Abstract:
There is described a simple two compartment electrochemical cell which may be associated in multiples in parallel for electrochemically freeing alkanolamines of the heat stable salt forming anions found or formed in acid gas conditioning thermal regenerative sorbent processes using alkanolamines as sorbents for acid gases, the cell having an anion exchange membrane separating the cell into two compartments and an anode comprised of an iridium coated electroconductive metal electrode.

Description:
BACKGROUND OF THE INVENTION 
     The prior art is aware that in the conditioning of natural and synthetic gases to remove the acid gases such as hydrogen sulfide, carbon dioxide, carbonyl sulfide and the like, other acids, such as formic, sulfuric sulfurous, thiocyanic, oxalic, chloric acids as well as other acids, are generally also present in these gases and these acids form heat stable salts with the amine sorbents. These salts build up in the amine treating solution and must periodically be removed to maintain the overall efficiency of the amine with respect to regeneration for reuse in the absorbing process. The conventional manner for renewal of an amine sorbing solution contaminated with the heat stable salts, of the aforestated acids, is to transport the amine solution to a caustic treater wherein the salts are decomposed to their respective amine and acid components, wherein the latter is recovered as the alkali salt of the acid. Such processes are time consuming, not readily adaptable to field unit operations on site of the absorber and are relatively expensive, particularly because of the need to transport the solution to be regenerated from the site to a caustic processing plant which to be economically viable must serve several absorber operations. 
     It has been known for considerable time that amine salts in general and those produced as a result of the gas conditioning of natural and synthetic gases could be regenerated by electrochemical action. For example, Shapiro, U.S. Pat. No. 2,768,945, teaches one method for separating acidic gases from aqueous alkanolamine solutions used as absorbing solutions in the gas conditioning field. The Shapiro technique uses an electrochemical treatment of a portion of the thermally regenerated sorbing solution, a side stream, in a cell which separates the anode and cathode compartments from each other by use of a porous diaphragm. The anode is graphite and the cathode is steel. The anolyte is a weak acid and the catholyte is the amine solution. In another patent, Kuo et al, U.S. Pat. No. 3,554,691, the electrolytic conversion of amine salts of the principal acid gases, such as hydrogen sulfide and carbon dioxide, is described without mention of the effect of such electrochemical conversion of the other heat stable salts, viz., the amine formates, thiocyanates, sulfates, sulfites, oxalates, chlorides and the like. This patent uses a multi-compartment cell having at least one ion exchange resin-water compartment separating the electrode compartments each from intermediate compartments, which intermediate compartments include an acid compartment and product compartment, respectively, and a central feed compartment, all defined by ion (cation or anion exchange) permeable membranes between compartments. 
     Neither of these processes is known to be used today, Shapiro being comparatively more expensive to operate than periodic purging of a portion of the sorbent and replenishment with virgin sorbent diluting the heat stable salt concentration to a level whereat the effect of the presence of the tied up (protonated) amine is minimized. Kuo et al is far too expensive to operate since a multiplicity of cells between electrodes increases the internal resistance of the cell increasing operating costs at least proportionally. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It has now been found that an economical electrochemical cell, and, thus, an economical conversion can be run in the field using the Shapiro scheme of side stream reclamation if the cell is designed as a simple two compartment cell employing specific materials of construction. Thus, an efficient cell can be produced when (a) a dimensionally stable transition metal oxide coated electrode is used as the anode (particularly iridium oxide coated materials, stable under the conditions of environment herein described, e.g., titanium or tantalum, commonly used as anodes in conventional cells); and (b) a single anion exchange membrane, quaternerized functionalized polymers such as aminated polystyrene (e.g.; sold under the trademark Ionics by Ionics, Inc. or Ionac, e.g., Ionac MA3475, by Sybron) separating the anode and cathode compartments. The cathode may be any suitable material having electroconductivity and stable under the use environment, e.g., porous graphite, nickel and the like. 
     Experimentation has established that a graphite anode, as used by Shapiro, is a poor material since the current densities are low, and, while nickel and steel have high current densities), they are unstable and will dissolve or corrode under the operating conditions. Other well known anode electrode materials, such as titanium and tantalum, have been shown to have low, on the order of graphite, operating current densities. Experimentation likewise has shown that an electrode coated with ruthenium oxide has the ability to operate at high current densities, but is not long-lived enough to be commercially viable under conditions normally found in the field, since the ruthenium is worn away in about 30 days. Iridium oxide on titanium is shown herein to operate at high current densities and to be sufficiently long lived to be commercially viable. Similarly, tantalum is expected to give equivalent results when coated with iridium oxide. The following data, collected from laboratory experimentation, illustrates the foregoing discoveries: 
     
                       TABLE 1______________________________________       CURRENT DENSITY (amps/ft.sup.2)ANDOE MATERIAL         3 V     5 V      7 V    9 V______________________________________Porous Graphite         0       2.9      20.1   37.3Nickel*       33.4    56.2     78.6   101.0Titanium      2.4     3.5      4.7    5.9RuO.sub.2 /titanium**         37.7    49.5     67.4   85.7iridium oxide/         current density not measured in thetitanium***   laboratory______________________________________ *Nickel dissolved **the ruthenium coating wore away during field trials in less than 400 hours; ***electrode still operated after 5000 hours of field trial operation. 
    
     Thus, the only metal oxide coated anode materials found useful to date in this electrochemical process on a commercially viable scale are the iridium oxide coated stable anode materials. To prove the commercial viability of the discovery a full scale gas treating plant had a side stream from the regenerator return line passed to a cell at a rate of about one gallon per minute. The absorbent side stream analyzed 3.53% heat stable salts entering the cell and 3.18% leaving the cell, representing a 10% reduction in heat stable salts. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following examples illustrate the experimentation and provide the data to demonstrate the improved and unexpected results achieved by employing electrochemical cells constructed in accordance with the present invention: 
     A series of laboratory experiments were run to collect data on cell construction material. The cell was approximately, 4×4 inches by 3 inches. The cathode was a 3×3×1 inch piece of porous graphite and the anode was a 3×3 inch piece of expanded nickel. The anode compartment was separated from the cathode compartment by an anion exchange membrane, Ionics, Inc. 103 PZL-386. The catholyte solution was prepared by mixing 5.95 g. of methyldiethanolamine with 2.61 g. of 88% formic acid in 500 ml. of water. The initial pH was 4.89. The anolyte solution was 500 ml. of a 0.10M aqueous solution of NaCl. The catholyte was pumped through the porous graphite cathode at a rate of 300 ml/m. A D.C. power supply was connected across the electrodes and the current density was measured. In the first control experiment the anolyte and catholyte were analyzed for formic acid content. This data is set forth in table 2 resulting from a control run employing an expanded nickel anode. 
     
                       TABLE 2______________________________________        ppm FORMATETime (hrs)     Anolyte  Catholyte______________________________________0.00             0      38920.50            538     33321.03           1347     27371.70           1904     21842.53           2618     15753.45           3224      9334.45           3649      612______________________________________ 
    
     The final pH of the catholyte was 10.34. The nickel anode was partially dissolved. 
     Employing the same cell, changing only the cathode material and using 0.5M sodium carbonate in both electrode compartments the results set forth in Table 1 above were obtained. 
     Table 1, demonstrates that nickel is capable of operating at a high current density but is unsuitable as an anode in this system because it is soluble in the anolyte. Graphite and titanium are not suitable because while stable in the anolyte, their low current density makes a cell too expensive to operate. Thus only rare earth coated metals are useful and practical for such an operation. 
     Comparative field test 
     Run 1, Stainless Steel anodes 
     A treating unit was constructed of four cathode compartments and three anode compartments each separated from the other by an Ionics anion membrane. The cathodes in each cathode compartment were 14×14 inches square expanded nickel sheets. The anodes were 316 stainless steel punched plate. The electrodes were connected in parallel. Lean methyldiethanolamine (MDEA) at 120° C. and 200 psig was filtered and metered to the treating unit cathode compartments. The anolyte solution was originally a 10% sodium carbonate solution which was circulated through the anode compartments. The pH of the anolyte was maintained between about 8 and 11 by periodic additions of 15% aqueous sodium hydroxide. The volume of the anolyte solution was maintained constant. The following table sets forth the results: 
     
                       TABLE 3______________________________________Run                    Amine            MDEAtime Current           Flow   Hss* (wt %)                                   Regen(hrs)(amps)   Voltage  (GPM)  in    out   #/hr______________________________________0.5  320      5.8      1.0    4.86  4.43  2.525.0  480      9.9      1.1    4.73  4.36  2.1117.5 391      16.2     1.1    4.67  4.35  1.8320.5 338      19.8     1.3    3.82  3.69  1.55______________________________________ *HSS heat stable salts 
    
     Although the heat stable salts were being regenerated, the stainless steel anodes proved to be a failure since under the use environment the electrodes (anodes) completely corroded in 20.5 hours. 
     Run 2. 
     Ruthenium Coated Anodes 
     A treating unit was constructed of four cathode compartments and three anode compartments each separated from the other by an Ionics anion membrane. The cathodes in each cathode compartment were 14×14 inches square expanded nickel sheets. The anodes were ruthenium coated expanded nickel sheets. The electrodes were connected in parallel. Lean methydiethanolamine at 120° C. and 200 psig was filtered and metered to the treating unit cathode compartments. The anolyte solution was originally a 10% sodium carbonate solution which was circulated through the anode compartments. The pH of the anolyte was maintained between about 8 and 11 by periodic additions of 15% aqueous sodium hydroxide. The volume of the anolyte solution was maintained constant. The following table sets forth the results: 
     
                       TABLE 4______________________________________Run                     Amine           MDEAtime  Current           Flow  HSS (wt %)                                   Regen(hrs) (amps)   Voltage  (GPM) in    out   #/hr______________________________________0.5   363      7.58     1.0   3.5   3.1   2.0748    495      9.18     1.761    500      9.26     0.8    3.24  3.17 0.2978.5  500      9.31     0.9   3.5   3.3   0.9392    310      7.81     0.995    300      5.80     1.3    3.17  3.01 1.08181   300      5.69     1.3    3.36  3.13 1.55288   405      6.46     1.3   3.1   3.0   0.67290   405      7.78     0.7   3.0   2.6   1.45292.5 450      7.13     0.9   3.0   2.6   1.87388   cell shut down, voltage exceeded 20 volts at 10 amps.______________________________________ 
    
     Anode analysis with electron microscopy showed all the ruthenium oxide coating had worn off after only 388 hours. This field trial illustrates that while ruthenium oxide coating of the anode material gave positive results, the coating was not a satisfactory coating for extended use such as required in the environment of gas conditioning. 
     EXAMPLE 1 
     Iridium oxide anodes 
     The replacement of the anode electrodes of the cell of Runs 1 and 2 with iridium oxide coated expanded titanium sheets was then field tested and proved to be satisfactory under the use environment. The data collected from this field trial is set forth in table 5. 
     
                       TABLE 5______________________________________Run                    Amine            MDEAtime Current           Flow   HSS* (wt %)                                   Regen(hrs)(amps)   Voltage  (GPM)  in    out   #/hr______________________________________ 1   360      4.8      1.13   5.3   5.2   0.59 22.5360      4.7      1.31   5.9   4.9   6.79 42  480      5.62     1.31   5.7   5.1   4.07 67  180      4.35     0.87169  405      5.3      1.13   5.29  4.94  2.05211  400      4.82     1.22   4.63  4.56  0.44255  400      4.65     1.18   4.67  4.29  2.32284  400      4.42     1.18   4.64  4.29  2.14308  400               1.15   4.37  3.96  2.44332  402      4.85     1.13   4.78  4.57  1.23356  405      4.26     1.22   4.72  4.67  0.32380  405      4.28     1.09404  410      4.2      1.00   5.34  4.76  3.01428  400      4.25     1.13   4.55  4.47  0.47452  &#34;        4.3      1.13   4.97  4.65  1.87476  &#34;        4.28     1.09   4.84  3.83  5.71500  &#34;        4.14     &#34;      5.20  4.88  1.81524  &#34;        4.31     &#34;      5.18  4.47  4.01572  &#34;        4.36     1.31620  &#34;        4.3      1.09   5.07  4.56  2.88688  &#34;        4.8      1.00   4.74  4.57  1.04692  390      4.5      1.09   4.71  4.57  0.79740  400      4.9      1.09   4.92  4.81  0.62788  &#34;        4.5      1.18   5.35  5.12  1.41812  &#34;        -        1.09   4.99  4.77  1.24______________________________________ *HSS heat stable salts 
    
     Analysis of the anode by microscopy at 700 hours showed less than 5% loss of coating. To date the cells have been operated over 5000 hours with no appreciable change in results.