DESCRIPTION OF EQUIPMENT USED IN THIS WORK
3.4 DESIGN AND CONSTRUCTION OF NEW APPARATUS
3.4.1 General considerations
The new equipment was designed based on the principles of the still of Raal and
to withstand these pressures the entire apparatus was constructed in stainless-steel (SS). SS is not the most robust metal but other factors need to be considered when choosing a material of construction (MaC):
• Most important is the strength of the material. As far as tensile strength is concerned, SS is not the best by weight. Metals such as tungsten have much greater tensile strength.
• Resistance to corrosion. SS is not resistant to acidic corrosion but as the still was designed for the purpose of measuring solvent VLE this was not deemed a serious setback as stainless-steel is fairly resistant to atmospheric oxidation and does not react with any of the well known solvents used in this work.
• Cost of materials. The cost of the materials used is a very important factor in deciding which MaC to use. Although there are other metals stronger than SS none of the stronger metals are cheaper than SS or as freely available.
• Ease of machining the chosen MaC. As all construction work for this project was undertaken in our workshop it was important to use a material that would not cause problems to the workshop, which could result in unnecessary delays. The workshop advised us to use SS as they felt that metals with a higher tensile strength such as tungsten could cause breakages to their machining equipment.
By considering all of the above factors the MaC chosen was 316 SS. 316 SS has good corrosion resistance and has good tensile strength and is a very common MaC in the processing industry.
316 SS has a maximum of 0.1 % carbon, approximately 16 to 18 % Chromium, 10 to 14 % Nickel and 2 to 3%Molybdenum (Fontana (1986)). 316 SS has a tensile strength of 520 N/mm2 and its design strength vs. temperature is plotted in Figure 3-4 (Sinnott (1998)). For the discussion on wall thickness used in the design of the apparatus it is important to note how the design stress decreases with temperature.
200 _._--- ~ ~ _ . _ - - - - - ----
0
~E 150 0
E 0
:i
0
en 0 0
en 100 0 0
~ 0
U5 0
cCl
'00
(1) 50 0
o
300 400 500
TIK
600 700 800
Figure 3-4:Design Stress vs. Temperature for 316 SS.
316 SS is resistant to rust which is important as no contamination is vital for accurate VLE measurements. Furthermore, 316 SS is an austenitic steel and nonmagnetic and cannot be hardened by heat treatment. Although hardening of the steel makes it stronger, it also makes it less ductile. As the still would be subject to larger temperature ranges the flanges would expand and contract. If the steel became hardedned by repeated heating and cooling it could possibly crack and this needed to be avoided at all costs as this would weaken the still (making it hazardous at high pressures) and allow for leaks.
As with the MOC, a general discussion of pressure vessel design is appropriate. Although all our designs are not technically pressure vessels4 they were designed by referring to recommendations for pressure vessels (Sinnott (1998» and with safety as the first concern. Pressure vessels can be divided into two classes depending on the wall thickness to vessel diameter ratio. Vessels with a ratio less than 1: 10 are classed as thin-walled vessels and those with a ratio greater than 1: 10 are classed as thick-walled vessels. Due to our intended maximum pressure (35 MPa), our vessels are classed as thick walled vessels.
Thick-walled vessel design is a very complex science and the scope of this project did not allow for an in-depth study. However, it is important to note that although the components designed and manufactured for this study were machined from a solid billet (monobloc), there are more complex designs for thick-walled pressure vessels. The overall affect of these more complex designs is to create a vessel with a higher maximum pressure limit than a monobloc vessel with the same wall thickness. Sinnott (1998) lists three compound vessel designs:
i.) Shrink-fitted vessels. These vessels are made by shrinking one vessel over another.
The outer vessel is made with an internal diameter slightly smaller than the outer diameter of the internal vessel. The outer vessel is then heated until it expands sufficiently to fit over the internal vessel. As the outer vessel cools it shrinks and places the inner under compression. This process can be repeated for more than one layer.
ii.) Multilayer vessels. These vessels are made by wrapping thin layers around a central tube. Thethinlayers are heated, fitted and allowed to tighten and then welded. This process gives the desired stress distribution in the compound wall.
iii.) Wound vessels. Cylindrical vessels can be reinforced by winding. Thin ribbons or wire are wound on the vessel under compression. The winding can be wound on hot to increase the prestressing.
These three methods can be used to increase the strength of pressure vessel, however, they require a great deal of expertise and experience in the field of pressure vessel design and manufacture. As mentioned previously, all our construction was done by our in-house workshop and it was not possible for them to construct any of the compound vessels described above. Thus, the monobloc design was adopted.
The general process of the DRVS was discussed and explained in Chapter Two. Inprinciple the still requires three main unit operations:
i.) A reboiler. The chemical mixture is heated (boiled) in the reboiler and the superheated mixture is transported to the equilibrium chamber.
ii.) The equilibrium chamber. In the equilibrium chamber the superheated mixture equilibrates. The two phases are disengaged and the liquid phase is returned to the reboiler and the vapour phase is transported to the condenser. Both phases are sampled before they are returned to the reboiler.
iii.) The condenser. Although the design of the condenser is simple it is an important part of the DRSV used in this work. The function of the condenser is to condense the vapour phase and return it to the reboiler. As the condenser is situated above the reboiler, the condensed vapour phase returns to the reboiler by the hydrostatic head.
This makes a vapour pump unnecessary and thus reduces costs and equipment complications.
In addition to these unit operations there are several auxillary components which are vital to the operation of the still. These include the heating of the reboiler, the cooling of the condenser and the maintaining of the pressureinthe still. (To a lesser degree the equilibrium chamber is also heated but only for short periods to speed up the heating up of the equipment.) The general outlay of the apparatus and auxillary equipment is illustrated in Figure 3-5. Different hardware is used depending on whether the apparatus is being operated either at low pressures (P < 100 kPa) or at high pressures (P > 100 kPa). Figure 3-6 illustrates the general principles of the still itself.
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,,I I I I I I I I I,
,
I,
I I I
I I
~---~---.
C)~---{IC
L
A: Equilibrium still; B: liquid sampling loop; C: vapour sampling loop; 0: cartridge heater; E: reboiler band heater;
F: equilibrium band heater; G: variable-l.IJltage power supply; H: main temperature sensor; I: pressure transducer;
J:pressure ballast; K: vacuum pump;L: inert high-pressure gas; M: GC; N: coolant for condenser; 0: cold finger;
P: coolant circulation pump
Figure 3-5:Schematic diagram of the equipment ofHarris et al. (2003b).
Figure 3-6 illustrates the general principles of the still itself. A liquid mixture is charged into the reboiler. Both phases are sampled before they are returned to the reboiler.
PC
LSL
CWO
VSL
R REBOLER
E B:lULElRU.1 CHA"IlER
C CXJNOENSOR
CP OO1RaLPlJM?
LR LOUDREnRN
VR VAroffiRETURN LSL LOUD SAM'\.NG LOOP VSL VAPOURSAM'\.NGLOOP
PC PRESSURECONTROl.PONT CWI COOlNGWA1ER IN CWO COOLING WA1ER OUT
LD UaUD DRAN
VD VAPOUR DRAIN FD FEED ORAN
FD
Figure 3-6: The still ofHarris et a1. (2003b).
The apparatus can be divided into six sections for discussion:
• The reboiler.
• The equilibrium chamber.
• The condenser.
• Thermal insulation.
• Temperature and pressure measurement.
• Vapour and liquid phase sampling.