| Abstract |
The deposition of silica from geothermal fluid was investigated as a function of temperature and flow rate in 6" nominal diameter packed gravel beds and 1" pipes. Fluid, at temperatures between 120 and 180?C and silica at supersaturation ratios of 2.7 to 1.4, was passed through the pipes and gravel beds at flow rates of 3 and 30 1/min. At the higher temperatures of 160 and 180?C, before the onset of rapid nucleation, the deposition rate was uniform across the length of the pipes at about 60 mg cm"2 year"1. This corresponds to a linear growth rate of 0.3 mm year"1. At the lower temperatures of 140 and 120?C where the silica was rapidly polymerising the deposition rate was found to decrease from about 150 to 12 mg.cnf2 year"1 between the pipe inlet and outlet. There appeared to be no substantial enhancement of scaling rate due to the high polymer concentrations at these lower temperatures and little difference between flow rates of 3 and 30 1/min. In contrast to the behaviour in the pipes deposition in the gravel beds at 140 and 120?C was significantly different at the two flows. At 30 1/min the beds rapidly blocked with a soft porous deposit right at the fluid inlet, while there was very little deposition anywhere in the gravel bed at 3 1/min. At the higher temperatures of 160 and 180?C where the particle concentrations were assumed to be low, and molecular deposition was assumed to be the dominant process, deposition appeared to be uniform across the length of the beds. |