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Ammonia is one of the chemical constituents of industrial waste that must be removed in a treatment plant before the waste can safely be discharged into a river or estuary. Ammonia is normally present in wastewater as aqueous ammonium hydroxide \(\left(\mathrm{NH}_{4}^{+} \mathrm{OH}^{-}\right) .\) A two- part process is frequently carried out to accomplish the removal. Lime (CaO) is first added to the wastewater, leading to the reaction $$\mathrm{CaO}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}^{2+}+2\left(\mathrm{OH}^{-}\right)$$ The hydroxide ions produced in this reaction drive the following reaction to the right, resulting in the conversion of ammonium ions to dissolved ammonia: $$\mathrm{NH}_{4}^{+}+\mathrm{OH}^{-}=\mathrm{NH}_{3}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$$ Air is then contacted with the wastewater, stripping out the ammonia. (a) One million gallons per day of alkaline wastewater containing 0.03 mole \(\mathrm{NH}_{3} /\) mole ammoniafree \(\mathrm{H}_{2} \mathrm{O}\) is fed to a stripping tower that operates at \(68^{\circ} \mathrm{F}\). Air at \(68^{\circ} \mathrm{F}\) and 21.3 psia contacts the wastewater countercurrently as it passes through the tower. The feed ratio is \(300 \mathrm{ft}^{3}\) air/gal wastewater, and 93\% of the ammonia is stripped from the wastewater. Calculate the volumetric flow rate of the gas leaving the tower and the partial pressure of ammonia in this gas. (b) Briefly explain in terms a first-year chemistry student could understand how this process works. Include the equilibrium constant for the second reaction in your explanation. (c) This problem is an illustration of challenges associated with addressing undesirable releases into the environment; namely, in developing a process to prevent dumping ammonia into a waterway, the release is instead made to the atmosphere. Suppose you are to write an article for a newspaper on the installation of the process described in the beginning of this problem. Explain why the company is installing the two-part process, and then explain the ultimate fate of the ammonia. Take one of two positions - either that the release is harmless or that it jeopardizes the environment in the vicinity of the plant. since this is a newspaper article, it cannot be more than 800 words.

Step by step solution

01

Understanding the removal process

Firstly, it's important to understand the process to be able to answer the questions. The process involves adding Lime to the wastewater which creates a reaction and forms hydroxide ions. These ions then interact with the ammonium ions leading to a production of dissolved ammonia. Air is then passed through, which strips out the ammonia.

02

Calculating flow rate and partial pressure

Given that one million gallons per day is the wastewater input, 0.03 mole NH3/mole ammonia-free water is the ammonia content and 300 ft3 air/gal is the air volume per gallon of wastewater, 93% of which is stripped from wastewater. The ammonia leaving the tower is thus \(0.03 * 1,000,000 * 0.93 = 27,900\) moles per day. The total volume of gas leaving the tower is the volume of air fed in plus the ammonia stripped out, which is \(300 * 1,000,000 = 300,000,000\) ft3. The partial pressure of ammonia in the gas can be found using the ideal gas law relation \(P = (nRT)/V\), where n = number of moles of ammonia, R = gas constant in proper units, T = absolute temperature in Kelvin, and V = total volume. Substituting known values should yield the answer.

03

Explaining the process in simple terms and its environmental impact

As far as explaining to a first-year chemistry student, make sure to focus on the basic concepts of chemical reactions and pressure, and demonstrate how the process works to take the harmful ammonia out of the wastewater and release it into the air. If writing a newspaper article, one will have to research on the impact of ammonia in the atmosphere and argue either that it is harmless or that it could be harmful, stressing the need for further treatment processes to remove it completely.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Ammonia Removal

Let's dive into the fascinating world of wastewater treatment, specifically, how ammonia—a compound found in industrial waste—is removed before the water is released back into the environment. Ammonia removal is crucial because if untreated, it can harm aquatic life and compromise water quality. In the scenario provided, we're dealing with alkaline wastewater containing ammonium hydroxide ((NH_4^+ OH^−)).

To kick off the removal process, lime (CaO) is added, which reacts with water to produce calcium ions (Ca^{2+}) and hydroxide ions (OH^−). The increased presence of hydroxide ions shifts the chemical equilibrium, converting ammonium ions (NH_4^+) into gas-form ammonia (NH_3) and water (H_2O). This change in the chemical state of ammonia—from ionic to gaseous form—is crucial as it allows for the subsequent separation step, where air strips away the gaseous ammonia, reducing the pollutant load in the wastewater.

Stripping Tower Operation

Moving on to the stripping tower operation, visualize it as a large column through which air and alkaline wastewater flow counter to each other—air moving upwards and water downwards. In the example, a tower is handling a massive one million gallons per day, using 300 cubic feet of air per gallon of wastewater.

During this process, the air 'grabs' the ammonia gas from the water, as the newly formed ammonia prefers to exist as a gas in air rather than remaining dissolved in water. The efficiency is impressive, with 93% of the ammonia transferred from the water to the air. This stripping process is governed by the principles of mass transfer and relies on a large contact surface area to achieve high removal rates.

Environmental Impact of Ammonia Emissions

While treating wastewater is essential for protecting waterways, the environmental impact of ammonia emissions from the treatment process cannot be ignored. Ammonia released into the air can react with other pollutants to form fine particulate matter, which is a health hazard. Additionally, when deposited onto soil or surface waters, ammonia can contribute to eutrophication—a process that leads to excessive growth of algae and a decrease in oxygen levels in water bodies adversely affecting aquatic ecosystems.

The choice of releasing ammonia into the atmosphere is a trade-off, where immediate water pollution is prevented at the cost of potential air quality issues. As such, it's essential that the stripping process in the wastewater treatment plant integrates with broader environmental management strategies to minimize the overall ecological footprint.

Chemical Reaction Equilibria

Understanding chemical reaction equilibria is crucial for comprehending how processes like ammonia stripping work. A chemical equilibrium represents a balance in a reversible reaction, where the rate of the forward reaction equals the rate of the reverse reaction. In the removal of ammonia, the addition of lime shifts the equilibrium of the reaction towards the production of ammonia gas. Equilibrium is governed by Le Chatelier’s principle, which states that a system at equilibrium will adjust to counteract the effect of a disturbance.

In our case, the presence of excess hydroxide ions 'disturbs' the equilibrium, thereby 'forcing' the reaction to produce more gas-form ammonia to re-establish balance. These equilibria are quantified by equilibrium constants, which signify the ratio of concentration of reactants to products at equilibrium. The entire concept is vital for designing processes like those in a stripping tower, ensuring the efficient removal of contaminants from wastewater.