Essay: Determining the best piping material for a chemical reactor (lab)

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  • Determining the best piping material for a chemical reactor (lab)
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Abstract

The purpose of the lab was to evaluate different metals in search of determining the best piping material for a chemical reactor created by Kauffman Enterprises. The reactor vessel is to be made out of stainless steel and will operate at 525C while containing highly corrosive hydrochloric acid. The primary metals that were evaluated were aluminum, brass, copper and stainless steel. The initial lengths of the rod were taken first, then they were placed into a heat exchanger apparatus where their initial temperature was taken by a thermometer. This apparatus was then hooked up by a hose to a steam generator to cause expansion of the metal rods. This caused a change in length and change in temperature which was recorded by a micrometer and thermometer respectively. The engineers used the change in length, initial length, and change in temperature to determine the metal’s coefficient of linear expansion. The researchers also had to take the cost and corrosion resistance of each metal into account when determining the best material for the pipes. The researchers were able to determine that the copper pipe was the best piping material for the reactor. This material had the second lowest coefficient of expansion, a low material cost, and no corrosion effect.

Introduction and Background

Kauffman Enterprises has hired a group of University of Iowa engineers who are knowledgeable in materials science to work on a chemical reactor being designed by the company. The reactor vessel that will be operated at 525C and will contain highly corrosive hydrochloric acid. The reactor will be created with stainless steel, but a significant amount of piping attached to the reactor will be needed. The job for the engineers is to determine the best material for the piping given four options of: Brass, Copper, Aluminum and Steel. The recommendation will take into account the expansion of the pipes, corrosion resistance and cost.

Since the process will be operated at 525C, there will be some type of expansion with the piping material. Each material that will be tested has a unique coefficient of linear expansion, and this is a property that indicates how much that material will expand when heated or retract when cooled. This coefficient is used in many engineering projects to determine how much stress a material can handle when heated or cooled. These types of stresses can lead to cracking or even deformation of the material depending on the temperature. To calculate this coefficient, you will need to apply the equation:

Figure [1]. Coefficient of Linear Expansion Formula (Kauffman)

The coefficient is represented by the symbol alpha (), L signifies the change in length, L_0 denotes the original length, and D_t is the change in temperature. It is ideal to have two metals having like coefficients of linear expansion, because it will allow for the expansion and retraction of both to be similar. For example, if a metal with a high coefficient has another metal wrapped around it with a low coefficient, the metal inside expands faster and could cause a fracture to the metal surrounding it. However, there are situations in which you may need to join two metals having unlike coefficients of linear expansion. For example, preparing an alloy requires combining metals with two different coefficients. The four metals being tested have theoretical coefficients of:

Table [1]. Theoretical Coefficient of Linear Expansion for 4 materials (Toolbox 2003)
Material Linear Temperature Expansion Coefficient
(10-6 m/ (m K))
Brass 18-19
Copper 16-16.7
Aluminum 21-24
Stainless Steel 11-12.5

These theoretical values are important because they will be a baseline example to compare our coefficients with from our experiment. We will be able to calculate our error percentage based off these values.

Another factor in determining the best material for the vessel is cost. Each material is listed with its cost per foot of pipe below. In terms of optimal price for the piping, the ranking would go as follows from lowest cost to highest: Aluminum, Brass, Copper, then Stainless Steel.

Table [2]. Cost per Foot of Piping Material (Kauffman)

Material Cost/Foot of Pipe
Brass $2.65
Copper $2.76
Aluminum $1.24
Stainless Steel $4.65

In addition to these factors in determining the best material for the piping, corrosion resistance is a key factor. It is brought into consideration, because hydrochloric acid is an extremely corrosive substance and higher temperatures increase the rate of corrosion. Given the chemical formula of each metal, they must be compared to hydrogen on the reactivity series to see how they will react.

Figure [2]. Reactivity Series for Metal (Chinmayi)

The metal that will react the most with the HCl will be the aluminum since it is located above hydrogen. This makes it the most corrosive and not a suitable choice for the pipe. The next most corrosive will be the common brass since it is composed of around 62-65% zinc and around 37% copper. (Bell) Almost the least corrosive, but still not corrosive resistant would be stainless steel. It is an alloy composed of chromium, nickel, iron and carbon. Given stainless steel’s carbon makeup, some corrosion will occur, however, it will occur at a very minimal rate. The least corrosive element will be copper, because it is located well below hydrogen. Being that this pipe will be solely composed of copper, the pipe will never corrode since the reaction cannot occur. This makes copper the optimal choice in terms of corrosion.

Experimental Methods

On February 14th, four groups of engineers put the metals of brass, aluminium, steel and copper to a test to determine which metal would be optimal in using for the pipes on the reactor. Two groups were selected to run the experiment using steel and copper, while the other two groups tested the brass and aluminium. Each group was given a rod of each of the material. Our first step was to fill our steam generator with an adequate amount of water and turn it on to allow the water to heat up. Weights were placed on top of the steam generator as to not allow any steam to escape while the generator was heating up. Heat resistance gloves must be worn when handling the steam generator in order to prevent severe burns.

Figure [3]. Steam Generator in Process of Heating Water

The next step was to measure the initial length of the rod as this would be later used to determine the coefficient of linear expansion for each metal.

Figure [4]. Brass and Aluminium Rods Measured for Initial Lengths

After measuring the rod, it had to be placed inside of the heat exchanger. Following the placement of the rod in the apparatus, the jacket had to be secured to the exchanger using the micrometer screw.

Figure [5]. Rod Placed Inside of Heat Exchanger

The micrometer screw attached to the apparatus had to be slowly tightened just to make contact with the rod to be sure of an accurate reading on the micrometer gauge.

Figure [6]. Rods Securely Placed into Apparatus by Micrometer Screw

Figure [7]. Micrometer Gauge Reading in mm

We then recorded the initial micrometer gauge reading and inserted a thermometer into the exchanger to get the initial temperature reading. Once the exchanger was ready, we connected the steam generator to the apparatus using rubber tubes and applying weights on the generator to allow minimal leakage of steam.

Figure [8]. Steam Generator Attached to Heat Exchange

Attached to the other end of the apparatus, another hose was led into the sink to allow drainage. On average, the groups waited several minutes for the steam to fully heat up each rod until it was stabilized. The final temperatures were recorded, and we were able to record the change in length based off the change in the micrometer (in mm).

Figure [9]. Overview of Fully Set Up Experiment

Following the final readings, the hose attached to the steam generator was pulled off and attached to a faucet. This allowed for cold water to flow through the apparatus and cool the rods which were returned to their initial lengths. This process was repeated for all rods until sufficient data was collected.

Figure [10]. Heat Exchanger Attached to Faucet to Cool Rod

Results and Discussion

Following the conclusion of our experiment, all the groups came together to discuss their results and compare what they found out about their given materials. The following table below will show all of the groups recorded data from the investigation including each metals coefficient of linear expansion.

Group Metal Ti (℃) Tf (℃) Li (m) △L (m) ⍺
10-6 m/m•K

1 Steel 1 27.8 93.9 0.598 0.00032 8.09
Steel 2 23.3 94.2 0.599 0.00045 10.6
Copper 1 21.6 94.2 0.599 0.0007 16.1
Copper 2 21.2 100.8 0.598 0.00091 19.1

2 Steel 1 26 97 0.603 0.0004 9.34
Steel 2 21.2 101 0.60 0.00048 10.0
Copper 1 22 97 0.599 0.00051 11.4
Copper 2 24 96 0.599 0.00054 12.5

3 Aluminium 1 20 99 0.599 0.00101 21.3
Aluminium 2 25 100 0.60 0.000965 21.4
Brass 1 24 99 0.5974 0.00081 18.1
Brass 2 22 100 0.598 0.000869 18.6

4 Aluminium 1 22 99 0.598 0.001 21.7
Aluminium 2 23 99 0.597 0.00103 22.7
Brass 1 23 99 0.599 0.00078 17.1
Brass 2 22 98 0.598 0.0007 15.4
Table [3]. Groups Results Including Calculated Coefficients

I averaged out all coefficients of linear expansion to be compared to the theoretical linear expansions, and to determine the error percentage for each material. The percent error is calculated by subtracting the experimental from the minimum theoretical, then taking the absolute value of this number. This is followed by dividing by the minimum theoretical and then multiplying by 100 to get the percentage.

Table [4]. Average Coefficients of Linear Expansions

Materials Average Coefficients of Linear Expansion

Brass 17.3
Copper 14.78
Aluminium 21.78
Stainless Steel 9.51

Table [5]. Percent Error of Coefficients of Linear Expansion
Materials Percent Error

Brass 3.89%
Copper 7.63%
Aluminium 0%
Stainless Steel 13.6%

The only average coefficient of linear expansion to be within its theoretical range was the aluminum rod. The one with the greatest error percentage was the stainless steel. This error could have been due to the old and faulty equipment. For example, two groups were sharing one steam generator, and the steam was leaking out of the sides, and the rods were not heating up as fast as the other two groups who had their own generator. Also, there could have been error when reading the initial micrometer, because if there is any slight movement of the heat exchanger when attaching the hose from the steam generator, it will change the position on the micrometer. In addition to these errors, if an adequate amount of water was not in the steam generator, the rod would not reach its stabilizing temperature to determine its coefficient of linear expansion.

Conclusion

This was a very interesting experiment to work with, because given the materials, you would believe that Stainless Steel would be the obvious choice considering it has the smallest coefficient of expansion. However, given the cost of material and corrosion resistance, I have determined that the best choice is the copper piping. To begin with, the copper piping has the second lowest coefficient of linear expansion compared to the three other materials. In addition to that, it is the only material that is not corrosive when it comes in contact with the hydrochloric acid. This will make it significantly cheaper in the long run, because there will be no need to repair the piping since there is no corrosion from the heated HCl. Compared to the other 3 materials, there will need to be replacement of piping due to corrosion over time. Finally, even though it is not the cheapest material compared to aluminum and brass, it is 41% cheaper than the stainless steel. It is worth it to spend some extra money in comparison to aluminum and brass, in order to create a higher quality pipe given these variables.

Table [6]. Ranking of Metals given Cost, Coefficient of Linear Expansion, and Corrosion Variables

Materials Ranking

Copper 1

Stainless Steel 2

Brass 3

Aluminium 4

References

Bell, Terrence. (2019, January 25). Composition of Common Brass Alloys. Retrieved from
https://www.thebalance.com/composition-of-common-brass-alloys-2340109

Chinmayi, H.B. (2016, August 18). What is the significance of the reactivity series? Retrieved
From https://www.quora.com/What-is-the-significance-of-the-reactivity-series-of-metals

Kauffman, Kenneth. (n.d.). Selection of a Piping Material for a Corrosive Environment.
Toolbox, Engineering. (2003). Coefficients of Linear Thermal Expansion. Retrieved from
https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Appendix A: Equations and Example Calculations
Coefficient of Linear Expansion:
α=(Lf-Li)/[Li(Tf-Ti)]

 = 8.09 m/m*K
Length Initial = Li = 0.598 m
Change in Length = L = 0.00032 m
Change in Temperature = Dt = 66.1C
Error Percent:
|Actual-Theoretical|/Theoretical * 100 = 3.89%
Actual = 17.3
Theoretical = 18

Appendix B: Equipment

Metal Rods: Aluminum, Brass, Copper, Stainless Steel

Heat Resistance Gloves

Heat Exchanger Apparatus

Steam Generator

Digital Thermometer

Micrometer Screw and Gauge

Meter Stick

Safety Glasses

Rubber Hoses

Weights

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