THE PRODUCTION OF LIQUID NITROGEN
FROM ATMOSPHERIC AIR USING
A GAS REFRIGERATING MACHINE
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT OP GEZAG VAN DE RECTOR MAGNI-FICUS DR. R. KRONlG, HOOGLERAAR IN DE AFDELING DER TECHNISCHE NATUUR-KUNDE, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 22 JUNI 1960 DES NA MIDDAGS TE 4 UURDOOR
JOHANNES VAN DER STER NATUURKUNDIG INGENIEUR GEBOREN TE VLAARDINGEN I / .
__
._---BIBliOTHEEK
DER
TECHNISCHE
HOGESCHOOL
DELFT
I. Het is niet mogelijk zuivere vloeibare zuurstof uit atmosferische lucht te winnen met een installatie analoog aan de in dit proefschrift beschreven installatie voor vloeibare stikstof.
2. Het is aan te bevelen om in condensor-verdampers, die in luchtsplitsingsapparaten voorkomen, de vloeistof in de pijpen te laten verdampen en de damp op de buitèn-wand van de pijpen te laten condenseren in tegenstelling met de methode die in de praktijk wordt toegepast.
(Ruhemann M. The separation of gases. Second edition. Oxford University Press 1949 blz. 69.)
3. De bedrijfstijd van de in dit proefschrift op blz. 18 beschreven water-en-koolzuur-afscheider wordt in zeer sterke mate bepaald door de manier waarop in het begin de sneeuwlaag op het gaas wordt gevormd.
4. Voor het oppompen en regelen van stromen van vloeibare lucht, zuurstof of stikstof is de dampbellenpomp tot nu toe ten onrechte verwaarloosd. (O.a. Scott R. B. Cryogenic Engineering. D. van Nostrand Company, Inc. New Yersey J 959 blz. 263.)
5. Naarmate de beschikbare relatieve insteekdiepte bij dampbellenpompen voor vloeibare gassen kleiner is moet men, als men met één-trapspompen wil volstaan, gebruik maken van stijgbuizen met kleinere diameter.
6. Voor het bepalen van de zuiverheid van kokende vloeistoffen is om principiële redenen in vele gevallen het gebruik van een areometer te prefereren boven het ge-bruik van een dampspanningsthermometer.
Tl. Voor het bepalen van de zuiverheid van kokende vloeibare zuurstof is in af-wijking van stelling 6 het gebruik van een dampspanningsthermometer te pre-fereren boven het gebruik van een areometer.
8. Het is nadelig voor de industrie dat de salariëring in de verschillende vormen van het technisch onderwijs niet concurrerend is met de salariëring in de industrie. 9. Bij het uitvoeren van processen bij lage temperaturen dient men er rekening mee te houden dat gifstoffen, die in ongevaarlijke concentraties met het te ver-werken gas worden aangevoerd, bij het opwarmen van de installatie in concen-traties vrij kunnen komen die wel gevaarlijk kunnen zijn.
Daarom dient men bij het opwarmen het vertrek waarin de installatie is geplaatst, goed te ventileren.
10. Vergeleken met het land wordt de zee op een zeer primitieve manier als voedsel-bron gebruikt. In verband met de sterk toenemende wereldbevolking is het ge-wenst dat naar nieuwe methoden van voedselwinning uit de zee wordt gezocht.
zijn werk op de hoogte wordt gebracht.
(Casimir H. B. G. Philips techno T. 20 (1958), 3, 66.)
12. Het is niet gewenst dat in wetenschappelijke publikaties uitspraken worden gedaan over nut of onbruikbaarheid van bepaalde processen, zonder dat daarbij argu-menten naar voren worden gebracht ter ondersteuning van de uitspraak. Dit geldt vooral wanneer door de uitspraak de ontwikkeling van de techniek kan worden geremd.
(Plank R. Kältetechnik II (1959), 5, 126.)
13. Ten onrechte wekt Hausen de indruk, dat men met installaties voor vloeibare lucht volgens Linde, Claude of Heylandt, geen vloeistof kan maken die dezelfde samenstelling heeft als de lucht waarvan men is uitgegaan.
FROM ATMOSPHERIC AIR USING A GAS REFRIGERATING MACHINE
1. General introduction
2. Theoretical part
2.1. Principle of the installation 2.2. The reflux ratio of the column. . 2.3. The specific power consumption . 3. Description of the installation
3.1. Introduction . . . . . . 3.2. The column. . . . . . 3.3. Heat-exchanger and reboiler .
3.3.1. Purification of the air. . . . 3.3.2. Design of heat-exchanger and reboiler 3.4. The condenser .
3.5. The reflux pump. . . 3.6. Insulation . . . . . 4. The automatic control
4.1. Introduction . . . . 4.2. The air feed. . . . . . 4.2. The removal of the oxygen . . . . 4.4. The removal of the liquid nitrogen . . . . .
4.4.1. Description of the control of the reflux 4.4.2. Statie properties of the control system . . 4.4.3. Dynamic properties of the control system .
5. Safeguarding the installation against explosion hazards of acetylene and other hydrocarbons 2 2 4 8 12 12 13 16 16 20 27 29 29 32 32 33 33 34 34 36 46 55 Appendix 1. Vapour bubble pumps for liquid nitrogen and liquid oxygen 60 Appendix 2. The transfer function of the refiux regulating device 71
Appendix 3. The use of a hydrometer for measuring the purity of liquid
nitrogen 77
List of symbols 85
Bibliography 88
Summary 90
THE PRODUCTION OF LIQUID NITROGEN FROM
ATMOSPHERIC AIR USING A GAS
REFRIGERATING MACHINE
1. GENERAL INTRODUCTION
In recent years there has been a marked increase in the use of liquid air and liquid nitrogen in laboratories and in industry. These extremely cold liquids, whose bolling points at 'a pressure of 1 atm (105 N/m2) are about -194°C and -196°C, respectively, are primarily employed for purposes of cooling. They find application, for example, in vacuum technique, in research on the proper ties of solids at low temperatures, in the making of shrink fits, and for delaying chemical reactions.
For particulars of the familiar methods by which liquid gases can be made, the reader is referred to the handbooks 1) and 2) in which these methods are described in detail. The installations commercially available, and which operate on conventional principles, are generally rather elaborate, and this has meant that the user of sm all quantities has hitherto been obliged to obtain liquid nitrogen and air from third parties.
With the gas refrigerating machine, described by Köhler and Jonkers 3), it has become possible to make small quantities of liquid air simply and economi-cally. In this machine the Stirling-process is performed with a certain quantity of hydrogen or helium. The gas is compressed at room temperature and the heat generated by the compression is removed by cooling-water. The gas flows through a regenerator, in which it is cooled still further, and thence to a cold space in which expansion takes place and where the cold is produced. This cold is transferred in a heat-exchanger to the air which is to be condensed. The gas then flows back through the regenerator, thereby being raised to room temperature again. When producing liquid air with a gas refrigerating machine it is not necessary to compress the air. As a re sult the installation
becomes particularly simple in design and the production of liquid air can be controlled in a very easy way.
When using liquid air, however, certain precautions are necessary, for it can be dangerous in certain circumstances if brought into contact with in-flammabie subsrances. For this reason it is sometimes better to use liquid nitrogen ins te ad of liquid air; liquid nitrogen is also preferabie if the tem-perature of a cooling bath is to be kept very constant.
atmospheric air using a gas refrigerating machine. Particular attention will be paid to the design and building of an installation employing a gas refrigerating machine whose refrigerating capacity at -196°C is about 650 W. This instaUa-tion produces liquid nitrogen at the rate of 0.036 mole per second (4.5 1 h-1).
In order to make such a small installation as attractive as possible, its opera-ti on must be made very simpie. Therefore an automaopera-tic control system has been introduced which reduces the manual operations required virtually to switching the installation on and off.
For separating water and carbon dioxide from the processed air no use has been made of chemicalor physical absorption methods, but to simplify opera-tion a new system of purifying the air by cooling is employed. By virtue of this system, the installation can operatc continuously for a week before it becomes necessary to defrost the trapped impurities.
In orthodox air-separating installations the air is compressed; one of the fèatures of the system described here is that no compressors are used. This contributes largely to the simplicity both of the installation and its method of operation.
In chapter 2 of this thesis the system employed will be analysed, and in chapter 3 structural details wiU be described. Certain investigations conducted with a view to making specific designs possible will be discussed, induding the new method of purifying air by cooling. In regard to the control system (chapter 4) particular attention will be paid to the stability of the reflux regu-lation. This system involves the use of a vapour bubble pump for pumping the reflux to the top of the rectifying column. An investigation has been made into the behaviour of this type of pump and is described in appendix 1.
A study has been made of the danger of explosions resulting from the accu-mulation of hydrocarbons in the air. Measures taken to prevent these explosion hazards are described in chapter 5.
For the purpose of measuring the purity of the liquid nitrogen product by simple means and with sufficient accuracy, a hydrometer has been developed. The work on this instrument is described in appendix 3.
2. THEORETICAL PART 2.1. Principle of the installation
With the advent of the gas refrigerating machine it has become possible to produce liquid nitrogen on a smalloscale from atmospheric air. For this purpose the refrigerating machine must be used in conjunction with a rectifying column which operates at a pressure of 1 atm. between the boiling l'oints of oxygen (90 OK) and nitrogen (77 OK).
A column ofthis kind is essentially a vertical tube in which liquid flows from top to bottom and a vapour flows from the bottom to the top. The flow of
vapour and liquid is maintained by heating the column at the base and cooling it at the top. Moreover, measures are adopted to ensure that intimate contact is established between the vapour and the liquid. When saturated air is fed into the column somewhere between top and bottom, the ascending vapour will become progressively richer in the more volatile component, nitrogen, and the descending liquid progressively richer in the heavier component, oxygen. With a sufficiently intensive supply and dissipation of heat it is possible in this way to extract pure nitrogen at the top and pure oxygen at the bottom.
The flow diagram adopted is shown in fig. 1. In this figure, A denotes the A 2 5 4 Liquid nifrog~n 8
--~
-.--J
c
1 3 Air OXY'lenFig. 1. Diagram of an installation with gas-refrigerating machine for producing liquid nitrogen from air.
gas refrigerating machine, B the column in which the air is separated into its components and C a heat-exchanger.
Air (1) is fed into the system, and the products extracted are virtually pure liquid nitrogen (2) and gaseous oxygen (3) which is slightly contaminated with nitrogen *). The air feed is pre-cooled by the cold bottom product in the
heat-*) Unless otherwise stated, it will henceforth be assumed that both products are completely pure. It will further be assumed that the air consists of 22 mol % oxygen and 78 mol %
exchanger C, where it is also partly dried. By heat exchange with the liquid
oxygen in the reboiler at the base of the column the air is then further cooled,
and at the same time the residu al water vapour and carbon dioxide are removed. The air then enters the rectifying column B at a suitable point between top and bottom. The vapour issuing from the top (4) is condensed by the gas refrigerat-ing machine A. Part of the condensate is taken off as product (2) andAhe remainder is returned to the column as reflux (5). The descending liquid becomes increasingly richer in oxygen by virtue of the rectifying action of the column, and enters the reboi1er at the base in the form of almost pure liquid oxygen. By beat exchange with the air the liquid oxygen is evaporated bere.
Part of tbe vapour leaves tbe column as gaseous bottom product (3), the
re-mainder is returned to ascend tbe column. The extracted cold oxygen vapour
is raised to room temperature in the heat-exchanger C by heat excbange with the air to be separated.
2.2 Tbe reDox ratio of the column
First of all we will demonstrate that the required fractionation of the air can in fact be accomplished by the system described. To tbis end it is necessary, and sufficient, to show that in the steady state the reflux ratio *) is higher than the minimum value required for the separation of the components according to the theory of rectification. It should be added that the separation is more easily effected the more the reflux ratio exceeds this minimum value.
It can be seen from tbe diagram ofthe system (fig. 1) that after the air (1) has
been pre-cooled in the heat-exchanger C, it is further cooled at tbe base of column B. The cold needed for this cooling is introduced to the column by the reflux (5), and therefore the reflux is coupled with tbe cooling of the air. In order to calculate the reflux ratio we must draw up the heat balance over the column and the beat-exchanger. In doing so we shall assume in the fust in-stance that losses are negligible. Tbe heat balance can be readily evaluated on the basis of tbe block diagram in fig. 2. As in fig. 1, A, Band C here denote respectively the gas refrigerating machine, the column and the heat-exchanger.
Air of, say, 20 °C is fed into the system and oxygen of the same temperature
is extracted from it. The oxygen from the air and the oxygen product balance each other thermally, and therefore they can be left out of account in calcu-lating the heat balance. The nitrogen fTOm the air passes, as it were, the heat-exchanger C without being coo1ed, but it leaves the column as top product
in a saturated state (-196°C). Somewhere between C and A then, the
nitro-gen has been cooled, which involves the extraction of 6390 joules from the va-pour per mole of nitrogen product. Cold is supplied to the column because liquid
*) The reflux ratio at the top is defined as the number of mols of reflux supplied to the column per mol of top product.
nitrogen is introduced into it as retlux, whilst this nitrogen leaves the column again as a saturated vapour. For a reflux of r moles liquid per mole product, this amount of cold is r times the molar heat of evaporation of nitrogen, that is r X 5600 joules. The heat balance for Band Cis thus :
6390 - r X 5600
=
0So we find for the retlux ratio the value r
=
1.14, which is roughly 1.4 times the minimum reflux of 0.8 tequired for separating air into 22 mole%
oxygen and 78 mole%
nitrogen 4). The retlux ratio, which in this case is determined by the ratio between the sensible and the latent heat of nitrogen, is so large that it is readily possible to separate air into its components.The amount of the nitrogen production mayalso be inferred from fig. 2.
, _.
_
.
_
.
_
._._,
. I H=OJ lol 1mo liquidn ifrogen I .I
I
II
H=5600J IH=H99OJi
I
i
I
L
.
_. _
H=11990J A -rmol H= 5600J I I I I I I I I I I I I I L ___ -.J I B ~---ï I I I I I I I I I I I I I Ic
_._ . _ .-I
I
I
i
I
H=O Ji
i
I
I
i
i
I
I
II
i
I
I
_.~
Nifrogen Oxygen Oxygen Air
Fig. 2. Diagram for ca1culating the nitrogen production and the reflux ratio.
The air flows into the region bounded by the dot-dash line. From this region come the products, liquid nitrogen and gaseous oxygen. The latter is thermally balanced with the oxygen in the air tlow. Thus it is as if only the nitrogen
is supplied to the instaUation, and the cold production of the gas refrigerating machine is entire1y utilized for cooling and condensing this nitrogen. The pro-duction is then equal to the propro-duction obtained when nitrogen supplied in boules is cooled and condensed by the refrigerating machine at normal pressure.
It will be evident that the foregoing gives a very idealized picture, since it takes no account of the cold losses. We shall now caIculate the reflux ratio taking the losses into account. These losses, with the symbols, are:
Insulation losses Recuperation losses
Losses arîsing fr om the freezing of the impurities out of the air Losses arising from the reflux pumping by means of a vapour bubble pump (enabling the machine to be set up at the side ofthe column) Also taken into account are:
q)water
The cold production ofthe gas refrigerating machine q)E The difference in enthalpy ofN2 at 20°C and liquid N2 at -196°C per
mole !:l.HN2
The useful cold, i.e. the production of the refrigerating machine af ter sub-traction of aH losses, can leave the installation only in the form of liquid nitrogen. The nitrogen production is therefore :
N
=
q)E - q)ins - ~~ - q)water-q)vbP mole S-1 (2.1) N2
In the reflux caIculation the correction for the losses is more complicated than in the caIculation of the production. This is evident as alllosses directed on the refrigerating machine itself cause a decrease in the cold production; the reflux ratio remains unaffected. Alllosses directed on the base of the column increase the reflux ratio, ho wever, because the reflux returned to the top of the column is not only required to cool the nitrogen of the air feed but also to cover the above-mentioned losses. The question is therefore important whether the losses are directed straight on the machine or whether they find their way via the column to the machine. To the first kind belongs q)vbP and the insula-tion losses of the refrigerating machine itself; the second sort incIude q)he and q)water. The insulation losses are distributed over the whole length of the column, and consequently it is not readily possible to say exactIy to what extent they influence the reflux. We shall as su me that one part b is purely directed on the base of the column, and the remainder (1 - b) is directed on the top of the column. As in the previous section, we caIculate the reflux ratio as follows:
Per mole of nitrogen product the quantity of heat supplied to the base of the column is:
Q
=
6390+
q)he+
q)water+
q)insThis must be absorbed by a flow of liquid consisting of r' moles of liquid nitrogen suppling 5600 r' joules of cold to the column. We then find for r' the value:
,
=
1 14+
ct>he+
q>water+
q>lnSr . 5600 N (2.3)
Substituting (2.1) in this expres sion gives : . !::.HN
r' = 1.14
+
560; (2.4)Obviously the losses are always smaller than the cold production, and there-fore r' is always greater than 1.14. So we see that rectification of the air is also possible when the losses are taken into account.
Generally speaking, it will be difficult to operate a liquid-nitrogen installa-tion of this kind in such a way that exactly 0.78 mole nitrogen and 0.22 mole oxygen are extracted as top and bottom products, respectively, per mole air feed. In order to be sure that the top product, the nitrogen, is constantly pure, it will always be necessary to extract more than 22% of the air feed as bottom product, which means of course that the bottom product will be impure oxygen. It is known from rectification theory that the minimum value of the reflux ratio at the top does not depend on the purity of the bottom product but solely on the purity of the top product 4). Thus there is no danger that the extraction of nitrogen together with the oxygen will cause the reflux to drop below the minimum value, which might reduce the purity of the top product. Summarizing, we may conclude that wh en using the system described in fig. 1 for making liquid nitrogen, there is in all cases an ample excess of reflux to ensure that the air will be properly separated into its components.
We will now calculate the value of r' for the column designed. Measure-ments have shown that the above-mentioned heat fluxes in the actual installa-tion have the following values :
q>E 650 W (at an average pres su re of 23 atm in the gas refrigerating machine).
q>lns 154 W q>vbP 20 W
q>water 42 W (at a wet bulb temperature of 14°C). q>he 8 W
The available cold for cooling and condensing the nitrogen is thus 426 W. This gives a productÎon of 0.035 moles of liquid nitrogen per second (4.4 1 h-1).
In fig. 3 the distribution of the cold production ct>E in the various parts is represented by splitting a current of width ct>E into a number of currents each having a width corresponding to their true magnitude. The shaded currents àre the heat and enthalpy flows flowing via the column and which
according-ly determine the reflux ratio. The reflux ratio can be calculated from formula (2.4). Putting <5 as 0.5, we find r'
=
1.78 for the reflux ratio, which is more~ins=154W=24%
For cooling down and condensafion
ol the nitrogen 421 W = 65 %
Fig. 3. Distribution of the cold-production of the gas-refrigerating machine. than twice the minimum value. Fig. 4 shows the build-up of the reflux ratio in a similar manner as in done in fig. 3.
From cooling down fhe nitrogiM product =64 %
FromfH.E.=2·3%
TM re/lux ratio = 100 % Fig. 4. Build-up of reflux-ratio.
2.3. Tbe specific power consumption
For the cold production of 650 W the gas refrigerating machine consumes
6.0 kW. The specific power consumption at the production of 4.4 l/h is thus equal to 1.36 kWh/I.
It might be asked what, in principle, is the minimum work to be performed
in order to produce at normal pressure llitre of liquid nitrogen from air. This
quantity of air into oxygen and nitrogen, and the minimum work required to cool and con den se this nitrogen. In both cases the minimum work is given by the increase in exergy. For I mol of air this can be written as :
Amin
=
D.Ex=
D.H - To D.S , (2.5)where To is the ambient temperature and D.H and D.S are respectively the in-crements in the enthalpy and the entropy per mole upon the transition from the one state into the other 6).
If the heat of association be disregarded, there will be no change in the enthalpy upon the separation of oxygen and nitrogen from a mixture, and if the process is isobaric the internal energy will also remain constant. The minimum work of separation can therefore be written as :
Amin
=
-
To D.S=
-D. U-I
PdV=
-I
PdV (2.6) When oxygen and nitrogen are separated from air at constant pressure they are both reduced in volume. The minimum work of separation now is the work required, to compress each component isothermally to its new volume. The re sult is the same as that obtained in the hypothetical experiment in which the air is separated into its components by means of two semi-permeable walls, followed by isothermal compression of the components 5).The minimum work is:
PI P pz P
AmJn
=
pv. (-
In - + - ln- )P PI P pz (2.7)
In this expres sion PI and pz are the partial pressures of the components where Pis equal to PI plus pz and V is the volume of I mole of the mixture. We can now calculate the minimum work needed to separate such a quantity of air of 20
o
e
into 78 mole % nitrogen and 22 mole % oxygen th at af ter separation and following liquefaction I litre of liquid nitrogen is obtained. This minimum work is:AmJn = 0.013 kWh/1
In order to calculate the minimum work required for the cooling and the condensation of the pure nitrogen, we again compute the increase in exergy (formula 2.5). The results are shown in Table I. Although the decrease in enthalpy is greater upon cooling than up on condensation, the minimum work is more than three times smaller. The reason for th is is that in the fust part the cold must be produced at all temperatures between 293 and 77 oK, whereas in the second part cold must be produced only at the lowest temperature (77 OK).
The total minimum work needed to obtain liquid nitrogen from air is 0.179 kWh/I, of which 7% is used for the separation, 23% for the cooling and 70% for the condensatjon of the nitrogen.
TABLE I
Calculated minimum work required for cooting from 20 °C and condensing nitrogen to obtain llitre ofliquid nitrogen at a pres su re of 105 Nm-2.
Bin Sin Ex in A min. in
State
J/mole J/mole °C J/mole kWh/1 N2 1 atm. 293 oK 11990 111 -20600
0.041 N21 atm.
n
O
K
5600 72 - 155000.l25
N2 1 atm. liquid 0 0 0
If the liquid nitrogen were produced without cold losses in a system as shown in fig. 1, in which the gas refrigerating machine were replaced by a Carnot process, the minimum work would be 0.269 kWh/I, which is 1.5 times as much as with the ideal process. In this temperature range the efficiency of the gas refrigerating machine is almost 30% of the efficiency of the Carnot process. The specific power consumption of a system as shown in fig. 1, without losses, would therefore be 0.89 kWh/I; as stated, this increases to 1.36 kWh/I when losses are taken into account. The processes mentioned are coUected with their specific power consumptions in Table Il.
TABLE II
The specific power consumptions of various processes capable of producing liquid nitrogen fr om air.
Specific power consumptions Process
compared with in kWh/1
ideal process
Ideal process 0.179 1.0
I
As in figAs in fig. 1 with gas refrigerating machine . 1 with Carnot process. 0.269 0.891 5.0 1.5(Without losses in the column and heat-exchanger).
As in fig. 1 with gas refrigerating machine, 1.36 7.8 taking losses into account.
We shall finally consider where the losses arise in the installation and what their magnitude is in terms of energy. To this end we first determine the amount
of energy expended on each heat and enthalpy flow and we then consider what each would have cost if it had been produced with an ideal process. The fust can be done quite simply by distributing the specific power consumption of
1.36 kWhjl proportionally over the various components offig. 3. The minimum work for each of these components can be found from the increase in exergy. For the flows of enthalpy, tb is is :
AmJn
=
/).Ex=
/).H - To /).S, and for the heat flows TlAmJn
=
/).Ex=
<P (To - 1).
In the latter expres sion <P is the relevant heat flow, To the ambient temperature
and Tl the temperature towards which the heat is flowing. The results of the calculation are given in Table lIl.
TABLE III
Analysis of the losses in the installation. Loss Workin min. work
kWh/1 in kWh/1 in kWh/1 in
%
in%
ofthe ofwork totalloss Cooling of N2 0.47 0.041 0.429 91 39.5 Condensation 0.41 0.125 0.285 70 26.2i
<Plns to 77 OK 0.16 0.048 0.112 70 . 10.3i
<Pins to 90 oK 0.16 0.038 0.122 76 11.2 <Pwater 0.09 0.001 0.089 99 8.2 <PvbP 0.05 0.016 0.034 70 3.1 <Pbe 0.02 0.004 0.016 76 1.5 1.36 0.273 1.087 100Compared with the ideal case the greatest losses occur in the cooling of the nitrogen and the freezing-out ofthe water. This is evident, since these processes are carried out with needlessly "expensive cold".
It would be possible to reduce the specific power consumption by precooling the air feed with an auxiliary machine operating at a higher temperature; this machine could be used at the same time to freeze out the greater part of the water. For reasons of economy and simplicity, however, this would only be worthwhile in installations larger than the one described here.
3. DESCRIPTION OF THE INSTALLATION
3.1. Introduction
A greatly simplified drawing ofthe installation built on the principle represent-ed in fig. lis shown in fig. 5. Here again, A is the gas refrigerating machine, B the rectifying column and C the heat-exchanger.
column packing insulation material
B
A
c
Owing to the condensation of the nitrogen in the condensor 6 of the machine, a subatmospheric pressure is produced so that the air to be processed is sucked in from atmosphere. It enters the instaUation at 1 and flows via the heat-exchan-ger C, where it is pre-cooled by the oxygen bottom product, to the reboiler 2. The air is further cooled by heat exchange with the liquid oxygen in the reboiler, after which it flows through the duct 3 to the air inlet port of the column. It then rises through the upper part ofthe column, where the oxygen is washed out. Nitrogen vapour leaving the column flows through pipe 4 and the connecting tube 5 to the condenser 6, where it is condensed. The condensate flows back through the tube 5 to the downcorner 7 of a vapour bubble pump. The latter pumps the reflux through the riser 8 to the top of the column. Via the overflow
9 and the trap 10 the nitrogen product is discharged. The vapour bubble pump is heated by conduction with the aid of a copper strip 11, one end of which is fixed to the pump, the other end being held at a temperature of roughly 15°C by a stream of water.
The liquid arriving at the base of the column is evaporated in the reboiler by the in-drawn air. Part of the vapour is .eturned to the column and the remainder is blown off via the heat-exchanger C as bottom product (12). The instaUation is insulated by three mutuaUy sealed spaces packed with insulating material. In this chapter we shaU deal at greater length with the design and construction of the various components and with the experience thereby gained.
3.2. Tbe column
If the reflux ratio of a rectifying column is known, as weU as the composition of feed, top product and bottom product, it is possible to calculate the number of theoretical plates required *). F or this purpose, use is made of phase diagrams of mixtures of the components present in the feed. A phase diagram of this kind, i.e. a diagram of temperature versus composition, has been made for oxygen and nitrogen mixtures by Dodge and Dunbar 7).
The simplest method of calculation is that devised by Mc Cabe and Thiele 8),
which assumes that the constituents of the feed have the same molar heat of evaporation and that no heat is supplied to the column or withdrawn from it between the top and bottom.
The first assumption is a rather rough one for the design of an air-rectifying column, the molar heat of evaporation of oxygen and nitrogen being respectively 6.8 and 5.5 kJ mole-1 at a pressure of 105 N m-2 (1 atm). An essentially more accurate method, whose only simplifying assumption is the absence of heat supply and withdrawal between the top and bottom of the column, has been put forward by Ponchon 9), Merkel 10) and Keesom 11). In this method the
*) A theoretical plate is a plate from which the issuing liquid and vapour are in equilibrium with each other.
number of plates is determined graphically in an enthalpy versus composition diagram. lts drawback is, however, that the graphical construction is not particularly accurate. Hausen 12) has indicated a method in which the caloric unit of mass is used. This unit is chosen such that the heat of evaporation for each sub stance is exactly equal to unity. The re sult is that, when all flows and compositions are expressed in these caloric units, the liquid and vapour flow is constant over the height of the column, as in the Mc Cabe-Thiele method. The difference, however, is that the molar heats of evaporation are not now assumed to be equal, and this makes the method more exact. This method is also useful when it is necessary to calculate the number of plates needed in a rectifying column in which mixtures of more than two components are to be separated.
Hitherto it has been assumed that the top and bottom products are both pure, in other words that for every 100 moles of air feed 22 moles of oxygen and 78 moles of nitrogen are extracted from the column. The ratio between the quantities of bottom product and air feed, which we shall refer to as blow-off ratio ~, was thus 0.22. Since nitrogen is the principal product of the installation, it must be as pure as possible. In order to prevent the purity of the nitrogen from being adversely affected by slight changes in the regulation ofthe column, the blow-off ratio is always chosen higher than 0.22, viz. between 0.25 and 0.32. The number of plates calculated for the upper part of the column *) is independent of the blow-off ratio if the latter is greater than 0.22. In that case the number of plates calculated for the lower part of the column is dependent on the blow-off ratio, such that as the ratio increases the necessary number of plates decreases. In the light of the above considerations, a blow-off ratio of 0.25 was chosen when calculating the number of plates for the column in fig. 1.
When air composed of 78% nitrogen and 22% oxygen is fed in and when a purity of the top product of 99.8 % is required it follows from the heat balance that the bottom product will contain 12% nitrogen. As we have seen, the reflux ratio is 1.8 (page 8). Calculation with the Mc Cabe and Thiele method then gives 8 plates for the upper part and slightly more than 2 plates for the lower part of the column. With the enthalpy-versus-composition diagram of Keesom one arrives at 7 and 3.5 plates, respectively. The difference between these meth-ods of calculation is most pronounced in the lower part of the column, where the greatest changes in concentration occur.
Till now we have disregarded the presence of argon in the air. It is however known that, in the rectification of air, the argon content of almost 1 % has an appreciabie influence on the process. Since argon has a boiling point of about -186°C it tends to accumulate in the lower part ofthe column, where the tem-perature at the base is -183°C (ifpure oxygen were made) and -191°C at the
*) The upper part of the column is the part between the top and the point of air entry; the bottom part is that between the boUom and the entry point.
entry port. This means that many plates are needed in the lower column in order to obtain the oxygen free from argon. In the column just described the bottom product contains as much as 12% nitrogen. It is therefore probable that the argon is extracted from the column together with the bottom product. To ascertain whether this really happens the number of plates for the column was again calculated, this time by the method proposed by Hausen and taking into account the argon content of the air. In this calculation Weishaupt's ternary diagram for oxygen-nitrogen-argon mixtures was used 13). Here again, 7 plates we re calculated for the upper part and 3.5 plates for the lower part of the column. Obviously, then, the effect ofthe argon in this column is very smal!.
We will now proceed in calculating the dimensions of the column. At the condensation temperature of nitrogen under a pressure of 1 atm., viz -196
o
e,
the refrigerating capacity of the gas refrigerating machine is 650 W. The heat of condensation of nitrogen at normal pressure being 5600 J mole-1 we find forthe rate of vapour flow through the upper column 0.11 mole s-1, and through the lower column 0.06 mole S-l. The rate of liquid flow in the column is 0.08
mole S-l (10 1 h-1).
In columns with such small volumetric flows it is often practi~e to use, instead of plates, a packing material with which the column is fiUed to the top. In the installation described, the packing material consists of small pieces of wire gauze of 8 X 8 mm2 bent into a saddle shape (Mc Mahon packing) 14). The
advantages of this packing material over plates are that it combines efIicient operation with a low resistance to flow; is simple to intro duce into the column and is relatively cheap; moreover its heat capacity is low, resulting in a short cooling time.
The diameter of the upper column was fixed at 48 mm, making the vapour velo city 0.4 m S-l (calculated with respect to the empty column). The diameter
of the lower column was fixed at 38 mm, resulting in a vapour velocity of 0.3 m S-l. These veloeities are low, as a result of which the pressure drop over
the column is also low. This is important in connection with the possibility of extracting the nitrogen product from the installation ; we shall return to this subject later (page 28).
We calculated above th at the upper column should contain 7 and the lower column
3t
theoretical plates. In the case of columns filled with packing it is usual to refer to the height ofthe column which, as regards its function, is equiva-lent to a theoretical plate. This H.E.T.P. (height equivaequiva-lent to a theoretical . plate) is not exactly known for the packing material in use; the data available on nitrogen-oxygen destillation show very considerable disparity. Mc Mahon himself gives 4 cm 14), but mentions that the measurement was not particularlyexact. Rushton gives 5 cm 15), Haselden 12.5 cm and Schuftan 7.5 cm 16). In order to make sure of a good product, the figure adopted for our column was about 9 cm per plate. This made the packing height of the upper column 59 cm,
and that of the lower column 36 cm. In this situation, pressure drops were measured over the upper and lower parts of the column of 600 and 300 N m-2,
respectively (6 and 3 cm water column). The measured purity of the nitrogen was 99.8 to 99.9%.
At the top ofthe column, and at the air inlet, distributors are fitted to ensure uniform distribution of the liquid. When conventional distributors were used, difficulties arose from the fact that the relatively small openings in these dis-tributors repeatedly became c10gged with pieces of ice or solid carbon dioxide. Another solution was therefore sought which would prevent this. The dis-tributors designed for the purpose consist essentially of a conical jacket having a serrated lower edge with 12 teeth (fig. 6). When the flow of liquid to be dis
-Fig. 6. Liquid distributor.
tributed is directed on to the apex of the cone, the excellent moistening proper-ties of liquid oxygen-nitrogen mixtures ensure that the surface of the cone is
covered with an almost uniformly thick film of liquid, so that virtually the same quantity ofliquid flows down over each tooth (approx. 0.81 h-1). Bending the teeth inwards does not disturb the individu al streams, thus making it possible to distribute the liquid uniformly over the surface of the packing by bending the points. With these distributors there is no danger of blockages since the liquid flows freely over the surface of the cone, carrying impurities with it. Oblique positioning of the distributor (up to an angle of 30°) is found to have scarcely any effect on its operation, nor is the directing of the liquid on to the apex of the cone particularly critica!. These distributors are only suitable for sub stances that moisten the surf ace thoroughly. A distributormade of phosphor bronze works excellently with liquid air but not nearly so weIl with water, for example.
3.3. Heat-exchanger and reboiler 3.3.1. Purification of the air
Since the air fractionation process takes place at extremely low temperatures it is necessary to remove all water and carbon dioxide from the air feed, as
otherwise blockages would be caused by the formation of ice or solid carbon dioxide in the low-temperature parts of the installation. The principal methods of separating water and carbon dioxide from air are :
1. Chemical and physical absorbtion whereby the carbon dioxide forms a bond with e.g. potassium hydroxide, and the water is absorbed in aluminum oxide, potassium hydroxide or silicagel.
2. The method whereby water and carbon dioxide are frozen-out by gradual cooling of the air.
The drawback of the first method is that it makes the operation of the system complicated, since it means that chemicals have to be supplied or regenerated at regular intervals.
The second method does not have this disadvantage, but it does mean that the freezing-out process costs expensive cold, resulting in a decrease in nitrogen production. In principle, most of the moisture could be frozen-out by means of an auxiliary refrigerating machine operating at, say, -20°C, but this is too complicated and therefore unacceptable for an installation as small as the one described here. Fig. 7 shows the decrease in nitrogen production y as a function
15r---,---r---,---~
101-- - - - + - - - - _ _ _ t - - -r-t7<C.-- - _ j
51--~~~~~-___t----+----_j
o 5 10 15 20
TWb(oe)
Fig. 7. Percentage drop yinproduction, due to freezing-out of moisture, as a function of wet bulb temperature Tw•b., for various values of blow-off ratio a.
of air humidity at the blow-off ratios <X
=
0.25 and <X=
0.32. Although inextreme cases the decrease in production may be as much as 15%, the freezing out of impurities was nevertheless chosen because of its simplicity. A dis ad-vantage of the method, however, is that the heat-exchanger must be warmed up after some time for defrosting.
The manner in which the moisture and carbon dioxide are frozen out of the air is analogous to the method described by Roozendaal17). According to this
kept at a temperature of about - 170
o
e.
The air is thereby cooled and a layer of sn ow forms on the gauze. One might expect that the gauze would soon be-co me clogged with snow and that be-consequently the pressure drop over the gauze would rise very rapidly. If, ho wever, at a given air flow, the surface area ofthe gauze is made large enough and the temperature ofthe gauze low enough, a situation can be reached whereby the gauze only becomes c10gged after a considerable time has eIapsed. It is in fact possible to select the conditions in such a way that the layer of snow grows counter to the direction of air flow. The layer becomes progressively trucker and yet the pressure drop over the snow and the gauze remains remarkably low for a long time.The physical explanation for this phenomenon is not known in detail, but in broad lines it may be explained as follows: When the damp air is cooled down by the cold gauze a layer of snow forms on the gauze. Now if the surface temperature of this snow is sufficiently low, crystals will form on the surface as a result of diffusional transfer of the water vapour in the air stream. The structure of these crystals appears to be fairly coarse. The air flows through the snow layer and is thereby cooled, and in the process the water vapour still present in the air settles in the form of fine crystals in the gaps between the
Fig. 8. "Sn ow-cake" formed on the water and carbon-dioxide separator of a gas-refrigerating machine used for producing liquid air. A section of the 6 cm fully-grown snow cake has been removed, showing clearly the difference in structure between the surface and the interior layers.
coarser crystals. Finally, these gaps become filled too, and therefore if the layer is not to cause clogging it is a necessary condition that the growth of the layer by diffusion should be a continuo us process. This is possible only if the surface temperature of the snow is sufficiently low. This temperature is determined by the temperature of the gauze, the thermal conductivity of the snow and the volume of air flowing per unit area of the gauze. The lower the gauze tempe-rature, the better the thermal conductivity and the smaller the volume of air flow, the longer wiU the snow trap remain in operation before the surface tempe-rature rises to such a value as to cause a blockage.
If the gauze temperature is lower than the sublimation temperature of the carbon dioxide in the air (-142°C), carbon dioxide will also settle in the snow layer. In this way a layer can build up close to the gauze consisting of ice crystals and solid carbon dioxide in roughly equal parts. .
Fig. 8 shows a "snow cake" formed on the ice and sn ow separator of a gas refrigerating machine used for making liquid air. In this experiment the tem-perature of the gauze was -170°C, the surface of the gauze 0.1 m2 and the
load 1.3 X 10-2 kg air m-2 S-l. The air velo city was 1.0 X 10-2 m S-l (at a temperature of 0 °C in the empty cross-section) and the wet-bulb temperature was 12°C. A section has been removed from the cake, showing clearly that the structure of the surface is much coarser than that of the parts farther inside. In this case the thickness of the snow layer is 6 cm and the mass of the snow 5 kg. The density of the snow layer is 400 kg m-3, that of the carbon dioxide layer 900 kg m-3.
15~--~~~---+---~+---~
o 50 100 150 200
((h)
Fig. 9. Pressure drop ,t",P across the snow cake as a function of time t.
I. Snow trap of liquid air machine.
11. Snow trap of nitrogen installation (cold oxygen coils not insulated).
In fig. 9, curve I represents the pressure drop over the gauze as a function of time. During the fust 100 hours from the start the surface temperature is so low that the snow layer can grow continuously; the pressure drop is therefore low and shows little increase with time. After about 110 hours a much sharper increase sets in. This might be thought to be attributable to the blockage of the sn ow layer owing to inadequate growth. At this stage, however, that is not yet the case. Measurements have demonstrated that at the end of the operating period the resistance is offered principally by the COz layer and much less by the water-snow layer. The latter is found to be primarily responsible for the increase in resistance only in the case of high humidities. The pressure-versus-time curve then resembles curve I, but the operating time is shorter.
During the fust hour af ter starting the machine, when no sn ow layer has yet formed on the gauze, the water vapour and carbon dioxide in the air is not completely trapped, and the liquid air produced looks rather cloudy. After one hour, however, the layer has grown sufficiently to trap virtually all the impurities, and the liquid air is then crystal-clear.
A sn ow trap built on the principle described combines excellent trapping properties *) with a low resistance to flow and very compact construction. Moreover, the trap is inexpensive to build and cannot be contaminated by dust, which is trapped in the sn ow layer and flushed away when the layer is melted. The construction of the snow trap designed for the liquid-nitrogen installation
will be discussed in the next section. .
3.3.2. Design of heat-exchanger and reboi/er
The air feed is pre-cooled in the heat-exchanger by the bottom product of the column, the gaseous oxygen. When this oxygen is raised in temperature from -183 °C to +20 °c, its enthalpy increase is 6426 J mole-1 which, at a
normal load and a blow-off ratio of 0.25, represents a cooling capacity of 56 W. (In that case oxygen is extracted at the rate of 0.088 mole S-I). This is a considerable percentage of the cold production of the refrigerating machine and therefore it is necessary to use the cold to pre-cool the air by the oxygen exhausted. Perfectly dry air of 20°C can be cooled, at a blow-off ratio of 0.25, to -31°C. If the air is moist, the degree of cooling is considerably less, owing to the condensation of the water vapour. For example, if the air feed has a wet bulb temperature of 20°C, it is cooled by the oxygen to an average temperature of 0 °C.
In the design of a heat-exchanger in which the air is to be pre-cooled in counter-flow by the cold oxygen, steps must be taken to ensure that the
heat-. exchanger does not rapidly become blocked by ice and sn ow formation. Even
*) Thls appears not only from the clarity of the liquid air but also from measurements described in chapter 5, in which the trapping of acetylene is dealt with.
at wet buib temperatures higher than 20°C, whereby the air has an average tem-perature higher than 0 °C upon Ieaving the heat-exehanger, these diffieulties still arise beeause it is virtually impossibie to utilize the coid from the oxygen without iee and snow settling on the walls of the heat-exehanger ehannels. Espeeially the requirement that the heat-exchanger has to funetion effieiently at widely varying air humidities is not easy to fuifil.
The eonstruetion of the heat-exehanger and reboiier is entirely governed by the new method of purifying the air by cooling, as deseribed in the previous seetion and whieh is also applied here. Fig. 10 shows the design of the
heat-l388l
column pachingI
~I
airoxygen OiquidJ insulafion material
w
Fig. 10. The heat-exchanger and reboiler combination.exehanger and reboiler eombination in simplified form. The gause G, through whieh the air is passed and on whieh the snow Iayer forms during operation, is soldered to a round eage eonsisting of a number of vertieally positioned
pipes H in which the liquid oxygen is evaporated. The reboiler, of which the cage is a very important component, is secured to a plate of phenolic laminated fabric P. Also fixed to this plate is a doubie-walledjacket B filled with insulating material, thus thermally insulating the space containing the reboiler.
The air enters through a large number of holes 1 uniformly distributed over the periphery of the plate P. It then fiows to the "snoiW chamber" 3 via a cylindrical gap formed by the wall of the insulating jacket Band a brass shield 2 fixed to plate P. Mter having been pre-cooled by the oxygen product in the
snow-chamber, the air fiows through the gauze G on which the snowcake will be built up. After passing the snowcake the air fiows via the filter F and the pipe 5 to the gap 6 surrounding the lower part of the column. In this gap the air is further cooled against the wall of the column, where at the same time any traces of hydrocarbons remaining in the air are removed. *) Thefilter F consists of a wire-gauze basket which, like the column, is fiHed with Mc Mahon packing. During the period when no snow has yet formed on the gauze, this filter intercepts part of the moisture in the air feed and thus prevents blockages inside the column.
The cold gaseous oxygen fr om the reboiler is passed through a fiat copper coil4 ofthree turns situated at the top ofthe snow chamber. This coil is shielded underneath by a glass-wool blanket and a perforated plate in order to insulate it to some extent from the air. The reason for this insulation wilI be dealt with on page 23. By free convection of the air (the coil is much lower in tempera-ture than the air in the snow chamber) the temperatempera-ture of the oxygen in the coil is raised from -183 °C to -60 °C. Mter emerging the coil the oxygen
fiows through a copper pipe which goes down inside the shield 2 in one turn and goes up outside it in one turn. At the position A, where the air enters the snow chamber, the temperature ofthe oxygen is -30°C, and is raised to about
+
10°C in the length of copper pipe contained in the gap between the shield and the wall of the jacket. In this construction, then, the major part (75%) of the cold contained in the oxygen is transferred to the air in the large snow chamber, where blockage by ice accretion is not possible. The remainder of the cold is transferred to the air in the above-mentioned gap. This gap, however, is wide enough not to become ice-blocked, partly because the oxygen coil consists of one turn only. Although a thick crust of ice and sn ow does form on the pipe in a certain humidity range, it has never caused a blockage. Shield 2 is cooled sufficiently by convection in the snow chamber to act as a water separator at high dew points. Water is then continuously tapped off at W at wet bulb temperatures higher than 13o
e.
Another advantage of this design is that the air feed, whose temperature is not very much below room temperature, fiows along the wall of the insulating jacket and thereby shields the jacketfrom the extremely cold parts. The insulation losses are therefore lower than they would be if no shield were present.
Sin ce the load on the gauze is 1.4 X 10-2 kg m-2 s-1, the sn ow layer might be expected to build-up in a manner similar to that described on page 19 in regard to the water and CO2 separator, for which the pressure drop over the sn ow
layer is given as a function of time by curve I in fig. 9. Tests showed, however, that this is by no means the case. The pressure-versus-time plot obtained, shown by curve 11 in fig. 6, is impermissible in connection with the regulation of the oxygen extraction. It was obvious that the cause of the rapid increase in the resistance of the snow layer should be sought in the pre-cooling of the air by the cold oxygen. It was thought that eddies might occur in the snow chamber, preventing the build-up of a snow layer with the desired structure.
Eddies might also be caused by the vertical position of the gauze. As the cage is more than twice as high as the sn ow trap in fig. 8, it may be expected that the eddies will also be stronger. Various tests were made with the flat coil positioned at different places in the snow chamber. The pressure variation measured was in all cases virtually the same as that shown by curve 11. It was noticed, however, that comparatively little snow had formed on the coldest parts of the coil. This led to the assumption that, whilst the air flowing along these parts was certainly cooled, the cooling gave rise to an extra dense mist of fine ice crystals swirling in the air. This would cause disproportion between the accretion of the layer of coarse cyrstals and the further filling of this layer, resulting in the measured rapid increase in flow resistance. The results were indeed much better when measures were taken to diminish the eddies by insulating the flat coil with cotton wool. The use of cotton wo ol is not advisable, however, for one thing because it is inflammabie and for another because its thermal conductivity is strongly dependent on the moisture content. Other suitable insulation was therefore sought, and good results were finally obtained by insulating the coil with a blanket of glass-wool supported by a perforated brass plate. The pressure-versus-time variation measured on this construction is represented by curve III in fig. 9. Measurements with a wet glass-wool blanket produced the same results. Although the pressure curve is not as favourable as that of the snow trap in fig. 8, the regulation of the blown-off oxygen is no longer upset and therefore this solution is acceptable. Measurements have proved that the heating of the oxygen is not reduced when the insulation is applied. The reason is that, al-though the heat transfer is on the one hand decreased by the insulation, it is promoted on the other by the increase in surface area. The latter also reduces the temperature difference between the air and the cold wall - the perforated plate - thereby diminishing mist formation.
The capacity of this heat-exchanger-reboiler to separate water and carbon dioxide in the form of snow is such that the installation can normally operate for a whole week without interruption. The trapped quantity of snow is then
8 kg. The behaviour in a very wide range of air humidities is fairly constant. At wet bulb temperatures higher than 13
o
e,
condensation water is continuously tapped oft' during operation.Inside the insulation of the jacket an electric element is fixed to the base plate of the snow chamber. When the sn ow chamber is filled up as far as the brass shield, the gas refrigerating machine is stopped, the electric element is switched on, the snow melts and the water is drained oft'.
Fig. 11 shows a view of the heat-exchanger and reboiler fr om underneath,
Fig. 11. View of heat-exchanger and reboiler combination from undemeath. The shield 2 and the insulating jacket B (fig. 10) have been removed.
the insulating jacket and the brass shield having been removed. The space between the reboiler and the coil entirely fills up with snow during a period of operation.
We will now describe the innerpart of the reboiler where the liquid oxygen from the column is evaporated. The construction will further be discussed with reference to fig. 10. The liquid oxygen flows from the lower part of the column .through the pipe 7 to the annular pipe 8. To this pipes the boiler pipes are connected thus constituting the above-mentioned cage, on which the gauze Gis soldered.
trans-ferred to the gauze, in consequence of which the liquid oxygen boils in the boiler pipes. The boiling causes a circulation of the vapourizing liquid through the boiler pipes, through vessel 9 and tube 10 back to the annular pipe 8. The vessel 9 serves as a buffer stage and plays an important role in the control
Fig. 12. Reboiler of liquid nitrogen column. The gauze has not yet been soldered to the boiler pipes.
of the reflux, described in chapter 4. To the bottom of 8 a plate is soldered that prevents air from flowing along the gauze to the column. In principle, gauze might also be fitted here, but its cooting would involve constructional difficulties. The oxygen to be exhausted flows from the vessel 9 through the pipe 11 to the coil 4, and the remainder of the vapour passes through the pipe 12 to the column. The partition 13 prevents drops of liquid, produced by the sputtering of the boiler pipes, from being carried along with the gas stream and causing pressure fluctuations in the column. Such pressure fluctuations must be avoided, because they would uspet the regulation of the oxygen product. Fig. 12 shows a photograph of areboiler with the boiler pipes not yet soldered to the gauze. The components indicated in fig. 10 can clearly be dis-tinguished. (The large conical plate welded to the reboiler is the base of the insulation space around the column.)
Sin ce the water and carbon dioxide are more readily frozen out of the air according as the temperature of the gauze is lower, the aim in designing the reboiler was to make this temperature as low as possible by ensuring good heat-conduction of the gauze and good heat-transfer in the boiler pipes.
The cage consists of 32 boiler pipes 20 cm long. The inside and outside diameters of the pipes are 4 and 6 mm, respectively; The centre-to-centre distance between the pipes is 15 mmo Soldered to these pipes is a piece of copper-wiregauze meauring 0.1 m2• The mesh width of the gauze is 0.84 mm, and the wire thickness is 0.34 mmo Calculations and measurements have shown that the maximum temperature drop over the gauze is about 2 °C at the normal heat flux of 320 W.
No data have been foundin the literature on theheat transfer to boiling liquid oxygen in boiler pipes as described above. Measurements made under other cÏrcumstances have been reported, however; Haselden and Peters, for example, performed measurements on a pipe of 10 mm diameter the exterior of which was flushed with oxygen 18). They found that the position of the pipe has a marked influence on the results, a greater heat transfer being measured in the horizontal position than in the vertical position. The results for the vertieal and horizontal positions of the pipe are represented by curves land Il, respectively, in fig. 13. Haselden and Prosad 19) have investigated the boiling heat transfer of liquid oxygen in a duct. Their measurements were made on a vertieal pipe 36 em long and having an internal diameter of 10 mm, inside of which a tube was inserted to form a duet of 4 mmo The liquid pumped up by the vapour bubbles in this duet flowed.down again through the inner tube. The results are represented by curve III in fig. 13. It is noticeable that the heat transfer on boiling in a duct is much better than on boiling on an open surface. The results of Haselden and Prosad are only applicable, as they themselves state, to those cases in which the circumstances (duct width,height and liquid flow) are exaetly the same as those in the experiment. The heat transfer of heated wires to liquid
oxygen has been measured by Weil 20). He found that the maximum heat flux is 15 X 1()4 Wm-2 and that it occurs at a temperature difference of 11°C.
20 10 8 6 4 2 10-" ~(Wm·~). 1-0 0'8 0'6 0·4 I
/
][~;/
]][;j,
~
I ~n I I I I/
I j j
/1
Ij1/jl
0'2 o o 2 4~
.p d . / 6 8 10 12 14 iJ /J(oCJFig. 13. The boling heat transfer of liquid oxygen as a funetion of tbe temperature differenee between the wall and the liquid.
I. Measurements by Haselden and Peters on a vertieally positioned pipe of 10 mm diameter.
Il. As I. Horizontal position.
111. Measurements by Haselden and Prosad on boiling oxygen in a duet 4 mm wide.
IV. Our measurements on a fin of 5 x 2 x 0.5 em3 •
P. Maximum heat flux measured by Weil on eleetrieally heated wires.
This is indicated in fig. 13 by the point P. We have performed heat transfer measurernents on a copper rib of 5 X 2 X 0.5 crn3 . It appeared that the position of the rib had no effect on the heat transfer. The results are represented by curve IV in fig. 13; a maximum heat flux of 17 X 104 Wm-2 was measured
at a temperature difference of 9.5
°
c.
In the construction adopted, about 10 Ware dissipated per boiler pipe. The surface load is then 0.4 X 104 Wm-2. On a working column a temperature difference of 1 °C was measured between the pipe wall and the liquid. The heat transfer is thus comparable with the data reported by Haselden and Ptosad.
The maximum gauze temperature was -180 °C. At this temperature the carbon dioxide and water contents are 10-2 p.p.m. and less than 10-3 p.p.m.,
respectively. For this installation, in which there are no expansion valves to get blocked up, this content is certainly low enough.
3.4. The condenser
The saturated nitrogen vapour that leaves the column is condensed in the condensor of the gas refrigerating machine. This gives rise to a subatmospheric pressure so that the air can be sucked in directly by the installation itself.
The air then flows via the heat-exchanger and snow trap to the column, in which the oxygen is washed out of the air. The pressure in the condensor is thus equa1 to the pres su re of the outside air minus the pressure 10ss caused by the flow resistance in the parts of the installation through which the air flows. This pressure loss, depending on the thickness of the sn ow 1ayer on the gauze, is (1-3) 103 Nm-2 (10-30 cm water column). The condensation temperature of
the nitrogen is accordingly about -196°C.
The head of the gas refrigerating machine is provided with 104 recessed fins, making the tota1 condensing surface 0.2 m2• The co1d production at the tem -perature referred to is 650 W. The surface 10ad is thus 3.2 X 103 Wm-2.
Hase1den and Prosad showed in their paper mentioned under 19) that Nusselt's theory on the heat transfer by condensation in the case of 1aminar flow of the condensation 1ayer is a1so applicable to the condensation of nitrogen. Calcu1ation indicates that the temperature difference between the vapour and the wall is 0.4 °c, at a 10ad of 3.2 X 103 Wm-2•
Un1ess special precautions are taken the traces of hydrogen a1ways present in the air, and other gases that cannot be liquefied at -196°C and a pres su re of 1 atmosphere, will accumu1ate in the condenser and cause appreciable deterioration of the heat transfer. To prevent this a water-jet pump is used for extracting gas continuously from the condenser. The gas is extracted after it has passed the condenser , when the amount of "uncondensable" gas is greatest; the flow rate amounts to 1 or 2% of the gas to be condensed, and is found to be sufficient in the cases normally encountered. When this exhaust system was not operated, a drop in cold production of ab out 50 W in an hours time was repeated1y ascertained.
Af ter the installation has been started, the parts that operate at 10w temperature must be cooled down before the normal operating situation is reached. During this starting period the air cannot be cooled in the reboiler because the latter as yet contains no liquid. To avoid contamination of the column during this period, the air is not sucked in via the normal channel but through a direct communication between the condenser and the outside air. Before the air enters the condenser, however, it flows through a auxiliary sn ow trap, of the same type as described above, which is mounted on the head of the machine. The gauze surface-area (0.025 m2) and the spa ce around it are both small, but adequate for the starting period of 1i hours.
The condensate formed must be divided during operation into reflux and nitrogen product. The way in which this is done is described in chapter 4, where the regulation of the system is dealt with.
The nitrogen extracted as product is tapped off via a trap. If the negative pressure in the condenser were very high, the nitrogen oudet wou1d have to be in a 10w position, which could mean that large vessels could not be placed under it. Measures were therefore taken to keep the negative pressure in the
