G.
m.
COOKE
A BARREL SIZE VACUUM STILL For removing maximum distillate, obtaining accurate data, in all sizes from laboratory scale to pilot plant made in several sizes suitable for A vacuum pilot . plantpotstill, operations, has been developed for batch vacuum distillations in petroleum laboratories. The largest unit is designed for charges from 100 to 400 liters (2 barrels) in size. Smaller units are available to handle charges of 50,20, 8,4, 2, and 1 liter conveniently. The various sfills are designed for almost identical performance. Thus data from all units are interchangeable. I n most petroleum distillations, the upper limit required would be removal of material with a normal boiling point of about 1100O F. The still described here operates in the region between 50 microns and 50 mm. of Hg absolute pressure. I t is thus suitable for removal of maximum distillate without thermal cracking, and for the preparation of heavy residua. The larger units can be operated continuously, if the job warrants it. I n gemral, the Sarnia Hivac pilot still is a versatile and light weight unit. The overhead equipment is largely of vacuum jacketed glass. Manipulation of equipment is easy, therefore operating costs are low. Time consumption per batch is usually two to four hours. The units are compact, requiring about 4 by 8 feet of floor space, with 8 to 10 feet of height. I n operation, we find that weight recoveries generally exceed 99.5%. Repeatability is excellent. Interchangeability of data between various sized units is very good, as shown in the table opposite. Four different operators performed these runs. Equipment
As shown in the sketch opposite, the major equipment parts are the distilling flask, an entrainment separator head, a vacuum adapter for product collection, a condenser, a cold trap for protection of the vacuum system, and the vacuum pumps themselves. Suitable measuring devices are required for data collection. Temperature and pnasure measurements are made simultaneously at the beginning of the condensing region. Perhaps the most important item is the entrainment separator, developed in our laboratories. This is an efficient separator of very low p m u r e drop, completely 36
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
enclosed in a silvered glass vacuum jacket. A permanent vacuum of 10-7 mm. of Hg is maintained in the jacket. Heat loss is reduced to negligible proportions without additional insulation or heated jackets-unique in a unit of such size. Two types of distilling head have been developed. The drawing opposite shows the Model A Hivac, while the photograph shows a ZOO-liter potstill with the improved Model B head. This unit is the largest capacity head made to date, and is stronger and less expensive than the Model A in the larger sizes. Flask and Hwt.r
Flasks are made of stainless steel because of the size and combustible nature of the usual charge. Spherical vessels are employed for stills having capacities from 100 liters down to 10 liters OT kss, and whkb stand in their heating mantles. Those designed for 200 and 400 liters are cylindrical and are mounted horizontally on legs. A pipe which leads to the bottom of the flask is used for charging, and also for discharging the residue while still hot. This line can also be used for stirring or for stripping of solvent by injecting a stream of inert gas. A thermowell is inserted through the upper surface of the flask to I/, inch from the bottom for detection of the temperature of the boiling liquid. An internal water quench coil of stainless steel tubing is included by means of which rapid cooling can be achieved before discharge of bottoms. Steam can also be used in the coil for thermostating at low temperatures and to assist in initial heatup of cold charges. The flask is heated by an electric heating mantle. Total load vanes from 12 kw. for the 100-liter still to over 25 kw. for the larger ones. Heat is applied by resistance heaters through a wire reinforced asbestos fabric mantle directly to the wall of the flask. Three to six 220-volt heaters are concentrated to the maximum tolerable
AUTHOR G.
M.Cooke, a chmical engineer, is a
&&on Head
in the Research Department of Imperial Oil Enterprises Ltd., Sarnia, Onfario, Canada.
heat density of about 15 watts per sq. in. which permits rapid heatup. Variable autotransformers control the heat during a run. Distilling Head
The large all-glass head surrounded by a vacuum jacket performs the essential job of entrainment separation. In addition, it ensures that the vapor stream arrives at the measuring point in a saturated condition but without excessive condensation. The diagram of the Model A Hivac head shows the elbow bend through which the vapors pass at high velocities under vacuum. Entrained liquid droplets are swung outward against the wall where they impinge and run back into the boiler, leaving entrainment-free vapor to pass on. The Model B Hivac head is used for units over 50 liters in capacity where the strength of unsymmetrical glass sections becomes critical. Model B design is, in fact, a spherical version of the Model A. Centrifugal separation of the entrained droplets occurs in a spherical annulus, a sphere within a sphere. Very high rates are possible in this version. The head has a standard taper joint at the top of the sidearm through which temperature and pressure meas-
COMPARISON OF HIVAC STILLS
(Charge-blend
of crude products)
Dirlillofs, Vol. 90
10 20 40 60 80 90 Av. std. dev.
Tampnohrrt, ’F.
819 863 914 962 1043
814 861 910 960 1036
816 862 911 958 1028 1092 4.2
-
-
3.5
2.1
808 854 908 959 1030 1098 2.1
814.3 860.5 910.5 959.6 1033.7 1093.7 4.5
Ao. Raconsy, W f .%
99.8
I
99.7
1
100.2
I
99.7
I
uring devices are connected. A McLeod gage or Tensimeter vacuum gage (2) is connected through an adapter which supports a special high response thermocouple at the proper point in the neck. This point is a the end of the entrainment separator but ahead of tht cold walls of the condensing area. Thus, the thermo couple is exposed to saturated vapor but is not influenced by superheated vapor or by cool surroundings. I t is important that the pressure measurement be made at the point where the temperature is observed. Pressure gradients can develop both upstream and downswam, causing errors in calculating the atmospheric equivalent temperature. The vapors flow out through the sidearm and into the condensing zone which is provided with a large surVOL. 5 5
NO. 3 M A R C H 1 9 6 3
37
face area to handle light materials when necessary. Circulating coolant at 130’ F. is also provided for use with waxy distillates to prevent solidification on the coils. A large cold trap at the temperature of dry ice is included ahead of the diffusion pump. Condensed liquid runs down the Y-shaped sidearm into a vacuum adapter, as shown. No interruption of the run occurs when segregating cuts. The head, condenser, traps, and flask are connected by newly developed Viton gasketed ball joints ( 4 ) . These hold their seal at extremely low pressures and at temperatures up to 600’ F. while maintaining articulation. A joint is provided directly above the neck for continuous addition of feed, as for instance in solvent stripping. A large volume of solvent is removed overhead while the bottoms are collected in the flask. By adapting a totally immersed pump to the flask for removal of bottoms, this unit can be converted to true continuous operation ( 7 ) . Pumping System
Large pumping capacity is essential to remove gases which occur due to thermal decomposition and incipient cracking a t the high temperature end of the distillation. Adequately low pressures can only be maintained by use of a diffusion pump backed by a good mechanical pump. Diffusion pumps of 3- to 6-inch throat are used with 450 to 3000 liters per minute of mechanical pumping capacity. Prime Measurements
Measurement of the prime variables of mass, density, temperature, and pressure is the main concern in establishing accurac) and repeatability. Actually, only repeatability can be studied because accuracy in such a complex petroleum mixture is not subject to analysis. Of the four prime variables, mass and density can easily be determined to an absolute accuracy of 1 or 2 parts in 10,000. However, temperature and pressure are much more difficult to measure; they are the chief concern in distillation. The simple thermocouple, properly used and maintained, appears to be the best routine temperature sensor. Because of the low heat content of gases undcr vacuum EQU I PM E N T USED
Safety
-Stainless steel flask and mantles, glass heads, condensers, traps, and accessories from H. S. Martin and Son, Evanston,
Ill. -Pump for 14-inch CMS Still from Consolidated Electrodynamics Corp., Pasadena, Calif. -Diffusion pumps type PMC 1440 Vacuum Corp., Rochester, N. Y.
from
Consolidated
-Mechanical Pump 2S4.50 and 1S3000 frotn Edwards High Vacuum, Burlington, Ontario.
-300-kg.
platform balance with tare f r o m G e o r g e Westphul C o . o f Can. Ltd., Box 511 Yonge Street, O a k Ridges, Ontario.
-Gardsmun, M o d e l JP controller from W e s t Instrument Co., Chicago, Ill. 38
and the requirement of following a rising temperature curve, fast response is essential. Low mass and high conductivity and surface of the sensor are indicated. The thermocouple designed for the Hivac stills is an iron constantan bead of 26 to 30 gage wire enclosed in a thin-walled glass tube to which it is fused at the tip. The lower end of the thermowell is turned up in the shape of a shepherd’s crook to prevent the occasional drop of condensate from above running down over the tip and momentarily cooling it. Thus, it indicates the condensing film temperature. Calibration of the thermocouple is important. Freezing points of pure substances or eutectic mixtures are employed as primary standards (5) to make a correction curve covering the distillation range for routine use. Thereafter, a weekly check is made at some frequently used temperature. Corrections are determined by following the cooling curve from above the freezing point and noting the temperature at the plateau. The recommended vacuum gage is the Tensimeter which is simple, foolproof, and produces a signal which can be recorded on a temperature recorder with the vapor temperature. I t employs the boiling point of a pure compound to indicate the pressure and requires a calibrated thermocouple. The Tensimeter does not function well at pressures below 100 to 200 microns Hg, so a McLeod gage is suggested in the lowest regions of pressure. A McLeod gage is the only primary standard for subatmospheric pressures in the distillation region. Its accuracy depends on careful calibration and thorough hot baking out at micron pressures. Thereafter it must be protected from exposure to atmospheric air by venting only with dry nitrogen or other gas. If a double range gage is used and the two scales agree during measurement, it is a good indication that the system is free of condensables ( 3 ) . Conversion of vapor temperature readings taken at pressures below about 100 microns Hg to atmospheric equivalent temperature is not recommended because accuracy will be uncertain. VVhile distillates can be removed, boiling points of pure compounds are indefinite at very low pressures, because they depend not only on the pressure but also on boiling rate (2).
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Since these devices are made of glass, they suffer from the normal fragility of this material. However, it has been demonstrated in these and other laboratories that large glass pieces can be handled dai1)- by properly trained personnel. N o significantly greater workiriq hazard is associated with them when tha proper precautions are observed. REFERENCES Cooke, G. M., IND.ENG.CHEM.54, 47-51 (April 1962). Cooke, G. M., Rev. Sci. Znstr. 32, 780-3 (July 1961). Flosdorf, E. A , , I N D . ENG.CHEIM. ( A N A L . ED.) 17, 198 (1945). Rev. Sci. Znstr., p. 708 (June 1962). (5) “Temperature, Its Measurement and Control in Science and Industry,” p. 289, Reinhold, 1941.
(1) (2) (3) (4)
