Process and Apparatus for Liquefying Gases or Gaseous Mixtures, and for Producing Cold, more particularly applicable for Separating Oxygen from Atmospheric Air.
Year: 1895

page_text: PROVISIONAL SPECIFICATION.
Process and Apparatus for Liquefying Gases or Gaseous Mixtures,
and for Producing Cold, more particularly applicable for
Separating Oxygen from Atmospheric Air.
I, CARL LINDE, of Munich, Bavaria, Professor, do hereby declare the nature of
this invention to be as follows:-
This invention relates to a process and apparatus by means of which more
particularly oxygen or a mixture of oxygen and nitrogen rich in oxygen is
produced, by first liquefying atmospheric air and by then separating the nitrogen
in part or entirely by evaporation from the liquid.
The process is based on the fact observed by physicists, that oxygen, though its
boiling point be higher than that of nitrogen, can only be liquefied together with a
portion of the nitrogen, but that nitrogen first separates from the liquefied mixture
on vaporising the same, so that the mixture becomes the richer in oxygen, the
longer the evaporation is continued.
The liquefaction of atmospheric air has hitherto been performed by physicists in
such a manner, that, substances of gradually increasing volatility, such as carbonic
acid, nitrous oxide, ethylene and the like, are consecutively liquefied and vaporised
again. According to my invention I effect such liquefaction at a single operation
under the following conditions. When a gas is caused to escape under a certain
pressure into a chamber, where the pressure is lower, the temperature of the escaped
gas, after it has come again to rest, is, according to the laws of thermodynamics, the
same as that before its escape, so long as the gas completely follows the laws of
Mariotte and Gay-Lussac, that is to say, so long as the inner attractive forces of
the smallest particles have no sensible influence on the thermal condition. But as
soon as there is combined with an increase of volume a consumption of latent heat
for overcoming these inner powers, the gas will after the discharge, be of a lower
temperature than before.
Now as is commonly known, the before mentioned laws are not quite correct with
respect to atmospheric air, even at the ordinary thermal condition and, in particular,
the well known experiments of Thomson and Joule have shown, that as a matter of
fact, at the discharge a certain cooling takes place. The more the specific volume
of the air is reduced (by increasing the pressure and reducing the temperature) the
more considerable is the cooling.
This physical law is utilised according to my invention in the following manner.
On the accompanying drawing C is a compressor, by means of which the
pressure of the air is increased from a pressure p¹ to p³, the temperature of the air
increasing from t¹ to t. The air then flows to the refrigerator K, where by means
of cooling-water or by any other convenient source of cold, heat is taken off, so that
the temperature is lowered from t* to t³, the pressure remaining constant at p³.
In this condition the air is led to a reversed current-cooler G', working as
perfectly as possible and being protected in the best possible manner against any
absorption of heat from without; in passing through the said cooler G' the
temperature of the air is still further reduced to t* for reasons hereinafter explained.
[Price]

page_text: At the end of the reversed current cooler G' a regulating valve R' is arranged,
through which the air escapes into the vessel V, which is also carefully protected
against any absorption of heat and where the pressure is equal to p'. After its
escape the air will have a temperature 5, which is the more inferior to t, the greater
is the pressure-difference p²-p and the lower is the temperature t. The air having
now the temperature to is led into the outer pipe of the reversed-current cooler Gi
through which it flows in the contrary direction to the arriving air, so that the
temperature of the latter before its escape is still further lowered and the two
temperatures and to decrease continuously until by a supply of heat from outside
or by an internal disengagement of heat, a permanent condition is established.
The latter will take place as soon as the cooling has arrived at the condensing
point of the atmospheric air, at which a liquefaction will commence, whereby latent
heat is set free. When the permanent condition has been established a definite and
self-regulating portion of the escaping air will be collected in a liquid condition in
the vessel V, while the remaining portion returns through the reversed-current-
cooler to the compressor C.
The liquid collected in the vessel V₁ passes through a second regulating valve R²
into the vessel V2, in which exists the ordinary atmospheric pressure (°). At the
reduction of the pressure from p₁ to p° a portion of the liquid evaporates, namely a
portion of the nitrogen. Heat being then imparted to the liquid contained in the
vessel V2 by the passage through the worms of a greater or less quantity of the air
compressed by the compressor C to the pressure, the evaporation of the nitrogen
can be continued at will, and then the remaining oxygen mixture (or, as the case
may be, pure oxygen) may be drawn off through the cock h.
The nitrogen
escaping at A exchanges heat in a reversed-current-apparatus G2 with the arriving
compressed air. When oxygen is to be obtained not as a liquid, but in a gaseous
condition, the liquid is caused to flow through the reversed current apparatus G³,
against a third portion of the compressed air, so that the heat necessary for
evaporating and re-heating the oxygen is taken from the compressed air. By this
means the cold employed for cooling and liquefying the air is completely recovered
and the machine only requires to produce the cold necessary for covering any losses.
The feed pump P serves to charge the machine with air and to replace the
discharged liquefied air.
The output of this machine depends essentially on the perfection of the reversed
current-action in the three apparatus G' G2 G3 and on the insulation of these
apparatus and of the vessels V₁ and V2 against any absorption of external heat.
As shewn in the accompanying drawing, these apparatus are formed by placing
two pipes of considerable length (say of about 100 metre one into the other, then
helically coiling up and mounting them on wooden supports, the several windings
being isolated one from the other, and externally by an effectual isolating material
(such as raw sheep's wool).
This machine may of course also be used for liquefying other gases as well as
atmospheric air, and for separating other gaseous mixtures by means of
liquefaction.
It is also obvious, that the low temperatures to obtained in the vessel V₁ may be
employed for other purposes as well as for liquefying gases and that this machine
may generally be used as a refrigerating machine for extraordinarily low
temperatures.
Dated this 28th day of June 1895.
ABEL & IMRAY,
Agents for the Applicant.

page_text: COMPLETE SPECIFICATION.
Process and Apparatus for Liquefying Gases or Gaseous Mixtures,
and for Producing Cold, more particularly applicable for
Separating Oxygen from Atmospheric Air.
I, CARL LINDE, of Munich, Bavaria, Professor, do hereby declare the nature
of this invention and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:-
This invention relates to a process and apparatus by means of which very low
temperatures are produced and atmospheric air or other gaseous mixtures or
gases are liquefied, mainly for the purpose of separating the constituents of gaseous
mixtures such as the oxygen and nitrogen of atmospheric air; it is however
generally applicable for the production of cold.
The method of effecting the above mentioned separation is based, for example,
with reference to atmospheric air, upon the fact known by physicists that oxygen,
although its boiling point is higher than that of nitrogen, can only be liquefied
together with part of the nitrogen, while on volatilizing the liquefied mixture, the
nitrogen will be first evaporated, so that the mixture will become richer in oxygen
the longer the volatilization is continued.
The liquefaction of atmospheric air has hitherto been effected by physicists by
producing successive liquefactions and volatilisations of liquids of an increasingly
volatile nature, such as carbonic acid, nitrous oxide, ethylene &c.
In the Specification of Solvay's Patent No. 13466 dated 6th November 1888
was described a different process for this purpose, which consisted in causing
compressed air to expand while doing work in a cylinder with piston whereby it
was deprived of an amount of heat equivalent to the work performed, and then
causing the cold thus produced, to he transferred to the compressed air passing to
the expansion engine, whereby, it was assumed, the temperatures at the commence-
ment and termination of the expansion would continue to sink until liquefaction
ensued.
Precisely the same process was described in C. W. Siemens' Provisional Specifi-
cation dated 29th July 1857 No. 2064, for the purpose of producing very low
temperatures.
This process has however not proved susceptible of practical application in so far
as regards the attainment of such low temperatures as are requisite for liquefying
air, because it was not possible to maintain an expansion engine in proper action at
temperatures below the critical temperature of air, namely-140° C., nor to protect
the engine-cylinder in which the expansion of the air was effected sufficiently
against the absorption of heat from the outside, to enable such low temperatures
to be attained.
According to my present invention however I am enabled, by a materially
different process, to effectually and economically produce liquefied air or other gas
in a continuous manner, the said process being based upon the following physical
acts.
If a gas under a certain pressure be made to issue into a space in which a lower
pressure exists, then according to the laws of thermodynamics the temperature of
the expanded gas, after it has come to rest, will be the same as that which it had
before expansion, so long as the gas complies perfectly with the laws of Mariotte
and Gay-Lussac, that is to say, so long as the internal attractive forces of the
molecules have no perceptible influence upon the temperature.
But as soon as the expansion causes a consumption of latent heat for overcoming
he said forces, the gas will, after expansion, show a lower temperature than before.
Now, as is known, the above law does not strictly hold good in the case of

page_text: atmospheric air, even at ordinary temperatures and in particular the experiments
of Thomson and Joule have shewn that as a matter of fact a certain reduction of
temperature does take place on expansion; such cooling, expressed in Celcius
2
degrees, is: 8=0·276 (p²—p¹) (273) * where p²-p² indicates the difference of
pressure and T the absolute temperature of the outflow.
The more the specific volume of the air is reduced (by increase of pressure and
decrease of temperature) the greater will be the cooling effect.
According to my present invention I apply the above known facts as I will
proceed to describe with reference to Fig. 1 of the accompanying drawings, which
shews a diagrammatic view of the apparatus I employ.
C represents any suitable compressor by means of which air is compressed from
a pressure to a higher pressure p², the air being thereby raised from a
temperature to that of 13. The compressed air is then passed through a cooling
apparatus K of any suitable arrangement in which it is cooled down from the
In this condition
compression temperature t² to f³, the pressure remaining at 2.
the air is led into a reversed flow heat interchanger G¹, which must be of very
perfect action and be protected to the greatest possible extent against transmission
of heat from the exterior; on passing through this interchanger, the compressed air
has its temperature farther reduced to t' by means to be presently explained. At
the other end of this interchanger is a restricted orifice, preferably constituted by a
regulating valve R', through which the air issues into a chamber V₁ the interior of
which is maintained at a lower pressure, say p' (and which is also protected as much
as possible against absorption of heat) so that the compressed air issuing from the
interchanger will expand from the pressure to such lower pressure pl. The
expanded air in the chamber will then have the reduced temperature to which will
differ from the temperature to an extent proportionate to the pressure
difference p-p². The air at this temperature t³ is now made to pass through
the second passage of the interchanger G1, in the contrary direction to the first
named current of air, so that the temperature of the latter will be farther reduced
before expanding into the chamber V. By such expansion a still farther reduction
of temperature will be obtained in V₁, and thus a continuous lowering of the
temperatures and to will be obtained until owing to the introduction of heat from
the outside, or to a liberation of latent heat from within, a state of equilibrium is
attained.
This state will ensue as soon as the cooling has advanced to the point of
condensation of the air, when liquefaction will take place, and latent heat will be
given off.
When this point is reached, a definite proportion of the expanding air
in the chamber VI will assume a liquid condition and will collect in V₁ while the
uncondensed portion will flow through the interchanger G' back to the compressor C
and will then pass through the same circuit as before.
The liquefied air in the collecting vessel V₁ is led thence through a pipe R with
a regulating valve R2 into the vessel V2 in which there is a lower atmospheric
pressure po. Owing to the reduction of pressure from pl to p a portion of the
liquid will evaporate, namely a portion of the nitrogen. In the vessel V is
arranged a pipe coil S through which a portion of the air compressed by the
compressor to the pressure p² is made to pass from the cooler K, the heat of
which air, being taken up by the liquid, causes a farther evaporation of nitrogen,
so that liquid oxygen, practically free from nitrogen will eventually remain in
the vessel, and can be drawn off through the cock h. The nitrogen gas passing
off at A is made to pass through a reversed flow interchanger G through which
the before mentioned body of compressed air passes to the vessel V2, so that such
air is thereby cooled more or less before entering V2, whereby the expenditure of
cuergy represented by the cold of the nitrogen will be economised.
If it be desired to obtain the oxygen in the gaseous form, the liquid is passed
through the pipe R³ into a third reversed flow interchanger G³, where it is made

page_text: to take up heat from a farther portion of the compressed air and becoming
volatilised thereby, it is led off to a receiver through pipe R³. The cooled
compressed air passing off from the interchangers G and G³, is led through pipe R'
to the air pipe of the interchanger G where it flows together with the body of air
from G into the chamber V₁; it will thus be seen that by the above described
means practically the whole of expenditure of energy represented by the degree of
cold employed for cooling and liquefying the air will be regained, and the only
actual expenditure of work will be that required for making good losses and for
the initial compression of the air in the pump P, which serves for supplying the
compressor C with air compressed from atmospheric pressure p' to the initial
degree of compression p¹, for charging the apparatus and for making good the
quantity liquefied.
Figures 2 and 3 shew vertical sections of the actual construction of two modified
arrangements of the above described apparatus. Figure 2 shews an arrangement
where the gas or gaseous mixture which is subject to the liquefying process, is
retained unseparated in the liquid condition, being discharged from the expansion
chamber V1 through the pipe h, or the low temperature produced in the apparatus
may be employed for refrigerating or liquefying another gas, for which purpose
a coil X is placed in the expansion chamber V₁ into which the said gas enters
at i and whence flows away again at k. The non-liquefied gas or the volatilised
gas in V passes thence through the interchanger G and escapes at N, after taking
up heat from the compressed gas entering the interchanger at A. Figure 3 shew's
an arrangement for separating a mixture of gases (such as atmospheric air), in
which the vessels V₁ and V2 described with reference to Figure 1, are combined in
one, as are also the reversed current interchangers G1 G2 of that arrangement.
The compressed air enters at A, and is distributed to the two reversed current
interchangers G and G³, and after uniting again at B on issuing from these, it
flows through the coil S in the vessel V₁ from which coil it expands through
valve R into the vessel V'.
The portion passing off in the gaseous condition (nitrogen) passes up through G'
and issues at N, while the liquid portion (oxygen) passes through the inter-
changer G³ and flows off at O.
The other letters of reference have the same significance as in Figure 1.
The degree of useful effect of the above described apparatus is dependent upon
the following conditions:
:-
1. As, according to the formula giveu above, the reduction of temperature by
expansion in the chamber V₁ only amounts to about 0.25° C. for a difference of
pressure of one atmosphere, the liquefying process can only be practically carried
out with the application of a very high difference of pressure p²-p'. Experience
has shown that for the mere making good of the losses in working, p²-p' must be
greater than 20 atmospheres.
1
2. In order to keep the work of compression within moderate limits notwith-
standing the employment of such great difference of pressure, the proportion p³: p¹
must be taken as small as possible. Thus if for example p³ 200 atmospheres
and p:
100 atmospheres, then a reduction of temperature of about 25° C. will
be obtained with an expenditure of energy that will not be greater than that
required for compressing air from 1 atmosphere to 2 atmospheres.
=
3. The entire apparatus must, for maintaining the requisite very low temperatures,
be perfectly enclosed in a good isolating medium, without having any parts
thereof, whether moving or fixed, exposed to atmospheric temperature.
The external connections must therefore only be at those parts of the reversed
flow-interchanger in which atmospheric or slightly lower temperatures exist.
4. Inside the apparatus, those parts which move with friction must not be subject
to the low temperatures required for liquefying the air, because the slightest
admixtures, such as moisture, lubricating material, carbonic acid &c., would be in a
frozen condition and would arrest or impede the motion.
5. The reversed flow interchangers must be formed of channels with metal walls

page_text: of very considerable length, which are only connected to each other at the ends
and which throughout their entire length must be protected in the most perfect
possible manner against the absorption of heat from the outside, as also against
conduction from one part thereof to another.
Now the process proposed by Siemens and Solvay, hereinbefore referred to,
when considered relatively to these conditions is in the following position.
As to Conditions Nos. 1 and 2 the conditions of pressure are entirely different,
as with the Siemens process only low pressures (from 1 to 4 atmospheres) are
contemplated, whereas with the present process at least 20 atmospheres are
required.
The Conditions 3 and 4, though requisite both for the Siemens and Solvay
processes and for the present one, cannot be practically fulfilled by the former
owing to the reduction of temperature being effected by doing work in an engine,
parts of which are necessarily in contact with the atmosphere.
A proof of this is given in the statement made by Solvay (see Comptes
Rendues 1895, p. 1141) to the effect that "the maximum lowering of temperature
which he had succeeded in obtaining was 95° C., the causes of loss of cold, after
-
that point was reached overbalancing the power of production thereof."
All heat interchangers that have hitherto been proposed in connection with the
Siemens process fail to comply with the Condition No. 5. In the present apparatus
this condition is fulfilled by constructing the interchangers of two tubes of different
diameters and of considerable length (such as 300 feet) of which one is inserted
into the other so as to leave an annular space between them and then both are
coiled up helically, the successive convolutions being separated from each other
and from the outside by a very perfect non-conductor of heat, such as raw sheep's
wool.
Although I have spoken in the above description, by way of example, only of
the liquefactiou of air, yet it will be obvious that the method and apparatus is also
applicable for the liquefaction of other gases or for the separation of other gaseous
mixtures.
It is also obvious that the low temperatures t³ obtained in the chamber V' may
be utilised not only for the liquefaction of gases, but also generally for producing
very low temperatures.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is :-
1. Producing low temperatures and effecting the liquefaction of air or other
gas or mixture of gases, by first compressing. such gas, then passing it through a
cooler for reducing its compression temperature, then passing it through a reversed
flow interchanger from which it issues under expansion to a considerably lower
pressure into a chamber, whence the air or gas, reduced to a lower temperature by
the internal work performed in expanding is made to pass through the reversed
flow interchanger on its way back to the compressing pump in order to take up
heat from the compressed air or gas passing to the said expansion chamber, the
same body of air or gas being made to flow repeatedly through the above-
mentioned circuit, until, by the continued reduction of temperature both before
and after issuing from the interchanger, a liquefaction of the air or gas takes place,
or a certain degree of cold is produced, substantially as described.
2. Producing gaseous oxygen from air by first liquefying air in the manner set
forth in the first claim then separating the nitrogen more or less completely
therefrom by causing the liquid to take up heat from a portion of the compressed
air passing to the expansion chamber the cold of the volatilised nitrogen being
also utilised for this purpose and then volatilising the liquid oxygen by causing it
to take up heat from another portion of the compressed air supply passing to the
expansion chamber by which means the work expended in producing the requisite
low temperatures is to a great extent regained, substantially as described.

page_text: 3. Apparatus for carrying out the process referred to in the first claim consisting
of the combination of a compressor C, a cooler K a reversed flow interchanger G¹
a regulating valve R₁ a receiver V₁ and a gas feed pump P, such parts being
combined and operating substantially as herein described with reference to the
accompanying drawings, and the parts G' R' V' being effectually protected against
absorption of heat from the outside.
4. The combination with the apparatus referred to in the 3rd claim of a coil s
arranged and operating as herein described for effecting the volatilisation of the
nitrogen from the liquefied oxygen, by causing it to absorb heat from a portion of.
the compressed air to be liquefied, substantially as described.
5. The combination with the apparatus referred to in the 3rd and 4th claims of
a reversed flow interchanger G³ arranged and operating as herein described for
converting the liquid oxygen into gaseous form by causing it to absorb heat from
a portion of the compressed air to be liquefied, substantially as described.
Dated this 25th day of March 1896.
ABEL & IMRAY,
Agents for the Applicant.