Final Report: Developing a Vacuum Distillation – Acid Absorption System for Recovery of Ammonia from Dairy ManureEPA Grant Number: SU835723
Title: Developing a Vacuum Distillation – Acid Absorption System for Recovery of Ammonia from Dairy Manure
Investigators: Tao, Wendong , Das, Jonathan Masih , Caven, Alexander , Ukwuani, Anayo , Abrams, Neal , Anwar, Sohaib
Institution: The State University of New York
EPA Project Officer: Hahn, Intaek
Project Period: August 15, 2014 through August 14, 2015
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2014) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Chemical Safety , P3 Challenge Area - Safe and Sustainable Water Resources , P3 Awards , Sustainable and Healthy Communities
The specific objectives are to 1) evaluate the combined effect of temperature and vacuum on ammonia volatilization from anaerobically digested dairy manure as well as liquid portion (filtrate) of digested and undigested dairy manure, which have different concentrations of total and dissolved solids; 2) design a vacuum distillation – acid absorption system for production of ammonium sulfate granules with dairy manure; 3) construct a pilot-scale vacuum distillation – acid absorption system and develop operational parameters; and 4) perform a farm-scale economic analysis of the developed technology. A laboratory vacuum distillation – acid absorption assembly was used to evaluate ammonia volatilization kinetics and ammonium sulfate formation in vacuum distillation of ammonia from digested dairy manure at four combinations of bubble point temperature and vacuum (absolute pressure), i.e., 100 oC 760 torr, 90 oC 557 torr, 80 oC 392 torr, and 70 oC 252 torr. Ammonia volatilization in distillation at the normal boiling point without vacuum was also compared between filtrate of digested and undigested dairy manure that have different concentrations of dissolved solids. A stainless steel kettle reboiler equipped with a demister and connected to a gas absorption column by a vacuum pump was designed for batch operation of the vacuum distillation – acid absorption process, featuring operational ease, technical simplicity, and low cost compared with other ammonia recovery methods. A pilot-scale vacuum distillation – acid absorption system is under construction, which can hold up to 30 L of digested dairy manure. This pilot-scale ammonia recovery system will be operated in the coming months to investigate ammonia volatilization kinetics and ammonium sulfate production capacity at four different feed depths in the still. Cost benefit assessment was performed to assess economic sustainability of the ammonia recovery technology, taking a largesize dairy farm as example. The base option was as it was, without ammonia recovery. Alternatives were to recover ammonia in the four combinations of vacuum and temperature, which allow different recirculation rates of digester effluent.
No significant differences in ammonia recovery were found between the distillation experiments with filtrate of undigested and digested manure. The higher dissolved solids content in the digested manure only slightly retarded ammonia volatilization. 96.5-99.5% of ammonia was stripped in 3 h with an ammonia half-life of 1.4-1.6 h. The cumulative ammonium sulfate formed upon complete volatilization of ammonia was 3.70-3.87 g/L manure filtrate. High-purity ammonium sulfate (≥98%) was produced as clear granules by cooling the acid solutions pre-saturated with ammonium sulfate.
Aqueous ammonium is converted to free ammonia with increasing temperature. In the experiments with anaerobically digested dairy manure, free ammonia concentration peaked after less than 1 h of heating. Free ammonia was stripped rapidly out of dairy manure, with an average liquid-phase ammonia mass transfer coefficient of 21 mm/h. After 3 h of vacuum distillation, ammonium sulfate formed was 6.13-7.92 g/L manure. Upon 5 h of vacuum distillation, ammonium sulfate formed was 6.46-8.13 g/L manure. Ammonia solubility was zero in all the combinations of temperature and vacuum. Ammonium sulfate production rate varied with total ammonia concentration initially in the digested manure samples. There were no significant differences in ammonia volatilization kinetics among the 4 combinations of temperature and vacuum. The insignificant differences among the combinations of temperature and vacuum could be attributed to the high fraction of free ammonia in total ammonia (more than 94%) in the tested ranges of temperature and vacuum.
Vacuum distillation at a lower temperature and stronger vacuum creates greater economic profitability. Ammonia recovery at the lowest bubble point temperature and vacuum generates the greatest net present value ($10.9 million) and benefit/cost ratio (1.3) for the example dairy farm at a usable life of 10 years and an 8% discount rate. With less than $9000 of initial capital cost, an ammonia recovery system can create 3 jobs at this large-size dairy farm. Labor cost accounts for 64-68% of the total operational cost.
It is technically and economically promising to recover ammonia from dairy manure by coupling vacuum distillation and acid absorption. Ammonia volatilization kinetics in distillation was similar across the 4 combinations of temperature and vacuum. However, distillation under lower bubble point temperature and vacuum will generate greater economic profitability because a larger amount of digester effluent could be drawn for ammonia recovery when the returned hot manure eliminates the need to heat anaerobic digesters. It is necessary to further test ammonia volatilization in distillation under even lower bubble point temperature and vacuum in Phase II.
When the stripped ammonia was absorbed to a sulfuric acid solution in the laboratory experiments, ammonium sulfate formed was either dissolved in the solution due to water vapor trapped in the solution or partially in granules. The design and operational parameters, especially for the absorption column, have to be further addressed in Phase II.
This P3 team has accomplished the anticipated outputs/outcomes specified in the original proposal and will continue to work on the remaining tasks as planned, including determining the optimum feed depth using the pilot-scale system and updating the economic analysis with experimental results from the pilot-scale system.
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|| Tao W, Ukwuani AT. Coupling thermal stripping and acid absorption for ammonia recovery from dairy manure: ammonia volatilization kinetics and effects of temperature, pH and dissolved solids content. Chemical Engineering Journal 2015;280:188-196.