High recovery reverse osmosis process and apparatus

Abstract

Processes and apparatus are disclosed for producing clear reverse osmosis retentates having silica concentrations which are substantially supersaturated in silica from feedwaters having silica concentrations, without substantial formation of alkali-soluble scale having substantial silica content in the associated reverse osmosis apparatus, by adjusting the pH of such feedwaters to an acidic pH range prior to reverse osmosis in accordance with this invention. Also disclosed are related processes and apparatus for periodically removing minor amounts of alkali-soluble scale having substantial silica content from the apparatus.

1 . A process for producing a clear reverse osmosis retentate having a supersaturated silica concentration substantially greater than about 125 milligrams per liter in a reverse osmosis apparatus from a feedwater having a silica concentration, without substantial formation of alkali-soluble silica-containing scale in said apparatus, said process comprising at least the following steps: 
 (i) adjusting the pH of said feedwater to an acidic pH range to form an acidic pH-adjusted feedwater;    (ii) providing a reverse osmosis apparatus having a reverse osmosis feed entrance conduit, a reverse osmosis retentate exit conduit, and a reverse osmosis permeate exit conduit;    (iii) passing at least a portion of said acidic pH-adjusted feedwater into said feed entrance conduit to produce: (a) a reverse osmosis permeate from said permeate exit conduit, said reverse osmosis permeate having a silica concentration substantially less than the silica concentration of said feedwater; and (b) a clear reverse osmosis retentate from said retentate exit conduit, said retentate having a silica concentration substantially greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater; and,    (iv) recovering permeate from said reverse osmosis permeate exit conduit and retentate from said reverse osmosis retentate exit conduit.    
     
     
         2 . In a process for producing a reverse osmosis retentate having a supersaturated silica concentration greater than about 125 milligrams per liter in a reverse osmosis apparatus from a feedwater having a silica concentration and at a pH which is not in an acidic pH range, said process tending to form alkali-soluble scale having a substantial silica content in said apparatus, an improvement to substantially reduce formation of alkali-soluble scale having a substantial silica content in said apparatus, the improvement comprising: adjusting the pH of said feedwater to an acidic pH range to form an acidic pH-adjusted feedwater; passing at least a portion of said acidic pH-adjusted feedwater into a feed entrance conduit of said reverse osmosis apparatus to produce: (a) a reverse osmosis permeate from a permeate exit conduit of said apparatus, said permeate having a silica concentration substantially less than said silica concentration of said feedwater; and (b) a clear reverse osmosis retentate from a retentate exit conduit of said apparatus, said retentate having a silica concentration substantially greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater; and, recovering permeate from said permeate exit conduit and retentate from said retentate exit conduit.  
     
     
         3 . The process of  claim 2  whereby the pH of said feedwater is adjusted such that there is substantially no formation of alkali-soluble scale having a substantial silica content in said apparatus.  
     
     
         4 . A process according to any of claims  1 ,  2  or  3  in which the reverse osmosis step is operated such that said clear reverse osmosis retentate has a silica concentration greater than about 278 milligrams per liter.  
     
     
         5 . A process according to any of claims  1 ,  2  or  3  in which the reverse osmosis step is operated such that said permeate has a silica concentration not greater than about 2.4 milligrams per liter.  
     
     
         6 . A process according to either  claim 2  or  claim 3  in which the reverse osmosis step is operated such that said permeate recovered from such permeate exit conduit is not less than about 85 percent by volume of said portion of said acidic pH-adjusted feedwater passed into said feed entrance conduit.  
     
     
         7 . A process according to any of claims  1 ,  2  or  3  wherein the pH of said feedwater is adjusted to an acidic pH range between about 1 and about 6.  
     
     
         8 . A process according to  claim 7  wherein the pH of said feedwater is adjusted to an acidic pH range between about 2 and about 5.  
     
     
         9 . A process according to any of claims  1 ,  2  or  3  further comprising a periodic reverse osmosis apparatus cleansing treatment comprising seriatim the steps of: 
 (a) stopping said passing of at least a portion of said acidic pH-adjusted feedwater into said entrance conduit;  
 (b) passing cleansing water having a substantially depleted silica concentration into said entrance conduit thereby producing retentate from said retentate exit conduit for a period until silica in said retentate has a concentration substantially equal to said silica concentration in said cleansing water;  
 (c) subsequently passing basic-adjusted cleansing water having a pH in the range of from about 9 to about 11 into said entrance conduit at a temperature and for a time sufficient to effect substantial removal of alkali-soluble scale having a substantial silica content from said apparatus;  
 (d) thereafter passing cleansing water having a substantially depleted silica concentration into said entrance conduit thereby producing retentate from said retentate exit conduit for a period until silica in said retentate has a concentration substantially equal to said silica concentration in said cleansing water;  
 (e) passing at least a portion of said acidic pH-adjusted feedwater into said entrance conduit to produce: (i) a reverse osmosis permeate from a permeate exit conduit of said apparatus, said permeate having a silica concentration substantially less than said silica concentration of said feedwater; and (ii) a clear reverse osmosis retentate from a retentate exit conduit, said retentate having a silica concentration substantially greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feed water; and,  
 (f) recovering permeate from said permeate exit conduit and retentate from said retentate exit conduit.  
 
     
     
         10 . A process according to any of claims  1 ,  2 , or  3  further comprising the step of recycling at least a portion of said retentate recovered from said retentate exit conduit to said feedwater or to said acidic pH-adjusted feedwater.  
     
     
         11 . Reverse osmosis apparatus for producing a clear reverse osmosis retentate having a supersaturated silica concentration substantially greater than about 125 milligrams per liter from a feedwater having a silica concentration, without substantial formation of alkali-soluble scale comprising substantially silica in said apparatus, said apparatus comprising at least: 
 a reverse osmosis portion comprising a reverse osmosis entrance conduit, a reverse osmosis system, a reverse osmosis retentate exit conduit and a reverse osmosis permeate exit conduit;    a feedwater pH control system for adjusting the pH of at least a portion of said feedwater to an acidic pH range;    a conduit for passing at least a portion of the acidic pH-adjusted feedwater into said reverse osmosis entrance conduit and through said reverse osmosis system to produce: (a) reverse osmosis permeate from said permeate exit conduit having a silica concentration substantially less than said silica concentration of said feedwater, and (b) a clear reverse osmosis retentate from said retentate exit conduit having a supersaturated silica concentration greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater; and,    a recovery system to recover permeate from said permeate exit conduit and retentate from said retentate exit conduit.    
     
     
         12 . Apparatus according to  claim 11  wherein said recovery system includes a recycle conduit for recycling at least a portion of said retentate recovered from said retentate exit conduit to said acidic pH-adjusted feedwater.  
     
     
         13 . Apparatus according to  claim 11  wherein said reverse osmosis system is of appropriate size and numbers of stages relative to the volume of said portion of acidic pH-adjusted feedwater to enable permeate recovery from said permeate exit conduit of not less than about 85 percent by volume of said portion of said acidic pH-adjusted feedwater.  
     
     
         14 . Apparatus according to  claim 11  wherein said reverse osmosis system is of appropriate size and numbers of stages relative to the volume of said portion of acidic pH-adjusted feedwater to enable retentate recovery from said retentate exit conduit having a silica concentration greater than about 278 milligrams per liter.  
     
     
         15 . Apparatus according to  claim 11  further comprising: 
 a feedwater flow control system for stopping and subsequently restarting the passing of at least a portion of said acidic pH-adjusted feedwater into said entrance conduit,  
 a water rinse system for passing cleansing water having a substantially depleted silica concentration into said entrance conduit instead of said acidic pH-adjusted feedwater to produce retentate from said retentate exit conduit, and,  
 a scale-removal system for passing cleansing water having a pH in the range of from about 9 to about 11 into said entrance conduit at a temperature and for a time sufficient to effect substantial removal of alkali-soluble scale having a substantial silica content from said apparatus.  
 
     
     
         16 . Apparatus according to  claim 11  wherein said reverse osmosis system is of appropriate size and numbers of stages relative to the volume of said portion of acidic pH-adjusted feedwater to enable recovery from said permeate exit conduit of permeate having a silica concentration not greater than about 2.4 milligrams per liter.  
     
     
         17 . Apparatus according to  claim 11  wherein said feedwater pH control system operates to adjust the pH of said portion of said feedwater to a pH range between about 1 and about 6.  
     
     
         18 . Apparatus according to  claim 11  wherein said feedwater pH control system operates to adjust the pH of said portion of said feedwater to a pH range between about 2 and about 5.  
     
     
         19 . A process according to  claim 10  comprising seriatim the steps of: 
 (a) recycling substantially all of said retentate recovered from said retentate exit conduit to said feedwater or to said acidic pH-adjusted feedwater until said retentate has a predetermined supersaturated silica concentration greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater;  
 (b) thereafter recycling at most a minor portion of said retentate recovered from said retentate exit conduit until said retentate has a predetermined silica concentration not substantially greater than said silica concentration of said feedwater; and,  
 (c) thereafter repeating steps (a) and (b) seriatim.  
 
     
     
         20 . A process according to  claim 19  in which said retentate is operated on a batch basis, each batch terminated when said retentate has a predetermined silica concentration greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater.  
     
     
         21 . Apparatus according to  claim 12  wherein said recovery system comprises a flow control and monitoring system: (a) to recycle substantially all of said retentate recovered from said retentate exit conduit to said feedwater or to said acidic pH-adjusted feedwater until said retentate has a predetermined silica concentration greater than about 125 milligrams per liter and substantially greater than said silica concentration of said feedwater; (b) after said retentate has said predetermined silica concentration, to recycle at most a minor portion of said retentate recovered from said retentate exit conduit until said retentate has a predetermined silica concentration not substantially greater than said silica concentration of said feedwater; and, (c) thereafter to alternate steps (a) and (b) seriatim.  
     
     
         22 . Apparatus according to  claim 21  in which said flow control and monitoring system regulates the recycle of said retentate on a batch basis.