System and method for pH control of lean MEG product from MEG regeneration and reclamation packages

Abstract

A lean MEG stream having a first pH level is contacted with a CO.sub.2-rich gas stream to yield a lean MEG product having a second different and lower pH level preferably in a range of 6.5 to 7.0. The system and method can be readily incorporated into a slipstream MEG recovery package, with a source of the lean MEG stream being a MEG regeneration section of the package. The CO.sub.2-rich gas could be a vented CO.sub.2 stream from the MEG reclamation section of the package. Unlike hydrochloric and acetic acid overdosing, CO.sub.2 overdosing of the lean MEG stream does not lead to rapid acidification of the lean MEG product to be stored or injected.

Claims

1. A MEG recovery system comprising a vessel for adjusting a pH level of a lean MEG stream exiting a MEG regeneration unit, the vessel including an inlet in fluid communication with the lean MEG stream and a port in fluid communication with a CO.sub.2-rich gas stream from a MEG reclamation unit, the lean MEG stream and the CO.sub.2-rich gas stream coming into contact with one another within the vessel, the vessel further including an outlet for a reduced pH lean MEG stream and a different outlet to vent a reduced CO.sub.2 gas stream.

2. A MEG recovery system according to claim 1 wherein an amount of the CO.sub.2-rich gas is at least equal to a stoichiometric quantity effective for achieving a desired reduced pH level.

3. A MEG recovery system according to claim 2 wherein the amount of CO.sub.2-rich gas exceeds the stoichiometric quantity.

4. A MEG recovery system according to claim 1 wherein the reduced pH lean MEG stream has a pH level-of at least 6.

5. A MEG recovery system according to claim 1 wherein the reduced pH lean MEG stream has a pH level of 7.

6. A MEG recovery system according to claim 1 wherein the lean MEG stream is mixed with a second lean MEG stream having a different pH level than a pH level of the lean MEG stream.

7. A MEG recovery system according to claim 6 further comprising the second lean MEG stream contains less salt than the lean MEG stream.

8. A MEG recovery system according to claim 6 wherein the second lean MEG stream has a pH level of 7.

9. A MEG recovery system according to claim 1 wherein a portion of the lean MEG stream exiting the regeneration unit is routed to the MEG reclamation unit.

10. A MEG recovery system according to claim 1 wherein the vessel is a gas-liquid contactor.

11. A MEG recovery system comprising a gas/liquid contactor vessel including an inlet in fluid communication with a lean MEG stream exiting a MEG regeneration unit and a port in fluid communication with a CO.sub.2-rich gas stream from a MEG reclamation unit, the lean MEG stream and the CO.sub.2-rich gas stream coming into contact with one another within the gas/liquid contactor vessel, the gas/liquid contactor vessel further including an outlet for a reduced pH lean MEG stream and a different outlet to vent a reduced CO.sub.2 gas stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) So that the manner in which the above recited features can be understood in detail, a more particular description may be had by reference to embodiments, some of which are illustrated in the appended drawings, wherein like reference numerals denote like elements. It is to be noted, however, that the appended drawings illustrate various embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

(2) FIG. 1 is a schematic of an embodiment of a system and method of this disclosure. A vessel located downstream of a MEG regeneration section receives a high pH lean MEG stream and allows the steam to come into contact with a CO.sub.2-rich gas.

(3) FIG. 2 is a graph illustrating a lean MEG stream with alkalinity present as sodium carbonate as the stream is treated with CO.sub.2, acetic acid, and hydrochloric acid.

(4) FIG. 3 is a graph illustrating a lean MEG stream with alkalinity present as sodium hydroxide as the stream is treated with CO.sub.2, acetic acid, and hydrochloric acid.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS

(5) 10 Vessel 15 Rich MEG steam 20 Lean MEG stream 21 Portion of lean MEG stream 20 30 MEG regeneration unit or section 40 CO.sub.2-rich gas 40 50 Lean MEG product exiting 10 60 MEG reclamation unit or section 61 Salt-free lean MEG stream 70 Calcium removal unit or section

DETAILED DESCRIPTION

(6) In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

(7) In the specification and appended claims: the terms “connect”, “connection”, “connected”, “in connection with”, and “connecting ” are used to mean “in direct connection with ” or “in connection with via one or more elements”; and the term “set” is used to mean “one element” or “more than one element”. Further, the terms “couple”, “coupling”, “coupled”, “coupled together”, and “coupled with” are used to mean “directly coupled together” or “coupled together via one or more elements”. As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and downwardly”, “upstream” and “downstream”; “above” and “below”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the disclosure.

(8) Referring to FIG. 1, an embodiment of a system and method for adjusting a pH level of a lean MEG steam includes a vessel 10 which receives a lean MEG stream 20 from a lean MEG source such as a regeneration unit or section 30 of a slipstream MEG recovery package. Typically, stream 20 has a pH level above 9.5, as does rich MEG stream 15 upstream of the regeneration section 30. Within vessel 10, this high pH lean MEG stream 20 comes into contact with a CO.sub.2-rich gas 40 (i.e., greater than 50% CO.sub.2 content). Vessel 10 can be a contactor vessel of a kind known in the art.

(9) The CO.sub.2 in gas 40 forms acidic solutions when dissolved in the MEG-water mixture of stream 20, thereby reducing the pH. A lean MEG product 50 having a second lower pH exits the vessel 10. Preferably, product 50 has a pH level in a range of 6.5 to 7. No inorganic acids such as HCl or organic acids such as acetic or citric acid is required for reducing the pH to this level.

(10) The CO.sub.2-rich gas 40 can be from any source preferable but is more preferably a vent stream from a reclamation unit or section 60 of the slipstream MEG recovery package. Similar to MEG regeneration section 30, MEG reclamation section 60 is of a kind well-known in the art.

(11) A salt-free lean MEG stream 61 which exits the reclamation section 60 can be mixed with the lean MEG stream 20 prior to stream 20 entering vessel 10. Additionally, a portion 21 of the lean MEG stream 20 which exits the regeneration section 30 can be routed to the reclamation unit 60.

(12) In slipstream MEG recovery packages that make use of a calcium removal unit or section 70 upstream of the regeneration unit 30, excess carbonate that finds its way into the reclamation section 60 degrades to form CO.sub.2 (and hydroxide) under the elevated temperature, low pressure regime of a flash separator (not shown).

(13) Referring to FIGS. 2 and 3, unlike hydrochloric and acetic acid overdosing, CO.sub.2 overdosing within vessel 10 does not lead to rapid acidification of the lean MEG product 50. In a CO.sub.2 overdosing condition, the pH level remains above 6 whereas in an acetic acid and hydrochloric acid overdosing condition the pH level falls below 4 and 2 respectively. Therefore, the system and method of this disclosure is less sensitive to overdosing conditions than prior art methods.

(14) As mentioned above, acidification with CO.sub.2 removes the risk which occurs with inorganic acids (HCl) and the absence of carboxylates (acetate), namely, overdosing to the point of potentially damaging pH levels. In addition, carboxylates are highly soluble in MEG and are difficult to remove once added to the MEG system. The accumulation of carboxylates can lead to operational problems as the density and viscosity of the MEG increases with increasing carboxylate content. Hydrochloric acid converts readily to salt plus water; carbon dioxide converts to bicarbonate which is much more easily managed in the MEG system than carboxylates. Although the CO.sub.2 reduces the pH, the ‘alkalinity’ (OH— plus HCO.sub.3— plus CO.sub.2) is not reduced.

(15) To examine the “scaling” potential for the system and method, the following software simulation was run employing OLI Analyzer v 9.1.5 (OLI Systems, Inc., Cedar Knolls, N.J.).

(16) Starting Solution:

(17) 90 wt % MEG (on salt-free basis) at 40° C. containing 30,000 mg/kg.sub.solvent sodium chloride, 250 mg/kg.sub.solvent of sodium carbonate and 25 mg/kg.sub.solvent of sodium hydroxide. The pH of this mixture was 10.053 or about 10 (see Table 1, col. A, below).

(18) Acidification:

(19) The MEG solution was neutralized to pH=7.0 and to pH=6.5 using HCl acetic acid and CO.sub.2. Quantities of HCl, CH.sub.3CO.sub.2H and CO.sub.2 added are shown in Table 1, rows 12-14, below.

(20) Scaling Test:

(21) Scaling potential of the acidified solutions was determined by adding in separate simulations MgCl.sub.2, CaCl.sub.2, FeCl.sub.2, SrCl.sub.2 and BaCl.sub.2 to the lean MEG solutions at the quantities shown in Table 1, rows 19-23.

(22) TABLE-US-00001 TABLE 1 Software Simulation of Scaling Potential. 1 A B C D E F G 2 3 TEMP 40 40 40 40 40 40 40 4 5 H2O g 100,000 100,000 100,000 100,000 100,000 100,000 100,000 6 MEG g 900,000 900,000 900,000 900,000 900,000 900,000 900,000 7 NaCl g 30,000 30,000 30,000 30,000 30,000 30,000 30,000 8 Na2CO3 g 250 250 250 250 250 250 250 9 NaOH g 25 25 25 25 25 25 25 10 11 ACIDIFICATION 12 HCl g 0 136 — — 160 — — 13 CHCO2H g 0 — 228 — — 279 — 14 CO.sub.2 g 0 — — 239 — — 478 15 16 pH — 10.05 7.01 7.01 7.01 6.50 6.50 6.50 17 18 SCALING TEST POST ACIDIFICATION 19 MgCl2 for Mg g precipitation as Mg(OH)2 20 CaCl2 for Ca g 1.4 840 880 250 4300 5300 800 precipitation as CaCO3 21 FeCl2 for Fe g 0.1 1.5 1.7 0.8 6.7 7.7 2.4 precipitation as FeCO3 22 SrCl2 for Sr g 0.5 650 660 200 3900 3950 620 precipitation as SrCO3 23 BaCl2 for Ba g 0.2 7 13.7 2.1 35 80 6.3 precipitation as BaCO3

(23) Results:

(24) For the starting solution (col. A, pH=10.0) precipitation of divalent cations as carbonate occurs on addition of 1.4 g of CaCl.sub.2. After acidification to pH 7.0 with HCl, the quantity of calcium chloride added before precipitation of CaCO.sub.3 increases to 840 g from 1.4 g. The effect with acetic acid is similar with precipitation starting at 880 g of CaCl.sub.2. The equivalent scaling point with carbon dioxide occurs at 250 g, less than that for HCl or acetic acid but a considerable improvement on the 1.4 g for the untreated sample.

(25) Similar trends are observed for the other divalent cations (Fe, Sr, Ba) although some are more insoluble than others. Iron, in particular, tends to precipitate out readily. At pH=6.5 (col. E-G) the trends agree with those shown at pH=7.0 (col. B-D), i.e. precipitation of divalent cations (Ca, Fe, Sr, and Ba) from the lean MEG is inhibited by addition of CO.sub.2 to the alkaline lean MEG mixture.

(26) Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.