Treatment of OTSG blowdown

Abstract

A method and system generate steam for SAGD operation wherein steam generator blowdown water is acidified, cooled and clarified before reuse. Acid Clarification Treatment, or “ACT”, mitigates organic fouling in Once-Through Stream Generators. Lab tests quantitatively and qualitatively show that ACT reduces Total organic carbon (TOC) and ‘bad actors’ TOC, respectively, in blowdown streams.

Claims

1. A method of treating once through steam generator (OTSG) blowdown water for reuse, said method comprising: a) providing OTSG blowdown water having acid insoluble organics therein; b) acidifying said OTSG blowdown water to pH 8 or lower; c) cooling said OTSG blowdown water to 30-40° C.; d) settling precipitants out of said OTSG blowdown water for at least 12 hours to produce an acid clarified blowdown water having more than 50% of the acid insoluble organics removed; and e) reusing said acid clarified blowdown water.

2. The method of claim 1, wherein the cooling step c) occurs at least partially before the acidifying step b).

3. The method of claim 1, wherein the settling step d) occurs in a sludge pond.

4. The method of claim 1, wherein the settling step d) occurs in a clarifier tank.

5. The method of claim 1, wherein the reusing step e) comprises use as feedwater in a steam generator.

6. The method of claim 1, wherein the reusing step e) comprises blending with clean feedwater and use as feedwater in a steam generator.

7. The method of claim 1, wherein reusing step e) comprises blending with clean feedwater and use as feedwater in an OTSG.

8. The method of claim 1, further comprising a softening step f) to remove calcium, magnesium and silica.

9. The method of claim 1, further comprising a softening step f) in a warm lime softener to remove calcium, magnesium and silica.

10. The method of claim 5, further comprising a softening step f) in a warm lime softener to remove calcium, magnesium and silica.

11. The method of claim 7, further comprising a softening step f) in a warm lime softener to remove calcium, magnesium and silica.

12. The method of claim 1, further comprising a step d-1) adjusting pH of said acid clarified blowdown water to pH 8 or lower.

13. An improved method of producing steam for oil production, the method comprising heating feedwater in a steam generator to generate steam for downhole use and blowdown water to reuse in said steam generator, the improvement comprising acidifying, cooling to 25-45° C. and passively clarifying said blowdown water by settling to produce an acid clarified water having about 50% of total organic carbon removed, before reuse of said acid clarified water in said steam generator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the operating principle for an OTSG.

(2) FIG. 2 Effect of acid addition on Total Suspended Solids (TSS)—a graph of TSS versus pH. A steep increase in TSS generation was noted after the pH drop to 7, which provided an indirect measure of accelerated organics precipitation at pH 7 and provided the motivation to investigate acid clarification as one possible method of cleaning blowdown water prior to reuse.

(3) FIG. 3 Effect of acid addition on organics removal—a graph of total organic carbon (TOC) versus pH showing increasing precipitation of TOCs with increasing pH.

(4) FIG. 4A and FIG. 4B Effect of acid addition and temperature on TSS generation—a graph of total suspended solids (TSS) versus pH under warm (FIG. 4A: 60° C., 15″) versus cooled (FIG. 4B: 22° C., O/N) conditions. This graph confirms that TSS increases with acid treatment and cooling. These TSS can be removed by allowing the suspended solids sufficient time to settle for e.g., at least 12 hrs. The effect of changing pH on the precipitation kinetics is immediate, while mixing and separation may take some time. However, mixing and separation may be optimized by appropriate vessel design.

(5) FIG. 5 One proposed embodiment of an acid clarification treatment system before reuse in an OTSG.

(6) FIG. 6 One proposed embodiment of an acid clarification treatment system before reuse in an OTSG.

DETAILED DESCRIPTION

(7) As noted in the introduction, blowdown water must be recycled in order to meet strict water regulations, but it is typically heavily contaminated water and use without pretreatment has resulted in shutdown and pigging to clean the OTSG every six weeks. This could result in downtime, equipment maintenance, and revenue loss. Thus, there is a need to pretreat the blowdown water in a cost effective manner so as to reduce the TOC level and increase the time between pigging operations.

(8) We investigated several possible alternatives for cleaning this particular dirty water. Ion exchanges resins were investigated, but even using the best resin according to our bench testing, the method was impractical because more water was needed to regenerate the resins than could be cleaned using the resins. Thus, the method actually worsened water usage, rather than improving it. For example, the anion exchange resin TAN-1 (Dowex™) capacity was found to be limited, and 75 volumes of 10% NaCl and 4% NaOH for resin regeneration was needed for resin regeneration whereas resin capacity, whereas the resin was already exhausted only at 20 bed volumes of dirty blowdown water.

(9) Another method investigated was an advanced oxidation process (AOP) employing ozone/peroxide. The AOP process showed effectiveness for acid insoluble organics removal. At 1000 mg/L ozone dosage, about 50% of the acid insoluble organics were removed from the system. However, AOP was rejected as cost ineffective because it required high CAPEX and OPEX to implement on the large scales needed for steam injection as a method of enhanced oil recovery.

(10) We also investigated evaporator technology, but this method also required high CAPEX and OPEX, and the treatment of evaporator blowdown, which will be at least three times that of OSTG blowdown, adds some uncertainly to the method. It may be of limited value to generate even dirtier water while cleaning the OTSG blowdown water.

(11) After considerable testing of several alternatives, it was determined that an acid clarification treatment was the most cost effective method of cleaning OTSG blowdown water. Acid alone treatment was ranked high because i) it is targeted to remove most sparingly soluble organics from the OTSG blowdown stream, ii) acid clarification removes more than 50% acid insoluble organics, which is about same as the 1000 mg/L ozone treatment, and iii) it is a minimal treatment approach, utilizing mostly the existing infrastructure and hence the CAPEX and OPEX are low.

(12) The acid clarification treatment method requires that OTSG blowdown be acidified to a pH between 7 and 8, or lower, and then allowed to cool to 30 to 40° C. before settling for at least 12 hrs (to complete the precipitation and allow the suspended solids to fall to the bottom). Any strong acid can be used, and HCl and sulphuric acid have been tested to date, although sulfuric acid is preferably avoided. Weak acids could also be used, although greater quantities are required, making them less preferred.

(13) The cooled acidified water can then can be clarified and/or filtered and recycled to the warm-lime softener (WLS). A large tank (or a clarifier) or the existing sludge pond can be used to provide adequate storage for cooling and settling of the precipitated solids.

(14) The treated clarified blowdown water stream may be re-heated to 80° C. and the pH adjusted if necessary prior to entering the WLS to avoid dropping the temperature of the WLS and thus negatively impacting hardness removal. This can be done with original hot blowdown water in a heat exchanger, though this preheating step may not be needed.

(15) The disclosure provides a novel method and systems for pretreating blowdown water by acid clarification treatment prior to recycling, but the method can be generally applied to any dirty water, such as produced water, OTSG blowdown, evaporator blowdown, and the like. The cleaned water may still have some dissolved organics, but any increase in time between shutdown for pigging of the OTSG is greatly beneficial. The method is very simple, and can be implemented with low CAPEX and OPEX because it can be easily performed with existing systems with only a few simple modifications. The clean water can be used as is, or can be blended with other waters, such as pond water or other relatively clean source water. The cleaned water can then be used in steam generation, e.g., as boiler feedwater for an OTSG, or for other uses.

(16) It is believed that by using the methods and systems described herein, fouling in the steam generator can be greatly reduced, thereby reducing the operational cost and downtime for repairing and maintaining the steam generator and at the same time meeting strict water usage regulations.

(17) In general, an improved method of generating steam for SAGD, and other heavy oil production uses is provided, wherein blowdown feedwater is acid clarified to remove about 50% of the TOC prior to use as a feedwater for a steam generator. This process is less relevant to CSS since that process typically does not recycle OTSG blowdown, but rather injects it along with the steam during steam stimulation process.

(18) A system for generating steam is also provided, comprising a steam generator for generating steam and blowdown, an acid supply for acidifying blowdown, a tank for acidifying and settling the blowdown water, and appropriate lines connecting the various components. Of course, all of the elements are in fluidic connection, such that fluid can travel from one part of the system to another.

(19) FIG. 1 illustrates the basic concept of an OTSG, wherein a single tube holds the feedwater, which winds back and forth and the feedwater therein is progressively heated by e.g., hot gas travelling in the reverse direction.

Method

(20) OTSG blowdown coolers have reported organic fouling issues in the recent past, indicating that organic molecules present in the boiler feed water stream are highly unstable and start precipitating even with a reduction in temperature. A set of systematic experiments was designed to evaluate if the unstable nature of organics present in the OTSG blowdown stream might be exploited to solve the organics fouling problem boilers.

(21) The first round of tests was conducted at the central processing facility using a fresh blowdown (BD) sample. OTSG blowdown was titrated with HCl to pH levels 10.75, 9.75, 8.5. and 7. These samples were allowed to react and cool after acid addition for about 60 minutes. A steep increase in TSS generation was noted after the pH drop to 7, which provided an indirect measure of accelerated organics precipitation at pH 7. See FIG. 2.

(22) A second round of tests was conducted at an offsite laboratory with the capability to measure TOC accurately. First OTSG blowdown water samples were heated to 60° C. to reflect the typical temperature of blowdown water. The warmed samples were then acidified with HCl to pH values: 8.5, 8, 7.5, and 7.0 from the initial pH of 11.3. The HCl amounts to reach each of the target pH were noted as 2667, 2889, 3111, and 3333 mg/L. After 60 minutes, samples were collected from each vial for TSS and TOC measurement.

(23) As shown in FIG. 3, about 20% TOC drop (˜500 mg/L) was noted in TOC concentration when the OTSG blowdown was acidified to pH 7-8. This is a significant drop in TOC and would indicate that some of the more problematic organics can be taken out of the process simply by acidification. The acid insoluble organics are measured at pH<2. The total amount of acid insolubles measured in blowdown was roughly 1000 mg/L. A 500 mg/L drop in acid insoluble at pH 7-8 suggest that about 50% of problematic organics can be precipitated by acid clarification treatment (target pH 7).

(24) FIG. 4A provides an estimate of immediate TSS generation upon acidification after 15 minutes. FIG. 4B data were collected overnight after the cooling the sample to 22° C. A 5 fold increase in TSS generation, following the overnight cooling, indicated that organics precipitation reactions are slow, and are favored at lower temperature and pH.

System

(25) Based on the above observations, the system 100 in FIG. 1 illustrates one possible embodiment for implementing the inventive methods. In FIG. 5, clean feedwater enters the OTSG 101 via feedwater line 13. A 70-80% steam exits the OTSG 101 to separator 103 where steam is routed e.g., for steam injection via line 1, and dirty blowdown water exits via line 3 to heat exchanger 105, which can be used e.g., for preheat of the feedwater or preheat of the acid clarified blowdown water before entering the WLS 111, or for any other use.

(26) In one embodiment, cooled blowdown exits heat exchanger 105 via line 5 to tank 109 where acid is added via line 7 from acid supply tank 107. If the volume of tank 109 is sufficient, the acidified blowdown water can clarify in this tank, but if not, the cooled acidified water can be routed to a sludge pond for a more lengthy clarification (not shown). The clarified supernatant travels to water lime softener 111 via line 9, and from there back to OTSG 101 via lines 11 and 15.

(27) In FIG. 6, clean feedwater enters the OTSG 201 via feedwater line 13. A 70-80% steam exits the OTSG 201 to separator 203 where steam is routed e.g., for steam injection via line 1, and dirty blowdown water exits via line 3 to heat exchanger 205, which can be used e.g., for preheat of the feedwater or preheat of the acid clarified blowdown water before entering the WLS 111, or for any other use.

(28) In another embodiment, cooled blowdown exits heat exchanger 105 via line 5 to tank 209 where acid is added via line 7 from acid supply tank 207 or may be transferred directly via line 11 to the WLS 211. If the volume of tank 209 is sufficient, the acidified blowdown water can clarify in this tank, but if not, the cooled acidified water can be routed to a sludge pond for a more lengthy clarification (not shown). The clarified supernatant travels to water lime softener 211 via line 9, and from there back to OTSG 201 via lines 11 and 15.

(29) Additionally, an anthracite filter may be provided between the WLS and OSTG. Using anthracite filter media has the advantages of not requiring chemicals for maintenance, durable with long life and temperature range, lower backwash rate, ideal for sub-fill requirements and hot process filtering applications, while containing no silica. The wide temperature range is especially beneficial for the OTSG operation.

(30) A weak acid cation exchange can also be added between the WLS and the OTSG for water softening. The weak acid cation exchange have a high affinity for the divalent cations that constitute hardness, and remove cations and associated alkalinity from water by converting alkaline salts of calcium and magnesium to the corresponding weak acid (dissolved CO.sub.2). The dissolved CO.sub.2 can later be removed by degasification. Weak acid cation exchange resins are typically made of co-polymers of acrylic acid with divinyl benzene or methacrylic acid as a crosslinking agent. Non-limiting examples of weak acid cation resins include Amberlite IRC-86 and IRC-50 by Rohm & Haas, Purolite C-105 by Purolite, Dowex MAC-3 by Dow Chemicals, Lewatit CNP 80 WS by Bayer, and Indion 236 by Ion Exchange (India) Ltd.

(31) The minimal treatment approach requires that the OTSG blowdown be acidified to pH between 7-8, or lower, and then allowed to cool to 30-40° C. before settling for at least 12 hrs. However, using the OTSG in a heat exchanger to e.g., preheat the boiler feedwater or heat the WLS, can accomplish much or all of this cooling before acid is added thereto.

(32) A large tank (like a clarifier) or an existing pond will be required to provide the adequate storage for cooling and settling of the precipitated solids. Alternatively or in addition, the cooled blowdown water supernatant can be routed through a filter, e.g. a sand filter, and routed to the warm-lime softener. From the WLS, which removes silica, the feedwater is routed to the OTSG.