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INDUSTRIAL AND ENGINEERING CHEMISTRY-
for the remaining oils. This is indirect confirmation t h a t these low polymers are not of the “aromatic-unsaturate” type of structure. The ratio of these synthetics is close to 1 , 5 , and 50 parts of benzene, toluene, and xylene, respectively. This is a fortunate and timely reversal of the usual ratio in which the standard aromatics have previously been produced. The tapping of this waste material and its conversion promises to provide industry with the more desired and higher boiling aromatics. The use of mixed cracking catalysts, to provide ring opening, offers some possibility of obtaining higher alkyl aromatic derivatives. The process in general is one of pseudosynthesis. It is not one of simple regeneration, and is synthetic to the extent t h a t the various fragments are subjected to a recombination and
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produce aromatics which are not identical with those initially involved in the washing step.
Acknowledgment The present development was completed by and for the Neville Company, in their laboratories, and we wish to express our appreciation for blanket permission to publish all articles dealing with this work from time to time; the developments are covered by patents.
Literature Cited (1) Carmody, W. H., IND.ENQ.CHEM.,29, 576 (1937). (2) Carmody, W. H. (to Neville Co.), U. S. Patents 2,143,474 (Jan. 10, 1939). and 2,149,577 (March 7, 1939). (3) Kraemer and Spilker, Ber., 123, 3169, 3269 (1890).
Effect of Pressure on the Enthalpy of Benzene E. R. GILLILAND AND R. V. LUKESl Massachusetts Institute of Technology, Cambridge, Mass.
I
r\’ C E R T A I S types of engineering design and calculations
on equipment operating a t elevated pressures, a knowledge of the effect of pressure on the enthalpy of the materials processed in that equipment is desired. Experimental data on this effect of pressure are meager, and for this reason considerable work has been done in calculating pressure-enthalpy changes from the P-T’-T relations of the materials in order to supply this need readily. Certain discrepancies appear between these calculations and the few data t h a t do exist, particularly in the critical region. The Fources of difficulty in the methods used become apparent on close examination. The purpose of this investigation was to obtain experimentally some of these data by a method and apparatus less subject to the difficulties inherent in the methods previously used.
Previous Methods CALCULATIONS FROM P-V-T DATA. These calculations involve the use of the thermodynamic equations of state. These equations of state furnish a method for calculating the change of enthalpy with pressure a t constant temperature using only P-V-T data. Lacey (6) and Lindsay and Brown (8) used the method for n-pentane. York ( l a ) calculated enthalpy values for a number of hydrocarbons using P-V-T data of Beattie and co-workers. Watson and Nelson (11) calculated such values b y the general reduced equation of state, PV = pRT and expressed p as an empirical function of PR. Lewis and Luke (7) used the reduced isometrics to calculate the change in the internal energy with pressure for methane, ethylene, isopentane, and n-pentane. The enthalpy changes computed from P-V-T d a t a must be derived from the data by differentiation. This inherently leads to a tremendous reduction in accuracy because of the uncertainties encountered in the differentiation. 1
Present address, Corning Glass Works, Corning, N. Y.
A n apparatus for measuring the isothermal enthalpy change accompanying the expansion of a vapor from an elevated pressure to atmospheric pressure is described. The results of such measurements are reported for benzene at pressures up to 190 atmospheres.
JOULE-THOXSOX EXPERIMENTS. The Joule-Thomson expansion involves the measurement of the change in temperature with pressure a t constant enthalpy, (dT/dP)x. This quantity can be related to the change in enthalpy with pressure a t constant temperature by the thermodynamic relation = -C , ($)H. Such experiments and calculations have been made‘ for various hydrocarbons b y Pattee and Brown (Q), Lindsay and Brown (8),and Lacey and eo-workers. This method suffers ( a ) from the fact that it is difficult to isolate thermally the expansion unit from the surroundings, due to the different temperatures involved within the unit and (6) from the inaccuracies in the values of Cp. The first difficulty can be overcome to a large extent b y increasing the rate of flow through the expansion unit, b u t this usually means that a larger quantity of the material being investigated must be on hand which is not practical for most hydrocarbons because of the difficulty of preparing them in pure form. The second source of error is more serious because of the relatively larger probable error in the experimental determination of the heat capacities, particularly at the higher pressures and temperatures. PARTIAL ISOTHERMAL EXPANSIONS. The quantity ( a H / bP) can be determined experimentally by measuring t h e change in enthalpy produced b y a small change in pressure a t constant temperature. This can be easily done by allowing the pressure of the material under investigation to decrease with the production of no useful work and by introducing sufficient heat into the system to keep it a t constant tem-
(%)*
INDUSTRIAL AND ENGINEERING CHEMISTRY
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n+------
4?
stant T R , the reduced absolute temperature, were considered most convenient.
INLET THERMOCOUPLE WELL
PR = P/P,; T R = T I T , where P , T = absolute pressure and temperature, respectively Po,T, = critical constants
k "
CHES
'
'/
