Neutralization Theory - W2 Systems

1 Neutralization TheoryW2 Systems (415) 468-9858 TheoryChemical Feed ControlThe controls, alarms and external contacts are shown on the drawings. As the amount of chemical needed to neutralize the wastewater varies exponentially from neutral pH 7, special consideration must be given to control overshooting the pH. Chemicals, especially viscous caustic soda, must be fully mixed in the compartment before more is than modulating the pump settings, the pump is fully on for a time proportional to the feed ratio needed. For example, turning on the pump for 10 seconds of the 30-second cycle, allows 1/3 of the full chemical flow to be added. This discussion also refers to injection Systems that stop and start small, motorized pumps.

2 The chemical feed valves are actuated as shown on below: Neutralization TheoryW2 Systems (415) 468-9858 injection of Neutralization ChemicalsThis timing diagram example plots pH against cycle time and valve open/close status. The pH axis runs horizontally from 0 to 14 as measured by the instrument in the tank. A line drawn from A1 to A2 will move left or right on the diagram according to the pH as read by the probe in the tank. Another line from B1 to B2 represents time. Moving this line upwards takes 30 seconds from the bottom to the top of the graph. It then resets to the bottom for the next pH measurement and associated valve cycle. The intersection of these two lines is the injection point.

3 The chemical feed will be on for the shaded time in the diagram and off during the un-shaded time. If the tank pH is , the feed will be on for 5 seconds (read up) and off for the remaining part in the cycle. 5s/30s is about 1/6: it is injecting chemical at a 16% of full rate, compared to maximum(100%). Below pH the feed is on 100%. system MonitoringAll Systems include effluent pH monitoring with local indication, alarm points and 4-20 mA outputs and flow indicating-totalizers with 4-20 mA output of flow rate. A chart recorder continuously records effluent pH and flow TheoryThe most common measurement made in water-pollution control, particularly in the area of industrial wastewater treatment, is pH.

4 Acidic and basic substances, which define pH, have a severe effect on aquatic life and, in turn, an effect on humans. Since acids and bases are used extensively in industry, it is important to understand these effects, because pH measurement is the essential key to study acid/base effects. Thus, you should understand and familiarize yourself with the concept of pH because of its important consequences to the industrial community and the will need a few basic definitions to fully comprehend pH: Neutralization TheoryW2 Systems (415) 468-9858 : An atom or molecule with a net positive or negative charge. The ions we are most concerned with are the positive charged hydrogen ion (H+) and the negatively charged hydroxide ion (OH-).

5 Concentration: A unit of measure defined as weight per volume. Our concentration has units of mole/liter or molarity (M). Logarithm: A mathematical operation used to express a number, by its powers of 10. For example, log 1,000,000 is log (10)6 or The logarithm is used expressly to create the pH scale . Please note that each unit of pH is equivalent to a 10-fold increase in H+ or OH- ionWith these basic definitions now familiar, we can easily handle the concept of we all know, the chemical formula for water is H2O, and some of us also know the H2O molecule can, and does breakdown into two ions, namely H+ and OH-. In pure water, the number of these free ions is very, very small, something like one H+ and one OH- ion in every half-billion water molecules.

6 If we define this ratio by our units of concentrations, [H+] = [OH-] = M for pure water. Now, pH is simply the negative logarithm of [H-]. Therefore, we can calculate the pH of pure water:pH = -log .0000001 M = will define this, pH = , as the neutral point, which means any solution with that pH has an equal amount of [H+] and [OH-]. Now, if we rearrange the concentrations we used above, we can define K (water), which is a constant used in many pH calculation:K (water) = [H+] [OH-] = 1 x (10)-14 Let s take the pure water from above and inject hydrogen chloride gas (HCl). The gas will breakdown into its respective ions, [H+] and [CI-], once it is injected. Let s say that we have added enough HCL to increase the overall [H+] by 100 times.

7 Let s calculate the pH:[H+] = .0000001 M X 100 = .00001 MpH = -log = 5By adding [H+], we have produced an acidic aqueous we will stir solid sodium hydroxide (NaOH) into pure water (pH 7). The solid, like gas, will breakdown into its respective ions, [Na+] and [OH-]. But this time, we have increased [OH-] by 1000 times. We will calculate [H+] using the aforementioned definition for K (water). Neutralization TheoryW2 Systems (415) 468-9858 (water) = 1 x (10)-14[H+] = [OH-] = 1000 x 1 x (10)-7 M = 1 x (10)-10 MpH = -log 1 X 10- 10 M = can now state the relationship between pH, [H+] and [OH-]:[OH-] < [H+], pH < acidic solutions[OH-] = [H+], pH = neutral solution[OH-] > [H+], pH > basic solutionsTo illustrate the uses of pH, we will look at two common scenarios.

8 In the electronics industry, strong acids are used in the manufacture of integrated circuits. The various acids flow through the process, they are then dumped into a Neutralization system where the pH is neutralized to a safe level. The neutralized solution is usually pumped into the public sewage system for further s look at the Neutralization system : we know for acidic solution [H+] > [OH-], and we want them to be equal; therefore, the Neutralization system adds [OH-], say in the form of a NaOH solution, that balances the [H+] and [OH-]. First, the system monitors the pH of the incoming acidic solution, it then calculates the amount of base needed to neutralize the solution, and finally it adds the necessary NaOH other often seen scenario is one of an environmental nature as well as an industrial nature.

9 In the northeastern USA and parts of Canada there is a serious problem known as acid rain. Acid rain is formed when toxic gases, which are unwanted by-products of various industrial processes, are spewed into the atmosphere where they are absorbed by rain clouds. Once absorbed, the gases react with the clouds. When precipitation occurs, acids are formed that are then returned to the land by the process known as acid rain. Environmentalists are constantly testing the pH of streams, lakes, and rivers to stud the effect on the wildlife. Even though acid rain is very weak it is still harmful. By keeping good pH data and rainfall records, these environmentalists can calculate the qualities of toxins in the air.

10 The and Canadian governments will use these analyses to create better air quality standards for industry in an attempt to keep both nature and industry TheoryW2 Systems (415) 468-9858 Usage - Example 1 This chart shows NaOH solution flowrate (gallons per minute) needed to neutralize (reach pH = ) 100 gpm of a solution at a given inlet pH. NaOH weight % = weight of NaOH / weight of solution x 100% Chart 1A: Flowrate Scale for 100gpmInlet pH10% NaOH20% NaOH30% NaOH40% NaOH50% 300 gpm flow rate solution has a pH = What flow rate of 30% NaOH needed to neutralize the solution?Using Chart 1A: with 100 gpm flow rate, at pH = , we need gpm of 30% Flow rate=(300 gpm /100 gpm) x gpm = gpmNeutralization TheoryW2 Systems (415) 468-9858 Usage - Example 1 Chart shows H2SO4 solution flow rate (gallons per minute) needed to neutralize (reach pH = ) 100 gpm of a solution at a given inlet pH.