US2937504A - Process for the vaporisation of liquefied low-boiling gases - Google Patents

Description

May 24, 1960 B. RIEDIGER 2,937,504

PROCESS FOR THE VAPORISATION OF LIQUEFIED LOW-BOILING GASES Filed Oct. 10, 1956 2 Sheets-Sheet 2 20C lat/n. gauge Methane 2,937,504 7 PROCESS FOR THE VAPORISATION F LIQUEFIED LOW-BOILING GASES Bruno Riediger, Frankfurt am Main, Germany,"assignor to Metallgesellschaft A.G., Frankfurt am Main, German Filed Oct. 10, 1956, s... No. 615,1ss I Claims priority, application Germany Oct. 10, 1955 7 Claims. (Cl. 62 -53) Stat s Pa ent 10 In the production of natural gas as well as in the production and processing of petroleum, large quantities of methane, ethane and similar low-boiling hydrocarbons, or gases rich in such substances, are produced. Since such' substances cannot always be put to good use in the areas where they are produced, they are frequently piped to distant consumers. These possibilities ofutilisation, however, are in many cases, e.g. the petroleum sources of the Middle or Far East, the Gulf Coast or Venezuela, not present, or only present to a limited extent. The transport of natural gas or the like in very long trunk supply pipes also involves considerable costs. w

Another possibility for taking methane, ethane, or similar gases from areas where there is a surplus and conveying them to consumer areas lies in the liquefaction of the gases and the despatch of these liquids in ships,

or other large-capacity containers. It is necessary, here,

that the temperatures employed for the liquefied gases should be below their critical temperature.

.It is only in this way that the liquefied gases can be stored or transported under the usual pressures for transport receptacles-where possible at atmospheric pressure or at a pressure a little above atmospheric. If the critical. temperature were to be exceeded, complete evaporation would take place, and the pressure would rise to such an extent that the walls of containers which can be economically produced would not be able to withstand the same.

In the areas of surplus there is a cheap supply of natural gas or products from petroleum processing. The liquefaction of the gases which have to be transported is, therefore, not subject to any high costs because the not inconsiderable energy requirements for this process can be met without much expenditure.

The present invention aims at recovering a major part of the energy which has been expended in the liquefaction of low-boiling substances such as natural gas, methane,

ethane, or the like, at the place of destination after-the liquefied substances have been transported in largecapacity-containers, especially ships. To this end, according to the invention, the vaporis'ation or heating of the liquefied substances is carried out in conjunction with a cyclic process in which a working medium is cooled and then heated again. It is effective ifthe liquefied substances, are utilised as cooling agents, preferably at fairly low temperatures, in one or more cyclic processes, e.g. inone of these in which a vaporous medium which'has done work is completely or partially condensed by cooling, the condensate formed from the vapour being subsequently vaporised again. As an example acondensate is vapourised, the vapours of which are supp1ied,'undei' the pressure generated by the evaporation,

. 2,937,504 Patented lVlay 24, I960.

pressure gradients, and possibly, before being utilised for the generation of energy, they can be heated further and their enthalpy raised, by the application of heat, to such a value that the gases, after expansion in the engine, are available with the required conditions (pressure and temperature) so that they are ready for further use.

When use is being made of the cold content of the liquefied gases in a cyclic process it is desirable to choose the working medium of the cyclic process so that the heat required simply for re-vaporisation can be extracted from the surroundings, e.g. the atmosphere or supplied from the heat content of water. Waste energy can also bemade available for this purpose. The cyclic process is, thus,

so arranged that the temperatures involved are for the greater part below the ambient temperature.

It is also possible to use the liquefied substances in succession in two or more cyclic processes, by so selecting the working media in the successive cyclic processes that they are condensed at temperatures which are higher from one cyclic process to the next. If, for example, it is desired to utilise the cold content of liquefied ethane which has been transported in ships in the liquid state at atmospheric pressure, this is available at the site of utilisation at a temperature the maximum of which is approximately- -88 C. In view of the need for a most favourable pressure gradient in the cyclic process it appears advisable to choose as working medium for this the hydrocarbon next above ethane as regards boiling, point viz. propane.

The condensation temperature for the propane will then lie, for example, above approximately -40 C. if, for reasons which will be understood, it is desired to avoid a vacuum in the system. The propane can then be vaporised at temperatures up to +96 C. and then superheated. The pressure to be applied is arbitrary and can i also be above the critical pressure. Thus there is an even greater enthalpy gradient available for the generation of energy.

In the case of liquefied methane the transport temperature at atmospheric pressure lies around -160 C. For

its utilisation at the destination it is expedient to use 7 cold of liquefied ethane;

Fig. 2 represents the utilisation of the cold of liquefied methane in the same manner; and

. Fig. 3 is a diagrammatic exemplification of another embodiment of the invention in which the liquefied gas itself is also utilised in an energy generating plant as the working medium.

In the process, represented in the diagram of Fig. 1,. the liquefied ethane passes at a temperature of -88,, C. and under a pressure of atmospheres gauge, through a conduit 17 into the condenser 1 where it serves forjthe; condensation of propane vapour coming through a couduit 12 from an expansion engine 2 at a temperature of 25 C. and under a pressure of 2 atmospheres absolute. In the condenser 1 the propane vapour is condensed; the condensate flows through conduit 13 to apump 5 which brings it to a pressure of about 65 atm. absolute and leads it through a conduit 14 to a vaporiser 3. The propane vapour which is drawn off from this vaporiser through conduit 15 is raised to a temperature of about 125 C. in the superheater 4 and passes under a pressure of about 60 atm. absolute through conduit 16 to the expansion engine 2. This preferably takes the form of a turbine for driving a generator 17 for electrical energy. After expansion, the propane starts its cycle afresh in the manner described. The ethane is heated in the condenser to about -40 C. and led through conduit 18 to the point at which it will be further exploited or utilised.

In the vaporiser 3 the heating agent in the first place may be air drawn from the ambient atmosphere but it must be absolutely dry so as to avoid the formation of ice on the heat-exchange surfaces. As soon as the propane has been heated to such an extent that the wall temperatures in the vaporiser are above C., the further heating can also be effected by means of waste energy, e.g. in the form of hot water from the condensers of steam turbines or the cooling water system of diesel engines or waste steam under low pressure. The heating agent enters the vaporiser through conduit 19 and leaves itv through conduit 20.

For the pressures and temperatures in the propane cycle other values may naturally also be chosen. It is desirable to choose them so that on one hand the lowest pressure in the cycle remains above the external atmospheric pressure, to prevent air penetrating in the event of a leak occurring, and on the other hand the working pressure is only chosen so high that, not only common design may be applied for the apparatus for the cyclic process but also an adequate enthalpy gradient can be made available in the expansion engine.

In the embodiment illustrated in Fig. 1, the highest working pressure in the propane cycle is given as 60 atm. absolute and the highest temperature 125 C. There is no reason why even higher working pressures should not be employed. In the last resort the criterion for a suitable choice of the maximum working pressure is the amount of the throughput, so that the blades in the first stage of the turbine have lengths with which favourable turbine efficiencies can be reached.

It is not necessary to avoid exceeding the critical conditions in the propane cyclic process, as is proved by the successful application of the Benson Principle in the construction of steam power plant, but allowance must be made for this condition in the construction of the vaporiser 3 and the superheater 4.

It is advisable so to select the maximum temperature in the cyclic process that, with the usual efficiencies of the expansion engines, the vapour issuing therefrom is just in the saturated state. However, any deviations from such a selection have only an insignificantly harmful effect,

on the economic efficiency obtainable. Other conditions being the same, a reduction in temperature at the outlet from the superheater 4 results in the wet steam range being reached at the outlet from the expansion engine 2, which in the case of turbines, for example, can lead to erosion of the blades. A rise in temperature has a similar efiect on the efficiency of the cyclic process as such, as does the use of superheated steam in the usual power plant process. It is advisable, however, in general to fix the maximum temperature so that waste heat can be utilised in the vaporiser 3 and in the superheater 4. The feed pump can also consist of two units if a storage tank for the circulating fluid has to be interposed between the condenser and the vaporiser, one of these units being located before the reservoir and one after it.

An example of the method for exploitation in stages of the energy gradient in case of liquefied methane em- 4 ploys ethane in a primary cycle and propane in the secondary cycle as the working medium, as shown in Fig. 2. The choice of pressures and temperatures is governed by the same considerations as those prevailing in the method of operation described with reference to Fig. 1.

Liquid methane passes through a conduit 31 at a temperature of about l6 0 C. and under a pressure of about 90 atm. gauge into a condenser 21. It flows out from said condenser 21 through a conduit 42 into a condenser 26, which it enters at about 100 C. and leaves at a temperature of -40 C. through conduit 43 to be conducted to the point of its further exploitation or utilisation. The pressure drop of the methane during its passage through the condensers amounts to about 10 atm. absolute, so that the vaporised methane in the conduit 43 is under a pressure of about atm. gauge.

In the primary cycle I the working medium is ethane, and in the secondary cycle H propane is used. Both cycles can be designed on the diagram of the cycle shown in Fig. 1 and in the same manner. For example, the primary cycle is provided with an expansion engine 22 together with the plant 37 for producing electrical energy, the feed pump 25, the vaporiser 23 and the superheater 24, and the cycle II with the same equipment 27, 41, 26, 10, 28 and 29, which are connected by means of the conduits 32, 33, 34, 35, 36 and 52, 53, 54, 55 and 56, respectively. As an example, the ethane in cycle I enters the expansion engine 22 at a temperature of 80 C. and a pressure of 60 atm. absolute and leaves it under a pressure of 2 atm. absolute and a temperature of 75 C. The condensate passes at the same temperature from the condenser 21 to the feed pump 25 in which its pressure is raised to approximately 65 atm. absolute. The propane cycle II shown in Fig. 2 can work with the same pressures as the cycle in Fig. 1 and corresponding temperatures.

With the operations according to the invention, the low temperature of a liquid hydrocarbon or mixture of hydrocarbons or of substances with similar physical properties can be utilised for producing energy, in a way which is particularly favourable from both the economical and technical standpoints, from amounts of heat which shall be drawn off at a low temperature and which represent either a desired cooling performance or a waste energy which cannot be utilised readily. The liquefied gas is thereby vaporised and brought to a higher tem perature.

If the temperature desired for the vaporised methane, etc. is near the ambient temperature and higher than can be attained by the examples described with reference to Figs. 1 and 2, this can be achieved, for example, by raising the pressure in the cyclic system and hence the temperatures in the condensers. Alternatively it can be effected by the addition of a third, for example an analogous cycle, for butane, for example, thus obtaining an outlet temperature for the gas leaving the condenser of, for example, +5 C.

According to Figs. 1 and 2 the liquefied gas is passed through the condensers under fairly high pressure. Since, initially, it is liquid which has to be conveyed, this conveyance under pressure only requires low energy rates. This conveyance under pressure can be utilised with advantage for the further recovery of energy from the liquefied and re-vaporised gas. It can also be of advantage in other cases, however, for example, when it is required to shift the start of vaporisation of the liquefied gas to higher temperature ranges. This again has the advantage that the flow cross sections in one part of the installation can be kept smaller and the heat transfer coefiicients which are more favourable with liquids can be utilised in a wider temperature range.

In Fig. 3 a pump 62 which is provided inside a ship 61 or other storage container pumps the liquefied gas first through an installation K which may take the form illustrated in Fig. 1 or 2 and in which the liquefied gas is vaporised and heated. From this installation K the gas passes through a conduit 63, for example with a temperature of a little over 0 C. and under a pressure of 80 atm. gauge, into the heater, for example the tubular heater 64 or into an apparatus of similar action. In

this heater the gas is brought, by means of external heat, to such a temperature that, after expansion in an engine 65, it reaches a desired condition, making it suitable, for example, for its further conveyance through a distribution system to the consumers. The inlet temperature to the heater 64 should preferably be chosen a little above freezing point so as to avoid icing of the tube system by the steam contained in the flue gases. The choice of this temperature point is more or less arbitrary, however, and has no great influence on the results obtainable. The higher the temperature point due to the supply of heat in the preceding part of the installation K, the less heat will be necessary to supply to the heater 64. If, for example, at the outlet from the energy generating plant 65, the gas is to be available at 20 C., 1 atm. gauge, it is advisable that, with a pressure of 60 atm. gauge before the expansion engine 65, it should have a temperature of about 200 C. There is nothing against the use of an even higher pressure. In this case a corresponding increase in the temperature now given as 200 C. as a result of the expansion process should take place. The other pressures employed in the form of embodiment according to Fig. 3 can be obtained as approximations from the anticipated pressure drop and can be calculated for each case individually according to the known technical rules.

The efiiciency of this production of energy from heat energy is very favourable as compared with the usual power plant processes because the heat which has to be supplied to the tubular heater 64 is simply the heat for superheating. To this extent it coincides with the gas turbine process. However, the considerable consumption of energy for the necessary work of compression, which markedly reduces the overall efficiency of the gas turbine process, is eliminated because, according to the invention, the working medium being in the liquid state can be brought to the necessary pressure with a much smaller energy input. As regards the last mentioned point it coincides with the steam process.

The process of the present invention has the great advantage over the latter, however, in that no vaporisation heat needs to be supplied to the working medium in the tubular heater since this heat can be withdrawn from the surrounding air in the preceding processes or can be made available from waste heat.

Iclaim:

1. A method of recovering energy from liquefied low boiling gases which comprises vaporising such liquefied low boiling gases in heat exchange relationship with a gaseous working medium which is condensed during such vaporisation, raising the pressure of the thus condensed working medium to a higher pressure, then vaporising said condensed working medium at said higher pressure, expanding said vaporised working medium to recover energy and then recycling the vaporized working medium to the condensation step where it is recondensed to provide a cyclic process.

2. The process of claim 1 in which the vaporisation of said low boiling gases is carried out under pressure.

3. The process of claim 1 in which at least two working media are provided, each Working medium being sequentially condensed, the pressure of the condensed working medium raised, the condensed working medium vaporised under the raised pressure, the resulting vapors; expanded to recover energy therefrom and then recon densed in a separate cyclic process and in which the liquefied gases are vaporised by passing them successively as cooling agents in heat exchange relationship with the working medium of each cyclic process to effect the condensation of the working medium in each cyclic process.

4. The process of claim 3 in which ethane is employed as the working medium of the first cyclic process and a hydrocarbon having a higher boiling point than the hydrocarbon of the next preceding cyclic process is employed in each successive cyclic process.

5. The process of claim 1 in which the vaporisation of .the condensed working medium is carried out below the ambient temperature.

6. The process of claim 1 in which the vaporised gas obtained by vaporisation of the liquefied gas in heat exchange relationship with the working medium is expanded to a predetermined pressure and temperature to recover energy therefrom.

7. The process of claim 1 in' which additional heat is supplied to the vaporised gas obtained by vaporisation of the liquefied gas in heat exchange relationship with the working medium and such heated gas is then expanded to recover energy therefrom.

References Cited in the file of this patent UNITED STATES PATENTS France Sept. 26,

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