Theory and practice of near critical pressure direct contact heat exchange. Final report

Abstract

The direct contact heat exchange (DCHE) system previously designed, developed, and tested, was operated successfully without scale deposition. Conceptual design, optimization, and cost analysis showed that an additional advantage is the reduction in cost due to the high capacity of the DCHE compared with conventional heat exchangers. The technical and economic feasibility of DCHE operated near or above the critical pressure of isobutane using isobutane in direct contact with geothermal brine was assessed. The program consisted of conceptual design with a preliminary process evaluation and feasibility analysis, design and construction of the experimental apparatus, experimental tests, a final revised process evaluation and feasibility analysis based on the experimental results, and preparation of a design procedure. No scaling problems were present in the direct contact heat exchanger or the test unit. Test runs show that no scale was deposited in either the packed column section or the sieve tray section for continuous runs of 76 and 160 hours using East Mesa well 6-2 brine. In fact, the sieve tray section after 160 hours operation was visibly cleaner after the run than before. Both the packed column section and the sieve tray section operated according to theoretical thermodynamic and hydraulic predictions. Operation of the supercritical isobutane direct contact heat exchange and associated test unit with geothermal brine was routine with automatic controls and without operator attention. A design procedure including the requisite equations for sieve tray direct contact heat exchange columns was developed. The procedure gives the optimum ratio of isobutane to brine, optimum number of trays and tray spacing, and the sieve tray layout variables, namely downcomer area, active hole area, bubbling area, and column cross sectional area.