Ana Iytica I Ap p I icatio ns a nd of Long-Tube Vaporizers 6.
Co nst ructio n
D. ASH
Research and Development Department, Union Carbide Chemicals Division, South Charleston, W. Va.
b An apparatus for the determination of residues in heat-sensitive samples is described. The equipment operates on the same principle as industrial-type, single-pass, long-tube, vertical evaporators. Residence times in the vaporizing zone of less than 5 seconds can b e easily achieved. Rapid phase separation in an efficient cyclone separator helps minimize polymerization and thermal decomposition. Reproducible residue determinations can b e performed routinely on mixtures which are virtually impossible to innalyze by conventional batch methods.
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challenging problems encountered in the field of analytical chemistry is the quantitative determination of residue products in highly reactive or heat-sensitive samples. The most common methods in current use employ some form of batch distillat,iori or evaporation. The apparatus is generally operated under reduced pressure to maintain the sample a t the lowest possible tcmperature so as to minimize decomposition or polymerization. However, a3 the distillation proceeds, the concentration of the nonvolatile portion increaf,es and as a result, the kettle temperature starts to If the a p p r a t u s is being increase. operated a t the lowest practical pressure, there is no choice left to t,he analyst but to allow the kettle temperature to increase indefinikly. Often, the high kettle temperature makes it necessary to terminate the anall while a large percentage of the volatile portion remains in the kettle. I n the case of heat-induced polymerizations, the analyst may find that in the course of analyzing the sample he has inadvertently made additional polymeric residue which will result in high and misleading residue figures. h further problem is encountercd in analyzing samples that are thermally unst,able. Vigorous decompositilm can occur through the effect of p:*olonged heating a t elevated temperatures. Consequently, accurate dptermination or isolation of residue products is often imposhible in conventional equipment. The need for developing a quantitative method for determining residues in heat sensitive mistures was a direct reK E OF THE MOST
sult of our study of the synthesis of tolylene diisocyanate. The isocyanate is produced by phosgenation of the corresponding diamine as indicated in the equation below. Minor concentra-
CHs I
All single-pass, long-tube evaporators operate on the same basic principle. The feed mixture, which is normally a t atmospheric pressure, is fed through a metering valve to the base of a long
CHa
hC0 tions of residue by-products are invariably produced during the reaction. In attempting to analyze these mixtures by conventional batch methods, the residue portion was reactive with tolylene diisocyanate and the residue values were consistently too high. Since it was evident that the long residence time (30 to 180 minutes) in batch equipment produced unsatisfact’ory results, a highly efficient long-tube vaporizer was developed in which the time that the sample remained in the vaporizing zone was reduced to 5 seconds or less. The concept of a single-pass, longtube evaporator for industrial applications is quite old but its use as a quantitative analytical device has not been previously recognized. The original idea for the technique was conceived by Kestner (3) of Lille, France. I n 1899 he patented a variet,y of industrial-type, long-tube evaporators. The evaporator was highly successful in Europe but was completely unknown in this country until 1930 ( 2 ) . In recent years it has been used fairly extensively by American indust,ry. The equipment has a variety of names including “Kestner evaporator,” “long-tube, natural-circulation evaporator,” “rising-film evaporator,” “long-tube, vertical evaporator,” and “long-tube vaporizer.” The abbreviation LTV is commonly used to identify this type of equipment. Bartholomew ( 1 ) designed the first all-glass, laboratory, long-tube evaporator. The apparatus described in this paper is a modification of his basic design. Our improved design has achieved increased efficiency, shorter reqidence time, and wider operating parameters. Even more important, the quantitative applications can now hv realized for the first time.
vertical or spiral tube which is heated by a suitable means. The entire system beyond the metering valve is maintained under reduced pressure by an adequate vacuum source. After the feed mixture passes through the metering valve, a series of complex events occur which have only been partially explained ( 2 ) . By carefully observing the action in glass equipment, it appears that, the volatile portion of the feed mixture is converted to vapor almost instantaneously. The rapid vaporization creates a high velocity gas &earn which immediately reduces part of the remaining liquid phase to very minute droplets. At the same t’ime a rapidly moving thin film of liquid can be observed flowing up the walls of the tube. Any solid phase which separat,es is carried along by entrainment through the combined effect of the high velocity gas stream and the rapidly rising liquid film. I n most feed mixtures, the low boiling components completely vaporize before reaching the mid-point of the vertical tube. The combined vapor, liquid, and entrained solid phase is fed tangentially into a n efficient cyclone separator. The heavier liquid and solid phases are thrown to the periphery of t,he separator by centrifugal force. The vapor phase concentrates in the middle of the re3ulting vortex and is carried out of the separator by a centrally-positioned, large-diameter vapor line. The liquid and solid phases follow a rapidly moving spiral path down the wills of the cyclone separator and into a suitable residue receiver. In brief, the rapid vaporization of the feed mixture followed by fast and efficient phare separation are unique advantages of the LTT‘ concept. The operation is “single-pass,” and, thereVOL. 3 6 , NO. 7 , JUNE 1964
* 1363
Figure 1.
Cyclone separator
NEEOCE VALVE
fore, each molecule spends the same length of time in the heated zone. Since the residence time in the vaporizing zone is on the order of a few seconds, adverse thermal effects are held to a minimum. EXPERIMENTAL
Apparatus. At the risk of belaboring the point, it should be emphasized t h a t the successful application of LTV’s in analytical work is very dependent on properly fabricated equipment. If too much latitude is taken with the dimensions and configuration of key components, the equipment can be rendered virtually inoperable for all practical purposes. The vaporizer design departs radically from that of Bartholomew ( I ) . The spiral tube has an internal diameter of 5 mm. and an overall length of 7 feet. The ratio of the length to diameter is on the order of 430:l. The internal volume is 47 ml. The glass jacket and resistance winding on the vaporizer allows a wide range of vaporizing temperatures to be used, depending solely on the availability of suitable reflux liquids, Vaporizers with shorter tubes can be substituted but the evaporating capacity is decreased proportionately The vaporizer is constructed from a 500-mm. spiral (Graham type) condenser. The drain stopcock and feed line must be attached as closely as possible to the bottom of the vaporizer. The heating element, which consists of about 20 feet of 0.5 ohm per foot Xichrome wire wound spirally over a layer of asbestos (or glabs) tape, should always be operated by means of a variable transformer. The cyclone separator is an integral and vital component in the successful operation of any LTV. The spherical 1364
ANALYTICAL
CHEMISTRY
CfWN S W C O C K
Figure 2.
Long-tube vaporizer system
design (Figure 1) is used primarily because it is readily fabricated from a 1-liter standard distillation flask. Several precautions must be observed during fabrication. First, the inlet tube to the cyclone must be held to a small diameter to maintain the vaporliquid feed a t a high velocity. Any increase in the inlet tube diameter u-ill cause a corresponding decrease in the vapor velocity and in the case of very large inlet tubes, the “cyclone effect” is destroyed completely. This will cause very inefficient phase separation. A second and equally vital part of the design is the angle a t which the inlet tube is attached to the wall of the cyclone. I t is imperative that the axis of the inlet tube form a true tangent with the curvature of the cyclone. Improperly positioned inlet tubes can direct the liquid phase to the center of the chamber where entrainment of liquid and solid in the evit vapor line can occur. In a properly designed unit, the high-velocity feed is directed against the wall of the cyclone by centrifugal force. The liquid spreads out in a circular pattern forming a very thin film from which further evaporation can take place. The last important factor to consider is the diameter of the exit vapor line. This line should be made from large diameter tubing to accommodate a large volume of vapor a t a minimum pressure drop through the system. The remaining components are of conventional debign. The infrared lamp minimizes condensation of the vapor phase in the cyclone separator. Operation. To monitor a stead) state tolylene diisocyanate pilot unit,
the equipment was assembled as illustrated in Figure 2. 13uty1 Cellosolve (b.1). 171’ C./’760 mm. of Hg) was charged to the jacket of the vaporizer and heated under total reflux. A 500ml., round-bottomed flask was tared to the neare3t 0.1 gram and attached to the base of the cyclone separator. The oil bath was maintained a t 200’ C. Brine (- 10’ C.) was circulated ihrough the inner coil of the distillate condenser. The system was maintained a t 5 mm. of Hg pressure. h 500-gram sample of crude tolylene diisocyanate was then poured into the feed reservoir. The sample n-as fed a t the rate of 1500 ml. per hour. After the sample had been fed, 100 grams of oi-tho-dichlorobenzene was immediately added to reservoir and fed to the vaporizer a t the same rate. After all of the distillate had been condensed, the system was vented to atmospheric pressure and the residue receiver was reweighed to determine the net weight of residue which had been collected. ri series of samples, analyzed a t three-hour intervals, produced the following typical residue values: 4.02, 4.12, 4.07, 4.23, 3.81, and 4.13 per cent by weight. T o determine the precision of the LTV method and to compare it with a standard thermogravimetric residue method currently in use, a large sample of a heat sensitive osidation mixture wa,q rollccted from a plant process stream and aliquot. were analyzed using both iirocedurei. The LT6- data was obtained using the same technique as that de,srribed for crude tolylene diisoryanate. ?’hi. result,s reportcd by three different analyst5 are summarized in Table I.
RESULTS AND DISitUSSION
The results obtained using the LTV technique for determination of the residue content of crude tolylene diisocyanate were very en:ouraging. The crude samples contained large concentrations of phosgene and HCl as well as solvent, tolylene diisocyanate and residue. Initial attempts to analyze these mixtures in corn-entional batch equipment produced \ ery erratic results which were two to four times higher than the LTV method. The data in Table I indicate a high degree of precision when the LTV method is employed. The thvrmogravimetric (TG) method which was used for comparison is actually another variation of batch evaporation during which the entire sample is heated for a period of 15 to 30 minutes. The formation of additional residue during the analysis is evident in the higher values consistently produced by thermogravimetry. I n using the LTV technique for analyzing a wide variety of mixtures, it is necessary to establish optimum operating parameters for each system. It is sometimes necessary to try a variety of temperatures, pressures, and feed rates in order to approach optimum conditions. As a rule, the lowest pressure is chosen that one can conveniently condense the distillate when using brine (or cold water) as the coolant of choice. Once the working pressure has been chosen, the average boiling point of the volatile portion of the feed misture (at the selected pressure) is established. If the atmospheric boiling point of the volatile phase is known in addition to one or more reduced pressure boiling points, the data can be easily plotted on 2 cycle log P vs. l / ( T o C. 230) vapor pressure graph paper and extrapolated, if necessary, to the desired operating pressure. The reflux liquid for the jacket of the vaporizer is then chosen by selecting a stable organic compound that boils at 50' to 100' C. higher than the average boiling teriperature of the feed mixture at the wlected pressure. For example, if the voltttile portion of a misture to be evaporated has an average boiling point of 70' C a t 100 mm. of Hg, then ethylbenzene (b.p. 136' C. a t 760 mm. of Hg) would be a logical choice for use in the vaporizu jacket, if the LTV is operated a t approhimately 10 mm. of Hg pressure. The problem of feed rate is sometimes the most difficult to establish. I t must be remembered that the successful
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operation of any high-velocity, longtube vaporizer is very dependent upon the rapid vaporization of a large portion of the volatile components in the feed mixture. If one attempts to feed material containing 90 to 95y0 nonvolatiles, the efficiency of the operation drops sharply. If the feed is not heatsensitive, the impaired efficiency may not be serious. However, in the case of heat-sensitive, relatively nonvolatile mixtures, it is sometimes essential that an inert, miscible, and easily volatilized diluent be added to the misture. The increased rate of vaporization and decreased residence time will then approach optimum conditions for LTV operation. The average feed rate recommended for small laboratory LTV's is generally between 1 liter and 5 liters per hour. The rate of feed must be fast enough to maintain a short residence time in the vaporizer. However, if the mixture is fed too fast, the cyclone separator and distillate condenser may be overloaded. I n difficult separation cases, it is recommended that the material be processed twice. Often, the more volatile portion can be removed in the first pass. The pressure in the system can then be reduced and the remainder of the volatiles removed during the second pass. Obviously, materials boiling closely together cannot be separated on an LTV. For comparison, the LTV will separate any mixture that can be resolved on a Claiaon-type still. Several factors determine the successful operation of a n LTV. These include the difference in boiling point between the low-boiling distillate and the high-boiling residue, the percentage of low-boiling fraction present in the sample, the feed rate of the mixture, the average pressure of the system, the temperature differential between the boiling liquid in the jacket of the vaporizer and the average boiling point of the material being separated, the ratio of the internal diameter of the vaporizer tube to the overall length of the heated section, the geometry of the c%ycloneseparator, the condensing capacity of. the distillate condenser, the capacity of the vacuum source for handling noncondensable gases, and the temperature of the residue receiver. Although the uqe of the LTV in quantitative residue determinations is quite new, a large variety of heatsensitive mktures have been successfully analyzed. In most of these
Table I. Comparison of the LTV with Thermogravimetric Analysis
Analyst S o . 1
Analyst No. 2
LTTa
TGb
0.83 0.91 0.94 0.98 0.87 0.82 1.18
5.31 5.12 5.82 5.38 2.14 2.30
1 08
iii
Analyst S o . 3
113 1.09 1.20 0.88 0.92 1.00 0.05
,
,
... ... ... . . ...
Mean 4.35 Std. Dev. 1.54 Long-tube vaporizer. b Thermogravimetric. This method involves type of batch evaporation in which recording microbalance is enclosed in suitable glass container that can be operated under reduced pressure. 20-mg. sample is placed on pan of balance. System is evacuated while heat is applied to sample. Weight loss is plotted continuously by strip-chart recorder. Residence time is usually 15 to 30 minutes.
determinations, the values have been consistently lower than conventional methods by a factor of 2 to 5. Aqueous samples containing inorganic salts have been easily resolved. Samples containing a very low level of residue (
