Measurements of excess enthalpies at high temperature and pressure using a new type of mixing unit

A flow mixing unit (calorimetric cell and auxiliary devices) has been designed for measuring the enthalpy of mixing or reaction of two fluids (gas+liquid or liquid+liquid). The indicator of the heat effect is a differential heat flux calorimeter, SETARAM C-80, allowing measurements at temperatures up to 300°C. The mixing cell is made of a stainless-steel capillary (o.d 1.6 mm, length 2.4m) which is coiled in a cylindrical form and tightly fitted in the thermopile well of the calorimeter. The fluids are delivered from the high pressure piston pumps and circulated through the system at controlled flow rates ranging from 100 to 1500 L-min^–1. The tests were carried out at pressures up to 20 MPa. Special care was taken to allow good thermostatting of fluids entering the mixing cell. Check measurements were made with one liquid-liquid system (C₂H₅OH+H₂O) and one gas-liquid system (CO₂+C₆H₅CH₃); our enthalpies of mixing agreed with the literature values in most cases to 2%. For the system ethanol+water the experiments have been also performed at temperature of 250°C and pressures of 15 and 20 MPa. The endothermal mixing effect was higher than expected indicating an increase in the excess heat capacity.