Corrosion in Aqueous Solution of Two Alkanolamines with CO2

4726

Ind. Eng. Chem. Res. 2008, 47, 4726–4735

Corrosion in Aqueous Solution of Two Alkanolamines with CO2 and H2S: N-Methyldiethanolamine + Diethanolamine at 393 K Rafael Eustaquio-Rincoń,† Marıá Esther Rebolledo-Libreros,† Arturo Trejo,*,† and Rene´ Molnar‡ A´rea de InVestigacioń en Termofı´sica, Programa de Ingenierıá Molecular, Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas Norte 152, 07730, Me´xico, D.F., Me´xico, and Departamento de Ciencias Ba´sicas, UniVersidad Autońoma Metropolitana-Azcapotzalco, AVenida San Pablo 180, 02200, Me´xico, D.F., Me´xico

In this work we have used an apparatus constructed in our laboratory to systematically study the corrosion rate on metals and evaluate specialty chemical products for preventing metal corrosion in industrial plants. We performed experimental studies on the effect of specific variables on corrosion rates over carbon steel, such as pressure, concentration of alkanolamines in aqueous solution, and amount of acid gases (CO2 and H2S). The technique utilized to evaluate the corrosion rate is the well-known weight loss method. With the experimental apparatus and method developed in this work we have determined the corrosion rate of AISI 1010 carbon steel in aqueous solutions of known concentration of N-methyldiethanolamine (MDEA) and diethanolamine (DEA), individually, with and without acid gases, and in different alkanolamine blends with total alkanolamine mass fraction in the range 15-60%, with mass ratios of MDEA/DEA of 3.5/1 and 2/1, with the addition of different amounts of H2S and CO2 at a pressure range of 276-5861 kPa (40-850 psig). Corrosion rates over carbon steel were determined in the liquid phase and also in some runs in both liquid and vapor phases, at 393.15 K. In the studied systems the corrosion rate decreases when the concentration of the alkanolamine increases. It was also possible to establish the effect of the amount of CO2 and H2S, in an individual way as well as in a mixture, on the corrosion rate of the carbon steel. The results show that corrosion rate of the alkanolamine solutions is not affected by CO2 in the range 0-0.35 mol; however, the presence of H2S drastically increases the corrosion rate as the amount of H2S increases. Introduction The removal of mainly hydrogen sulfide and carbon dioxide, also known as acid gases, from a great variety of hydrocarbonrich gas and liquid streams, such as natural, refinery, and synthesis gases and LPG streams has as the main goal to increase the industrial and commercial utility of the hydrocarbon streams, reducing contaminant emissions into the environment during the combustion of such streams, and reducing the corrosion problems in equipment and pipelines due to the presence of such acid gases.1 The alkanolamines are the most commonly accepted and widely used of the many available chemical solvents for the efficient removal of H2S and CO2, because of their reactivity and availability at low cost. Monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) are the most used alkanolamines in aqueous solution for hydrocarbon sweetening processes. An important advance in gas-treating technologies has been the availability of sterically hindered amines, such as 2-amino-2-methyl-1-propanol (AMP), for greater CO2 removal from industrial hydrocarbon-rich streams.2 In the past few years, aqueous solutions of blends of alkanolamines (a primary or secondary alkanolamine with a tertiary alkanolamine) have received increasing attention for the simultaneous removal of CO2 in the presence of H2S from different gas streams since those solutions combine the advantages of each individual amine with the aim to produce a considerable improvement in absorption capacity,3–5 as well as selective reactions and easier regeneration of the acid gas loaded mixed amine solutions. Overall, their use results in great savings in energy requirements in the gas sweetening processes.6 * To whom correspondence should be addressed. Tel.: +55 91758373. E-mail: [email protected]. † Instituto Mexicano del Petro´leo. ‡ Universidad Autońoma Metropolitana-Azcapotzalco.

In the search and evaluation of new solvents, to be applied industrially in absorption or purification of gas and liquid hydrocarbon streams, the most important facts to be considered are the high absorption capacity toward acid gases and the low absorption capacity for hydrocarbons. Simultaneously, it is desirable to have selectivity toward one of the acid gases, furthermore that they do not generate undesirable products, e.g., thermally stable salts or carbamates, and that they considerably reduce foaming during the absorption step, corrosion, and fouling problems in the whole process. In Figure 1 a schematic of the method for solvent selection is presented. Considering all these aspects, as a whole it is highly desirable to carry out research on the phase equilibria and thermophysical properties (e.g., density, viscosity, surface tension), on the corrosion rate, and on the foaming index of new solvents with a view to create an accurate and complete body of knowledge to set the scientific basis to develop industrial processes for the

Figure 1. Method for solvent selection.

10.1021/ie071557r CCC: $40.75  2008 American Chemical Society Published on Web 06/24/2008

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4727

purification of gas and liquid hydrocarbon streams that are technically, environmentally, and economically more efficient. Looking at the physicochemical characteristics of alkanolamine aqueous solutions gives the impression that corrosion would not be a problem. As a matter of fact, many amines are used as corrosion inhibitors. However, the situation changes drastically when the acid gases take part in the corrosion process. MEA and DEA solutions in the absence of acid gases are less corrosive toward steel than water; however, the presence of acid gases in the alkanolamine solutions as constituents causes corrosion directly.7 In industrial alkanolamine sweetening systems corrosion can take place in eight different forms:8 general, galvanic, crevice, pitting, intergranular, selective leaching, erosion, and stress corrosion cracking. Hydrogen sulfide corrosion is the most critical type of corrosion, causing failure of high-tensile steels under stress. The corrosion reactions of H2S give rise to the presence of both atomic and molecular hydrogen, which are able to penetrate the structure of the steel resulting in destruction of the material. Worldwide inspection programs indicate that pressure vessel cracking in wet H2S environments is a major refining problem.9 Thus, in order to have reliable information on the corrosion phenomena present in gas sweetening processes that employ aqueous solutions of alkanolamines, we have undertaken a systematic study on corrosion due to the presence of both H2S and CO2 on carbon steel. We present in this work the development of a new experimental device of stainless steel, constructed in our laboratory,10 based on the well-known weight loss method, which is useful for studying corrosion rates and for eventually evaluating corrosion inhibitors under conditions of pressure and temperature used in industrial hydrocarbon sweetening plants. Additionally, we report in this work a method for determining with high precision the amount of pure and mixed acid gases added to the aqueous solutions of alkanolamines. The experimental corrosion rate results included in this work were obtained for various aqueous solutions of industrial interest: (a) 30 and 40 mass % DEA without acid gases, (b) 30 and 40 mass % MDEA without acid gases, (c) four different blends of MDEA + DEA without acid gases, (d) seven solutions in the range 15-50 mass % DEA with 0.2 mol of H2S, (e) seven solutions in the range 15-50 mass % MDEA with 0.2 mol of H2S, (f) nine different blends of MDEA + DEA with a mass ratio of 3.5/1 with 0.2 mol of H2S, (g) eight different blends of MDEA + DEA with a mass ratio of 2/1 with 0.2 mol of H2S, (h) an aqueous solution in mass fraction of 15.6% MDEA + 4.4% DEA with CO2 in the range 0-0.35 mol, (i) an aqueous solution in mass fraction of 15.6% MDEA + 4.4% DEA with 0.1 mol of H2S and CO2 in the range 0.05-0.18 mol, and (j) an aqueous solution in mass fraction of 15.6% MDEA + 4.4% DEA with 0.1 mol of CO2 and H2S in the range 0-0.20 mol. All the measurements were carried out at 393.15 K, which is the highest temperature present in hydrocarbon sweetening plants. The different aqueous alkanolamine solutions studied here have been demonstrated to be efficient to remove CO2 and H2S from hydrocarbon streams.4,5 The material of all the coupons was carbon steel since it is the material with which the equipment of industrial plants for hydrocarbon sweetening is constructed. Corrosion rates over AISI 1010 carbon steel were determined in the liquid phase, and also in some experimental runs they were determined simultaneously in both the liquid

Figure 2. Semistatic weight loss test apparatus: (1) constant temperature bath, (2) circulator and temperature control, (3) digital thermometer, (4) thermocouple, (5) cell holder, (6) electric motor, (7) pulley and gears arrangement, and (8) connecting rod.

and vapor phases for some of the studied solutions of blends of alkanolamines. Experimental Section Materials. The chemicals are the same as those used in previous work:11 99.9 mol % DEA was obtained from J. T. Baker and 99+ mol % MDEA was obtained from Aldrich. CO2 was obtained from Infra Mexico with a purity of 99.5 mol %, and H2S was obtained from Linde with a purity of 99.5 mol %. Alkanolamines were distilled under vacuum with a stream of dry nitrogen in order to remove any possible traces of moisture and other impurities, and stored over a molecular sieve. Chromatographic analyses were carried out on each sample after distillation, and impurities were not found with a lower limit of detection of 0.05 mol % using a Supelcowax 10 capillary column and a thermal conductivity detector.12 The aqueous solutions of alkanolamines were prepared with doubly distilled and deionized water with a conductivity of 17 µΩ/cm. Apparatus. The measurements of corrosion rates were carried out in an apparatus constructed in our laboratory10 which is based on the well-known weight loss method.13 The electrochemical methods give corrosion rate results different from those of the weight loss method; hence we have used the latter because industry still uses such a method to monitor corrosion.14 In this apparatus, shown in Figure 2, it is possible to evaluate corrosion rates as a function of temperature, pressure, alkanolamine concentration, and acid gas amount. The apparatus to study corrosion rates consists mainly of a homemade constant high temperature oil bath, with a total volume of 60 L, which was constructed of galvanized sheet, a digital immersion circulator and temperature control (Polyscience), an electric motor (Erweka, Model ADM) which by means of a pulley and gears system produces an oscillating movement, giving a low velocity rocking movement to the solutions inside the cells, and a digital thermometer (Fluke, Model 2180A) with calibrated thermocouples type K with a precision of (0.01 K. Ten stainless steel high-pressure corrosion cells can be collocated simultaneously in a metal base inside the oil bath. The apparatus works in the temperature and pressure ranges of interest for gas sweetening plants, that is, 313-393 K and up to 6845 kPa (1000 psig).1 The temperature of the corrosion cells was controlled within (0.05 K, at 393.15 K. Readings of the thermocouples used were compared with a thermometer (Automatic System Laboratories (ASL)) that have an uncertainty of (0.005 K, traceable to the National Institute of Standards and Technology of the United States (NIST) with an uncertainty of (0.02 K.

4728 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

Figure 4. System to quantify the amount of CO2 and H2S fed into the corrosion cells: (1) CO2 commercial cylinder, (2) H2S commercial cylinder, (3) calibrated CO2 storage cylinder, (4) calibrated H2S storage cylinder, (5) digital thermometer, (6) platinum resistance probe, (7) pressure transducer, (8) digital pressure indicator, (9) precision pressure gauge, (10) constant temperature bath circulator, (11) vacuum pump, (12) corrosion cell, and (13-17) needle valves. Table 1. Experimental Results of Corrosion Rate of Aqueous Solutions of Alkanolamine, over AISI 1010 Carbon Steel, at 393.15 K corrosion rate/mpy

Figure 3. Schematic diagram of the stainless steel corrosion cells and position of the specimens during the test: (1) cell body, (2) coupon holder, (3) thermowell, (4) acid gas bubbling tube, (5) coupon, (6) PTFE washer, (7) O-ring, (8) manometer, and (9) stainless steel needle valve.

Figure 3 shows a diagram of one corrosion cell constructed of stainless steel, which can accept up to 10 corrosion specimens through a coupon holder that consists of two stainless steel tubes, one of which is the gas inlet and the other is a thermowell in which a thermocouple is placed. The coupons are mounted on these tubes using polytetrafluoroethylene (PTFE) insulating O-rings in order to avoid contact between the coupons and the tubes. Each corrosion cell has a stainless steel needle valve (Swagelok, Model SS-1R4) to feed the acid gases into the cells and a manometer (Ashcroft) to measure the internal pressure in the range 0-6845 kPa (0-1000 psig), which was compared with the readings from a calibrated dead weight balance (Ruska, Model 2400-736-00). The pressure in the cells is measured with an accuracy of (10 psig. In these cells, whose total volume is 150 cm3, the corrosion rate can be obtained for coupons in the vapor phase, in the liquid phase, or in the vapor-liquid interface of the studied solutions. Experimental Method. The experimental method consists of the following steps: a. Preparation of Specimens or Coupons. All the coupons used in this work were made from a single sheet of AISI 1010 carbon steel with a thickness of 1 mm. They were cut into rectangular shapes and marked with a consecutive number. The coupons were sandblasted during 100 h, degreased with hexanes, dried in a fumes hood, and stored in a vacuum desiccator until needed to determine their dimensions and mass. Fingerprints were avoided by handling the coupons with clean gloves. The mass of the coupons was obtained with a calibrated analytical balance (Sartorius, Model 2006 MP) to the nearest 0.0001 g, and the area was determined with an uncertainty of (5 mm2.

Calkanolamine/mass % 30% 40% 30% 40% 10% 10% 10% 20%

DEA DEA MDEA MDEA DEA-15% DEA-20% DEA-35% DEA-10%

MDEA MDEA MDEA MDEA

mass loss/g

this work

0.0016 0.0014 0.0018 0.0014 0.0015 0.0018 0.0018 0.0015

0.24 ( 0.01 0.21 ( 0.01 0.27 ( 0.01 0.21 ( 0.02 0.23 ( 0.01 0.28 ( 0.01 0.28 (0.02 0.23 ( 0.01

ref 22

n,a this work

0.10b 0.12b

6 6 6 5 5 6 6 6

a Number of coupons in liquid phase in each corrosion cell. 373 K.

b

At

Table 2. Experimental Results of Corrosion Rate of Aqueous Solutions of Diethanolamine and N-Methyldiethanolamine, Individually, with 0.2 mol of H2S, over AISI 1010 Carbon Steel, at 393.15 K CDEA/ mass %

mass loss/g

15 20 25 30 35 45 50

0.0755 11.51 ( 0.63 0.0599 9.13 ( 0.76 0.0407 6.20 ( 0.56 0.0296 4.51 ( 0.40 0.0264 4.03 ( 0.34 0.0307 4.68 ( 0.03 0.0388 5.92 ( 0.81 a

corrosion rate/mpy

CMDEA/ na mass %

mass loss/g

corrosion rate/mpy

na

6 6 4 4 5 6 4

0.0257 0.0248 0.0236 0.0205 0.0178 0.0133 0.0159

3.92 ( 0.11 3.78 ( 0.85 3.59 ( 0.37 3.13 ( 0.10 2.71 ( 0.30 2.02 ( 0.30 2.43 ( 0.10

6 3 5 6 4 4 6

15 20 25 30 35 45 50

Number of coupons in liquid phase in each corrosion cell.

The steel test specimens were carefully placed on the coupon holder of each corrosion cell. b. Preparation of Alkanolamine Solutions. The aqueous solutions of known concentrations of alkanolamines were prepared by batch using a calibrated digital balance (Mettler, Model PC 2000) with a precision of (0.01 g. For simplicity the concentration values for the studied solutions are reported up to the first decimal digit, although it should be noted that


Figure 5. Comparison of experimental corrosion rates between binary and ternary aqueous alkanolamine (DEA and MDEA) systems, over AISI 1010 carbon steel, at 393.15 K. The solutions with H2S were loaded with 0.2 mol. (0) Without H2S; (9) with H2S.

Figure 6. Experimental corrosion rate of aqueous mixtures in mass fraction of one alkanolamine with 0.2 mol of H2S, over AISI 1010 carbon steel, at 393.15 K. (0) DEA; (9) MDEA.

they are accurate within (0.002 mass %. The different solutions were homogenized by stirring with a magnetic bar and later poured into the cells; approximately 85 g of aqueous solution of alkanolamine was used for each cell. c. Degassing of Alkanolamine Solutions. It is well-known that oxygen accelerates corrosion rates15,16 because it produces heat-stable salts; hence in order to avoid this effect each of the studied alkanolamine solutions was degassed in situ by freeze-evacuation-thaw cycles.17,18 The stainless steel corrosion cells were connected to an all-glass vacuum system, then the solutions were frozen with solid CO2, and the noncondensable gases contained above the solution were extracted; after that the solutions were melted. d. Quantification of Acid Gases. We have determined with high precision and accuracy the amount of each acid gas introduced into each test cell with a method developed in our laboratory.18,19 The method is based on the measurement of PVT data. Figure 4 shows a diagram of the experimental apparatus, which consists essentially of two stainless steel gas-storage

cylinders whose volume has been determined with high accuracy, a pressure digital indicator (Validyne, Model CD23) with a differential transducer with a resolution of (3.5 kPa, a Bourdon tube gauge (Mensor, Model 2793) with a resolution of (3.5 kPa ((0.5 psig), a digital thermometer (Systemteknik AB, Model S1220) with platinum resistance probe with a resolution of (0.01 K, and a constant temperature bath circulator (Hetofrig, Model Birkerod). Knowing the pressure of the acid gas in the storage cylinder, at a constant temperature and volume, before and after the gas injection into the corrosion cells and applying the virial equation of state truncated after the second coefficient,20 it is then possible to know the amount of substance, ∆n/mol, of each acid gas that is fed into each cell, according to ∆n )

PaVa PbVb RTb+BPb RTa+BPa

(1)

where P, V, and T are the pressure, volume, and temperature, respectively, of the acid gas storage cylinder, R is the universal

4730 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Table 3. Experimental Results of Corrosion Rate of Aqueous Solutions of Blends of MDEA + DEA (Mass Ratio MDEA/DEA ) 3.5/1) with 0.2 mol of H2S, over AISI 1010 Carbon Steel, at 393.15 K Calkanolamine/ mass % 15 20 25 30 35 40 45 52 60 a

CMDEA/CDEA/ mass %

mass loss/g

corrosion rate/mpy

n

11.7/3.3 15.6/4.4 19.4/5.6 23.3/6.7 27.2/7.8 31.1/8.9 35.0/10.0 40.4/11.6 46.7/13.3

0.0723 0.0482 0.0309 0.0253 0.0247 0.0227 0.0183 0.0192 0.0165

11.01 ( 0.33 7.34 ( 1.53 4.71 ( 0.04 3.86 ( 0.07 3.76 ( 0.17 3.46 ( 0.69 2.79 ( 0.03 2.92 ( 0.18 2.52 ( 0.02

6 6 6 6 3 4 3 3 6

gas constant, B is the second virial coefficient, and ∆n is the amount of acid gas loaded into the cell; subscripts “b” and “a” mean before and after the addition of gas, respectively. The final or combined uncertainty of the amount of each acid gas loaded into the corrosion cells was evaluated through a complete statistical analysis on the propagation of uncertainties for all the known variables involved in the experimental work which considered the use of the so-called Student’s t distribution. Therefore, the uncertainty was always determined with 95% confidence. The estimated combined uncertainty of the amount of each acid gas loaded into the corrosion cells is (0.0005 mol. The method used in this work to quantify the amount of acid gas fed into each cell is clearly an improvement over other methods reported in the literature, since the majority of works have reported that the gas is introduced by bubbling into the cell and the amount of added gas is established in a qualitative way relative to a time period of bubbling or to the pH of the solution.21 After the acid gases have been added to each test cell, the latter are placed in the holder plate in the constant temperature bath in order to initiate the experimental run at constant temperature and with a regular rocking movement. The average test time was 350 h (approximately 14.5 days). e. Cleaning of Specimens or Coupons. After each experimental run all the coupons are rinsed with different solvents: first with water, benzene, acetone, inhibited hydrogen chloride solution in mass fraction 5% SnCl2 + 2% Sb2O3 and 93% HCl, and finally with an aqueous solution of Na2CO3. The coupons are then washed with soap and a soft brush in order to remove the corrosion products. The coupons are immersed in each solvent mentioned before for 1 min with ultrasonic agitation to accelerate the cleaning process. The coupons are rinsed at the end of this process with distilled water. They are then dried in an electric oven at 373.15 K and weighed in the analytical balance to the nearest 0.0001 g. f. Quantification of the Corrosion Rate. The mass lost during the corrosion test is determined in the same analytical balance mentioned above, and the corrosion rate in mil per year (mpy) may then be obtained through eq 2: Kw Atd

Calkanolamine/ mass %

CMDEA/CDEA/ mass %

mass loss/g

corrosion rate/mpy

na

10.0/5.0 13.0/7.0 16.7/8.3 20.0/10.0 23.3/11.7 30.0/15.0 34.7/19.3 40.0/20.0

0.0685 0.0434 0.0276 0.0217 0.0204 0.0168 0.0180 0.0195

10.44 ( 0.10 6.61 ( 0.90 4.21 ( 0.26 3.31 ( 0.05 3.11 ( 0.24 2.56 ( 0.04 2.75 ( 0.20 2.97 ( 0.05

5 6 5 6 3 4 3 4

a


corrosion rate/mpy )

Table 4. Experimental Results of Corrosion Rate of Aqueous Solutions of Blends of MDEA + DEA (Mass Ratio MDEA/DEA ) 2/1) with 0.2 mol of H2S, over AISI 1010 Carbon Steel, at 393.15 K

(2)

where K is a constant, 3.45 × 106 mpy h/cm; w is the mass loss in grams; A is the coupon area in cm2; t is the time of exposure in hours; and d is the density of the studied carbon steel (7.861 g/cm3). The corrosion rate results reported in this work are the averages of values from several coupons in a given cell, as indicated in each table. We have also obtained values of the

15 20 25 30 35 45 52 60 a


standard deviation, σjx, of the mean corrosion rate values. This standard deviation has been taken in this work as an estimate of the uncertainty of the corrosion rate measurements and is reported for each mean value. The standard deviation of the mean of the reported corrosion rate values was obtained through eq 3: σx )

σx

(3) √n where σx is the standard deviation and n is the number of coupons evaluated. The average of all the values of the standard deviation of the mean of the corrosion rates obtained in this work, considering 54 values, was (0.2 mpy in the σjxj range of 0.01-1.53 mpy. In each run a match of six new sandblasted coupons (blanks) were washed together with the test coupons in order to determine the weight loss due to the cleaning process and an average value obtained from the new coupons was used to correct the weight loss of the test coupons. Results and Discussion Table 1 contains experimental corrosion rates of AISI 1010 carbon steel in aqueous solutions of a single alkanolamine, MDEA and DEA without acid gases, and also in aqueous blends of MDEA and DEA at different concentrations, at 393.15 K. As mentioned above, each mean corrosion rate value is given with the corresponding standard deviation of the mean. Table 1 also includes the number of coupons used in each corrosion cell, the phase in which the coupons were placed, and the average mass loss of such coupons. From the results it is observed that the corrosion rate is very low and that it is almost independent of the type of alkanolamine and also of concentration, whether in single amine solutions or in blends of the two studied alkanolamines. Chakma and Meisen22 studied two aqueous systems composed of 30 and 40 mass % DEA, at 373 K, and their results for AISI 1020 carbon steel are included in Table 1. A comparison of our results with those of Chakma and Meisen shows that both sets of results are of the same order of magnitude, although it is to be noticed that at 393.15 K the corrosion rate is higher than that obtained at 373 K. The effect of increasing corrosion rate with increases of temperature can also be observed in the work by Veawab et al. for aqueous solutions of AMP.23 This difference is in agreement with the observation made by Maddox,24 that the corrosion rate in aqueous amine systems increases as the temperature increases. It has been mentioned that the corrosion rate is increased by the presence of acid gases absorbed in the sweetening solutions.24 In order to quantify it, we have investigated the corrosion behavior of seven aqueous solutions of DEA and MDEA,


Figure 7. Experimental corrosion rate of aqueous mixtures in mass fraction of one and two alkanolamines with 0.2 mol of H2S, over AISI 1010 carbon steel, at 393.15 K. ([) MDEA; (b) DEA; (0) MDEA-DEA (mass ratio MDEA/DEA of 3.5:1), (O) MDEA-DEA (mass ratio MDEA/DEA of 2:1). Lines are drawn to show tendencies.

individually, in the concentration range of 15-50 mass % with the addition of 0.2 mol of H2S, at 393.15 K. This amount of H2S in the DEA solution is equivalent to an acid gas partial pressure range of 827-4344 kPa (120-630 psig), and corresponds to a mole fraction solubility range of H2S in the solution of 0.045-0.083.25 For the MDEA solutions 0.2 mol of H2S gives a partial pressure range of 1517-5309 kPa (220-770 psig), which corresponds to a mole fraction solubility range of H2S in the solution of 0.049-0.093.26 Table 2 shows the mean corrosion rates as a function of amine concentration together with the number of studied coupons in each corrosion cell, and the mean mass loss of the coupons. The results show that the presence of H2S increases 1 order of magnitude the corrosion rate compared with the amine solutions of MDEA and DEA without acid gas (see Table 1). For example, the aqueous solution with 30 mass % DEA presents a corrosion rate of 0.24 mpy, whereas the same solution with H2S presents a value of 4.51 mpy. Similar behavior is observed for the aqueous solution of MDEA with 30 mass %. It is possible to observe from the results in Table 2 that the corrosion rate decreases as the amine concentration increases for both DEA and MDEA solutions. Also, the solutions of MDEA present lower values for the corrosion rate than the DEA solutions in the concentration range studied. Figure 5 shows a comparison between the experimental corrosion rates of several aqueous solutions of MDEA and DEA, individually, aqueous solutions of blends of MDEA + DEA with different concentration ratios, and aqueous solutions of MDEA and DEA with 0.2 mol of H2S. The figure clearly shows the large effect on the corrosion rate of adding H2S. Figure 6 presents the plot of the experimental results included in Table 2. Several of the features already discussed above are clearly observed: Aqueous solutions of DEA are more corrosive than the aqueous solutions with MDEA, as shown by DuPart et al.27,28 and confirmed by Yang et al.21 This behavior is in agreement with results on the thermal degradation of alkanolamines by Reza and Trejo,29 who found that MDEA degrades more slowly than DEA, and that such degradation products produce corrosion on the metal. All these different results are congruent with the fact that MDEA sweetening plants present smaller corrosion rates than DEA sweetening plants. The figure shows that the corrosion rate for both types of solutions presents

a minimum near a mass fraction of 40% of the alkanolamine. Therefore, the results of this work indicate that the best solution to be used in plants is a solution with a concentration of MDEA around a mass fraction of 40%, and this proposal is highly congruent with the absorption capacity of this solution since the mole fraction solubility range of H2S, at 313.15 K, is 0.013-0.171 in a pressure range of 1-2360 kPa, and is selective toward H2S with respect to CO2.26 Aqueous solutions of mixed amines have increasingly been used in industrial plants because they can provide greater absorption performance.3–5 These blends offer higher separation capacity with higher energy efficiency in the regeneration step and a smaller required size for the regeneration section. Then, in this work we have studied the corrosion behavior of aqueous blends of MDEA + DEA, at 393.15 K. The corrosion rate results of nine blends, at constant mass ratio of MDEA/DEA ) 3.5/1 and a pressure range of 690-3723 kPa (100-540 psig), are reported in Table 3, where the mean mass loss of the coupons has been given. Further results for eight blends at constant mass ratio of MDEA/DEA ) 2/1 and a pressure range of 1310-2758 kPa (190-400 psig), are reported in Table 4 and plotted in Figure 7 together with the results for the aqueous solutions of MDEA and DEA, individually. Some mole fraction solubility values of H2S in these solutions are known; e.g., for the solution of 45 mass % alkanolamine with a mass ratio of 3.5/1 it is 0.056,30 and for the ratio 2.6/1 it is 0.077,31 both at 1000 kPa. The experimental results indicate that for a given MDEA/ DEA mass ratio the corrosion rate decreases as the overall concentration of the alkanolamines increases from 15 to 60 mass %. It is observed that this behavior can also be related to the increase of the amount of MDEA present in each blend. For example, for the blend with the amine ratio of 3.5/1 having an overall amine concentration of 15 mass %, with 11.7 mass % MDEA, the corrosion rate is highest, whereas for the blend with overall concentration of 60 mass %, with 46.7 mass % MDEA, the corrosion rate is the lowest of the nine studied blends. Similar behavior is observed for the eight blends with a constant amine mass ratio of 2/1. Additionally, it is observed that at low overall amine concentration, e.g., 15-30 mass %, the corrosion rate of the blends is very close to the values observed for the aqueous solutions of DEA of the corresponding concentration,

4732 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Table 5. Experimental Results of Corrosion Rate of Aqueous Solutions in Mass Fraction of 15.6% MDEA + 4.4% DEA (Mass Ratio MDEA/ DEA )3.5/1) as a Function of the Amount of CO2, over AISI 1010 Carbon Steel, at 393.15 K nCO2/mol

P/kPa

mass loss/g

0 0.10 0.15 0.20 0.25 0.30 0.35

0 827 1379 2551 3310 4757 4551

0.0022 0.0031 0.0026 0.0030 0.0025 0.0031

a

liquid phase corrosion rate/mpy 0.34 ( 0.01 0.47 ( 0.03 0.39 ( 0.01 0.45 ( 0.07 0.38 (0.03 0.47 ( 0.08

na

mass loss/g

4 5 6 4 6 6

0.0016 0.0020 0.0016 0.0018 0.0012 0.0020 0.0015

vapor phase corrosion rate/mpy 0.24 ( 0.04 0.31 ( 0.04 0.24 ( 0.01 0.27 ( 0.04 0.19 ( 0.06 0.30 (0.09 0.23 ( 0.10

nb 3 4 4 3 3 3 4

Number of coupons in liquid phase in each corrosion cell. b Number of coupons in vapor phase in each corrosion cell.

Figure 8. Experimental corrosion rate of aqueous mixtures in mass fraction of 15.6% MDEA + 4.4% DEA as a function of the amount of CO2, over AISI 1010 carbon steel, at 393.15 K. (9) Liquid phase; (0) vapor phase. Lines are drawn to show tendencies. Table 6. Experimental Results of Corrosion Rate of Aqueous Mixtures in Mass Fraction of 15.6% MDEA + 4.4% DEA (Mass Ratio MDEA/ DEA )3.5/1) with 0.1 mol of H2S as a Function of the Amount of CO2, over AISI 1010 Carbon Steel, at 393.15 K nCO2/mol

P/kPa

mass loss/g

liquid phase corrosion rate/mpy

na

mass loss/g

vapor phase corrosion rate/mpy

nb

0.05 0.10 0.15 0.18

276 3310 4551 5516

0.0114 0.0117 0.0093

1.73 ( 0.04 1.78 ( 0.04 1.42 ( 0.05

6 5 6

0.0129 0.0125 0.0117 0.0131

1.97 ( 0.10 1.90 ( 0.13 1.79 ( 0.02 1.99 ( 0.08

4 4 2 2

a


whereas at high concentration, e.g., 35-60 mass %, the corrosion rate is similar to the values determined for the aqueous solutions of MDEA. It is interesting to note, Figure 7, that the corrosion rates for both sets of aqueous blends of alkanolamines achieve an asymptotic value at around 45 mass % total amine concentration. In order to further assess the performance of any formulation of alkanolamines, it is necessary to evaluate the corrosion rate of carbon steel at industrial plant conditions and in the presence of two of the most common acid gases. Then, in order to establish clearly the effect of H2S and CO2 over the corrosion behavior, we selected for the study the system of concentrations in mass ratio of MDEA/DEA of 3.5/1 with 15.6% MDEA + 4.4% DEA + 80% H2O, at 393.15 K. In the evaluation of the corrosion rate of this formulation, specimens were located in the liquid and vapor phases. In Table 5 and Figure 8 the corrosion rate is reported for the formulation mentioned above in the presence of different amounts of CO2. The CO2 was studied in the range 0-0.35 mol, which gives the total pressure values included in Table 5. It can be observed that the effect of CO2 on the corrosion rate is negligible in both phases compared with any of the alkano-

lamine blends reported in Table 1. Nonetheless, the results show that the corrosion rate is slightly lower in the vapor phase. For industrial purposes the corrosion rate of carbon steel, in the studied formulation of two alkanolamines, in all the range of CO2 studied is essentially constant. In order to determine the effect of having two acid gases present on the corrosion behavior of both liquid and vapor phases, a fixed amount of 0.1 mol of H2S was added to the same solvent formulation whereas CO2 was added in the range 0.05-0.18 mol, at 393.15 K. The results of corrosion rate are reported in Table 6 and plotted in Figure 9. Table 6 also includes the values of the total pressure in each of the corrosion cells as well as the mean mass loss of the coupons. It can be observed that the presence of H2S increases the value of the corrosion rate, and it is also evident that the corrosion rate is not affected by the increase in the amount of CO2 in the studied formulation of solvents. The vapor phase corrosion rates are slightly higher than those in the liquid phase. From a comparison with the results in Table 5 and Figure 8, it can be concluded that the increment in the corrosion rate is clearly due to the addition of H2S into the formulation of solvents.


Figure 9. Experimental corrosion rate of aqueous mixtures in mass fraction of 15.6% MDEA + 4.4% DEA with 0.1 mol of H2S as a function of the amount of CO2, over AISI 1010 carbon steel, at 393.15 K. (9) Liquid phase; (0) vapor phase. Lines are drawn to show tendencies. Table 7. Experimental Results of Corrosion Rate of Aqueous Mixtures in Mass Fraction of 15.6% MDEA + 4.4% DEA (Mass Ratio MDEA/ DEA )3.5/1) with 0.1 mol CO2 as a Function of the Amount of H2S, over AISI 1010 Carbon Steel, at 393.15 K nCO2/mol

P/kPa

mass loss/g

0 0.05 0.10 0.15 0.20

827 2068 3310 4551 5861

0.0061 0.0117 0.0566 0.0858

a

liquid phase corrosion rate/mpy

na

mass loss/g

vapor phase corrosion rate/mpy

nb

0.93 ( 0.09 1.78 ( 0.04 8.62 ( 0.20 13.07 ( 0.15

6 5 4 6

0.0020 0.0044 0.0125 0.0354 0.0730

0.31 ( 0.04 0.67 ( 0.11 1.90 ( 0.13 5.39 ( 1.04 11.13 ( 0.17

4 2 4 4 2


Figure 10. Experimental corrosion rate of aqueous mixtures in mass fraction of 15.6% MDEA + 4.4% DEA with 0.1 mol of CO2 as a function of the amount of H2S, over AISI 1010 carbon steel, at 393.15 K. (9) Liquid phase; (0) vapor phase. Lines are drawn to show tendencies.

In order to clearly establish the effect of H2S on the corrosion rate, we studied the same solvent formulation with a constant amount of CO2, 0.1 mol, and varyied the amount of H2S in the range 0-0.2 mol. The results are shown in Table 7 and Figure 10. It is very clear that the corrosion rate in both phases has a strong dependence on the amount of H2S loaded into the cells. These results confirm that the contribution of H2S to the corrosion rate is more important than the contribution due to CO2. Several important features observed on the test coupons should be mentioned: the adherence of the corrosion products to the metal surface was more tenacious when the CO2 was

present in the different alkanolamine solutions studied. The coupons that were in contact with both acid gases (CO2 and H2S) were covered with a dark film. It was also observed that when such film was thicker the corrosion rate was smaller. The presence of that film is the result of H2S reacting with iron to form a protective iron sulfide layer, and consequently the general corrosion effect is diminished. Micrographs of a clean coupon, of a coupon from an experimental run in an aqueous blend in mass fraction of 15.6% MDEA + 4.4% DEA without acid gases, at 393.15 K, and of a coupon from a run in the same alkanolamine formulation with

4734 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

in the coupon from the experimental run that included the presence of H2S. According to Ren et al.32 the coarse grain in Figure 12 could be mackinawite (FeS1-x) and the fine grain could be pyrrhotite (FeS1+x). In this study as well as in the work by Ren et al. mackinawite was the main product. From a comparison of Figures 11, 12, and 13 it is possible to establish that the alkanolamines produce a protective film, observed in Figure 12, and the H2S attacks the film, cracking it, as observed in the micrograph in Figure 13. Conclusions

Figure 11. Micrograph of a clean coupon of AISI 1010 carbon steel.

With the semistatic apparatus constructed and the method used in this work, it was possible to obtain corrosion rates as a function of different parameters that are important at the industrial level, such as pressure, temperature, and concentration of both studied alkanolamines, and two acid gases. The results obtained in this work, which are of great interest for sweetening processes, showed that H2S is the acid gas that contributes in an important way to the corrosion phenomena, either alone or in the presence of CO2. The corrosion products form a film on the metal surface, and this film acts as a protective barrier against further corrosion. Acknowledgment This paper is dedicated to Professor Ian A. McLure (Sheffield University, U.K.) to commemorate his 70th birthday. We thank the late Mr. Rau´l Bocanegra and Mr. Apolinar Jimeńez for their continuous help to set up the experimental technique. This research was performed under Research Projects D.00338, D.00406, I.00348, and I.00385 of the Mexican Petroleum Institute. Literature Cited

Figure 12. Micrograph of a coupon of AISI 1010 carbon steel from an experimental run in an aqueous mixture in mass fraction of 15.6% MDEA + 4.4% DEA, at 393.15 K.

Figure 13. Micrograph of a coupon of AISI 1010 carbon steel from an experimental run in an aqueous mixture in mass fraction of 15.6% MDEA + 4.4% DEA with 0.2 mol of H2S, at 393.15 K.

0.2 mol of H2S, at 393.15 K, are shown in Figures 11, 12, and 13, respectively. These micrographs were taken with a Philips environmental scanning electron microscope, Model XL30 ESEM. From the elemental analysis sulfur element was observed

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ReceiVed for reView November 15, 2007 ReVised manuscript receiVed March 4, 2008 Accepted May 3, 2008 IE071557R

Corrosion in Aqueous Solution of Two Alkanolamines with CO2

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