Evaluation of Batch Vs Continuous Ammoniacal Nitrogen Stripping
Arvind Madalgi
AGM Process Engineering,
Coromandel International Ltd, Sarigam.

Samarpita Chakraborty
Asst. Manager Process Engineering
Coromandel International Ltd, Sarigam.

In this work, the case of a real industrial problem is selected to evaluate the efficiency, operability and economic performance of NH3-N Stripping by Batch vs Continuous methodology. The removal of ammoniacal nitrogen(NH3-N) is investigated by both the methods. The alternative method of continuous stripping by packed bed column was first modelled in the professional software environment of ASPEN Plus. Real time data was collected from plant trials for both the methods for comparison. The same is considered for design of continuous packed bed stripping column. It is proved that continuous steam stripping by packed bed shows better operability and economic performance than batch-wise steam stripping operation. Moreover the ammoniacal nitrogen can be recovered in the distillate making it a more environment friendly method thus improving the sustainability. The work presented in this paper was performed at Coromandel International Ltd, Sarigam. This study presents pilot-scale testing and commercial scale design of continuous stripping column performed at the facility.

Why Stripping?
Ammonia stripping is a simple desorption process used to lower the ammonia content of a wastewater stream. Some wastewaters contain large amounts of ammonia or nitrogen containing compounds that may generate ammonia. It is often easier and less expensive to remove nitrogen from wastewater in the form of ammonia than to convert it to nitratenitrogen before removing it.[1]

Ammonia (a weak base) reacts with water (a weak acid) to form ammonium hydroxide. In ammonia stripping, lime or caustic is added to the wastewater until the pH reaches 10.8 to 12 which converts ammonium hydroxide ions to ammonia gas according to the following reaction:


where [NH3] is the molecular ammonia concentration, [NH3 + NH+4] is the total ammonia concentration, [H+] is the hydrogen ion concentration, and Ka is the acid ionization constant. Besides that, pKa can be expressed in terms of temperature. [2]

General Stripping Process
Ammoniacal nitrogen stripping can be performed as either a batch or a continuous process. Recent development in ammonia removal by ammonia stripping fall into the following categories: ammonia stripping reactor modifications like Packed bed stripping column (Air or Steam), Semi batch Jet loop reactor, Water-sparged aero cyclone reactor, Rotating packed bed reactors as well as membrane contactor, membrane distillation, ion exchangestripping loop, and microwave assisted ammonia stripping. [3]

Ammonia stripping with air works well with wastewater that has ammonia content between 10 to 100 mg/l. For higher ammonia content (more than 100 mg /l), it maybe more economical to use alternate ammonia removal techniques such as steam stripping or biological methods.[1]

In the stripping process, the wastewater is brought into contact with high amount of hot gas, vapor or steam to bring the volatile organic and/or inorganic contaminants into the gas/vapor phase from the aqueous phase. In the case of air stripping, water is usually also transferred into the gas phase, which lowers the temperature of the hot air, and therefore lowers volatility of the impurities. Impurities are removed from the gas phase used for stripping and the gas/air can be used again.[4]

Packed or tray columns can be properly used to remove the polluting compounds. The process wastewater is fed at the top, and the air or steam is fed at the bottom. The pollutants can be typically found in the top product that can also be called distillate. The treated wastewater is removed at the bottom. It is advisable to use the heat content of the removed warm, treated wastewater to preheat the cold fed wastewater.[5] The same technology of packed bed stripping column with random packing has been incorporated at the facility.

Operational parameters affecting stripping process:

1] Temperature: Temperature has been proven to have a significant impact on the performance of ammonia stripper. This is because the solubility of ammonia in water is governed by Henry's law. In Henry's law, the constant of gas relies on solute, solvent, and temperature. Generally, higher efficiency ammonia removal can be obtained at higher temperature

2] pH: An optimum pH is required to strike a balance between process efficiency and economic cost. It was found that when the pH exceeded 10.5, the removal efficiency was insignificant because pH no longer be affected by the ionization balance between molecular ammonia and ionic ammonium, but the cost incurred rose significantly due to the additional caustic consumption required to increase the pH levels.

3] Steam to water ratio: Steam to water ratio is an important parameter that has an impact on the removal rates of ammonia in water. Mass transfer of ammonia into the steam is affected by the variance between ammonia concentration level in liquid form and vapor phase.


Figure1: Pilot trial observations with Packed bed Steam Stripping column

The case study described below aims at focussing on the aspects of batch vs continuous ammonia stripping and how packed bed columns are more efficient for controlling the temperature, pH, steam to water ratio, and the best selection of these optimized operating parameters to achieve higher efficiency.

Conventional treatment method - Batch treatment by direct steam purging in agitated vessels.
The batch treatment of ammoniacal nitrogen stripping consisted of agitated vessels for receiving the waste water to be treated. Direct steam purging was done using low pressure steam at 3-3.5 kg/cm2 and the NH3-N concentration was decreased. The temperature and steam flow rate exhibited a significant impacts on the ammonia removal rate. But this method appeared to be less effective than packed bed columns.

From the analysis of the batch-wise data, it was found that the average reduction in NH3-N concentration was only 81 percent. Moreover it was found that the temperature of the discharged water was quite high and a huge scope existed for optimization of energy for the same. The average steam norms for condensate treated in batch-wise stripping was 0.27 kg steam/kg condensate which was much high and had huge scope for optimization.

Continuous treatment of NH3-N by using packed bed stripping column by counter current contact of steam
On basis of the model simulated using ASPEN Plus, pilot setup by inhouse system was designed and scheme was finalized. The pilot column fabricated was 1.7 meters in height with internal diameter of 200 NB and packed with lessing ring(surface area of 600 m2/m3 and dry packing factor of 6500 m-1). It was observed during the pilot trials that the pilot model satisfied the design intent and could be replicated in commercial scale.

Scale up, erection and commission of equipments for large scale was done treatment for NH3-N removal by packed bed NH3-N stripping column. The commercial scale ammonia stripping column was 7 meters in height with internal diameter of 600 NB and packed with lessing ring (surface area of 600 m2/m3 and dry packing factor of 6500 m-1). The schematic of the setup is as shown in Fig.3. Commissioning of the stripper column was conducted at different flowrates and the design intent was achieved. Based on the monitoring of continuous packed bed stripping column, the following results were obtained as shown in Fig.2.


Figure 2: Continuous observation and monitoring of Packed Bed Stripper Column


Figure3: Scheme for NH3-N removal by Packed bed Steam Stripping column

Results
From the observations as shown in Fig.2, the steam norms for the condensate treated in continuous packed bed stripping column decreased to 0.124 kg steam/kg condensate i.e. a reduction of 0.15 kg steam/kg condensate as compared to batch process. The space requirement has decreased as the stripping column has been retrofitted in the existing facility. A total savings of 1.03 crores/annum has been achieved considering the steam savings.

Conclusions
Ammonia stripping performance is highly dependent on the column temperature and steam to wastewater ratio. Efficiency decreases significantly as column temperature decreases. The maximum ammonia transfer efficiency of about 96 percent was achieved at pH 12 in continuous packed column which was much higher compared to 81 percent efficiency achieved in batch-wise stripping of ammonia. The success of an ammonia stripping process is greatly dependent on temperature, pH, and steam to water ratio which was easier to control in continuous packed bed reactor. As such, the selection of optimized operating parameter is vital for the ammonia stripper to achieve higher efficiency. Thus concluding continuous steam stripping column for NH3-N removal is highly efficient, easy to operate, and leading to cost effective remediation.

References
1. Stripping, Ammonia. "Technology Fact Sheet." (2000).
2. Wastewaters, Ammonia Removal from Highstrength. "G. B. Wickramanayake, Ph. D., PE Senior Engineer ENVIRON Corporation Princeton, New Jersey."
3. Kinidi, Lennevey, Ivy Ai Wei Tan, Abdul Wahab, Noraziah Binti, Khairul Fikri Bin Tamrin, Cirilo Nolasco Hipolito, and Shanti Faridah Salleh. "Recent Development in Ammonia Stripping Process for Industrial Wastewater Treatment." International Journal of Chemical Engineering 2018 (2018).
4. Toth, A. J., and P. Mizsey. "Comparison of Air And Steam Stripping: Removal Of Organic Halogen Compounds From Process Wastewaters." International Journal Of Environmental Science And Technology 12.4(2015): 1321-1330.
5. Hassan, Sardar Q., and Dennis L. Timberlake. "Steam Stripping And Batch Distillation For The Removal And/Or Recovery Of Volatile Organic Compounds From Industrial Wastes." Journal of the Air & Waste Management Association 42.7 (1992): 936-943.