CONCENTRATION OF SALINE PRODUCED WATER FROM COALBED METHANE GAS-WELLSIN A MULTIPLE-EFFECT EVAPORATOR USING WASTE HEAT FROM THE GAS-COMPRESSOR AND COMPRESSOR DRIVE ENGINE

The use of heat of compression from the gas compressor and waste heat from the diesel compressor drive engine in a triple-effect feed forwar d evaporator was studied as a means of concentrating saline produced w ater to facilitate its disposal. The saline water, trapped in deeply b uried coal seams, must be continuously pumped from coalbed natural gas wells so that the gas can desorb from the coal and make its way to th e wellbore. Unlike conventional natural gas which is associated with p etroleum and usually reaches the wellhead at high pressure, coalbed na tural gas reaches the wellhead at low pressure, usually around 101 kPa (1 atm), and must be compressed near the well site for injection into gas transmission pipelines. The water concentration process was simul ated for a typical 3.93 m(3)/s (500 MCF/h*), at standard conditions ( 101 kPa, 289K), at the gas production field in the Warrior Coal Basin of Alabama, but has application to other coalbed gas fields being brou ght into production throughout the world. The baseline conditions of t he study were: feed water dissolved solids content: 0.25 wt% ppm (most ly NaCl); feed water flow rate: 0.84 kg of produced water per m(3) (50 lb/MCF) of produced gas; product water dissolved solids content: satu rated brine (approximately 26 wt%) at 293 K (68 F); gas pressure in/ou t of compressor: 101 kPa/3.35 MPa (1 atm/35 atm). The study results sh owed that a fuel heat input to the diesel compressor drive engine of 6 .13 MW (20.9 x 10(6)Btu/h) was required. Heat transfer surface areas o f 47.5, 69.1, and 71.0 m(2) (511, 744, and 764 ft(2)) were required in effects I, II, and III, respectively. Approximately 47% of the energy in the fuel supplied to the compressor drive engine was utilized by t he evaporator for the base case. When the approach temperatures in all evaporator heat exchangers was specified to be 5.55 K (10 F) (as a li miting case), the evaporator system could handle a produced water-to-p roduced gas ratio of 1.43 kg/m(3) (89 lb/MCF). The capital cost of the evaporator system for the base case was estimated to be US$6.8 x 10(6 ), and the total (capital and operating) unit cost was estimated to be $3.53 per 1000 m(3) gas ($0.10 per MCF) of coalbed natural gas produc ed, with 90% of this being the cost of capital. This compares with the current U.S. wellhead price of natural gas of about $70.6 per 10(3) m (3) ($2.00/MCF). It is thus demonstrated that this process can be cons idered for concentrating saline water produced with natural gas in cas es where the gas must be compressed near the wellhead for transportati on to market.