FRANK A. BIRIBAUER, HOWARD T. OAKLEY, CARTER E. PORTER, H. STAIB, and JOSEPH STEWART
JOHN
Esso Research and Engineering to., Chemicolr Research Division, linden,
N. J.
Bench Scale Continuous Distillation Techniques Useful "know-how" on obtainina - .d o t Dlant data with bench scale continuous distillation units
A
NEW pmcess generally pama through sewd atages of development before it cmcrgca a8 a full-scale plant operation. Thcw indude the exploratory stage, the pilot plant, and the semiworks unit. Of course, this stepwise growth invokes successively larger and more expensive equipment. The size of the pilot plant, semiworks unit, and full-scale plant varies from indusvy to industry and process to pmces. In peaoleum refining, for example, a pmcem may be developed through a 5-baml-per-day pilot plant, a 100-baml-perday semiworks unit, and into a 10,000-barrel-perday fullscale unit. Each step, however, costs more in time and money. Therefore, the smaller the scale on which reliable data can be obtained, the less will be the cost in terms of investment, operating upenaea, manpower, and time. This paper describe8 the application of bench scale pilot unit techniques to the field of continuous distillation. The 0.5to 5-titer-per-hour throughput range of thcw units gives. an indication of how economical they can be for obtaining pilot plant data, They simulate certain plant operations very closely, operate continuously and automatically, and can be asembled readily on a single laboratory bench.
Ground glass joints make it possible to stack these section8 as q d .For the bench d e continnoun d i s a t i o n studies, the 1-inch diameter column waa found to be the most convenient size. A section of an Oldashaw column is Shown.
Two types of d u x sptiners are available for controlling the reflux ratio in the bench scale columns (see drawings). In the liquid-dividing bead, the separation between product and reflux is made on the condensate from the overhead condenser in the liquid phase. In the vapordividing head, this scparation is made on the vapor rising from the column. Both 'ypes of beads are actuated by a solenoid which is turned on and off hy an electrical timer. The
type of timer g a t frequently uscd can be set for reflux ratios as high as 120 to 1. For the bench d e distillation studies, a 3aecond on-timc was found to give smooth column operation with accurate reEux contml. For mariy distiUadon operations, either tvpc of reflux splitter may be used. Under certain eirCumtances, however, the vapordividing head is preferred. For example, whenever the overhead condenacs as two liquid phasea, the vapordividing head is used to avoid phase segregation between d u x and disrillate. A h , when column boil-up rates are low, the vapordividing head is somewhat more accurate. Both types of reflux splitter are equipped with ground glass joints for
Distillation Equipment Is Simple
and V e r d I e A typical laboratory setup for bench scale continuous distillation is pictured. Badcally, it compriacs a glass tower or towers with suitable acmories. The towers selected for this work are perforated plate, vacuum jacketed Oldershaw columns (4). They were c h a e n for two reasons. Firaq the plate efficiencies can readily be measured and correlated with plate efficienciesin plant rowers (2). Second, the columns are available in a wide range of diametem (0.5 to 3 inches) and plate sections (two to 30).
w
liquid-dividing head VOL. 49, NO. 10
W O W R 191
models used in the bench scale continuous distillation studies were not driven by constant speed motors. Consequently, to ensure uniform pumping rates, the current was supplied from a constant voltage regulator. Gear pumps, on the other hand, are equipped with constant speed motors so that they can be connected directly to the available power supply.
Typical laboratory setup For bench scale continuous distillation
easy connection to column sections and overhead condensers. In addition, the heads are vacuum jacketed and have suitable openings for thermometers or thermowells. At the top of the water-cooled condenser, an opening is provided for removal of noncondensable gases. These gases are generally passed through a dry ice trap to collect any product carryover. The feed section (shown) has ground glass joints at both ends so that it may be joined with plate sections in any desired fashion. Thus. it is possible to introduce several feed or solvent streams a t suitable points in a column. The distillate collector is usually a water-cooled, graduated receiver with a stopcock at the bottom. Gear Pumps Effective for l o w Pumping Rates
Feed is withdrawn from a graduated vessel so that flow? rates may be readily measured. Small rotameters are also employed for this purpose. Three types of pumps have been used in the bench scale continuous distillation studies. One type, the gear pump. has proved to be especially versatile in this service. It can handle a wide range of feed rates from 50 to more than 4000 ml. per hour. The pumping rate settings are reproducible within 0.3%. Very low feed rates can be pumped
1 674
accurately by setting up two pumps to operate differentially. For example, a 50-ml.-per-hour pump can be used to cycle liquid from the outlet to the inlet of a 70-ml.-per-hour pump. This technique results in a differential rate of 20 ml. per hour. One limitation of these pumps is that the liquid being pumped must be somewhat self-lubricating and also relatively noncorrosive to the metal of the pump. Many hydrocarbons and some aqueous solutions fulfill these requirements and can be pumped without difficulty. In the bellows pump, a motor driven cam compresses and releases a metal bellows. thus imparting a pulsating motion to the liquid in the bellows. The feed stream is pumped by the pulsations, which operate a set of check valves. Nonlubricating or corrosive feeds may be handled lvith the bellows pump, if suitable precautions are taken to isolate the pump from the system-. e.g., by a liquid seal. The piston-type pump will handle liquids that are not corrosive to the pump metal. Packing must be selected to be resistant to the liquid being pumped. Light hydrocarbons such as propane or propylene can be pumped by using a back-pressure regulator between the pump and the distillation equipment. The pulsating flow developed by bellows and piston pumps does not limit their usefulness. However, the
INDUSTRIAL AND ENGINEERING CHEMISTRY
Feed is usually passed through a preheater, a wire wound, insulated glass tube, before entering the tower in order to heat it to the required temperature. The small thermoelectric switch shown is not used for heat control because the intermittent on/off action would be too erratic to give constant feed temperature. Instead, the rate of heat input is controlled by a variable transformer fed from a constant voltage regulator. The thermoelectric switch is set to cut off the power at some temperature above that desired in the preheater but below the safe operating temperature for the unit. Thus, it serves to prevent overheating in case of an upset in the system. The temperature of the feed entering the column may be read or recorded with a thermometer or thermocouple inserted into the tee joint shown. The system residence time is closely
Preheater
C O N T I N U O U S DISTILLATION TECHNIQUES controlled in bench scale continuous distillation by adjusting still pot holdup. This control is especially important when the material being distilled is heat sensitive. To obtain a desired residence time, the liquid holdup in the distillation flask is adjusted to the proper level. The adjustment is accomplished, first, by selecting the right size flask and, second, by adding glass beads to the bottom of the flask until the liquid holdup is exactly that needed for the desired residence time. The glass flask which serves as the still pot is usually heated by means of a heating mantle. Occasionally, it is not possible to put all the required heat into the flask via the mantle. Cartridge heaters can then be used. They are inserted into glass wells blown into the flask below liquid level. Flaked graphite is used as a heat transfer medium between the cartridges and the glass wells. Both mantles and cartridge heaters operate through a variable transformer which receives its power from a constant voltage regulator. The transformer controls the heat input while the regulator assures that it will be constant for any given setting. Sometimes, it is desirable to vary the heat input to the still pot as a function of the temperature at some point in the distillation column. Under these conditions, about 90% of the total heat is put into the system via the mantle a t a fixed rate. The remaining 10% is added via the cartridge heaters at a variable rate determined by a temperature controller which in turn is actuated b y the particular system temperature. Bottoms Removed by Gravity Overtlow or Pumps
Liquid level must be maintained constant in the still pot to assure uniform flow rate and residence time. The simplest method of accomplishing this is by means of gravity overflow through a U-tube seal. This method, however, is generally not applicable in multicolumn distillation setups where the bottoms from the first column are charged to an intermediate point in the second column. Under these conditions, there is often not enough headroom available for a gravity overflow U-tube. When headroom is so limited, the overflow device shown is frequently used. By adjusting the height of the mercury bulb, enough back pressure can be maintained on the still pot liquid so that it flows smoothly through the cooler and into the graduated bottoms product receiver or, if desired, into a second column. Gear pumps have also been used for withdrawing bottoms from a still pot through a dip tube. The pumps,
however, do not maintain a uniform pot level when operating conditions change or when minor upsets occur in the system. The overflow devices are self-controlled under these conditions and so are
such as light hydrocarbons are never processed without continuous, full-time supervision.
usually preferred.
Careful Attention
Long Operating Periods without Supervision
An important advantage of bench scale continuous distillation equipment is that it can be operated for long periods of time with little or no supervision. For example, runs are frequently made with no overnight supervision. To run this way, however, requires that the equipment maintain steady state conditions. I t is especially necessary that the heat input and flow rates be maintained constant. To accomplish this, a constant voltage regulator has been found essential for smoothing out line voltage fluctuations. I t has been found desirable to keep a continuous record of temperatures throughout the system, especially when the equipment is running unattended. High speed automatic temperature recorders have been used for this purpose and are preferred as frequent temperature readings are often required, particularly when starting up a unit. The bench scale continuous distillation equipment is assembled largely with ground glass joints. Conventional glass joint lubricants have proved fairly satisfactory for sealing these joints and preventing sticking. When the stock being handled leaches out the lubricant and causes excessive leakage, a graphited grease has been used with good results. Often, simply putting an additional spring clamp on a ball joint will prevent leakage. The laboratory in which the bench scale continuous distillation studies have been carried out is equipped with an automatic carbon dioxide fire extinguishing system. However: volatile materials
18/9 BALL J O I N T S
Still pot level control device
Starting up Requires
When a bench scale continuous distillation unit is started up, the operator “rides” the equipment very closely so that he can change transformer settings, pump rates, and reflux ratios promptly as needed. First, the still pot is heated up until liquid is refluxing in the column. Feed is then charged to the column at the desired rate, as established from previous pump calibrations. The preheater is adjusted to give the proper feed temperature. The reflux timer is turned on, and the boilup rate is adjusted to give the desired distillate take-off rate. The unit is run until flow rates and temperatures “line out” a t steady state conditions. It is necessary to allow from three to five volume changes of the key or critical component in the system before steady state is attained. Typical Studies
The examples that follow show how bench scale continuous distillation equipment has been used to collect pilot plant data in the field of petroleum refining. These examples illustrate the effectiveness of the equipment for simulating plant operations and providing process information quickly, economically, and with small manpower and investment requirements. Hydroformate Fractionation Problem. The bench scale equipment has been used to obtain pilot plant data required for designing the fractionation section of a hydroformer unit. It was necessary to show experimentally that the design octane number would be achieved in the plant. The design specifications for the required two-tower system are given in Table I. The laboratory setup for simulating the design conditions in terms of reflux ratios and numbers of plates is shown in Figure 1. A comparison of the bench scale results with required design values is shown in Table 11. Agreement is good for all streams. The unit was operated with a man present at all times because of the volatile, flammable nature of the feed stock. Where there is no safety hazard, however, this type of operation has been carried out with little or no supervision for periods u p to 16 hours. Obtaining this type of data was once considered a major pilot plant job. Today it is a reasonably routine laboratory operation which can be carried out rapidly and cheaply. With this particular VOL. 49, NO. 10
OCTOBER 1957
1675
.
,
.. ,.
Toble I.
charged to a stripping column where the d&ed benzene p d n c t was taken overhead and the stripped phenol was recovered and recycled to the extractive column. The phenol solvent used in this study melts above mom temperature. Coasequently, all phenol lines were wound with Nichrome w i n and insulated so they could be electrically heabcd. Becaw phenol is a hazardous material, the phenol lines werr constructed of stainles steel instead of glans. The operating conditions used for the phenol extractive distillation of benzene in the bench scale equipment were as _ .
Hydrofwmote Distillation Design Specifications Theoretical between Still and Feed Point
Plates
Theoretioal Plates above Feed Point
Total Theoretical Plateso
16 9
9 17
26 27
Actual Plates between Still and Feed Point
Actual plates
Renu Rstia O/D 0.625 6.54
column T-1 T-2
Total Actual
above Feed
Point*
Plstes15 T-1 0.625 25 41 27 6.54 T-2 43 15 * Using a tots1 reboiler sssumat to be equivalent to m e theoretical plsta a Olderahaw 1-inch perforated plate laborstorv columns. Plate etiicisnoy MI% (2).
-
Toble
II.
Hydroformate
Sitream Pwd T-1 overhead T-1 bottom@ T-2 overhead T-2 bottoms
Distillation Liq. Vol. % Repuestad Obtained '100
100
90.1
88.7
9.9
11.3 31.6 57.1
31.2 58.9
pmblem, for example, it took less than one month from the time the data were requested until the study was completed. In addition, several combinations o f ' plates and feed point location were tested to confirm alternate designs. Extnetive DLtilLtion of Benzene with PhenoL With some minor modif i c a h , the same bench scale equipment has been used to shcdy extractive dist2iation (5). Large volumes of
follows:
solvent are often required to alter the activity c d c i e n t s and relative volatilities of the components in the extractive tower feed. Hence, it is convenient to strip the solvent continuously and recycle it to the system. The bench scale continnous distillation equipment can be set up to do this very effectively. A typical system of this type was set up to confirm design data for the recovery of benzene from a refinery s m a m using phenol as the selective solvent. Figure 2 shows the equipment layout. The vapor phase feed was introduced into the extractive column on the 15th plate. Solvent phenol was added on the 30th plate, with five plates above the solvent feed point as the solvent-stripping section. Overhead from the extractive column was collected as r h a t e , and bottoms were
VENT TO DRY ICE TRAP
Phenol charge to syntem, ml. Hydrourbon feed rate, ml./hr. Phenol recirculation rate. ml./hr. Phenol1frs.b HC feed. by rt. Hydrourbon holdttp in system. ml. Bxhlctire column reflux r.tI0 Stripper column ream ratio
VENT TO DRY ICE TRAP
DRY ICE
ONDENSER AND COOLER
T-ZOVERHEAD
,VENT
7s
.
.
I T-i0OTTOMS RECEIVER
Figure 1.
3 3
MERCURY
Hydroformote distillation
Laborahwy smvp fw simulating design cadilions in terms of reflux ratios and numbersof p h t r
1676
/.
.
500 1700 511
100-300 2/1 311
Solvent rate, feed rate, solvent/feed ratio, reflux ratio, and system holdup were readily controlled in the laboratory unit. Typical data obtained in this unit are shown in Table 111. Based on the data, the original design for the commercial unit was somewhat revised. The revised design was also tested in the bench scale equipment to check the predicted benzene purity and recovery. Some collateral laboratory data from this operation were used to predict plant performance with regard to sludge formation. Based on the bench scale
CONDKNSER
MERCURY
2wo
INDUtTllAl AND ENOINEERINB CHEMISTRY
'
I PRODUCT RECEIVER
.
C O N T I N U O U S D I S T I L L A T I O N TECHNIQUES Table 111.
Naphtha Feed BenCut. oc.
50-85
54-85 54-85 54-85
F
*
m e , wt.%
49.1 45.7 45.7 45.7
Continuous Extractive Distillation of Naphtha Steady State Conditions
EXtrSC-
tive Raffin.de pat Vapor Bentemp., temp., Wt.% sene. oc. O C . offeed % 70 70 70 70
150 149 152 153.5
59.8 53.7 57.9 61.2
rtudies, the rate, type, and source of sludge formation in the commercial unit was predicted and suhquently confirmedin plant performance tests.
Continuous Vacuum Distillation of High Boiling Alcohols. I n the examples described so far, all the operations were canied out at atmospheric pressure. The bench scale equipment is also readiiy adaptable for continuous vacuum distillation. The use of this equipment in vacuum operation will
.
-
I
-
1
... 6.6
11.2 13.8
Product Vapor Bensene temp., Wt. % Bensene, rwovery, oc. offeed % wt.% 80 80 80 80
40.2 46.3 42.1 38.8
97.1 93.0 95.2 96.4
79.5 94.2 87.7 81.7
be described by meam of an actual laboratory study. In connection with some work done on higher boiling alcohols produced .by the Oxo reaction (3),it was found necessary to determine how distillation conditions affect the color forming tendencies of these alcohols. The bench scale setup used for this study is shown in Figure 3. T o allow for increased vapor volume at low pressure, a 2-inch diameter column was used for the vacuum tower while a
conventional 1-inch diameter tower served as the atmospheric tower. In the operation of a vacuum tower, good pressure control is esential. If the pressure suddenly d r o p below that in the tower because of poor control, liquid on the plates will Bash and upset column operation. Conversely, if the pressure' suddenly rises, the liquid on the plates will dump and also upset column operation. If such u p t s occur, it is necesary to stop the run and bring the unit to steady state conditions again. Commercially available manostam are well suited for pressure control in bench scale continuous vacuum distillation. The preferred type makes use of an automatic vacuum bleed to control pressure; it is effective in maintaining vacuum within 0.1 mm. of mercury. In this study, two receivers were used to collect bottoms and two for overhead product. Thus, it was pogsible to remove samples from the system without interrupting the run by shutting off
d-f? PRO UCT RECEIVERS
II
A
PHENOL
Figure 2. Recovery of benzene from refinery stream using phenol as solvent
Figure 3.
Continuous vacuum distillation
Effee of dirtillation mnditioru on color forming tendencia of higher boiling alcohols
4
4
BOTTOMS RECEIVERS
Table IVi Hydrolysis of C I ~Acetals and Continuous Separation of Butyraldehyde and Butanol Aldehyde Product Solvent Water Fed ' Alcohol produot AldeAldeSolubili.er Acetd Rate, hyde Rate. Alcohol Rate. hyde conen.. Rats. Appmx. RUn ea ml./ e a ml./ Reflux weid, as ml./ Reaur ml.1 mole. wt. NO. Ca hr. Wt. % 0 hr. ratio w t . % c 4 hr. % ratio % hr. %
-
1 2 3 4
43.4 47.1 44.0 44.0
250 300 378
300
33.5 25.6 17.3 12.4
91.3 89.5 88 W
83.5 77 65.5 72
15/1 1511 2/1 5/1
94 92 65
..
74.1 71.6 60.5 40.6
76.9 79.5 75.3 68.0
185 215 229 2
