mixtures of Carbon Dioxide + Toluene and Carbon Dioxide + m-Xylene. J. Chem. Eng.
Data, 25,246-248.
45. Lm. H.M.. Kim C.H., Let, W.A. and Chao. K.C. 1985. New Vapour-Liquid Equilibrium
Apparatus for Elevated Temperatures and Pressures. Ind. Eng. Chem. Fundam., 24, 260262.
46. Inomata, H., Tuchiya. K., Atai. K. and Saito. S. 1986. Measurement of Vapour-Liquid
Equilibria at Elevated Temperatures and Pressures using a Flow-Type Apparatus. J.
Chem. Eng. (Japan), 19(5). 386-391.
47. Niesen. V.. Palvara, A., Kidnay, A.J. and Esavage. 1986. An apparatus for Vapour-Liquid
Equilibrium at Elevated Temperatures and Pressures and Selected Results for the WaterEthanol and Methanol-Ethanol Systems. Fluid Phase Equilibria, 31,283-298.
48. Roebers. J.R. and Theis, M.C. 1990. An equilibrium View Cell for Measuring Phase
Equilibria at Elevated Temperatures and Pressures. Ind Eng. Chem. Re.. 29,1568-1570.
49. Fleck, R.N. and Prausnitz J.M. 1968. Apparatus for Determination of Liquid-Liquid-Gas
Equilibria at Advanced Pressures. Ind. Eng. Chem. Fundam.. 7(1), 174-176.
50. Meister, K.H. 1985. Apparatuses for the Determination of High-Pressure Vapour-Liquid
Equilibrium Data. Lmde Reports on Science and Technology, 39.23-29.42
51. Bae, H.K.. Nagahama, K. and Hirata, M. 1981. Measurement and Correlation of High
Pressure Vapour-Liquid Equilibria for the Systems Ethylene-1-Butene and EthylenePropylene. J. Chem. Eng. (Japan), 14(1). 1-6.
52. Dorau, W:. Kremer, H.W. and Knapp, H. 1983. An apparatus for the Investigation of
Low-Temperature, High-Pressure, Vapour-Liquid and Vapour-Liquid-Liquid Equilibria.
Fluid Phase Equilibria, 11.83-89.
53. Chou, C.F.. Forbert, R.R. and Prausnitz, J.M. 1990. High Pressure Vapour-Liquid
Equilibria for COJn-Decane, COfletralin and Cob-Decanereealin at 71.1 and 104,4"C.
J. Chem. Eng. Data, 35.26-29.
54. Raal, J.D. 1993. Unpublished work, contact author.
55. Shah, N.N., Pozo de Fernandes, M.E., Zollweg, J.A. and Streett, WB. 1990. VapourLiquid Equilibrium in the System Carbon Dioxide + 2.2-Dimethylpropane from 262 to
424 K at Pressures to 8.4 MPa. J. Chem. Eng. Data, 35,278-283.
Received 4 M a y 1993;Accepted after revision: 31 January 1994.
87
Документ
Категория
Без категории
Просмотров
3
Размер файла
1 024 Кб
Теги
measurements, part, high, method, pressure, vapour, dynamics, equilibrium, liquid
1/--страниц
Пожаловаться на содержимое документа