Generation Of Power Using A Rankine-cycle Engine With Tetrachloroethylene As The Working Fluid

Abstract

Tetrachloroethylene has excellent physical properties as the working fluid in a Rankine-cycle engine. Thermal degradation of the working fluid can be reduced to acceptable levels by contacting the fluid with at least ten sq. cm of ferritic iron per cc of tetrachloroethylene when at temperatures greater than 200.degree.C.

Parent Case Text

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 68,740, filed Sept. 1, 1970, now abandoned.

Description

BACKGROUND OF THE INVENTION

Rankine cycle, high power, multistage, vapor turbines are well known as highly efficient heat engines for the production of power. When steam is employed, it is essential to employ multiple stages to obtain power at usable speeds and at useful efficiency. Superheat is also required, since, on expansion, saturated steam forms a mixture of vapor and liquid water which erodes turbine blades and is difficult to handle. Accordingly, steam turbines are generally complex pieces of equipment which are ill adapted to manufacture and operate on the small scale needed for portable engines of modest power, i.e., less than 1,000 h.p.

Small engines adapted for portability should preferably employ a single stage impulse turbine. This requires the use of working fluids having a higher molecular weight in order to reduce the efflux velocity at the nozzle and the speed of the turbine for a single stage. It is also highly desirable to operate below the critical temperature of the liquid in order to minimize the pressure requirements.

Many organic liquids have been proposed for use as working fluids for engines. For example, Fulton U.S. Pat. No. 795,761 has suggested such fluids as alcohol, ether, carbon disulfide or chloroform as working fluids. Govers U.S. Pat. No. 870,507 teaches the use of chlorides of carbon such as carbon tetrachloride, chloroform, ethylidene chloride or trichloroethane. Norton et al. U.S. Pat. No. 3,511,049 and Minto U.S. Pat. No. 3,479,817 have taught the use of chlorofluorohydrocarbons, and McEwan U.S. Pat. No. 3,516,248 has taught the use of a variety of non-halogenated organic fluids having the property that the change in entropy from the maximum saturated enthalpy point on a Mollier diagram to the entropy of the saturated vapor at atmospheric pressure is less than 0.1 BTU/(lb..degree.F).

Tetrachloroethylene has also been proposed as a working fluid for Rankine-cycle engines by Paxton U.S. Pat. No. 3,512,357. Although tetrachloroethylene has desirable thermal properties and is relatively inexpensive, the thermal stability at elevated temperatures needed for Rankine-cycle engines is inadequate.

SUMMARY OF THE INVENTION

The present invention is directed to a method of generating power wherein tetrachloroethylene is vaporized at a temperature greater than 200.degree.C, the vapor is expanded and does work, and is thereafter condensed and recycled, wherein the tetrachloroethylene is stabilized in the liquid phase by contacting the liquid with ferritic iron in an amount sufficient to provide a surface of ferritic metal/volume of liquid tetrachloroethylene of at least 10 cm.sup..sup.-1.

THE DRAWINGS AND DETAILED DESCRIPTION OF THE INVENTION

It has long been known that tetrachloroethylene has relatively low stability. Stabilizers suitable for stabilizing this compound have been described in many patents including U.S. Pat. Nos. 2,492,048; 2,997,507; 2,947,792; 2,094,367 and 2,096,735. While such stabilizers are effective for moderate temperatures such as those employed in dry cleaning, they are not effective in preventing the degradation of tetrachloroethylene at higher temperatures such as 246.degree.C (475.degree.F) employed in external combustion engines, even in sealed systems. Surprisingly, it has been found that the degradation of tetrachloroethylene can be inhibited by the presence of ferrous metals in the system. The term "ferrous metal" refers to metals or alloys in which metallic iron is the major component such as 1020 steel, 1018 steel, 303 stainless steel and the like. While stainless steel suffers the least corrosion and does effect some degree of stabilization, best results are obtained with ferritic steels such as 1018 steel or ordinary iron. The amount required to achieve significant stabilization should provide at least 10 sq cm of surface per cubic centimeter of liquid tetrachloroethylene and should be in contact with the liquid. This amount is greater than is found in conventional boilers. For example, a tube-type boiler with tubes having an inside diameter of 0.5 inches provides surface/volume ratio of 3:1 cm.sup..sup.-1. Accordingly, it is preferred to pack the boiler with steel wool or to employ iron metal grids or fine iron metal powder to achieve the desired stabilization.

Table I shows the results of tests wherein 1018 cold rolled steel, 430 stainless steel (17% Cr) and P11 steel (a low alloy steel containing 1.3% Cr and 0.5% Mo) are heated to 300.degree.C for 28 days in sealed glass tubes with tetrachloroethylene.

TABLE I

Decomposition of Tetrachloroethylene and Corrosion in the Presence of --------------------------------------------------------------------------- Ferrous Metals: 28 days at 300.degree.C

1018 Steel Corrosion Other Corrosion % C.sub.2 Cl.sub.4 S/V cm.sup.-.sup.1 Mg/cm.sup.2 S/V cm.sup..sup.-1 Mg/cm.sup.2 after test 0.8 260 0 1.5 129 0 3.0 33 14.7 6.0 11 23.8

P11

1.7 85 17.0 2.4 68 5.1 0.8 100 1.7 54 41.7 1.5 12 1.7 53 20.7 3.0 10 1.7 42 23.9 6.0 6 1.7 24 27.9 12.0 4 3.4 7 46.6

430 SS

1.15 11.4 13.2 2.65 7.5 12.4 5.3 5.8 21.1 3.1 6.8 12.2 0.8 165 1.15 7.1 4.6 1.5 42 1.15 6.0 10.4 3.0 46 1.15 10.4 8.4 6.0 16 1.15 5.5 35.0 12.0 5 2.3 1.8 75.6 12.8 10 5.6 1.2 75.8 __________________________________________________________________________

in another experiment 0.3 gm of iron powder having an estimated surface area of 700 cm.sup.2 was combined with 0.3 cc. of tetrachloroethylene in a 2 cc tube and heated for 28 days at 300.degree.C. At the end of this period the tetrachloroethylene was analyzed by gas chromatography and found to be 99.1% pure.

The reason for the stabilizing action of iron is not known and it is possible that a compound derived from iron such as FeCl.sub.2 rather than the metal itself is the effective stabilizer.

In the accompanying drawings:

FIG. 1 shows the entropy-temperature diagram which has been determined for tetrachloroethylene.

FIG. 2 is a schematic view of a turbine power generator adapted for use with tetrachloroethylene as a working substance.

Referring now to FIG. 1, the critical temperature of tetrachloroethylene is 662.9.degree.F and the critical pressure is 568.75 psia. The value of ds/dT for the saturated vapor line is very slightly positive, i.e., the change in entropy in passing from the critical temperature to the boiling point is less than 0.02 BTU/(lb..degree.F). Accordingly, vapor produced by expansion of saturated vapor from a temperature below the critical temperature to the pressure of the condenser will be only slightly superheated so that an efficient Rankine-cycle engine can be operated without the problems encountered with vapor/liquid mixtures after expansion, e.g., at the turbine wheel, while the degree of superheat after expansion is so slight that it can be neglected. Thus, regenerative cooling of the vapor prior to condensation is unnecessary.

A typical cycle employed in Rankine cycle engines is shown in FIG. 1. The vapor is produced essentially saturated at a temperature of 475.degree.F corresponding to point 1 of FIG. 1 at a pressure of 178.4 psia. The gas is expanded essentially isentropically to a pressure of 3 psia, the condenser pressure, and cooling to a temperature of 205.degree.F indicated by point 2 in FIG. 1 whereupon the gas does work in, for example, a turbine. The gas is then cooled to 160.degree.F (3) and condensed to a liquid at 160.degree.F (4) in the condenser. The condensed liquid is pumped back to the boiler and heated to 475.degree.F (5) at which temperature the liquid is vaporized to vapor at 475.degree.F and 178.4 psia, thus completing the cycle. With the above cycle, the Rankine cycle efficiency is 26.6%. The small amount of superheat can be recovered by a heat exchanger or regenerator and employed to heat the boiler feed. However, with 70% regeneration, the efficiency is only increased to 27.4 %.

FIG. 2 is a schematic diagram of a turbine engine operating on the cycle described above in connection with FIG. 1 using tetrachloroethylene as the working fluid. The boiler indicated by 10 heats the liquid and vaporizes it to saturated vapor. The vapor is expanded in the nozzles of turbine 11 and cooled, the expanded vapor doing work on the blades of the turbine. The vapor at about 205.degree.F is transferred to the condenser 12, cooled and condensed and the liquid pumped back to the boiler 10 by pump 13. In addition to the substantial efficiency which can be obtained in a Rankine cycle engine as described above, tetrachloroethylene has other desirable properties.

Molecular Weight

The molecular weight of tetrachloroethylene is 166. Since the efflux velocity on expansion in a nozzle is roughly in inverse proportion to the square root of the molecular weight, sufficiently low efflux velocities can be achieved to render a single stage impulse turbine feasible. The low efflux velocities and the relatively low operating temperatures make the use of light metals such as aluminum or even plastics feasible for construction of the turbine blades.

Melting Point and Boiling Point

The melting point of tetrachloroethylene of -19.degree.C (0.degree.F) makes the use of the liquid as a working fluid feasible even when relatively low ambient temperatures are encountered. The boiling point of 121.degree.C (250.degree.F) provides reasonably efficient condensing with gaseous or liquid coolants at ambient temperatures.

Liquid Density

In the copending commonly assigned applications of William A. Doerner, U.S. Ser. No. 110,478 filed Jan. 28, 1971 as a continuation-in-part of U.S. Ser. No. 25,857 filed Apr. 6, 1970 and now abandoned, U.S. Ser. No. 231,232 filed Mar. 2, 1972 and U.S. Ser. No. 227,902 filed Feb. 22, 1972; also U.S. Pat Nos. 3,590,786 and 3,613,368 to William A. Doerner there are disclosed rotary engines, rotary boilers and rotary regenerators for low-pressure application. In the case of the rotary boilers and regenerators, high density liquids are particularly desirable to minimize the speed of rotation required to maintain a film of liquid in the desired uniform condition. Tetrachloroethylene liquid has a density of 1.61 at 25.degree.C and is well suited for use in such high density, low weight equipment.

Flammability

Tetrachloroethylene is not flammable and therefore presents no fire or explosion hazard, in the event of damage to the heated engine.

Toxicity

In the event of leakage or in the case of accident when the power fluid escapes from the heat engine, low toxicity is highly desirable. Tetrachloroethylene has low toxicity: inhalation of the vapor at a concentration of 100 ppm per 8 hour day is permissible. Tetrachloroethylene is not considered a contributory cause of smog.

The foregoing detailed description has been given for clarity of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described for obvious modifications will be apparent to those skilled in the art.

Claims

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. The method of generating power in which tetrachloroethylene is vaporized at a temperature greater than 200.degree.C, the vapor is expanded and does work and is thereafter condensed and recycled, wherein liquid tetrachloroethylene is contacted with ferritic iron in an amount sufficient to provide at least 10 sq. cm. of iron per cc of tetrachloroethylene while at temperatures greater than 200.degree.C.