Static mixer assembly for ph-modification in geothermal power plants

Abstract

A static mixer assembly for pH-modification in geothermal power plants. The static mixer assembly includes a first tube extending along an axis, and a plurality of primary baffles extending from the first tube. The static mixer assembly further includes a second tube positioned within the first tube, and a plurality of secondary baffles extending from the second tube. An inlet tube includes a first end positioned outside the first tube and a second end positioned within the second tube.

Description

STATIC MIXER ASSEMBLY FOR pH-MODIFICATION IN GEOTHERMALPOWER PLANTSTECHNICAL FIELDThe present disclosure relates to a static mixer for the dilution and mixing ofconcentrated acid in water for use in, for example, a pH-modification system in geothermalpower plants.BACKGROUNDConventional geothermal power plants include multiple stages of flash orsubcooling through Organic Rankine Cycle (ORC) heat exchangers. Heat exchangers extractheat from geothermal brine to a point where dissolved silica becomes supersaturated and willprecipitate solid scale deposits in the power plant equipment, pipelines and reinjection wells.A pH-modification (pH-mod) process inhibits scale deposition. The pH-modification process includes the injection of a concentrated acid (typically sulfuric acid orhydrochloric acid) into the geothermal brine to reduce the pH to within a range ofapproximately 4.5 to 5.0 pH units. See, for example, U.S. Patent No. 4,500,434 entitled“Inhibiting scale precipitation from high temperature brine,” and U.S. Patent No. 5,190,664entitled “Brine heat exchanger treatment method.”SUMMARYThe disclosure provides, in one aspect, a static mixer assembly including a firsttube extending along an axis; a plurality of primary baffles extending from the first tube; asecond tube positioned within the first tube; and a plurality of secondary baffles extendingfrom the second tube. The static mixer assembly further includes an inlet tube with a firstend positioned outside the first tube and a second end positioned within the second tube.In some embodiments, the first tube 1s configured to convey a brine; the inlet tubeis configured to convey an acid to the second tube; and the second tube is configured to mixthe brine and the acid.In some embodiments, the second tube includes an inlet portion that is funnel-shaped and positioned within the first tube.In some embodiments, the second end of the inlet tube is positioned in the inletportion of the second tube.In some embodiments, the static mixer assembly further includes a supportassembly connected to the first tube and configured to support the second tube within the firsttube.In some embodiments, the inlet tube is coupled to the support.In some embodiments, the second tube is aligned with the axis.In some embodiments, the first tube has a first length and the second tube has asecond length, and wherein a ratio of the first length to the second length is within a range of5:1 to 10:1.In some embodiments, the inlet tube extends along an inlet axis that intersects theaxis at an inlet angle.In some embodiments, the inlet angle is within a range of 35 degrees to 55 degrees.In some embodiments, the static mixer assembly further includes a tab positionedwithin the second tube.In some embodiments, the tab includes a planar surface that extends along a planethat intersects the axis at a tab angle; wherein the tab angle is within a range of 20 degrees toAO degrees.In some embodiments, the inlet axis intersects the planar surface.In some embodiments, the tab is positioned between the inlet tube and the pluralityof secondary baffles.In some embodiments, the first tube includes a high-nickel alloy, the second tubeincludes tantalum, and the inlet tube includes tantalum.In some embodiments, the first tube includes a first circular cross-section and thesecond tube includes a second circular cross-section.In some embodiments, the static mixer assembly further includes a brine flowingthrough the first tube and an acid flowing through the inlet tube.The disclosure provides, in one aspect, a method of reducing scale build up inapower plant. The method comprising: moving a brine through a first tube; moving a firstportion of the brine through a second tube positioned within the first tube, and moving asecond portion of the brine around the second tube. The method further includes injecting anacid into the second tube through an inlet tube; mixing the acid and the first portion of the2brine in the second tube to create a first mixture; and mixing the first mixture exiting thesecond tube with the second portion of the brine in the first tube to create a second mixture.In some embodiments, the method further includes moving the second mixture to apower plant.In some embodiments, the acid has a density of at least 1.07 kg / L.In some embodiments, the acid is at least 10% by weight H2SO4 or HCl.In some embodiments, the method further includes moving the acid between anacid supply and the inlet tube with an acid supply tube, wherein the acid supply tube is ahigh-nickel alloy, stainless steel, or tantalum.Other aspects of the disclosure will become apparent by consideration of thedetailed description and accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGSThese and other features, aspects, and advantages of the present technology willbecome better understood with regards to the following drawings. The accompanying figuresand examples are provided by way of illustration and not by way of limitation.FIG. 1 is a schematic of a prior art system implementing a pH-modification processfor a binary bottoming cycle.FIG. 2 is a perspective view of a static mixer assembly with a pre-mixer, with anouter tube shown transparently.FIG. 3 is a partial perspective view of a cross-section of the static mixer assemblyof FIG. 2.FIG. 4 1s a partial cross-section view of the static mixer assembly of FIG. 2.FIG. 5 is a partial perspective view of a cross-section of the static mixer assemblyof FIG. 2.FIG. 6 is a partial cross-section view of the static mixer assembly of FIG. 2.FIG. 7 is a cross-section of the static mixer assembly of FIG. 2, illustrating acomputational fluid dynamics analysis of a mass fraction of acid.FIG. 8 is a flowchart of a method for reducing scale build up in a power plant.Before any embodiments are explained in detail, it is to be understood that theinvention is not limited in its application to the details of construction and the arrangement ofcomponents set forth in the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced or of being carried outin various ways.DETAILED DESCRIPTIONUnless otherwise defined, all technical and scientific terms used herein have thesame meaning as commonly understood by one of ordinary skill in the art. In case ofconflict, the present document, including definitions, will control. Preferred methods andmaterials are described below, although methods and materials similar or equivalent to thosedescribed herein can be used in practice or testing of the present disclosure. All publications,patent applications, patents and other references mentioned herein are incorporated byreference in their entirety. The materials, methods, and examples disclosed herein areillustrative only and not intended to be limiting.99 GCs 99 66 99 66The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” andvariants thereof, as used herein, are intended to be open-ended transitional phrases, terms, orwords that do not preclude the possibility of additional acts or structures. The singular forms4 99 66a,” “an” and “the” include plural references unless the context clearly dictates otherwise.99 66The present disclosure also contemplates other embodiments “comprising,” “consisting of”and “consisting essentially of,” the embodiments or elements presented herein, whetherexplicitly set forth or not.For the recitation of numeric ranges herein, each intervening number there betweenwith the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, thenumber 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.The term “coupled,” as used herein, is defined as “connected,” although notnecessarily directly, and not necessarily mechanically. The term coupled is to be understoodto mean physically, magnetically, chemically, fluidly, electrically, or otherwise coupled,connected or linked and does not exclude the presence of intermediate elements between thecoupled elements absent specific contrary language.The term “brine,” as used herein, refers to geothermal brine (e.g., hot salty wateroriginating underground).With reference to FIG. 1, a geothermal system 10 implementing a pH-modificationprocess for geothermal brine in combination with a power plant 14 (e.g., a binary bottomingplant, an Organic Rankine Cycle (ORC), a steam flash plant, etc.). To implement the pH-modification process, the geothermal system 10 includes an acid tank 18, an acid pump 22, anacid valve 26, and a pH control system 30. The geothermal system 10 further includes a pre-mixer 34 that is separate and spaced from a main mixer 38. Conventional systems requirethat the acid is pre-diluted in a continuous side-stream piping system 42. The fluid used todilute the acid is typically a side-stream of untreated geothermal brine. The acid is pre-dilutedfrom the concentrated form (e.g., 98% solution) stored in the acid tank 18 to a dilute solutionof approximately 1% at an outlet 46 of the pre-mixer 38.The dilute acid from the pre-mixer 34 then travels through an intermediate pipingsystem 50 before being injected where scale control is needed (e.g., upstream of ORC heatexchangers, downstream of 2"4-flash vessels in a multi-stage flash plant, etc.). In FIG. 1, thedilute acid from the pre-mixer 34 travels through the intermediate piping system 50 to themain mixer 38. See “Silica Scale Control in Geothermal Bottoming Cycle Plants by pH-Modification and Thermal Quenching,” Hirtz, GCE, 2018, which is incorporated herein byreference in its entirety.However, pre-dilution of concentrated acid is limited by available materials due tothe corrosive nature of the hot, dilute acid. As such, the dilute acid needs to be handled inTeflon-lined or Tantalum-lined pipes, fittings and valves. For example, the intermediatepiping system 50 in conventional systems is made of Teflon-lined or Tantalum-linedcomponents. Teflon-lined and Tantalum-lined components are expensive, unreliable, have alimited life, long lead times to source, and cannot always meet the geothermal processpressure or temperature requirements. After the pre-diluted acid is injected into the mainmixer 38, the diluted acid becomes mixed in the main brine stream and is no longer corrosive(pH 4.5 — 5.0). The main mixer 38 is typically fabricated from a high-nickel alloy (e.g.,Hastelloy C-276) and cannot handle concentrated acid injection directly. However, it is notpractical to line the main mixer 38 with Tantalum or Teflon due to the cost, pressure, andtemperature limitations of Tantalum and Teflon.With reference to FIG. 2, a static mixer assembly 54 includes a main mixer 58and a pre-mixer 62 positioned within the main mixer 58 (e.g., the pre-mixer 62 is “in-situ”’).The static mixer assembly 54 includes the pre-mixer 62 for the pre-mixing concentrated acidwith a portion of the brine and is integrated inside of the main mixer 58. The pre-mixer 62 is5configured for pre-dilution of concentrated acid within the main mixer 58. As detailedfurther herein, concentrated acid is injected to the pre-mixer 62 through an inlet tube 66. Incontrast to dilute acid, concentrated acid is not corrosive. In some embodiments, the staticmixer assembly 54 is implemented in the geothermal system 10 and replaces the pre-mixer34, the main mixer 38, the side-stream piping system 42. In the illustrated embodiment, thepre-mixer 62 is fabricated from Tantalum and is centered within the main mixer 58. In someembodiments, the inlet tube 66 is formed of Tantalum and connects externally to stainless-steel tubing that supplies the concentrated acid from the acid supply (e.g., the acid tank 18).Advantageously, the static mixer assembly 54 eliminates the need for Tantalum-lined (orTeflon-lmed) piping and components for handling dilute acid (e.g., the intermediate pipingsystem 50).With continued reference to FIG. 2, the static mixer assembly 54 includes a firsttube 70 extending along an axis 74, and a second tube 78 positioned within the first tube 70.In the illustrated embodiment, the second tube 78 is aligned with the axis 74. In other words,the second tube 78 is positioned co-axially with the first tube 70. In some embodiments, thesecond tube 78 is positioned offset from the axis 74. In the illustrated embodiment, ageothermal brine flows through the first tube 70. As detailed further herein, the first tube 70is configured to convey a brine, and the second tube 78 is configured to mix a brine and anacid. In the illustrated embodiment, brine and concentrated acid are pre-mixed in the secondtube 78 before being further mixed with additional brine downstream in the first tube 70. Insome embodiments, the second tube 78 is configured to pre-mix the brine and the acid to anacid concentration less than approximately 1% by weight. In some embodiments, the firsttube 70 is configured to complete the mixing process, to result in a final acid concentration atthe outlet of the first tube 70 less than approximately 0.01% by weight. The overallperformance of the static mixer assembly 54 is expressed as a Coefficient of Variance (CoV).In some embodiments, the static mixer assembly 54 is configured such that the final acidconcentration at the outlet of the first tube 70 has a CoV of less than approximately 5%. Insome embodiments, the static mixer assembly 54 has a final acid concentration with a CoV ofless than approximately 2%.In the illustrated embodiment, the first tube 70 and the second tube 78 are circulartubes. The first tube 70 includes a first circular cross section and the second tube 78 includesa second circular cross-section. In other embodiments, the first tube or the second tube 70,78 have non-circular cross-sections. The first tube 70 has a first length 82 (FIG. 2) and the6second tube 78 has a second length 86 (FIG. 4) that is shorter than the first length 82. Insome embodiments, a ratio of the first length 82 to the second length 86 is within a range ofapproximately 5:1 to approximately 10:1. In some embodiments, the ratio of the first length82 to the second length 86 is within a range of approximately 5:1 to approximately 9:1.With reference to FIGS. 3 and 5, the static mixer assembly 54 further includes asupport assembly 90 to position the second tube 78 within the first tube 70. In the illustratedembodiment, the support assembly 90 includes radial supports 94 and an axial collar 98. Theradial supports 94 are connected to the first tube 70 at a first end 102 and connected to theaxial collar 98 at a second end 106, opposite the first end 102. In the illustrated embodiment,the axial collar 98 is aligned with the axis 74 and is configured to receive the second tube 78.As such, the support assembly 90 is connected to the first tube 70 and configured to supportthe second tube 78 within the first tube 70. In some embodiments, radial supports 94 arepositioned circumferentially around the axial collar 98 at more than one axial location alongthe axis 74.With reference to FIG. 2, the static mixer assembly 54 includes a plurality ofprimary baffles 110 (e.g., vanes, tabs, protrusions, fins, etc.) extending from the first tube 70.In the illustrated embodiment, the primary baffles 110 extend radially inward from the firsttube 70. With reference to FIG. 4, the primary baffles 110 are spaced a distance 114 from aninlet 118 of the first tube 70. In some embodiments, the main mixer 54 includes the first tube70 and the primary baffles 110.With reference to FIGS. 3 and 4, the static mixer assembly 54 further includes aplurality of secondary baffles 122 extending from the second tube 78. In the illustratedembodiment, the secondary baffles 122 extend radially inward from the second tube 78. Thesecond tube 78 includes an inlet portion 126 positioned within the first tube 70. In theillustrated embodiment, the inlet portion 126 is funnel-shaped with a larger cross-sectionpositioned upstream (e.g., toward the inlet 118 of the first tube 70). With reference to FIG. 4,the inlet portion 126 is spaced a distance 130 from the inlet 118 of the first tube 70. In theillustrated embodiment, the distance 130 is smaller than the distance 114. In other words, theprimary baffles 110 are positioned downstream of the inlet 126 of the second tube 78.With reference to FIGS. 5 and 6, the static mixer assembly 54 further includes a tab134 positioned within the second tube 78. The tab 134 is positioned between the inlet portion126 and the plurality of secondary baffles 122. In the illustrated embodiment, the tab 134includes a planar surface 138 that extends along a plane 142 that intersects the axis 74 at a tabangle 146 (FIG. 6). In some embodiments, the tab angle 146 is within a range ofapproximately 20 degrees to approximately 40 degrees. In the illustrated embodiment, thetab angle 146 is approximately 30 degrees. In some embodiments, the pre-mixer 62 includesthe second tube 78, the secondary baffles 122, and the tab 134.With reference to FIGS. 3 and 4, the inlet tube 66 includes a first end 150positioned outside the first tube 70. The inlet tube 66 further includes a second end 154,opposite the first end 150, positioned within the second tube 78. In the illustratedembodiment, the inlet tube 66 is at least partially positioned within a nozzle body 158coupled to one of the radial supports 94. As detailed further herein, the inlet tube 66 isconfigured to convey an acid (e.g., a concentrated acid) to the second tube 78. In otherwords, an acid (e.g., a concentrated acid) flows through the inlet tube 66. In the illustratedembodiment, the second end 154 of the inlet tube 66 is positioned in the inlet portion 126 ofthe second tube 78. In other words, the inlet tube 66 discharges concentrated acid directlyupstream of the pre-mixer 62.With continued reference to FIG. 4, the inlet tube 66 extends along an inlet axis162. In the illustrated embodiment, the inlet axis 162 intersects the axis 74 at an inlet angle166. In some embodiments, the inlet angle 166 is within a range of approximately 35 degreesto approximately 55 degrees. In the illustrated embodiment, the inlet angle 166 isapproximately 45 degrees. In some embodiments, the inlet axis 162 intersects the planarsurface 138 of the tab 134 positioned in the second tube 78. As such, the tab 134 deflectsconcentrated acid exiting the second end 154 of the inlet tube 66.With reference to FIG. 2, an acid supply tube 170 is fluidly coupled to the inlettube 66 and the acid supply tube 170 conveys concentrated acid from an acid supply (e.g., theacid tank 18). The acid supply tube 170 is a high-nickel alloy, stainless steel, or tantalum.Advantageously, the concentrated acid moving through the acid supply tube 170 is notcorrosive.In some embodiments, the first tube 70, the primary baffles 110, the supportassembly 90, and the nozzle body 158 are made of a high-nickel alloy (e.g., Hastelloy C-276). In some embodiments, the second tube 78 and the inlet tube 66 are made of Tantalum.In some embodiments, pipe upstream and downstream from the static mixer assembly 54 ismade of a carbon steel (e.g., ASTM A106, ASME SA106 Grade B). Advantageously, theamount of tantalum-lined components is minimized when utilizing the static mixer assembly54, while still achieving pH-modification of the brine for a geothermal power plant.With reference to FIG. 7, a computational fluid dynamic analysis of the staticmixer assembly 54 illustrates how concentrated acid injected by the inlet tube 66 into the pre-mixer 62 is pre-mixed with a portion of the brine. The pre-mixture of acid and brine exitingthe pre-mixer 62 is then mixed with the remaining brine in the main mixer 58.With reference to FIG. 8, a method 174 of reducing scale build up in a power plantis illustrated. The method 174 includes (STEP 178) moving a bring through a first tube. Themethod 174 further includes (STEP 182) moving a first portion of the brine through a secondtube positioned within the first tube, and moving a second portion of the brine around thesecond tube. In other words, some of the brine entering the first tube flows through thesecond tube and the rest of the brine flows around the second tube.The method 174 further includes (STEP 186) injecting an acid into the second tubethrough an inlet tube (e.g., the let tube 66). In some embodiments, the acid is aconcentrated acid with a density of at least approximately 1.07 kg / L. In some embodiments,the acid is at least 10% by weight sulfuric acid (H2SO4) or hydrochloric acid (HCI). In someembodiments, the method 174 further includes moving the acid between an acid supply (e.g.,an acid tank) and the inlet tube with an acid supply tube (e.g., acid supply tube 170). Theacid supply tube is a high-nickel alloy, stainless steel, or tantalum.The method 174 further includes (STEP 190) mixing the acid and the first portionof the brine in the second tube to create a first mixture. In other words, the acid is pre-mixedwith a portion of the brine in the second tube. In some embodiments, the acid concentrationleaving the second tube within a range of approximately 0.1% to approximately 1% byweight.The method 174 further includes (STEP 194) mixing the first mixture exiting thesecond tube with the second portion of the brine in the first tube to create a second mixture.As such, the method 174 mixes acid in two stages. In other words, the pre-mixture leavingthe second tube is mixed with the remaining portion of the brine to modify the pH of thebrine exiting the first tube. In some embodiments, the method further includes moving thesecond mixture (pH-modified brine) to a power plant (e.g., a binary bottoming plant, anOrganic Rankine Cycle (ORC), a steam flash plant, etc.).Various features and advantages are set forth in the following claims.9

Claims

CLAIMSWhat is claimed is:

1. A static mixer assembly comprising:a first tube extending along an axis;a plurality of primary baffles extending from the first tube;a second tube positioned within the first tube;a plurality of secondary baffles extending from the second tube; andan inlet tube with a first end positioned outside the first tube and a second endpositioned within the second tube.

2. The static mixer assembly of claim 1, wherein the first tube is configured to convey abrine; wherein the inlet tube is configured to convey an acid to the second tube; and whereinthe second tube is configured to mix the brine and the acid.

3. The static mixer assembly of claim 1, wherein the second tube includes an inletportion that is funnel-shaped and positioned within the first tube.

4. The static mixer assembly of claim 3, wherein the second end of the inlet tube ispositioned in the inlet portion of the second tube.

5. The static mixer assembly of claim 1, further comprising a support assemblyconnected to the first tube and configured to support the second tube within the first tube.

6. The static mixer assembly of claim 5, wherein the inlet tube is coupled to the support.

7. The static mixer assembly of claim 1, wherein the second tube is aligned with theaxis.

8. The static mixer assembly of claim 1, wherein the first tube has a first length and thesecond tube has a second length, and wherein a ratio of the first length to the second length iswithin a range of 5:1 to 10:1.

9. The static mixer assembly of claim 1, wherein the inlet tube extends along an inletaxis that intersects the axis at an inlet angle.

10. The static mixer assembly of claim 9, wherein the inlet angle is within a range of 35degrees to 55 degrees.

11. The static mixer assembly of claim 1, further comprising a tab positioned within thesecond tube.

12. The static mixer assembly of claim 11, wherein the tab includes a planar surface thatextends along a plane that intersects the axis at a tab angle; wherein the tab angle is within arange of 20 degrees to 40 degrees.

13. The static mixer assembly of claim 12, wherein inlet axis intersects the planar surface.

14. The static mixer assembly of claim 11, wherein the tab is positioned between the inlettube and the plurality of secondary baffles.

15. The static mixer assembly of claim 1, wherein the first tube includes a high-nickelalloy, the second tube includes tantalum, and the inlet tube includes tantalum.

16. The static mixer assembly of claim 1, wherein the first tube includes a first circularcross-section and the second tube includes a second circular cross-section.

17. The static mixer assembly of claim 1, further including a brine flowing through thefirst tube and an acid flowing through the inlet tube.

18. A method of reducing scale build up in a power plant, the method comprising:moving a brine through a first tube;moving a first portion of the brine through a second tube positioned within the firsttube, and moving a second portion of the brine around the second tube;injecting an acid into the second tube through an inlet tube;mixing the acid and the first portion of the brine in the second tube to create a firstmixture; andmixing the first mixture exiting the second tube with the second portion of the brine inthe first tube to create a second mixture.

19. The method of claim 18, further comprising moving the second mixture to a powerplant.

20. The method of claim 18, wherein the acid has a density of at least 1.07 kg / L.

21. The method of claim 18, wherein the acid is at least 10% by weight H2SO4 or HCl.

22. The method of claim 18, further comprising moving the acid between an acid supplyand the inlet tube with an acid supply tube, wherein the acid supply tube is a high-nickelalloy, stainless steel, or tantalum.