Initial Hardness Removal From High-TDS Waters With or Without Silica Removal

摘要

Recycle of high hardness, high TDS(total dissolved solids)rnwaters (hardness>1000 parts per million (ppm) as CaCO3 andrnTDS>10,000 ppm) water for steam generation or other reusernsuch as irrigation or drinking water is very expensive. Silicarncontent is usually above 250 ppm in such waters which can causernproblems in steam generation and with desalination. Many of thernthese high hardness waters are oil field produced waters but therernare other processes which generate waters which require additionalrntreatment. For those high hardness waters, hot lime or hotrncaustic, followed with strong acid/weak acid resin or just weakrnacid resin softeners are used in conjunction with oil field steamrngeneration. Treatment of these waters for irrigation or drinkingrnwaters involves thermal desalination or reverse osmotic(RO)rntreatment with biological control.rnSilica removal is not required for normal wet steam generarntion. However for desalination operations, whenever the feedrnwater concentrate is to be used for the steam generator feedrnand the fresh water sold, silica removal is required to aid inrnscale formation in desalination plus silica level in the concenrntrate water. The known limits on the concentrate feed water torna wet steam generator are 500 ppm silica and 25,000 -rn30,000 ppm TDS of total water ions, based on soluble saltsrnsolubility. Silica removal is also required in other waters suchrnas 210,000 ppm TDS water containing sodium carbonaternwhere silica removal is required prior to crystallization ofrnsodium carbonate crystals.rnIn the softening test work, the high solids addition and disposalrnassociated with a hot lime system was not desired sornalternatives were investigated. In addition,　better silica removalrnthan silica absorption on magnesium hydroxide orrnalumina or aluminum was required. Steam stripping of thernhigh pH water and removal of the precipitates using a ceramicrncrossflow filter for which a special crossflow filter back pulsernunit for cleaning was developed. Temperature and pH werernincreased prior to the steam stripping to decrease steam condensationrnand drive the reaction. When silica removal wasrnrequired, a bed of alumina or aluminum was used at the highrnpH and temperature to put aluminum into solution so aluminumrnsilicates were removed with the hardness precipitates.rnThe solids often contain some oil when using oil field watersrnso an odor chemical was also developed for the microbiologicalrnsoil remediation site.rnThe steam stripping was tested first in a countercurrent modernin a stainless steel column clad with a Hastelloy C 22, packedrnwith stainless steel packing. The second test was by injectingrnthe steam on the outside circumference of a crossflow ceramicrnmicrofilter in a cocurrent mode with flashing in a exit vessel.rnIn the countercurrent tower operation, the control of the equilirnbrium of the carbon dioxide, the carbonate and bicarbonate atrnthe top of the tower was more difficult than when contactingrnwith the microfilter. With the microfilter, the equilibriumrnapproach was not a large concern as fresh steam was contactingrnthe water and was then flashed. However the exact controlrnof the steam to water ratio was more difficult in the secondrncase.rnBoth thermal desalination and RO were pilot tested with watersrnfrom 10,000 to 36,000 ppm TDS to produce potable waterrnwith a quartz ultraviolet light for biologicalrn(disinfection) control. For wet steam generation, the fieldrnproduced waters(10,000-24,000 TDS) were tested using strongrnacid/weak acid resin softening with no silica removal in a 1rnMM BTU/Hr wet steam generator.1rnThe overall operational costs were less than normal sequencernof processes mentioned in the literature while the capital costsrnwere in the same range. Patents were obtained on the(1) steamrnstripping softening, (2)silica removal,(3) back pulse on thernmicro filter and(4) the odor chemical. A patent on the sour gasrntreatment is pending.