By using an isothermal throttling calorimeter, built for recirculating vapor operation, the effects of pressure on the enthalpy of methane and two methane-propane (93.9 and 86.2% methane) mixtures were measured. Pure methane was run a t 150°F. and the two mixtures were studied a t 90, 150, and 200°F. The pressures for these experiments were 500, 1,OOO, 1,500, and 2, OOO Ib./sq.in.abs.The resulting data were compared with similar data from other sources for these same syr- I n a previous experimental investigation of the isothermal effect of pressure on the enthalpy of hydrocarbon mixtures, an isothermal throttling calorimeter was built and operated on the propane-benzene system (16, 17). For the investigations described in this paper ( 4 ) , the previously built apparatus was modified and used to study the methane-propane system.(1). EXPERIMENTAL APPARATUS A N D PROCEDUREThe e uipment we used is a modification of that constructed %y Yarborough (16). The details of its design and operation are described elsewhere ( 16, 17). The calorimeter portion of the equipment used in this study is identical to Yarborough's; only the peripheral apparatus has been changed. Thus, only the modifications will be described.The equipment was originally designed for liquid-phase feed to the equipment, once-through operation, and effluent collection by condensation. For the present study, each of these features proved impractical, and modifications were made to yield the apparatus shown schematically in Figure 1. The first modification was to arrange for continuous recycle operation by installation of a closed loop employing two stages of com ression, 1 and 2. Long line-out times re uired consumption) Ld to abandonment of once-through operations and the adoption of the recycle operation. The second modification of the apparatus was a change in method of flow rate measurement. Existing facilities for condensing the calorimeter efBuent proved inadequate for the to reach steaf state operation (with subsequently hig R gas methane-propane system. Thus, the condensation method of flow rate determination was replaced by the equipment to the right of the solenoid valve, 14. After steady state conditions were reached in rec cle operation, flow was diverted to high pressure aluminum Eombs immersed in liquid nitrogen, 16, and makeup gas was supplied above the surge tank, 15. By using the valve directly above the high pressure bombs, the calorimeter effluent pressure was adjusted to approximately 1 atm. Valve 14 was then actuated to divert flow into originally evacuated glass collection bombs, 19. Flow into this collection system was continued for a measured time, until the calorimeter effluent pressure returned to the initial pressure as indicated by manometer, 12. Flow was then returned to the aluminum bombs. From the known pressure, 21, and temperature, 18, of the gas in the calibrated volume of the sample system, the mass of gas was calculated by using the known low pressure volumetric behavior of the gas. The mass and flow time served to e...