SOUR WATER ANALYSIS USING A DUAL- COLUMN HEADSPACE SAMPLING SYSTEM

Transcription

1 SOUR WATER ANALYSIS USING A DUAL- COLUMN HEADSPACE SAMPLING SYSTEM Andy Burgess, David Greaves, Daniel Murphy and Sam Rolley Applied Analytics, Inc. 29 Domino Drive, Concord, MA KEYWORDS Sour Water, Headspace, H 2 S, NH 3, Spectroscopy ABSTRACT In the petroleum refining process one of the main byproducts is wastewater rich in hydrogen sulfide and ammonia, commonly referred to as sour water. These chemicals must be removed to prevent contamination and corrosion. A process called Sour Water Stripping accomplishes this removal. Accurate measurement of hydrogen sulfide and ammonia concentrations at the outlet of the stripper provides critical process validation information and allows for corrective action in the event of stripper failure. To carry out this measurement in real time, a dual-column headspace sampling system is utilized in conjunction with a UV-diode array based spectrometer. This system allows for the simultaneous measurement of hydrogen sulfide and ammonia with a single device. A detailed discussion of sour water measurement benefits, the unique features of headspace sampling, and experimental results are presented. INTRODUCTION Sour water is defined as any refinery process water that contains hydrogen sulfide (H 2 S). With such a broad definition, sources of sour water vary significantly. Atmospheric crude columns and vacuum crude towers produce sour water from condensed stripping steam removed by overhead condensing systems. Steam cracking and fluid catalytic cracking units produce sour water as condensates from steam used in injection stripping and aeration [1]. Refineries create sour water as a byproduct in many of their processes. Unfortunately, sour water contains ammonium salts such as ammonium bisulfide and ammonium Session 7.3: Page 1

2 carbonate. These ammonium salts can cause several problems. In an extensive study of corrosion rates in sour water, a number of variables were considered including ammonium bisulfide concentration, velocity, H 2 S partial pressure, temperature, chloride concentration, hydrocarbon content, and chemical treatments. It was confirmed that the concentration of ammonium bisulfide is the most influential variable contributing to corrosion [5]. Ammonium salts can also settle on orifices causing plugging and increase maintenance concerns. As a result, refineries need to remove the hydrogen sulfide and ammonia (NH 3 ) from the water to avoid the formation of these ammonium salts when the water is reused for other plant processes. To remove the H 2 S and NH 3, a sour water stripper is often utilized. The plant operators need to know how efficient the stripping process is and if the water is suitable for reintroduction. TECHNOLOGY To address the need brought to light in the introduction, Applied Analytics developed a system utilizing the OMA-300 for the measurement of H 2 S and NH 3 in sour water. It utilizes an ultraviolet-visible full-spectrum spectrometer and a dual column headspace sampling approach. The combined system offers many benefits. It allows a large dynamic range in the measurement of the analytes due to flexibility in wavelength selection. Cross interference from moisture is avoided, because the measurement is being done in the UV- VIS region where water doesn t absorb light. Finally, consumables and/or reagents are not necessary for operation. The technology is broken down into two sections for ease of explanation: an analytical section and a sample conditioning section. ANALYTICAL The analytical configuration consists of the following equipment: a xenon flash lamp, an ultraviolet-visible full-spectrum spectrometer, collimating lenses, fiber optic cables, and a flow through cell. This layout can be seen in Figure 1. Absorbance is defined using the following equation: A = log 10 (I 0 /I) (1) Where: A = Absorbance I 0 = Light In I = Light Out Session 7.3: Page 2

3 FIGURE 1. ANALYTICAL HARDWARE The measurement algorithm used for analysis is a classical least squares regression of Beer s Law [2]: A l = e l bc (2) Where: A = Absorbance l = Wavelength e = Molar Absorptivity b = Path Length c = Concentration The spectrometer measures I 0 during its zero calibration and I when the analyte is in the flow through cell of path length b. e l is found by introducing a known concentration of the analyte into the flow-through cell and is saved for use in subsequent readings to determine unknown concentrations of the analyte. This would be adequate if the absorbance at a given wavelength could be attributed to only one component. However, this isn t the case with H 2 S and NH 3 in the ultraviolet region. At each wavelength in the UV where NH 3 absorbs, so does H 2 S. This is illustrated in the following spectra (Figures 2 & 3). Session 7.3: Page 3

4 H2S (gas) Absorbance Curve ) U (A e c n a rb o s b A Wavelength (nm) FIGURE 2. H 2 S ABSORBANCE CURVE NH3 (gas) Absorbance Curve ) U 0.7 (A e c 0.6 n a rb 0.5 o s b 0.4 A Wavelength (nm) FIGURE 3. NH 3 ABSORBANCE CURVE Session 7.3: Page 4

5 Therefore, in this case the equation must be extended to the following: A lx = e 1lx * b * c 1 + e 2lx * b * c 2 (3) A ly = e 1ly * b * c 1 + e 2ly * b * c 2 (4) Where lx could be 217nm and ly could be 220nm, for example (the over determination is omitted for simplicity). The final concentration readings are determined by solving for c 1 and c 2 using the previous equations. Accordingly, before the gas can be analyzed it must be coerced into the vapor phase. This is accomplished by the headspace sample conditioning system. SAMPLE CONDITIONING The heart of the headspace system is the stripping column (Figure 4). Its dimensions are 24 in height by 2 in diameter. The column is sealed with gaskets that are held together by sanitary clamps. The column is filled with Pall rings for efficient mass transfer between the liquid sample and the carrier gas. The column is heated with three band heaters and controlled with a thermocouple and a temperature controller. The flow rates of the carrier gas and the sample liquid are controlled by variable area flow meters with integrated flow control valves. A backpressure regulator regulates the pressure in the stripping column and the flow through cell. During operation, the liquid sample enters through a fitting on the top of the column and exits through a fitting on the bottom of the column. A carrier gas is introduced through another fitting on the bottom of the column and exits through a fourth fitting on the top of the column. This establishes a countercurrent flow between the liquid sample and the carrier gas. The Pall rings allow for efficient mass transfer between the liquid sample and the carrier gas by greatly increasing the total internal surface area of the column. After the carrier gas leaves the column it flows to the flow through cell for measurement. The headspace system is based on the principle of Henry s Law: C = k*p gas (5) Where: P = Partial Pressure k = Henry s Law Constant c = Concentration Henry s Law states that the amount of a gas dissolved in a solution at a given temperature is directly proportional to the partial pressure of the gas above the solution [3]. Session 7.3: Page 5

6 FIGURE 4. HEADSPACE SAMPLE CONDITIONING SYSTEM Therefore, given constant conditions such as temperature, pressure, carrier gas flow rate and liquid sample flow rate the headspace gas being measured can be correlated to the concentration of the liquid sample. Although the headspace sampling method can be used in a variety of applications, it is most advantageous in measurements of components with high volatility in opaque samples that have interfering components with low volatility. Sour water is a good example of such a stream, since it can often contain phenols that were formed from reactions between steam and cyclic hydrocarbons. Phenols have a very strong absorbance in the low UV region ( nm), which would make measurement of the H 2 S or NH 3 in the liquid phase very difficult. Fortunately, phenols also have a much higher boiling point (see Table I) than H 2 S or NH 3. As a result, only very low phenol concentrations can exist in the headspace gas, thus making the measurement easier by minimizing the interfering component. Session 7.3: Page 6

7 TABLE I. BOILING POINT COMPARISON Phenol H 2 S NH 3 Boiling Point EXPERIMENT The objective of the experiment is to quantitatively demonstrate the typical accuracy and stability that the headspace sampling system offers when correlated with a liquid sample of H 2 S and NH 3 in water. PARAMETERS A number of parameters need be determined before starting the experiment. The proper liquid sample flow rate shall be constant. The liquid sample flow rate was experimentally determined by filling the column with water and draining it with just gravity as a motive force. The column was packed with Pall rings, as it would be in normal operation, during this test. The time in which it takes the water to fully evacuate the column was recorded. The volume of water as it leaves the column was captured in a graduated beaker, thus allowing the determination of the volume of the column with Pall rings installed. That information was then used to find the volumetric flow rate (V/t). Since Henry s Law is dependent upon temperature, it was controlled and kept constant at a temperature that promotes low solubility of the gas in the water. The dependence of the solubility on temperature is described by the following equations [4]: k = k t * exp [ (-enthalpy/r) * ((1/T) (1/T t )) ] (6) (-d*ln(k))/(d*(1/t)) = enthalpy/r (7) Where k = Henry s constant of a specific gas (M/atm) k t = Henry s constant of a specific gas at a temperature of K (M/atm) R = universal gas constant (8.314 (J/mol*k)) T = System temperature (K) T t = K (-d*ln(k))/(d*(1/t)) = Temperature Dependence (K) TABLE II. HENRY S LAW CONSTANT & TEMPERATURE DEPENDENCE H 2 S NH 3 Henry's Law Constant (M/atm) Temperature Dependence (K) Session 7.3: Page 7

8 Table II gives a value for temperature dependence, which according to Equation 7 is equal to enthalpy/r. That value was substituted into Equation 6 to find the Henry s Law Constant at different temperatures. Then Equation 5 was used to predict the concentration of the gas in the water. TABLE III. HENRY S LAW CONSTANT FOR H 2 S AT DIFFERENT TEMPERATURES k (M/atm) k t (M/atm) E/R (K) T (K) T t (K) TABLE IV. HENRY S LAW CONSTANT FOR NH 3 AT DIFFERENT TEMPERATURES k (M/atm) k t (M/atm) E/R (K) T (K) T t (K) Tables III and IV show that as temperature increases, the Henry s Law constant and the concentration of the dissolved gas in the water decreases. Therefore, it is beneficial to choose a high temperature for the column set point because it forces more of the dissolved gas from the liquid phase into the vapor phase. A larger concentration in the vapor phase results in more absorbance in the path length of the flow cell. Furthermore, the accuracy of the analyzer increases as absorbance increases. Keeping this in mind, along with practical limitations of the setup, 50 degrees C was chosen for the set point of the column. The carrier gas flow rate was also kept constant. The ideal carrier flow rate was set based on the highest flow rate that was achieved without decreasing the concentration of the analyte in the stripping carrier gas. This was determined experimentally by analyzing the carrier gas at several carrier gas flow rates and recording the results. Session 7.3: Page 8

9 The analyzer (UV absorbance) was calibrated on gas standards of H 2 S in N 2 and NH 3 in N 2. These spectral standards were used to establish an extinction coefficient for each gas at each wavelength in the range of interest. This calibration was done prior to water testing. It is also important to note that ph plays an important role in sour water stripping. At low ph levels near 5.5, NH 3 tends to be held in solution and H 2 S tends to be more volatile. However, at higher ph values near 9.0, NH 3 is more volatile and H 2 S tends to stay in solution [1]. For the purposes of these experiments a constant ph was assumed. PROCEDURE A sample of water without H 2 S or NH 3 was introduced into the column. The temperature of the column, the carrier gas flow rate and the liquid sample flow rate were held constant at their optimal values. This sample was used as a zero reference by the analyzer. A sample of water with a known concentration of H 2 S was prepared in a reservoir large enough to hold the volume required to run a 1-hour stability test. The temperature of the column, the carrier gas flow rate and the liquid sample flow rate were held constant at their optimal values. The concentration log function on the analyzer was activated and the system was left to run for 1 hour. The start time and the end time were recorded. At the end of the 1-hour run, a reservoir of clean water was connected to wash the system. Second, third, and fourth samples of water with different known concentrations of H 2 S were prepared and introduced to the system in the same manner as the first. A 1-hour run was not required as these samples were for the purpose of testing accuracy, not stability. After the system was washed with clean water again, a sample of water with a known concentration of NH 3 was prepared in a reservoir large enough to hold the volume required to run a 1-hour stability test. The concentration log function on the analyzer was activated and the system was left to run for 1 hour. The start time and the end time were recorded. At the end of the 1-hour run, a reservoir of clean water was connected to wash the system. Second, third, and fourth samples of water with different known concentrations of NH 3 were prepared and introduced into the system in the same manner as the first. RESULTS The data retrieved from the concentration log file was used to show the stability and accuracy of the headspace system using H 2 S and NH 3 concentrations in water. Session 7.3: Page 9

10 TABLE V. H 2 S STABILITY TABLE Test Duration (hours) 1 Reading Stored (1/sec) 60 Average Value (PPM) Maximum Value (PPM) Minimum Value (PPM) Standard Deviation (PPM) 6.54 Slope (PPM/reading).36 Slope * 60 (PPM/hour) 2.35 H 2 S Stability Concentration (ppm) Time (seconds) FIGURE 5. H 2 S STABILITY GRAPH The first set of data is the stability of H 2 S in water over 1 hour. The average value over the 1-hour run was determined to be , with the maximum reading at and the minimum reading at The standard deviation of the data set is 6.54 and it has a slope of.36. TABLE VI. NH 3 STABILITY Test Duration (hours) 1 Reading Stored (1/sec) 60 Average Value (PPM) Maximum Value (PPM) Minimum Value (PPM) Standard Deviation (PPM) 4.09 Slope (PPM/reading) 0.02 Slope * 60 (PPM/hour) 1.10 Session 7.3: Page 10

11 NH 3 Stability ) m p (p n tio tra n e c n o C Time (seconds) FIGURE 6. NH 3 STABILITY GRAPH The second set of data is the stability of NH 3 in water over 1 hour. The average value over the 1-hour run was determined to be , with the maximum reading at and the minimum reading at The standard deviation of the data set is 4.09 and it has a slope of TABLE VII. H 2 S ACCURACY TABLE Liquid Concentration Sample 5 (% of Cal Sample) 43.5 Liquid Concentration Sample 6 (% of Cal Sample) Liquid Concentration Sample 7 (% of Cal Sample) Liquid Concentration Sample 8 (% of Cal Sample) 100 Gas Concentration Sample 5 (PPM) 1300 Gas Concentration Sample 6 (PPM) 2600 Gas Concentration Sample 7 (PPM) 3800 Gas Concentration Sample 8 (PPM) 4200 Linearity R 2 Value.9996 The third set of data is the accuracy of H 2 S in water. The gas concentration reading for Sample 1 was 1300ppm. The gas concentration reading for Sample 2 was 2600ppm. The gas concentration reading for Sample 3 was 3800ppm. The gas concentration reading for Sample 4 was 4200ppm. The linearity (R 2 value) of the gas concentration readings was Session 7.3: Page 11

12 Gas Concentration (PPM) H2S Accuracy R² = Liquid Concentration (% of Cal Sample) FIGURE 7. H 2 S ACCURACY GRAPH TABLE VIII. NH 3 ACCURACY TABLE Liquid Concentration Sample 5 (% of Cal Sample) 25 Liquid Concentration Sample 6 (% of Cal Sample) 50 Liquid Concentration Sample 7 (% of Cal Sample) 75 Liquid Concentration Sample 8 (% of Cal Sample) 100 Gas Concentration Sample 5 (PPM) Gas Concentration Sample 6 (PPM) Gas Concentration Sample 7 (PPM) Gas Concentration Sample 8 (PPM) Linearity R 2 Value The fourth set of data is the accuracy of NH 3 in water. The gas concentration reading for Sample 5 was ppm. The gas concentration reading for Sample 6 was ppm. The gas concentration reading for Sample 7 was ppm. The gas concentration reading for Sample 8 was The linearity (R 2 value) of the gas concentration readings was Session 7.3: Page 12

13 Gas Concentration (PPM) NH3 Accuracy R² = Liquid Concentration (% of Cal Sample) FIGURE 8. NH 3 ACCURACY GRAPH CONCLUSIONS Refineries create sour water as a byproduct in many of their processes. Sour water has been proven to cause corrosion and plugging due to the presence of ammonium salts, such ammonium bisulfide. Therefore, it is beneficial for refineries to remove the hydrogen sulfide and ammonia so the water can be reused or discarded. This is often done through a stripping process. Measurement of the H 2 S and NH 3 in sour water occurs afterwards to ensure the effectiveness of the stripper before the water is sent for reuse. The measurement is not particularly easy without advanced sampling techniques. Therefore, a sampling technique was developed that used some of the same principles of stripping that the refineries themselves use to remove H 2 S and NH 3 from the sour water stream. After the concentrations of each of these chemicals are measured in the gas phase, they are correlated back to a liquid concentration. In conclusion, the results from the stability test demonstrate a drift of less than 0.5% full scale. Also, the headspace system yields a linearity of or better when comparing carrier gas to sample stream analyte concentrations. Coupled with the benefits mentioned in the introduction, these analytical results make the UV-VIS full-spectrum spectrometer Session 7.3: Page 13

14 with the headspace sample conditioning system an ideal solution for the measurement of H 2 S and NH 3 in sour water. REFERENCES 1. Armstrong, Tim, Scott, Bruce, Taylor, Kin, and Gardner, Art, Sour Water Stripping, Today s Refinery, June, Mark, Howard, Workman, Jerome, Classical Least Squares, Part 1: Mathematical Theory, Spectroscopy, May, Zumdahl, Steven S., Zumdahl, Susan A., Chemistry 6 th Edition Houghton Mifflin, Boston, Massachusetts, Sander, Rolf, Compilation of Henry s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry, Max Planck Institute of Chemistry, April Horvath, Richard J., Lagand, Vishal V., Srinivasan, Sridhar, Kane, Russel D., Prediction and Assessment of Ammonium Bisulfide Corrosion Under Refinery Sour Water Service Conditions Part 2., NACE International, Session 7.3: Page 14