ChE 111 Elements of Chemical Engineering

Transcription

1 ChE 111 Elements of Chemical Engineering February 2015 M.A.A. Shoukat Choudhury, PhD Professor Department of Chemical Engineering, BUET 10/9/16 1

2 Units and Dimensions Unit: A measured quantity has a numerical value and a unit Dimension: A property that can be measured such as length, time, mass, temperature, etc. Conversion Factor: - used to convert a quantity expressed in one unit to its equivalent in another unit - Multiply the given quantity by the conversion factor Chemical Engineering, BUET Dr. M. A. A. Shoukat Choudhury 2

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4 Conversion Factors

5 Example 2.2.1: Convert 1 cm/s2 to km/yr2

6 System of Units

7 System of Units Type Length Mass Time SI Unit m Kg s CGS cm gm s American Engineering or FPS I lbm s

8 System of Units

9 System of Units

10 Example 2.3.1

11 Force and Weight

12 Force and Weight

13 Dimensional Homogeneity and Dimensionless Group

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17 Scientific Notation

18 Significant Numbers and Significant Figures The significant figures of a number are the digits from the first nonzero digit on the left to either a) The last digit (zero or nonzero) on the right if there is a decimal point b) the last nonzero digit of the number if there is no decimal point For example:

19 3 Rules to Find Significant Figures There are three rules on determining how many significant figures are in a number: 1. Non-zero digits are always significant. 2. Any zeros between two significant digits are significant. 3. A final zero or trailing zeros in the decimal portion ONLY are significant.

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21 Significant figures for Multiplication or division

22 Significant figures for Multiplication or division When multiplying or dividing numbers, count the NUMBER OF SIGNIFICANT FIGURES. The answer cannot CONTAIN MORE SIGNIFICANT FIGURES THAN THE NUMBER BEING MULTIPLIED OR DIVIDED with the LEAST NUMBER OF SIGNIFICANT FIGURES. Example: (8 significant figures) x (5 significant figures) (on calculator) (rounded to 5 significant figures)

23 Significant figures for Addition or Subtraction

24 Significant figures for Addition or Subtraction When adding or subtracting numbers, count the NUMBER OF DECIMAL PLACES to determine the number of significant figures. The answer cannot CONTAIN MORE PLACES AFTER THE DECIMAL POINT THAN THE SMALLEST NUMBER OF DECIMAL PLACES in the numbers being added or subtracted (6 places after the decimal point) (4 places after the decimal point) (2 places after the decimal point) (on calculator) (rounded to 2 places in the answer)

25 Rounding Off Finally, a rule of thumb for rounding off numbers in which the digit to be dropped is a 5 is always to make the last digit of the rounded off number even: 1.35 è è 1.2

26 A student once needed a cube of metal which had to have a mass of 80 grams. He knew the density of this metal was 8.67 g/ml, which told him the cube's volume. Believing significant figures were invented just to make life difficult for chemistry students and had no practical use in the real world, he calculated the volume of the cube as ml. He thus determined that the edge of the cube had to be cm. He took his plans to the machine shop where his friend had the same type of work done the previous year. The shop foreman said, "Yes, we can make this according to your specifications - but it will be expensive." "That's OK," replied the student. "It's important." He knew his friend has paid $35, and he had been given $50 out of the

27 He returned the next day, expecting the job to be done. "Sorry," said the foreman. "We're still working on it. Try next week." Finally the da came, and our friend got his cube. It looked very, very smooth and shiny and beautiful in its velvet case. Seeing it, our hero had a premonition of disaster and became a bit nervous. But he summone up enough courage to ask for the bill. "$500, and cheap at the price We had a terrific job getting it right -- had to make three before we got one right." "But--but--my friend paid only $35 for the same thing!" "No. He wanted a cube 2.1 cm on an edge, and your specifications called for We had yours roughed out to 2.1 that very afternoon, but it was the precision grinding and lapping to get it dow to which took so long and cost the big money. The first one w made was on one edge when we got finished, so we had to scrap it. The second was closer, but still not what you specified. That's why the three tries." "Oh!"