conservation of mass (review)

1. living with the lab conservation of mass (review) accumulation of mass in the system if chemical reactions occur, new system components may be generated . . . while others are consumed © 2011 David Hall

2. living with the lab batch & rate problems http://mrg.bz/UWUXAC BATCH - start with nothing in system & end with nothing in system • RATE – continuous flow of inputs and outputs • steady state – the amount or type of mass in the system does not change with time • non steady state – the amount or type of mass in the system changes with time

3. living with the lab problem solving tips Draw a picture of the system. Sometimes it’s not easy to determine the boundaries of your system. (a large river flowing into the ocean for example . . . where does river end and ocean begin?) Label all inputs and outputs, listing all known quantities & concentrations and assigning variables to the unknowns. This key step is where errors usually occur. Think about the problem a little bit . . . determine if the process is a rate or batch problem. Are components generated or consumed? Revise (1) and (2) if needed. Write conservation of mass (or weight) for each component and for the entire system. Modify the diagram as new information is uncovered. Solve for the unknowns. Reflect on your solution. Do the concentrations or quantities make sense? WARNING avoid trying to just solve these problems in your head . . . use the systematic approach above

4. living with the lab paper machine problem A paper mill has equipment that uses heat, chemicals, and mechanical forces to break wood chips down into wood fibers that are used to make paper. The wood fibers and other ingredients are mixed with water to form “stock,” a wet mixture that contains 99.5% water and 0.5% dry material. The stock is first discharged onto a moving “wire” that is made of woven brass or bonze cloth that may be traveling at 2,000 feet per minute. The moving wire passes over several rolls were water is drained or sucked from the forming sheet. When the wire passes over the final roll, which is called the “couch roll,” it contains 82% water. The paper is then transferred to a traveling woolen felt where it passes through several presses, reducing the water content to 72%. The paper, which can then support its own weight, passes into the long dryer section where the paper winds around a large number of heated drums. When the paper exits the dryer section, the final concentration of water is 3% by weight. Class Problem: If 1000 lbs. of paper is produced each minute, then find the rates of water removed from the wire, felt and dryer sections as well as the total rate of water and solids that enter the system at the wire section.

5. living with the lab paper machine problem www.asia.ru/en/ProductInfo/908460.html stock felt dryer wire paper headbox(stock dumped on moving wire) felt wire dryer section water water water www.globalpapertrade.com www.sdxuancheng.cn

6. living with the lab solution • label all known concentrations • label any know mass flow rates • assign variables to unknown mass flow rates felt dryer wire • solve for unknown flow rates: (1) dryer (2) felt (3) wire (4) overall paper stock water water water