Design Specifications For A Bed Seal System Of A Moisture and Carbon Dioxide Adsorber Vessel
SUBMITTED TO:
Air Products and Chemicals, Inc.
ADDRESS: 7201 Hamilton Boulevard
Allentown, PA 18195
CONTACT: Vincent D’Imperio, Jr.
PHONE: 610-481-6652
FAX: 610-481-2400
EMAIL:
SUBMITTED BY: :
Amy Boyce
J.R. Daversa
Ben Gatto
Jake Mattern
Brian Smith
TEAM CONTACT: Brian Smith
ADDRESS: 120 Greary Hall
University Park, PA 16802
TELEPHONE: 814-862-2265
EMAIL:
DATE: March 14, 2002
EXECUTIVE SUMMARY:
This report provides the details of the final design of the bed seal system and the layout for the working prototype. Calculations are included which back up the claims made, showing that the thermal expansion of the different materials should not go beyond the confines of the vessel, nor lead to significant gaps through which absorbent material can escape, and that the movement of the different components should not cause the bolts holding them together to shear. Slight alterations were made from the previous design, based on preliminary calculations, leading to this final design.
The governing equations upon which our test prototype is based are given, explaining how thermal cycling can be simulated mechanically using a motor attached to life-size prototype. The prototype will be tested as to whether it can endure a 20-year life with six thermal cycles per day.
Based upon the information contained in this report and the future testing plans, implementation of this system as a solution for the leakage problem incurred by Air Products is a feasible solution.
TABLE OF CONTENTS
Introduction……………………………………………………………………………………. 4
Objectives……………………………………………………………………………………… 6
Nomenclature…………………………………………………………………………………. 7
Design Calculations…………………………………………………………………………... 8
Conclusions………………………………………………………………………………….. 13
References……………………………………………………………………………………14
Appendicies
Appendix A………………………………………………………………………………15
Appendix B………………………………………………………………………………18
Appendix C………………………………………………………………………………20
Appendix D………………………………………………………………………………22
Appendix E………………………………………………………………………………24
INTRODUCTION:
Pure gases play a vital role in a great many production processes in commercial applications. For instance, in the petrochemical segment they serve to keep oxygen at bay, displace or dispense combustible liquids or gases, protect sensitive goods against oxidation, and purge pipelines. Gases and chemicals such as pure oxygen, nitrogen, and argon serve many uses and applications in industry (7).
Oxygen:
The discovery that oxygen promoted combustion has been the basis for most industrial uses of oxygen. In the glass industry, oxygen/fuel combustion is used to reduce particulate and NOx emissions in melting operations. For metal fabrication, oxygen is burned with acetylene, propane, and other gases used in welding and cutting torches to provide a secure joint, stable fixture and fastening. In the pulp and paper industry, oxygen in used for limekiln enrichment.
In addition to industrial applications, oxygen is also recognized for its respiratory aid attributes. Traditionally, used for life saving applications, oxygen has been supplied to hospitals and through bulk tank installations, or larger cylinders, where the patient is confined to the institution. More recent advances have been concerned just as much with the quality of life, rather than just the saving of life.
Such quality can be enhanced if the patients can be allowed to return to their own homes, and even a more normal way of life, rather than being dependent on oxygen only available through the hospital or fixed installations.
As pure oxygen contains 5 times the concentration of oxygen in air, it can be used very successfully to provide ample oxygen to living systems that may be short of oxygen under normal atmospheric conditions. Such systems include biological wastewater treatment plants, rivers, lakes and fish farms.
Nitrogen:
Gaseous nitrogen is used in the chemical and petroleum industries for storage tank blanketing and vessel inerting applications. It is also used extensively by the electronics and metals industries for its inert properties. Liquid nitrogen, produced by the cryogenic air separation process, finds wide use as a refrigerant in applications such as cryogenic grinding of plastics and food freezing.
Argon:
Argon is used primarily for its properties as an inert gas in applications such as arc welding, steel making, heat-treating, and electronics manufacturing.
Air Products:
Air Products and Chemicals, Inc. is a company that produces a wide array of gases and chemicals for commercial use. Oxygen, nitrogen, and argon are taken directly from the air and separated using advanced technology and materials.
A common way to acquire pure oxygen and other gases is to cryogenically distill them from air. Air is cooled to the point where it liquefies, and then it is separated through distillation into its major parts. Before the air can be distilled, the water and carbon dioxide present in it must be removed so that it does not freeze and contaminate the distillation equipment.
Air Products and Chemicals, Inc. uses Temperature Swing Adsorption (TSA) technology to remove the troublesome water and carbon dioxide. In this process, air is forced through a vertical vessel, where it passes through an adsorbent bed of granular material. The adsorbent granules are able to remove the unwanted water and carbon dioxide from the air, so that the air is able to leave the vessel without taking its fouling components with it. The problem with using the adsorbent granular material is that it becomes saturated with water and carbon dioxide and is no longer able to remover after a few hours. To undo this saturation of the granules, a clean dry regeneration gas, such as nitrogen, is forced through the adsorbent bed in the direction opposite to that which the air is passed. While the regeneration gas is passing through the bed, it is heated, which in turn heats the granules to more effectively remove the water and carbon dioxide from them. After the undesirable agents are adequately removed from the adsorbent granules, the gas temperature cools down, cooling the bed to its appropriate operating temperature. At this point, the operation begins again.
The granules themselves, about 2mm in diameter, rest in the vertical pressure vessel on top a flat support screen. This screen supports the weight of the adsorbent material as well as the forces due to the pressure differential as gases pass through it. The screen must be able to sustain the massive temperature swing associated with the cryogenic distillation and the regenerative process. This temperature swing produces thermal expansion between the vessel wall and the screen.
Problem:
Thermal expansion creates a way for the adsorbent material to move about and travel into places in the assembly where it should not be. As seen in the attached photographs, the current setup of the screen region consists of a grating surrounded by a packing material. The screen is then bolted over this grating. The packing material seals the screen against the vessel wall preventing the adsorbent granules from entering into the chamber on the other side of the grating.
Currently, brass wool is used as the packing material. The adsorbent particles are able to travel through the brass wool and leak past the support screen. Thermal expansion moves the screen in such a way that the particles are able to move past the screen more easily. Another factor involved in passing the adsorbent granules past the screen is the pressure difference on opposite sides of the screen while air is forced through it.
This failure of the packing material to retard the progress of the granules past the screen is a consistent and long-term problem present in the process. Adsorbent leakage is disruptive to the process, difficult to clean, and a source of process down time.
The problem of leaking adsorbents is a consistent problem that has been attempted to correct in the past with little or no satisfaction. Other research has been done on this problem by teams from other schools. According to the sponsor, while these teams have had some interesting ideas, there is no adequate solution yet available to merit the investment in modifying the current equipment.
OBJECTIVES:
1. Determine the feasibility of the seal system design by calculating:
· The thermal expansion of the different pieces of the system in relation to one another, keeping in mind the varying materials used
· The shear stress on the bolts holding the seal system together
2. Specify the design of the prototype that will:
· Simulate the thermal cycling to which the system will be subjected
· Use a motor to approximate the physical motions that the system components would undergo
· Calculate the motor size and speed necessary to produce similar conditions that would test for a 20-year, six cycle per day life
NOMENCLATURE:
a Coefficient of linear thermal expansion ºC (ºF)
c Damping coefficient N•s/m (lb•s/ft)
d Deflection mm (in)
E Modulus of elasticity Pa (psi)
Fo Excitation amplitude N (lb)
I Moment of inertia m4 (in4)
k Spring coefficient N/m (lb/ft)
N Factor of safety
NAct Actual number of cycles needed cycles
NFail Number of cycles till failure cycles
s Thermal stress Pa (psi)
sTot Total stress Pa (psi)
P Load N (lb.)
Se Endurance limit Pa (psi)
Sut Ultimate tensile strength Pa (psi)
Sys Yield strength Pa (psi)
t Shear Pa (psi)
ω Excitation frequency rad/s
Bolt Force lbs
Screen Plate Force lbs
Bolt Torque lb-ft
Summation of Moments about pt 2 lb-ft
Max Deflection in y-direction in
Second moment of area about y-axis in4
DESIGN CALCULATIONS:
Bending Calculations
The group’s assumption as to how the beads are leaking through the current assembly was that they were passing underneath the hold down ring. The beads passed through gaps created by the ring’s bending/buckling between the bolts that fasten it down. The bending/buckling was caused by a bending moment created by torquing the bolts down onto the ring, which is supported by an uneven surface as seen in drawing 3 and 4 in Appendix A, and the free body diagram seen in drawing 2 Appendix E. The prove that this was the weak link in the assembly and the reason why it was leaking, the group would have to create a design that would eliminate or drastically reduce the bending/ buckling created in the ring by the grate and torquing down the bolts. Therefore, before further action could be taken the group must first verify this assumption. To determine if there was significant bending in the ring a preliminary “hand” calculations were used. After proving a significant bending in the ring, a computer simulation and analysis of the system were then done to verify the “hand” calculation and to gain more exact results. This computer simulation was done in Pro Mechanica using Finite Element Analysis (FEA), Drawing 3 in Appendix E.
The “hand” calculation was done using a bolt torque of 115 ft lb, and a bolt diameter of 0.75 in. The distance between bolts is 12.2 in as seen in Drawing 2 in the Appendix. This figure was obtained using the pressure vessel dimensions provided by the sponsor. The distance between bolts is 18°, which on a 7 ft pressure vessel is 12.2 in. On a larger pressure vessel the distance between bolts would obviously be larger, and the larger distance between bolts would result in a greater deflection in the area of the ring between the bolts. The result of the calculation for the 7 ft vessel was that the deflection in the middle of the ring was 0.0212 in. The mean diameter of the adsorption beads is only 2mm (0.0787 in), which indicates that some of the beads will be smaller, or may be broken into smaller pieces that will fit through the gap created between the bed and the ring. Therefore, the deflection in the ring was significant and was further analyzed using FEA as seen in Drawing 3 in the Appendix E. The tabulation of the “hand” calculations can be seen in the Appendix B.
Thermal Expansion
From the given graph (Appendix), the temperature change for the shell is 60 degrees Celsius. From the same graph, the temperature change for the screen plate is 150 degrees Celsius. This is for maximum temperature change.
From the proposed prototype, a motor would be placed at one specific point, and used to simulate the linear thermal expansion. In order to determine what deflection the motor must simulate, the linear deflection of the system due to thermal expansion must be calculated. This can be calculated using the following equation:
= (1)
The deflection of the pressure vessel’s shell due to thermal expansion was figured out using equation 1. The range of given values for the thickness of the shell was from ½ in to 1 in. The maximum thickness for the shell was used to obtain the maximum deflection of the shell. The thickness of 1 in was converted to the metric measure of 25.4mm for use in the calculation. Using the linear coefficient of thermal expansion for carbon steel (1) and equation 1, which can be found in the Appendix, the deflection of the shell due to thermal expansion equals:
dshell=0.01646mm = 6.48e-4 in
For the proposed redesign of the screen plate, the diameter is 84 in. To determine the best conditions, two calculations were made for the screen plate. One calculation was made assuming the screen plate was a solid disc. This would result in the maximum possible deflection by using the entire diameter for point linear thermal elongation. The diameter of 84 in was converted to the metric measure of 2133.6mm for the calculation. Using the linear coefficient of thermal expansion for carbon steel and equation 1, which can be found in the Appendix, the deflection of the screen plate as a disc due to thermal expansion equals:
dscreen plate (disc) = 3.456mm = 0.131 in
The second calculation was done assuming the screen plate as a ring. This was a more realistic calculation. It was assumed that the grating thermal expansion would be negligible compared to the expansion of the banding and ring on the outside of the screen plate. The length used was 4 in, which was then converted to the metric measure of 101.6mm for the calculation. Using the linear coefficient of thermal expansion for carbon steel and equation 1, which can be found in the Appendix, the deflection of the screen plate as a ring due to thermal expansion equals: