In Situ Raman Spectroscopic Analysis of the Regeneration of

Jun 8, 2007 - Department of Chemistry, Central Michigan UniVersity, Mount Pleasant, Michigan 48859. The generation of ammonium hydrogen sulfate from ...
0 downloads 0 Views 142KB Size
Download PDF
4900

Ind. Eng. Chem. Res. 2007, 46, 4900-4905

PROCESS DESIGN AND CONTROL In Situ Raman Spectroscopic Analysis of the Regeneration of Ammonium Hydrogen Sulfate from Ammonium Sulfate Maheshwari Jariwala, Jessica Crawford, and Dale J. LeCaptain* Department of Chemistry, Central Michigan UniVersity, Mount Pleasant, Michigan 48859

The generation of ammonium hydrogen sulfate from ammonium sulfate is of current interest because of its potential as a process component of bio-based organic acid production from biomass. An organic acid, for example succinic acid, is a chemical product that is used in food, cosmetics, deicing applications, etc. and a potential key carbon building block for numerous other materials. Its production from biomass (corn, etc.) makes it a renewable chemical resource, and the regeneration process described here minimizes the use of additional acid and base value reagents and byproduct (low value salt) waste. In the present work ammonium hydrogen sulfate (acid) and ammonia (base) were regenerated from ammonium sulfate using a thermal cracking method. Raman spectroscopy was used for primary analysis of the ammonium hydrogen sulfate product formed to determine the optimal reaction rate (80 °C/min) and to study the role of ammonium pyrosulfate in this reaction. 1. Introduction Development of greener chemical processing for bio-based organic acid production is important for continued environmentally viable alternative chemical production development. Succinic acid is an example of an organic acid that can be produced through the fermentation of renewable biomass.1 It is a naturally occurring constituent in plant and animal tissues with broad industrial value. Its production from biomass (corn, etc.) makes it a renewable chemical resource. The regeneration process developed here is a potential step in making the bio-based production of succinic acid an overall greener process. This step would minimize the use of mineral acid and base reagents for the organic acid production and decrease the salt byproduct waste. In the present work, ammonium hydrogen sulfate was regenerated from ammonium sulfate using a thermal cracking method. The principle analytical technique, Raman spectroscopy, was used to determine process conditions and analyze the product formed. The application of this regeneration is illustrated by considering succinic acid production as an example. Berglund et al. put forward an efficient process for the production and recovery of pure succinic acid from succinate salt fermentation broth. Cornderived carbohydrates, such as glucose, are anaerobically fermented with a microorganism that is designed to produce succinic acid. Fermentation of sugar is done at biological pH ∼7.0 (requires buffering with caustic because production of succinic acid lowers the pH). After completion of fermentation the succinate salt is crystallized from the broth by protonation (addition of sulfuric acid) because of the low solubility of succinic acid in water. The solid succinic acid is filtered and sent on to application processing.2 The fermentation broth contains spent microbial material, indigestible starches, and spent salt (CaSO4). Being a near zero value waste, this salt would be * To whom correspondence should be addressed. E-mail: lecap1dj@ cmich.edu.

disposed via a landfill. Potentially, ammonia could replace the caustic in the fermentation and ammonium hydrogen sulfate could replace the sulfuric acid in the subsequent purification. Ammonium sulfate would then be the resultant “spent salt”, which could be regenerated to ammonia and ammonium hydrogen sulfate as described here. A method to make “green” the processing step for the organic acid production is to regenerate the acid and base values from the waste salt. Using ammonium hydrogen sulfate (NH4HSO4) as the acid and ammonia (NH3) as the base, the salt would be ammonium sulfate ((NH4)2SO4). By the following reaction (eq 1), the production of the acid and base is accomplished by heating and removal of ammonia gas.

(NH4)2SO4 f NH4HSO4 + NH3(g) v

(1)

The thermal decomposition of ammonium sulfate takes place between 200 and 400 °C. Although numerous studies dating back to the mid-1940s reported this reaction, the results from our preliminary experiments revealed several inconsistencies pertaining to conditions for the process. The literature had some ambiguities about this conversion: some authors reported intermediates (ammonium pyrosulfate) and differing temperatures of reaction. Moreover, the rate of heating necessary for the reaction was not reported. As a background to our current work, we provide a brief literature review supporting our approach. Dixon et al. reported the decomposition of ammonium sulfate at 400 °C and formation of pyrosulfate. The thermal decomposition of ammonium sulfate resulted in formation of sulfamic acid, NH2SO3H, with evolution of ammonia gas. Sulfamic acid then undergoes partial elimination of water, forming ammonium pyrosulfate.3 Halstead et al.4 used infrared spectroscopy for analysis of the products and explained the formation of ammonium pyrosulfate. A platinum apparatus was designed to carry out the experiment; that could possibly bear a catalytic effect on the reaction. The sulfate and hydrogen sulfate ions

10.1021/ie070350v CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4901

may be adsorbed at the platinum surface.5,6 The authors also mentioned the formation of ammonium hydrogen sulfate as a reaction intermediate and further stated that hydrolysis of ammonium pyrosulfate produces ammonium hydrogen sulfate, which is shown in eq 2.4

2NH4HSO4 f (NH4)2S2O7 + H2O

(2)

Kiyoura et al. studied the process of thermal decomposition using thermogravimetric analysis (TGA), differential thermal analysis (DTA), gas chromatography, and X-ray diffraction.7,8 They concluded that ammonium sulfate on heating decomposes to triammonium hydrogen sulfate or ammonium hydrogen sulfate, which subsequently decomposes into ammonium pyrosulfate or sulfamic acid (a brown melt at 205 °C). Hydrogen sulfate is first formed, which then combines with unreacted sulfate giving triammonium hydrogen sulfate. At 150 °C water is released from the ammonium hydrogen sulfate, producing pyrosulfate according to eq 2. At 205-210 °C, they observed formation of a brown melt, thought to be sulfamic acid, NH2SO3H, which on combination with hydrogen sulfate gave pyrosulfate (eq 3).

NH2SO3H + NH4HSO4 f (NH4)2S2O7

(3)

Kambali et al.9 proposed the use of electrical energy to carry out the decomposition reaction. In this scenario, decomposition of ammonium sulfate was carried out in an electrically heated reactor; the rate of heating was accelerated by bubbling carrier gas through the melt at 360 °C for 2 h. It was mentioned that the presence of carrier gas reduces the formation of sulfamic acid and increases the formation of pyrosulfate. They state that pyrosulfate immediately dissolves in water to give hydrogen sulfate. To confirm the formation of acid, Kjeldahl’s method was done and gravimetric analysis was carried out for determination of the sulfate. A differential scanning calorimetry study of the ammonium sulfate has also been done, attempting to indicate the formation of pyrosulfate as the intermediate of the decomposition reaction.10 Several patents are also available on the design of apparatus for the thermal decomposition of ammonium sulfate to ammonium hydrogen sulfate.11-13 All these patents mention the thermal decomposition of ammonium sulfate taking place in the temperature range 230-550 °C, with the optimum temperature being 360-400 °C. The reactant (NH4)2SO4 was generally taken in the form of slurry droplets, which were then sprayed into the thermal reactor in order to achieve the removal of ammonia gas and formation of ammonium hydrogen sulfate. Some of the authors have argued that ammonium pyrosulfate is formed as an intermediate compound.4,7,10 Ammonium sulfate is first transformed into ammonium hydrogen sulfate, which later on dehydration forms ammonium pyrosulfate. Most of the patents have iterated the importance of water in the formation of ammonium hydrogen sulfate.11-13 Water was used in the form of superheated steam as well as in the preparation of ammonium sulfate slurry as the reactant. As evident in the review above, gaps in the literature reports of the process and omissions from the patent claims made the full study of the process necessary. The optimum heating rate at which the reaction takes place is not cited in any of the previous literature. Also, it is not clear whether ammonium pyrosulfate is formed as an end product or as an intermediate. The role of sulfamic acid is also not clear. To study the reaction regenerating ammonium hydrogen sulfate from ammonium

Figure 1. Experimental apparatus for continuous Raman analysis of the decomposition of ammonium sulfate salt by electric heating. The quartz pan was of 5 mm bottom diameter, with a 7 mm opening, and ∼2 cm tall.

sulfate, the in situ analysis of this reaction was carried out. In the present work, we put forward an in situ Raman spectral analysis for the regeneration of ammonium hydrogen sulfate and ammonia from ammonium sulfate. 2. Experimental Section Ammonium sulfate (g99.5% pure, enzyme grade) and potassium sulfate were purchased from Fischer Scientific Company, Hampton, NH. Ammonium hydrogen sulfate (g98% pure), potassium hydrogen sulfate (99.99% pure), and potassium disulfate (pyrosulfate) (99% pure) were purchased from Sigma Aldrich, St. Louis, MO. All were used as purchased without further purification. The in situ Raman reactor design is shown in Figure 1. A milligram quantity of sample was placed in the quartz pan, and then electric current was passed through the Nichrome wire for heating. The current flow used for heating the Nichrome wire was kept constant between 2.30 and 2.60 A using the variable autotransformer (rheostat). The rate of heating was kept constant within the limits of the apparatus, and the temperature reported for a given spectrum is the temperature at the start of data collection. The Raman spectra were collected with a Hololab 5000/RX1 near-infrared (NIR) dispersive Raman spectrometer (Kaiser Optical Systems, Inc., Ann Arbor, MI) with an Invictus 785 nm NIR diode laser operating at 785.714 nm and 450 mW (Kaiser Optical Systems, Inc., Ann Arbor, MI). The system was equipped with single grating f/1.8 Holographic Imaging Spectrograph with a holographic notch filter and HoloPlex grating. The detector was a multichannel CCD array detector. All the temperatures were measured using a temperature probe (Model K, OMEGA Engineering, Stamford, CT; Model No. HH11). The tip of the temperature probe was placed in contact with the substance being melted and/or decomposed to measure the temperature during the experiment. For each of the experiments the individual spectrum was collected after 1 s exposure and the average of five accumulations. Unfortunately, the NH mode at ∼3400 cm-1 is on the edge of the dynamic range of the spectrometer detector. For Raman identification of the products formed on thermal decomposition of ammonium sulfate, the Raman active vibrations are provided in reference here for pure ammonium sulfate and ammonium hydrogen sulfate. The Raman spectrum of ammonium sulfate shows the presence of ν2(NH4+) at 1666

4902

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 2. Ammonium sulfate Raman spectra at 60 °C produced at selected heating rates.

cm-1 and ν4(NH4+) at 1415 cm-1 due to the ammonium ion. The ν1(A1) at 974 cm-1, ν2(E) at 451 cm-1, ν3(T2) at 613 cm-1, and ν4(T2) at 1067 cm-1 are characteristic peaks of the sulfate ion (SO42-) in ammonium sulfate. Without further purification, pure ammonium hydrogen sulfate powder was taken from the bottle and analyzed. The ammonium hydrogen sulfate spectrum has the characteristic peaks ν1(A1) at ∼1010 cm-1, ν2(E) at ∼440 cm-1, ν3(A1) at 1420 cm-1, ν3(E) at 1205 cm-1, ν4(A1) at 878 cm-1, and ν4(E) at ∼600 cm-1 due to hydrogen sulfate ion (HSO4-1). A Perkin-Elmer Pyris 1 thermogravimetric analyzer (TGA) (Perkin-Elmer Life and Analytical Sciences, Inc., Wellesley, MA) was used for heating rate control. Perkin-Elmer N5190283 ceramic pans were used for the experiments. The ammonium sulfate sample was heated at 150 °C for 1 min, increased in temperature at the respective linear heating rate to 525 °C, and then held for 1 min. The program then terminated and the oven was removed, allowing the sample to cool to room temperature. 3. Results and Discussion The goal of this project was to determine the optimum heating rate at which the reaction (eq 1) takes place and to determine the presence and/or role of ammonium pyrosulfate. To study the reaction of regenerating ammonium hydrogen sulfate from ammonium sulfate, an in situ Raman spectroscopic analysis for this reaction was carried out. The present work describes an in-depth study for determination of the reaction rate and explains the role of pyrosulfate. 3.1. Determination of the Importance of Heating Rate on the Reaction. The importance of the rate at which the heat is provided to solid ammonium bisulfate for conversion was evaluated. Various heating rates starting at 20, 60, 80, and 120 °C/min were evaluated. It was observed that, at lower rates, the reaction did not reach completion, and the conversion of ammonium sulfate to ammonium hydrogen sulfate took place at higher temperature. Different gaseous environments such as dry air, water, nitrogen, and ammonia were tested with no conclusive differences observed. Furthermore, the high temperatures that were needed for this conversion process eventually made the role of water (i.e., starting with a solution) insignificant. Thus, all the experiments were performed on dry solid under a nitrogen purge. The four heating rates were selected as a practical range for the apparatus shown in Figure 1. Comparison of the four heating rates is broken down into profiles by temperature. Figure 2 shows the equal temperature spectra of 60 °C obtained at the

Figure 3. Ammonium sulfate Raman spectra at 180 °C produced at selected heating rates.

Figure 4. Ammonium sulfate Raman spectra at 300 °C for selected heating rates.

different overlaid heating rates. All the samples remained in the solid state at this temperature. The S-O bonds of the hydrogen sulfate are evident at 1020 and 1080 cm-1 in both 80 and 120 °C/min. These peaks are shifted to lower energy and correlate to the liquid form of hydrogen sulfate. Shifts in the ∼620 cm-1, ν3(T2), and ∼450 cm-1, ν2(E), peaks also indicate liquid hydrogen sulfate. The main S-O peak of the SO42-, ν1(A1), is also decreasing in these two spectra while remaining prominent in both the 20 and 60 °C/min spectra. From the spectra in Figure 3 taken at 180 °C with the heating rate of 20 °C/min, the structure of ammonium sulfate remained unchanged. Change is evident at the rate of 60 °C/min, but the relative peak intensity of the H-O-S bond (1020, 1080 cm-1) is substantially less than that in the spectra of the 80 and the 120 °C/min samples. This indicates that the 80 and 120 °C/min samples have hydrogen integrating into the S-O bonding. At 300 °C the samples are in the liquid state (melting point of ammonium sulfate reported at 280 °C), but it was found at even at such a high temperature, if the heating rate is low (20 °C/min), the spectral changes were different compared to the spectra taken at a higher heating rate (80 and 120 °C/min) (Figure 4). The H-O-S ν1(A1) and ν1(HSO4-) peaks are present, but the S-O peak at 980 cm-1 is still present. The lower rate samples still contained peaks at ∼980 cm-1 characteristic of the SdO bond in SO42-. The higher heating rates show loss of this peak. Similar results were obtained at 390 °C (Figure 5). Upon cooling of these samples it was confirmed by Raman spectroscopy (described below) that the samples produced at

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4903

Figure 5. Comparing the ammonium sulfate spectra taken at different rates at 390 °C.

higher heating rates of 120 and 80 °C/min predominantly contained the hydrogen sulfate peaks. Similarly, samples produced at the lower heating rates of 20 and 60 °C/min contained the peaks of (NH4)2SO4 as seen in the relative intensities of the 980 cm-1, ν1(A1)/ν1(SO42-) SdO peak and the 1100 cm-1, ν1(A1)/ν1(HSO4-) hydrogen sulfate peak. This experiment confirmed that the rate of heating was a key determinant in this conversion process. At lower heating rates of 20 and 60 °C/min, ammonium sulfate melts at its melting point of 280 °C, but it does not release ammonia and undergo conversion to hydrogen sulfate. An analytical challenge that remained upon completion of this process was the identification of the products that formed during this conversion. Due to lack of commercial availability of ammonium pyrosulfate, potassium salt of pyrosulfate was used to identify the products obtained by decomposition of ammonium sulfate. The potassium salt of pyrosulfate was used to study and understand the Raman spectrum of pyrosulfate since the cation sizes of K+ (∼1.33 Å) and NH4+ (∼1.43 Å) are similar.14 Also, on comparing the Raman spectra, it was found that the key sulfate peaks for the potassium salt and the ammonium salt (at ∼975, 625, and 450 cm-1) were quite similar. Similar comparisons were made for potassium and ammonium hydrogen sulfate peaks. Based on the results obtained, it is established that the pyrosulfate response for both potassium and ammonium salts would be similar. Figure 6 compares Raman spectra of potassium pyrosulfate and the product obtained on decomposition of ammonium sulfate. As seen from the figure, the spectrum of the product matches the potassium pyrosulfate (K2S2O7) except for the peak at ∼3400 cm-1 that is due to ammonium ion. The peak broadening seen in the case of ammonium sulfate decomposition product can be attributed to hydrogen bonding, lattice environment (pure crystalline compound ∼99.9% pure, while the product obtained is glassy and noncrystalline in nature), and opacity of sample (potassium pyrosulfate is a transparent crystal in pure form while the product of decomposition of ammonium sulfate is partially opaque). As previously mentioned, a number of authors have put forward an argument that ammonium pyrosulfate is one of the intermediates of this thermal decomposition reaction, and eventually gets transformed into ammonium hydrogen sulfate upon hydration. This was confirmed when the Raman spectra of the aqueous solution of ammonium hydrogen sulfate and the product of ammonium sulfate decomposition were taken: the

Figure 6. Overlaid potassium pyrosulfate and the decomposition product Raman spectra.

Figure 7. Peak area ratio of the amount of ammonium pyrosulfate produced over the ammonium sulfate that remains as a function of the heating rate. The Raman spectra peak areas are proportional to the quantity of material present in the Raman sampled spot. The 975 cm-1 peak of sulfate and the 742 cm-1 peak of pyrosulfate were selected.

principal peaks were found to be identical. Further, the pKa2 value for the decomposition product at 25 °C calculated from the pH titration curve is 1.9, similar to the pKa2 value for H2SO4, which is 1.92 at 25 °C. It was observed that these compounds have identical S-O peaks (∼980 and ∼1010 cm-1). This information combined with the previous acidic data obtained from pH titration confirms the presence of the acid. All solid samples used for Raman analysis were subjected to Fourier transform infrared (FTIR) spectroscopy for additional confirmation. The data obtained from various techniques corroborate that the end product obtained from the decomposition of ammonium sulfate is ammonium pyrosulfate. Having qualitatively established the mechanism and the importance of heating rate, quantifying the product formed was next. The quantitative description of the reaction shows a steady increase in the amount of pyrosulfate produced as the heating rate is increased. Figure 7 is the plot of peak area ratio from the Raman spectrum of solid product following subjection of solid ammonium sulfate to the temperature program described in the TGA experimental section. Ammonium sulfate was loaded into the TGA, run at the rate prescribed, allowed to cool to room temperature, and then analyzed using the Raman spectrometer. The Raman spectra were baseline corrected and then integrated over the peaks for the S-O bond of sulfate (975 cm-1) and a pyrosulfate band (742 cm-1). The 5 °C/min heating did not produce pyrosulfate above the Raman detection limit and therefore was set to 0% conversion. At 95 °C/min, the 100% conversion was not obtained, because of the physical limitation of the TGA. However, based on the data from in situ Raman apparatus, it could be extrapolated to being complete at 120 °C/min as described.

4904

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 8. Stacked Raman spectra of the conversion of ammonium sulfate to ammonium pyrosulfate (heated at 80 °C/min). The stages are relative time gaps starting from room temperature solid through the melt and back to room temperature at the end of stage 3. Some key peak identities are 750 cm-1 (hydrogen sulfate), 300 cm-1 (melted pure hydrogen sulfate), 850 cm-1 (found in pyrosulfate), 970 cm-1 (ν1(A1) SO42- peak), and 1060 cm-1 (ν1(A1)/ν1(HSO4-)).

3.2. Role of Pyrosulfate. All the data obtained indicated the formation of ammonium pyrosulfate at the end of this decomposition reaction. Thus it became necessary for us to establish the presence of pyrosulfate and to check for what role pyrosulfate plays in this thermal decomposition of ammonium sulfate (eq 1). An in situ Raman spectroscopy method was developed to study this reaction in detail. From our previous findings, we used a heating rate of 80 °C/min for our experiment. The whole process was divided into three stages, starting from the reactant stage (solid ammonium sulfate), going through the melt, and finally ending with the product obtained after cooling. The apparatus as described previously enabled us to detect the whole reaction in a short span of time (5 s) and to monitor the changes directly in the reactor as it happens. The whole conversion process starting with pure ammonium sulfate crystal followed by subsequent heating and cooling of the sample was continuously traced by taking Raman spectra at each individual step during this process. As seen in Figure 8, starting at stage 1, the major peak for ammonium sulfate at ∼980 cm-1, ν1(A1)/ν1(SO42-), decreases in intensity gradually as the heating is increased. At the end of stage 1 the sulfate peak diminishes and, simultaneously, there is the formation of the HSO4- peak at ∼1060 cm-1, ν1(A1)/ν1(HSO4-). This gradual formation of the broad ν1(A1)/ν1(HSO4-) peak was seen after the melting started at about 275 s (melting started at 128 °C). The ν1(A1)/ν1(HSO4-) peak formed is the appearance of S-O with and without the H attached. Also, the peaks of ammonium sulfate at ∼450 and ∼620 cm-1 shifted to higher wavenumbers. The peak at ∼350 cm-1 increased due to the fluid dynamics in the melt at about 65 s after the reaction started. Two new peaks were also observed at ∼750 cm-1 (found in hydrogen sulfate) and ∼850 cm-1 (found in pyrosulfate) that were due to the rapid change from the solid to liquid state. As the decomposition progresses, the sulfate peak at ∼980 cm-1, ν1(A1)/ν1(SO42-) gradually decreases and the peak at ∼1070 cm-1, ν1(A1)/ν1(HSO4-) increases. As shown in Figure 8, the hydrogen sulfate peak ν1(A1)/ν1(HSO4-) is very prominent and the sulfate peak ν1(A1)/ν1(SO42-) is almost extinct. During the second stage, the semisolid ammonium sulfate becomes liquid. There is visible bubble formation around 280300 °C where the solid melts. The Raman spectra obtained during this instance show the formation of ν1(A1)/ν1(HSO4-) peak at ∼1100 cm-1, indicating the hydrogen-bonded S-O (H-

Figure 9. Stacked ammonium hydrogen sulfate Raman spectra of pure crystals at 25 °C (solid), 180 °C (liquid melt), and 210 °C (melt) and at 25 °C (solid) after melting at 185 °C and cooling.

O-S). Simultaneously, the SdO singlet at ∼980 cm-1 (ν1(A1) of SO42-) decreases. After cessation of bubble formation (removal of ammonia gas), but prior to browning of the melt or substantial gas loss due to complete degradation, the system was allowed to cool. The broad H-O-S peak ν1(A1)/ν1(HSO4-) at 1060 cm-1 in the liquid gradually changed to a single sharp narrow peak as solidification occurred. The third and final stage in continuous Raman analysis is cooling. During the cooling process, it was found that the intensities of all the peaks decrease due to the dense packing and overall loss of background intensity. Also, the final product obtained is noncrystalline (glassy) and opaque. The monitoring of the (thermal decomposition) conversion of ammonium sulfate to ammonium hydrogen sulfate provided some very specific information about the structural changes that occur during the reaction. During the experiment, as the melting of the sulfate began, the peak at 970 cm-1 (ν1(A1)) decreased in intensity and the peak at 1097 cm-1 also shifted to 1105 cm-1. The two peaks at 610 and 450 cm-1 also shifted toward higher wavenumbers by about 10-20 cm-1. Around 200 °C the major hydrogen sulfate peaks (broad) started becoming prominent at ∼1100 cm-1 ν1(A1)/ν1(HSO4-). Also, a new peak was formed at ∼300 cm-1 (also found in melted pure ammonium hydrogen sulfate). These changes were attributed to the breaking of the sulfate bonds (S-O) and the formation of the new bonds (H-O-S and S-O-S found in pyrosulfate). The product formed from the thermal removal of ammonia from ammonium sulfate is ammonium pyrosulfate. Ammonium pyrosulfate can be converted to ammonium hydrogen sulfate by reaction with water. Finally, to corroborate one of our initial hypotheses that “the Raman spectrum of a compound changes when it is transformed from one physical state to another”, we collected Raman spectra of ammonium hydrogen sulfate in its different physical forms. The melting of ammonium hydrogen sulfate was also studied in situ with Raman spectroscopy. Pure ammonium hydrogen sulfate was used as received from Aldrich. The melting point of ammonium bisulfate is around 180 °C. The initial melting up to 180 °C does not involve any evolution of ammonia gas; only removal of H2O(g) was observed. The removal of water shows the formation of ammonium pyrosulfate according to eq 2. The Raman spectra of the ammonium bisulfate at different temperatures are shown in Figure 9. In Figure 9, the change is seen in the spectra at wavenumbers around 1020 cm-1. The broad peak seen at ∼600 cm-1 (ν4(E))

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4905

converges into a sharp narrow peak. This is characteristic of H-O-S. The major peak in pure ammonium hydrogen sulfate is a relatively broad peak at 1020 cm-1 with a shoulder (ν1(A1)/ν1(HSO4-)). This peak is transformed into a broad peak at ∼1100 cm-1, and progressively on cooling the original peak of HSO4-1 is reduced to a shoulder formed at 1140 cm-1. The formation of a new peak at 350 cm-1 and the convergence of the two broad peaks at ∼600 cm-1 (ν4(E)) and 420 cm-1 (ν2(E)) into narrow peaks confirms the transformation of solid ammonium hydrogen sulfate into its liquid form. Upon cooling back to solid, the new peak formed at 1140 cm-1 again dissolves to a sharp narrow peak. The products from the rate-determining work and the melt of hydrogen sulfate were the same. 4. Conclusion A temperature controlled minireactor was designed and constructed that enabled rapid solid sample heating and simultaneous Raman spectroscopic analysis. The apparatus can heat solids up to 120 °C/min and collect the entire Raman spectrum in less than 5 s. The thermal decomposition of ammonium sulfate to ammonium hydrogen sulfate and ammonia takes place above the melting point (180 °C) and before decomposition at temperatures greater than 320 °C. Critical to the process is the rate at which heat is provided to the reaction. The most effective heating rate was estimated to be greater than 80 °C/min. Ammonium hydrogen sulfate is formed during this conversion, but since it is formed at a temperature higher than its melting point, it immediately gets converted into ammonium pyrosulfate by giving off water. To summarize, we successfully developed an in situ Raman spectroscopic method to monitor the regeneration of ammonium pyrosulfate and ammonia from ammonium sulfate by thermal decomposition. Ammonium pyrosulfate thus formed gets converted to the desired ammonium hydrogen sulfate when mixed with water. Acknowledgment This project was financially supported by the Michigan Economic Development Emerging Technology Transfer Fund (Gr-335). Project insight was provided by Dr. Dilum Dunuwila,

Diversified Natural Products, Inc., 145 W. 57th St., New York, NY 10019 ([email protected]), and manuscript preparation assistance was provided by Ankur Desai. Literature Cited (1) Diversified Natural Products, Inc., 145 W. 57th St., New York, NY 10019. http://www.dnpworld.com/ (accessed January 2007). (2) Berglund, K. A.; Yedur, S. K.; Dunwila, D. D. Succinic Acid Production and Purification. U.S. Patent 5,958,744, 1999. (3) Dixon, P. Formation of Sulphamic Acid during the Thermal Decomposition of Ammonium Sulfate. Nature 1944, 154, 706. (4) Halstead, W. D. Thermal Decomposition of Ammonium Sulfate. J. Appl. Chem. 1970, 20, 129. (5) Savich, W.; Sun, S.-G.; Lipkowski, J.; Wieckowski, A. Determination of the sum of Gibbs Excesses of Sulfate and Bisulfate Adsorbed at the Pt (III) Electrode Surface using Chronocoulometry and Thermodynamics of the Perfectly Polarized Electrode. J. Electroanal. Chem. 1995, 388, 233. (6) Kolics, A.; Wieckowski, A. Adsorption of Bisulfate and Sulfate Anions on a Pt (III) Electrode. J. Phys. Chem. B 2001, 105 (13), 2588. (7) Kiyoura, R.; Urano, K. Mechanism, Kinetics, and Equilibrium of Thermal Decomposition of Ammonium Sulfate. Ind. Eng. Chem. Process Des. DeV. 1970, 9 (4), 489. (8) Konkoly, T. I. DSC studies of binary inorganic ammonium compound systems. J. Therm. Anal. Calorim. 1983, 27, 275. (9) Kambali, V. R.; Padalkar, S. R.; Dorai, C. S.; Lobo, J.; Damodaran, V. Thermal Decomposition of Ammonium Sulfate. Indian J. Technol. 1979, 17, 352. (10) Fouda, M. F. R.; Amin, R. S.; Abd-Elzaher, M. M. Characterization of Products of Interaction Between Kaolin Ore and Ammonium Sulfate. J. Chem. Technol. Biotechnol. 1993, 56, 195. (11) Welty, A. B., Jr.; Westfield, N. J. Process for decomposing ammonium sulfate into ammonium bisulfate and ammonia. U.S. Patent 3,674,427, July 4, 1972. (12) Bretherick, O. Method for Converting Ammonium Sulfate to Ammonium Bisulfate. U.S. Patent 3,911,092, Oct 7, 1975. (13) Brennan, E. D. Method for the thermal conversion of ammonium sulfate to ammonium bisulfate. U.S. Patent 3,929,977, Dec 30, 1975. (14) Dyekjaer, J. D.; Berg, R. W.; Johansen, H. Ab Initio Calculation of Conformation and Vibrational Spectrum for the Pyrosulfate Ion. J. Phys. Chem. A 2003, 107, 5826.

ReceiVed for reView March 7, 2007 ReVised manuscript receiVed April 27, 2007 Accepted May 1, 2007 IE070350V