Class III cosolvent processes

Class III cosolvent processes are opposite to and different from the Class 11 cosolvent processes. With Class III cosolvent processes, both cosolvents are miscible, and the flammability rating of the mixture is not specified. ... 

One might explain the Class III cosolvent process by noting that it is a "failed" Class II cosolvent process. 

In any proper Class II cosolvent application, the SA and RA cosolvents are separated from one another in "HSP space" by at least 10 to 12 MPa" (or more) so that fmmiscibility is maintained. In all Class III cosolvent processes, the separation (Ra ) is less than 10 MPa " hopefully, the Ra between Class III SA and RA cosolvents is as close to zero MPa as possible — so that complete miscibility is maintained. 

Class III cosolvent processes address a limiting problem inherent with Class II cosolvent systems — that the SA cosolvent can only be heated to the hoUing point of the RA cosolvent . 

It is not necessary to operate with the SA being boiled in a Class III cosolvent process Obviously, cold deaning may be done with the SA in either Class II or Class III cosolvent processes... 

One may choose Class III cosolvent processes not just because they allow high-temperature solvation cleaning, but because of the type of rinsing their use provides ... 

Fluid Management with Class III Cosolvent Processes... 

A study of Figure 3,26 shows that there are several instances where the two cosolvents must be properly managed, and treated in a Class III cosolvent process. The five primary issues about treatment are (1) separation of soil materials from the SA cosolvent and elimination of the soil materials from the cleaning process, (2) separation of the SA and RA cosolvents from one another so each can be reused, (3) achievement of the desired rinse quality, (4) use of energy in an efficient manner, and (5) removal of water from both cosolvents ... 

The Class III cosolvent process requires considerably more energy than a Class II cosolvent process. Energy is also used in a different way, because it s a different process. 

In either case, cleaning with a single solvent may be the appropriate process. That single solvent must be chosen to not only share HSP values with the soil components, but to also have a volatility intermediate between the SA and RA cosolvents being considered for a Class III cosolvent process. 

It s expected that Class III cosolvent systems will require more heat energy than do Class II cosolvent systems. After all, two major characteristics of the Class III cosolvent processes are that their SA sump can be raised to an elevated temperature, and that the RA cosolvent is a volatile and expensive material. 

When using the Class III cosolvent process described in Figure 3.26, it is neither necessary nor desirable to immerse the parts basket into the RA sump. This is because the liquid in the RA sump below the primary condenser is less clean of soil and SA cosolvent materials than is the pristine distilled RA cosolvent vapor. The RA sump exists to collect the liquid dragout material (SA cosolvent and dissolved soil materials) rinsed from the parts and basket surfaces, as well as RA cosolvent. This collection is not retained in the RA sump but is immediately fed to the distillation column. Immersion in the RA cosolvent sump would dilute one of the advantages of the Class III cosolvent process. 

The basis for the calculation is simple the sensible heat is calculated by multiplying the appropriate specific heat for each SA and RA cosolvent, times the temperature difference between the normal boiling point for the RA cosolvent and an assumed ambient temperature of 70°F, times the appropriate solvent density (to express the energy demand in energy units per volume). The latent heat is calculated by multiplying the latent heat of vaporization for the RA cosolvent times the solvent density (for the same reason). The latent contribution for the RA cosolvent is doubled because of the belief that excellent rinsing is required. The total energy use is the sum of the two. The SA cosolvent is assumed to not be vaporized in Class III cosolvent processes, and can t be vaporized in both Class II cosolvent processes. 

The calculation is made for 13 pairs of solvents listed in Table 3.8. Each pair has been chosen to clean a specific soil material with both the Class II and Class III cosolvent processes. 

The need for an increase in energy consumed in a Class III cosolvent process, above that for a Class II cosolvent process, can also be seen to be that expected — to provide the additional sensible heat to raise the SA cosolvent to its high boiling point. This can easily be recognized by examination of Figure 3.30 . ... 

All organic reactions are enhanced by an increase of temperature. So use of oxygenated solvents in a Class II cosolvent process raises less concern about reaction with water than does use of the same solvents in a Class III cosolvent process. 

At operating temperatures above 212°F (100°C), which are unlikely to occur in a Class II cosolvent process, and more than likely to occur in a Class III cosolvent process, reactivity of any solvent with water should not be surprising. But two other factors must be considered ... 

Nonetheless, if one is to practice the Class III cosolvent process with some SA solvents at their normal boiling points (which may range up to 400 to 500°F ( 200 to 250°C)), prevention is the order of the day. 

These four criteria play the crucial role in selection of the RA cosolvent, and hardly any role in selection of the SA cosolvent. This is because in the Class II cosolvent processes the RA cosolvent Is the only fluid In direct contact with the ambient environment In the Class III cosolvent process the same Is so, plus the SA cosolvent Is chosen for minimum volatility so fugitive emissions will be low. 

A minimum level of immisdbility (smallest Ra) between the SA cosolvent and the RA cosolvent for all Class III cosolvent processes. 

Recall that the SA cosolvent In the cleaning sump can t be heated above the normal boiling point of the RA cosolvent in a Class II cosolvent process because there is always (by intent) liquid RAcosolvent present- and the two sumps are physically connected. Recall also, that the SA and the RA cosolvent sumps are physically not connected in a Class III cosolvent process - so any desired temperature can be achieved. 

It is this temperature increment which can be employed in solution cleaning of the same soil if one chooses a Class III cosolvent process vs. a Class II cosolvent process . 

One may thus craft a Class III cosolvent process which removes a low molecular weight water-soluble wax used as a protectant for optic components — a highly valued deaning operation. 

Or one may craft another Class III cosolvent process which removes coking residue (coal tar pitch) from semi-porous dectrodes used in an electrolysis process — an extremely difficult but finandally necessary deaning task. 

Within the capabilities of a Class III cosolvent process, there is no mandate to heat parts and soils to the maximum temperature for which the process is capable. One might only do cold cleaning in the SA cleaning sump. Alternately, one may do vapor degreasing at temperatures over 250°C (approaching 500°F) should that be necessary to dislodge thick viscous tar materials from rugged industrial parts. 

These two examples illustrate the broad range of cleaning applications which can be considered by cosolvent processes — especially Class III cosolvent processes. 

To gain the most value from Table 3.8 one needs a sense of the ridiculous, and/or of the practical. After all, a few of the choices made to clean certain soils with either Class II or Class III cosolvent processes do force recognition of the ridiculous. Not all of the choices listed in Table 3.8 are good, some are poor a few are impractical. The reason all are included is to illustrate use of the selection method. After all, when confronted with an impractical choice one must conclude either another choice is necessary or that the selection method doesn t apply to the situation being considered — just as in life. 

When the Class III cosolvent process is considered for use with some soils, the RA best matched to the SA is HFC-365mfc. To use a cosolvent whose boiling point is 89°F (32°C) in an open-top machine is ridiculous. But a dual-chamber machine with one chamber enclosed might be an entirely practical choice for certain parts. 

Soil materials such as auto brake fluid, auto transmission fluid, MIL-H-5006 (petroleum), mineral oil (white refined), motor oil SAE 20W, pine oil, and silicone oil offer excellent potential for cleaning by the Class III cosolvent process. The Ra values between soil and SA cosolvent are acceptable, the Ra values between RA and SA cosolvent are also... 

Soil materials thought to be difficult to dean such as natural mbber, coal tar pitch, Mil-H-8446 (silicate), sperm oil, and Mil-L-7808 (ester) appear to be excellent applications for the Class III cosolvent process. 

The second comparison between Class II and Class III cosolvent processes can be made for some of the soil materials listed with candidate cosolvent processes in Tables 3.3 (Class IIA — flammable) and 3.4 (Class IIB nonflammable). Calculated solubility parameters for both classes of cosolvent processes when used with the common soil materials are foimd in Table 3.9 . At least three observations can be made ... 

Dilution rinsing in Class III cosolvent processes may be as acceptable as desired . 

The reason that not all of the soils noted in Tables 3.3 and 3.4 are listed In Table 3.9 is that cosolvent pairs could not be found for them which meet the requirements of a Class III cosolvent process. 

Remember the differences in the Class II and Class III cosolvent processes. The former is designed to operate at lower cleaning temperatures with immisdble SA and RA cosolvents it s the opposite for the SA with the latter. 

The Ra value between soil materials and the SA cosolvent for both classes averages about 5 MPa, with the Class III cosolvent process being about ]/2 unit better (lower Ra) than the Class II cosolvent process. 

If the SA cosolvent is miscible with the RA cosolvent, the limitation on rinsing quality is the amount of contact provided by the user. And the separation between SA and RA cosolvents for Class III cosolvent processes listed in Table 3.9 averages about 5.5 MPa. ... 

As noted in Chapter 3.7.3 and Figure 3.26, Class III cosolvent processes use solvents which could be present as a single phase, but are conducted in separated (split or dual) chambers so that cleaning and rinsing can be done at different temperatures. 

The answer has two parts it s not needed and wasn t considered. First, almost without exception the SA cosolvent in a Class III cosolvent process will have low volatility so parts can be heated to a high temperature and so there is no concern about flammability, and often the RA cosolvent will be a volatile fluorinated material without a measured flash point so there is also no concern about flammability. Second, when this author created the classification scheme in the early 1990s, the focus was on Class II systems and the author did not adequately give Class III systems due consideration. 

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