CONDENSATE
P. T. 100 BULB
2.7 Development of LPVLE methods for higher pressures and temperatures
2.7.1 Designs for elevated pressures
The first type of recirculating equilibrium still used for high-pressure determinations was based on the vapour condensate recirculation method and a detailed discussion ofthe apparatus will be presented below, together with the later designs.
Equipment ofScheeline and Gilliland (1939)
Scheeline and Gilliland considered the two most common HPVLE methods in that time in the form of the static method and the "dew and bubble-point" method to be undoubtedly associated with a superfluous amount of experimental difficulties. The success of the use of equilibrium VLE stills at atmospheric pressure it was believed would allow it to be also applicable to high- pressure measurements up to the critical region.
Initial considerations that arose for the design of the equipment related to the viewing of the cell contents and the accurate control of the system pressure. The former design consideration was
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
necessitated by the need to ensure that the complete flashing of the liquid phase or the partial condensation of the vapour phase did not occur, together with the need to observe the critical region behaviour of the system studied. With regards to pressure control, the use of an inert gas to pressurize the system contents was not favoured due to concerns over the solubility of the gas in the mixture (creating a ternary mixture) and consequently pressure control was achieved with controlled heat input into the still with the aid of a mercury switch.
The equipment is shown in Figure 2.26. The equilibrium VLE still was constructed from gauge- glass tubing which was closed at one end. A bottom heater was coiled around the base of the equilibrium still to bring the mixture to a boil. The top or open end of the tubing was sealed with a packing gland with a rubber and Neoprene® composite gasket. The Neoprene® portion of the gasket was in contact with the hydrocarbon vapours; a necessary consideration for chemical compatibility.
The opposite closed end of the glass tubing was slotted into a steel base, where lead wool and rubber sheet was used to prevent glass-steel contact. To secure the glass in the packing gland, three tie rods were used. A steel ruler was placed adjacent to the still to measure the liquid level.
The packing gland, which was machined from a steel hexagon, featured four silver-soldered tubes that entered the equilibrium still through the steel top. These were the still sampling line (steel hypodermic tubing), a suitable thermowell (copper tube sealed at one end), vapour exit line (copper tube) and the liquid return line from the trap (seamless steel tubing), which only extended only halfway into the steel top.
To prevent any partial condensation of the vapour on the inner walls of the still, a stream of electrically heated air was blown into the annular space between a loosely fitted Pyrex® jacket, used to surround the still, and the outer wall of the still.
The condenser was constructed from copper tubing and did not contain any internal cooling jackets or tubing and was instead cooled by "natural air convection". The vapour condensate trap was also constructed from glass tubing, sealed at one end, and the packing gland, steel base and the tie rods were arranged in a similar fashion to that of the equilibrium still, except for the inverted positioning.
TOP
Hl:i\TER
.
NaCY\JNO~RTO
Nor TO SCAle
Figure 2.26. Apparatus of Scheeline and Gilliland (1939).
The system investigated by the researchers was that of isobutylene and propane. The authors reported equilibrium times of 15 minutes. The equilibrium still (liquid) and the trap (vapour condensate) samples were withdrawn into glass sampling bulbs. The pressure capacity of the entire apparatus was 10.3 MPa with the failure of the glass body described as being "infrequent"
and being attributed to the over-tightening of the packing gland. Tests for possible entrainment of liquid droplets in the vapour phase were performed and no entrainment was observed even for conditions of vigorous boiling.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
Equipment of Griswold et al. (1943)
The modified high-pressure VLE apparatus of Griswold et al. was based on the operational principles of the low pressure Othmer vapour condensate recirculating still, as in the earlier work of Scheeline and Gilliland (1939), whose equipment was principally constructed from glass, for which sealing was achieved through the use of gaskets and packing glands. Griswold et al. considered the above two design criteria as undesirable, who opted for an all-metal construction to ensure that there would be no concerns about the structural integrity of the material of construction and the chemical compatibility of the gasket material with the systems studied.
The final design of the still was the culmination of three prior attempts and is shown in Figure 2.27. The entire apparatus was constructed from metal in the form of extra-heavy and double- extra-heavy pipe, machined steel monoblocs, sheet iron and steel tubing. The entire apparatus was insulated with 85% magnesia lagging. The reboiler consists of a circulation pipe and a collar on one end, which was essentially a welded sheet iron ring. The purpose of the circulation pipe and the collar was to ensure that there was sufficient mixing of the vapour condensate return stream and the boiler liquid contents.
The main body, or what can be described as the equilibrium chamber, had the largest volume of the sections in the apparatus. This design criterion was in accordance with the objectives of obtaining sufficient vapour space above the liquid contents to minimize any entrainment of liquid droplets in the exiting vapour stream. To compensate for any heat losses that might occur on the walls of the equilibrium chamber i.e. to prevent refluxing, a winding of Nichrome wire around a layer of asbestos, was used to heat the equilibrium chamber. The equilibrium chamber was heated at a temperature that was 3 - 4 K higher than the equilibrium temperature.
The vapour take-off tube had a vent valve for the evacuation of the apparatus and directed the vapour into the condenser. The variable lengths of the condenser sections allowed for a variable surface area available for the total condensation of the sample. The design allowed for both the condensing surface and the coolant fluid to be varied to suit the system and the operating temperature to ensure that excellent control over the system temperature could be achieved.
Since the use of an inert gas to pressurize the system was not favoured, isothermal conditions were maintained in the apparatus by balancing heat input and heat removal from the system by adjusting the reboiler heating voltage (from the variable-voltage transformers) and the flow rate of the coolant fluid, respectively.
I-
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M
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r:T
T 54
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FIGURE 1 A-Heater leg of .tlll.
~rculatloJlpip•.
e--Draln.
D-tlquld lompl. outlet.
E-Thermo....lIs.
F-Wall thermocouple..
G-Main body.
H-CompensatlJlg hIGI., wlnellng.
I-End plug! of .tlll and ...apor condensate cllamber.
J-I'ocition of alllneluftl IllGter winding holel.r.
(Horton Ho. MDU271,Alulldum RA 9'.) K-E...acuatlon ....nt.
L-Vapor lift••
M-Cond'nstr llIell.
N-Coplllary .t,.1 prlSSUII gaulIe tubing.
o-lA'".xtro h.avy pipe nlppl•.
P-Vapor cond.nsate chamber.
Q-8aU chede ...01.... stop.
R-3/16N .t..1 ball ch.de 01...
5-Body and Hat of chcck alv•.
T-Vapor condensate re'ur" 11"•.
U-VlIpor sompl. outl.t.
All val... artlA'" size drop-forg.d Iteel, 3000 p.I.1 mCllC.
workl"g prll.UII. (Henry Vogt Madln. Co., Loul....iileKy.)
Figure 2.27. Apparatus of Griswold et al. (1943).
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
The vapour condensate sample was withdrawn at the base of the condensate chamber through a sampling line passing through the threaded and welded end-cap. The condensate return line contained a vertically mounted ball check valve, to prevent the backflow of the boiler contents into the condensate chamber.
The vapour condensate and liquid sampling lines terminated in female connections to allow for the attachment of sampling bombs, which have a capacity 000 cm3and 60 cm3,respectively.
Equilibrium times of around five hours were reported by the researchers, who showed that the rate of circulation had a minimal effect on the equilibrium phase compositions.
Equipment of Othmer and Morely (1946)
A high-pressure version of the original design of the condensate recirculating Othmer apparatus (1928) was developed by Othmer, together with Morely in 1946, for operation at pressures up to 3.5 MPa. The means of pressurizing the system was in the form of an inert gas (nitrogen), since the pressure range of the experiments (below 1.4 MPa) would not introduce any significant errors (estimated below 0.7 %) into the measurements.
The apparatus of Othmer and Morely essentially consisted of a reboiler, condenser, condensate reservoir, trap and surge. The equipment, shown in Figure 2.28 was constructed from stainless steel and the individual parts were welded together.
When the mixture in the reboiler was brought to a boil, the generated vapours jacketted the walls of the vapour tube after the trapped air had been removed through the vent valve. Liquid samples, in the usual fashion for Othmer-type equipment, were withdrawn at the base of the reboiler through a liquid sample valve. The equipment was insulated through the use of a one- inch layer of 85% magnesia insulation. The vapours that passed up through the vapour tube and the top of the boiler entered a condenser consisting of several turns of stainless steel pipe housed in an iron pipe condenser box. The vapours were totally condensed and collected in the condensate receiver after passing through a drip indicator. The reservoir was constructed from cast stainless steel and had two sight glasses (front and back) to allow for visual observation and control of the circulation rate. A pressure equalizer tube was used to ensure that there were no pressure gradients between the condensate reservoir and the trap due to the hydrostatic head of the accumulating liquid.
-1NNER VAPOR
I TUBE
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cm
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VENT VALVE
BO'LER - (4"00 -
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- T1£R"10-Wf..~L
(3/8"0.0. - 16 US.C,· IS"'-cnq)
Ie'T!!!!DCONDENSATE - RESERVOJR
Figure 2.28. Apparatus of Othmer and Morely (1946).
There was a union tee at the base of the condensate reservoir, where one leg allowed for the withdrawal of a sample through the vapour sample valve and the other leg (at an angle of 900) allowed for the recirculation of the condensate into the trap. The trap was water-jacketed to cool the condensate. The purpose of the trap was to maintain a constant liquid level in the reservoir and as an overflow vessel to smooth out any fluctuations in the boiling action or liquid flow.
The trap was connected to a surge tank via a valve located in the vent line of the trap. The surge tank was constructed from stainless steel and was slightly inclined to allow any condensate to drip back into the trap.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
The authors reported equilibrium times of around 2 - 3 hours. A double valve metal bomb was attached to the sample valve and was used for the rapid and simultaneous liquid and condensate sample withdrawals so as to minimize the effect of flashing as a result of the pressure reduction.
Equipment of Gelus et al. (1949)
Gelus et af. (1949) followed on from the work of Scheeline and Gilliland (1939) in attempting to negate the apparent uncertainties associated with the vapour condensate recirculation method in its applicability to higher pressures.
The equipment was principally constructed from steel due to concerns over the high- temperature strength of glass and problems due to sealing difficulties (packing glands, gaskets, etc.). The maximum operating pressure was 3.5 MPa. The recirculation apparatus is shown in Figure 2.29. Steel pipe was used throughout for the body, stainless steel valves being used. All parts of the equipment were heavily insulated except for the ballast vessel and the vacuum system. The boiler was constructed from a standard steel pipe section with the ends being plugged, welded and fitted with pipe connections. The lower end was covered with baked ceramic cement, around which two heating elements were mounted. The upper section of the boiler was insulated with asbestos cloth around which a heating element is wound. A baffle system was used in the boiler to promote good mixing of the liquid contents.
A magnified internal section ofthe boiler is shown in Figure 2.30 to provide details of the baffle system. The annular space served as the "vapour-lift pump", as being analogous to the low- pressure design of Raal et al. (1972), which propelled a vapour-liquid mixture upwards and the mixture then hit against the upper baffles. The upper baffles (annular and conical), in addition to preventing liquid droplet entrainment, also served to allow for contacting between the hot rising vapours and the liquid, which returned to the base of the boiler by dropping through the centre of the baffles. The baffle at the base of the boiler at the condensate return line inlet served to prevent the returning stream from interfering with the circulation rate of the boiler contents and to some extent promote good mixing with the boiler contents. The vapour take-off line from the boiler and the condenser are heated, which can be used to reduce the circulation or boil-up rate if desired.
TO t\.... AS.
vtSSf.L
PRESS E·
<;C'~fttC""
hl:.~,t:r€p
!J
» :
...
Figure 2.29. Apparatus of Gelus et al. (1949).
The condenser was principally divided into three sections in accordance with the heating and cooling requirements of systems of varying volatilities and for control over the boiling rates.
The pipe union cross and the still side of the controlV-tube were heated to prevent any partial condensation of the vapour in the vacuum line or mercury lines.
The condensate sample was collected in a trap with 20 cm3 capacity with a sight glass. In the normal operation of the still, the boiler was charged with sufficient material (at least 150 cm3)
such that the still contained sufficient charge corresponding to the centre of the sight glass. A sample was taken from the trap through the use of an isolating valve between the condensate return line and the boiler.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
THER~~'ELL
SAMPLE: LINE
STEEL PLUG
CO,~JOENSATE
RETURN LINE
"
\
3/4" SGHEQ 40 PIPE.
CONICAL. BAFFLE
..w----ANNOLARBAFFlE CONVECTION CYLlNOER
\
- - t + - -ROUND BAFFlE
~/ELD METAL
Figure 2.30. Magnified section of the boiler of the apparatus of Gelus et al. (1949).
Equipment ofZieborak(1964)
The equipment of Zieborak represented the first attempt at the design of an ebulliometer based on the original design of Swietolawski (1924), to withstand conditions of elevated temperatures and pressures. Although, as mentioned previously that there is undoubtedly a clear distinction between ebulliometry and VLE recirculation methods, the analogous operating principles of the two warrants the inclusion of this equipment design in this review. The temperature range of the apparatus was 347 - 495 K and the equipment could withstand pressures up to 2.5 MPa.
The metal ebulliometer design of Zieborak is shown 111 Figure 2.31. Since the latter is a traditional ebulliometric-type design, there are no provisions for sampling since the measurement of boiling points is the desired outcome.
B
A
Figure 2.31. Ebulliometric apparatus of Zieborak (1964): A, boiling chamber; B, Cottrell tube; C, thermowell;D,condenser.
The design and principle of operation is analogous to that of the Swietolawski ebulliometer (1924) design, however, with a few modifications. One of these modifications was the use of steel capillaries (O.5mm diameter) in the boiling chamber (A) to smooth the flow of the return liquid and to limit backflow of the boiler contents. The contents of the boiling chamber were heated with a heating mantle mounted onto outside of the boiling chamber. The equilibrium temperature was measured with a platinum resistance thermometer housed in the thermowell (C) in the equilibrium chamber.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
Equipment ofNagahama and Hirata (1976)
The equipment of Nagahama and Hirata had combined features of both the Othmer-type and the liquid and vapour recirculating-type equilibrium stills. There were two types of pressure control strategies that were used, in the form of compressed nitrogen and a heat input control, which of course depended on the pressure range that was studied. The equipment design is shown in Figure 2.32.
A
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D B Cl 11 1 JLt
K 12 13 M-liquidcontl<:ti.o& cell
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Vwbt~ ft4istQc
Bmricwire
Tempc.ra.ture xeoordet
Pmuure tta:lt, r· COl1lroUtt Ml, tic Impliti~
Potentiometer
Figure 2.32. Apparatus of Nagahama and Hirata (1976).
As for a typical Othmer-type apparatus, the boiler and the equilibrium chamber were combined into a single vapour-liquid contacting cell with appropriately positioned sight-glasses and the liquid phase samples would be withdrawn from the cell. However, unlike typical Othmer-type apparatus, both phases were recirculated in separate lines and then combined in their re-entry into the cell. The cell also featured an internal stirrer to assist in ensuring the generation of equilibrium vapours.
Equipment of Olson (1989)
Later developments in the use of ebulliometers for the determination of boiling points and as well as modified ebulliometers for VLE determinations included the efforts of researchers such as Olson (1989) to demonstrate the industrial applicability of the ebulliometric VLE method.
Olson demonstrated the use of the technique for pressures up to 2 MPa with the design of a metal apparatus which had appropriately positioned sight glasses. The design of the apparatus is shown in Figure 2.33.
B
/
----1I-+--+--H_+_-A
E
D
C'
Figure 2.33. Apparatus of Olson (1989): A, boiling chamber; B, twin-arm Cottrell tube; C, the sight glass section; D, condenser; E, pressure gauge.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
As can be observed from the design, the vapour lift pump or Cottrell pump consisted of two arms i.e. a "twin-arm design". The boiled mixture first passed up through the two arms of the ebulliometer and impinged upon the thermal sensor in the thermowell. The vapour is condensed and mixed with the liquid as it returns to the reboiler through the section fitted with the sight glass, which allows for visual observation of the circulation rate. The pressure connections were made through the top of the condenser and pressure control in the system was achieved through the use of compressed nitrogen and a backpressure regulator.
Data was obtained for the dichlorosilane +trichlorosilane binary system for the pressure range of 0.7 - 2 MPa. This pressure range comes into question as the concerns over the solubility of the nitrogen in the binary system, forming a ternary system, were expressed by early researchers such as Scheeline and Gilliland (1936) and Othmer (1946), who opted for alternative pressure control strategies or diminished pressure ranges, respectively.
Equipment of Wisniewska et al. (1993)
The equipment of Wisniewskaet al. was based on the design of the Rogalski-Malanowski glass ebulliometers (Rogalski and Malanowski, 1980) that were modified for VLE measurement. The equipment was developed to withstand pressures up to 3 MPa but was checked up to a pressure of 5 MPa. Its development, as was the case for Olson (1989) was also inspired by a dire need for VLE data in the range of 0.1 - 3 MPa by a fast and reliable VLE measurement method.
The entire medium pressure ebulliometer apparatus, shown in Figure 2.34, was constructed from stainless steel, with the exception of the drop counter, which was constructed of thick- walled glass tubing to allow for visual observation of the circulation. As for the design upon which it was based i.e. the Rogalski-Malanowski still (1980), the boiler and the Cottrell tube were considered as a single entity (A). Special attention had to be paid to the design of the drop counter, the sampling valves and the feed valve in superatmospheric determinations.
The needle valves were constructed to minimize dead volumes where the body of the valve was filled with the condensate or liquid mixture at all times during the operation. As in the design of Rogalski and Malanowski (1980), a suitable mixing section in the return line to the reboiler was incorporated; however, there was still no mechanical agitation in the reboiler or the sample traps. Also, the sideways entry of the Cottrell tube in the equilibrium chamber, the design of the equilibrium chamber itself and the transport of the vapour to the condenser at the disengagement interface were unchanged.