Richard P. Rhodes ESSO Rkseorch and Eng~neer~ng Company
High Pressure .Techniques for Microscale Reactions
Linden, New Jersey
I n recent years great advances have been made in microscale analytical techniques. These techniques as well as the high cost of laboratory equipment and chemicals have made the use of microscale reactions both practical and economically necessary. Laboratory equipment exists for (10 g scale) ordinary organic reactions but no suitable equipment or techniques are available for very small high pressure reactions. We have therefore had occasion to develop high pressure techniques for microscale organic synthesis studies. These procedures were developed with a particular interest in obtaining complete material balances and in utilizing only inert construction materials. This paper outlines the design criteria and gives specific examples of pressure vessels which have been designed and tested. Pressure vessels constructed of glass are not new to chemists. Heavy walled glass tubes sealed .at one end and drawn out to a fusible tip a t the other (Carius tubes), have been used for many years as small scale laboratory autoclaves. There are, however, several drawbacks to these Carius tubes. The seal where the tube is fused cannot be annealed and so may crack under stress. The opening of such tubes with a glass cutter is a difficult and sometimes dangerous operation. I n addition there are problems in transferring materials out of contact with air. Simple glass bodied Teflon valves have recently become available from laboratory supply houses.'
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This resenrah- was sunnorted bv the Advanced Resmrah Projects Agency, Propellant Office,and was monitored by Army Ordnance under Contract No. DA-30-069-ORD-2487. * Fisher Porter Valve Cetalogue no. 795-609. ~~
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These valves can be used as closures on Pyrex pressure vessels as well as valves to control the flow of material into the reactor. Nonvolatile materials can be placed in the reactor by completely unscrewing the stem of the valve and injecting the material in with a hypodermic syringe. Volatile materials can be transferred into the reactor after evacuation, simply by cooling. Design of Glass Reoclors
The maximum gas pressure which the valve shown in Figure 1 will withstand will depend upon whether the pressure is applied to the stem side or the seat side (when the valve is closed). The maximum seat pressure PI, is equal to the maximum force which the stem threads will withstand divided by the area of the closed seal. (The seat diameter of the glass valves previously referred to are either 1'/4mm or 4 mm.) P,
4F
= ---i = =
2100 psig (for F = 4 lb, Dl = 1.25 mm) 205 psig (for F = 4 lb, D, = 4 mm)
A value for F of four pounds was experimentally determined as a safe working force by loading the valve stems with sufficient weights to tear out the threads. I n addition, the large and small diameter valves have been tested a t their working pressures numerous times without failure. Valves could be designed which would have stronger threads and a thicker body. These valves would, however, h v e to be custom made. The maximum pressure which the stem side of the valve can stand is Pz or 4 pounds, divided by the sectional area of the stem. This is approximately 80 psig for a 5 m m diameter stem. Thus, when the 1l / r mm valve is closed it can hold hack several thousand pounds, yet when it is open the maximum working
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423
pressure is only 80 psig. The pressure limits of the glass valves can be clearly seen. There remains, however, the specification of the dimensions of the vessel to which the valve is attached. Figure 2 shows a section of wall of a cylindrical pressure vessel. The pressure against the inside of this section is balanced by the tension in the wall. Using a maximum allowable tension in the glass of 10,000 psi (70°F) the maximum pressure containable is given by the relationship below. . .
Where 1 is wall thickness and R is radius, both 1 and R should have the same units. Pressure P, (psig) will be less for temperatures higher than 70'F.
NYLON STEM GLASS T "0" RLNG SEAL
R RADIUS OF CURVATURE
Figure 1.
Figure 2.
Testing of Glass Vessels
The types of glass pressure reactors which were built and tested were all of the same general design, i.e., cylindrical body with one or two 1-cm diameter heavy walled inlet tubes attached (Figure 3). In some cases the valves were blown onto the inlet tubes of the reactor before annealing. I n other cases, the valves were attached after annealing of the reactor, so that a Teflon coated magnetic stirrer could be inserted. I n this case the junction between the valve and the extension tube from the reactor was flame annealed after joining. Reactor bodies of 4-cm diameter, 11-cm length, and wall thickness 0.4-cm were checked a t 950 psig by condensing in a n excess of CO, using liquid nitrogen and allowing the reactor to warm to room temperature. No leakage was experienced. Reactors of 2-om diameter and 0.3-cm wall thickness have been regularly used at 200°C and 800 psig with, no difficulty. The 0.125-om seat diameter valves were used in both these cases.
Shielding of Reactors
The types of shielding devices used for pressure reactors can be divided into two classes, those that shield from blast (pressure release or detonation) alone and those that shield from blast and noise. A suitable shield from blast alone would consist of a section of 4-in. diameter schedule 80 stainless steel pipe, or a 4-in. diameter rectangular box made of steel plate. A window of 1-in. thick Plexiglass could be attached by screws inside a slit cut in the pipe or box. Providing the window slit in the steel does not run more than one-half the length of the shield, such a device has been tested to contain a detonation of 4 g of metal-encased high explosive centered in the shield. A suitable noise and fragment shield (Fig. 4) has been constructed from a 55-gallon oil drum. The reactor to be shielded is placed inside a pail containing a Dewar flask for liquid nitrogen, suitable thermocouples and a heating element. The pail is sealed with tape and buried close to the bottom of the drum under 1.5 feet of coarse gravel or rock salt. The feed lines and valve handles extend through the packing. Volatile material can he transferred into the reactor after evacuation simply by pouring liquid nitrogen through a feed line into the buried Dewar. Theliquid nitrogen is vaporized and temperature of the reaction controlled by the heating element and thermocouples surrounding the reactor. Volatile products can be removed through the feed line. The noise level next to such a packed shield is almost zero when 6 grams of high explosive are detonated inside the pail. FEED LINE B A L L JOINT HIGH PRESSURE VALVE
IUt-
u GLASS R m C T D R
STEEL REACTOR Figure 3.
NOISE AND BLAST SHIELD
Metal Vessels
Pressure vessels analogous to the previously described glass ones have been constructed of stainless steel using high pressure unions and valves (Fig. 3). These vessels are relatively leak free providing care is taken in assembling the systems. I n general the stainless systems were used only for reactions where the glass would be attacked by the reactants, where the reaction conditions were too severe for glass, or where reactants had to be charged a t high pressur.e The virtues of glass, i.e., freedom from leaks, cleanliness, and ability to see the reaction make it the preferred construction material for most reactions. 424
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Journal o f Chemicol Educotion
STAINLESS STEEL FINGER REACTOR
-
LIQUID NITROGEN FEE0
VALVE
REACTOR DEWAR SALT ON GRAVEL HEATER
Figure 4.
