the ultimate concentration in the liquid oxygen will , therefore , depend on the equilibrium constant for the impurity when present in low concentration in liquid oxygen . also , if the solubility is low , precipitation may occur before the concentration in the exit gas reaches the required value , and accumulation of the impurity as a solid will occur . table 3 shows the results of calculations for a number of trace impurities in which , for different assumed concentrations in the inlet air , the concentration in the liquid oxygen in the evaporator for steady state operation has been determined for gaseous oxygen production . it must be appreciated that the figures for the concentration build up are dependent on the accuracy of the equilibrium data , which are uncertain , but the table does give an indication of the order of magnitude to be expected . it is important to note that where high concentrations are theoretically possible in the plant evaporator the time required to build them up may be considerable , thus easily allowing steps to be taken to prevent such accumulations occurring . before discussing the methods which are adopted in practice to achieve this , we shall consider in a little more detail the effect of impurities in the air intake . effect of trace impurities in the air feed . there has been a considerable amount of work carried out in recent years on the effect of trace impurities in the air feed . not all of it has been convincing , and certain aspects are still by no means clear . it is impossible to do more than briefly review available information and data here . in considering the relative significance of trace impurities , particularly hydrocarbons , it should be borne in mind that small concentrations of hydrocarbons dissolved in liquid oxygen do not necessarily present a hazard . this depends on the susceptibility to detonation of the hydrocarbon solution , and on the explosive limits . data are incomplete for such solutions , but generally if the percentage by weight of the hydrocarbon in homogeneous solution is less than 2 % , detonation can not be initiated . in practice , it is obviously undesirable to operate near such a limit . light hydrocarbons , carbon monoxide , and hydrogen . in general , the C1 and C2 hydrocarbons such as methane , ethane , and ethylene , ( but excluding acetylene ) which have relatively low boiling points , do not normally present any hazard if present as traces in the air intake to a plant since they are appreciably soluble in liquid oxygen , and their equilibrium constants in admixture with this are relatively high . this means that they will not tend to accumulate in , for example , the oxygen evaporator to any dangerous concentration under likely operating conditions . carbon monoxide and hydrogen in trace quantities present no hazard since hydrogen is incondensible at the temperatures involved and is removed with the atmospheric inert gases , helium and neon , at a suitable vent-point in the plant . carbon monoxide is similar to nitrogen in properties and is , in fact , more volatile than oxygen . it therefore presents no hazard in trace concentrations . higher hydrocarbons and acetylenics . hydrocarbon impurities under the rather arbitrary classification of higher hydrocarbons and acetylenics can arise from three possible sources . the first is physical carry-over of hydrocarbon oil from , for example , an oil-lubricated expansion engine . this can accumulate as a solid in an oxygen evaporator unless provision is made in the plant design to prevent such an occurrence . the second source is from atmospheric contamination . the third is oxidation or thermal cracking of compressor lubricating oils where a reciprocating compressor is used . by the use of relatively low interstage pressure ratios ( 3/1 or less ) and by the use of lubricating oils of high stability , contamination from this source can be reduced to very small proportions . the problem does not arise with turbocompressors . the higher hydrocarbons and acetylenics have low vapour pressures at liquid oxygen temperature and are , therefore relatively non-volatile . when combined with a low solubility , as in the case of acetylene , accumulation as a precipitated solid can occur . it is for this reason that acetylene is one of the most dangerous of hydrocarbon contaminants . its solubility in liquid oxygen at its normal boiling point is approximately 6 parts per million and its k-value ( defined as the ratio of the mol-fraction of hydrocarbon in the gas phase to the mol-fraction in the liquid phase under equilibrium conditions ) is between 1/15 and 1/70 , depending on the data used , for oxygen evaporator conditions . whilst solid acetylene itself is more stable than usually realised , when mixed with liquid oxygen it is detonated relatively easily . it has also been shown that when a solid acetylene &amp;sol; liquid oxygen mixture contains fine inert solid particles , then the susceptibility of the mixture to detonation as measured by an impact sensitivity test is high . it has also been stated by Karwat that an incrustation of solid acetylene on oxygen evaporator tubes , which can be wetted by splashing with liquid , represents a particularly dangerous condition . it should be appreciated that whilst the amount of acetylene which can accumulate in a plant may not always in itself be sufficient to cause a serious explosion , it can , however , act as a trigger or detonator for the explosion of larger amounts of carbonaceous material if these should be allowed to accumulate . it is interesting to note that recently it has been pointed out by Karwat that propane may , under certain conditions , present a rather greater hazard than has perhaps hitherto been recognised , mainly due to the fact that although its solubility in liquid oxygen is relatively high ( circa 50000 parts per million of oxygen ) its equilibrium constant is very low . even traces in the air feed can , therefore , accumulate in the oxygen evaporator unless removed . the higher molecular weight hydrocarbons do not normally cause appreciable difficulty because they are almost completely non-volatile at low temperatures and are removed in the purification or heat exchanger system . non-hydrocarbon impurities . the main non-hydrocarbon impurities which are likely to pass through the heat exchanger system and initial purification on air separation plants are nitrous oxide , ozone , and oxides of nitrogen , in particular , nitric oxide . these impurities will also tend to concentrate in the oxygen evaporator , in particular nitrous oxide because of its low equilibrium constant . it has a low solubility ( circa 100 v.p.m ) and there have been suggestions that mixed crystals of nitrous oxide and acetylene may form from saturated solutions arising in air separation plants which can be easily detonated when the acetylene content of the mixed crystals is high enough . whilst fully conclusive evidence is not available , there are indications that the presence of ozone or oxides of nitrogen , ( or both ) in the presence of acetylene or other hydrocarbons may increase the susceptibility to explosion . further information is required to elucidate fully the possible role of these contaminants . solid particles . a factor that is not always mentioned when discussing the safety of air separation plants is the importance of strict cleanliness during plant assembly to avoid the introduction anywhere into the low temperature system of possible carbonaceous material , i.e carbonaceous dust , cloth fibres etc , since they can constitute a hazard if they accumulate in sufficient quantity at a particular point where a high oxygen concentration exists . safety measures . we shall now briefly review the various methods which have been or are used to control impurity build up in air separation plants . it is important to stress that the degree of protection which is employed may frequently be influenced by the amount of contamination of the atmosphere in the vicinity of the plant . purification of the air entering the plant . an obvious method , if practicable , is to eliminate impurities in the air entering the air separation unit . one approach to this problem is the use of catalytic purifiers after the air compressor in which the heat of compression is used , partly at least , to raise the air to a temperature at which the hydrocarbon impurities present can be catalytically oxidised . for example , in an installation in America a Hopcalite catalyst has been used . whilst for heavily contaminated atmospheres this initial purification may have advantages , it is relatively costly and also will not completely remove all trace impurities which will , therefore , nevertheless require treatment and removal at a later stage . a further disadvantage is that if used with a reciprocating compressor , oil contamination of the catalyst and loss of activity can occur if it is used directly after one of the compression stages . a different method of reducing the contamination in the air intake , particularly in industrialised areas , is the use of alternative suction lines , sometimes of considerable length , leading outside the contaminated area . these may be changed over depending on wind direction and the intensity of local contamination . this again does not eliminate contamination but it can reduce it appreciably . removal of impurities in the heat exchanger system . the removal of the less volatile trace constituents in the air-feed can take place in the heat exchanger system to some extent depending on their concentration , physical properties , the type of heat exchanger system , and the air pressure . for example , in a plant using regenerators in which the air is cooled at approximately 5 atm abs almost to its dew point ( approximately -173 &amp;deg; C ) , acetylene may be condensed in the regenerator packing and resublimed in the nitrogen of the next cooling cycle if the concentration in the inlet air exceeds approximately 0.6 v.p.m . this is a valuable safeguard of such plants . in plants operating with higher pressures , e.g fluid-producing plants , the effect of the superimposed air pressure in raising the vapour pressure of condensible impurities reduces , or may eliminate the possible deposition of impurities in the heat exchanger system . removal at low temperatures . one of the simplest methods of reducing the build-up of contaminants in the oxygen evaporator , which is the crucial part of the plant , is to provide a continuous purge of liquid . this has been practised since early days , but by itself is not an entirely satisfactory operation since it is only palliative , can easily be misapplied , or not operated , and also imposes an additional refrigeration load due to liquid withdrawn and rejected . the next step was to use an additional small condenser built away from the main plant condenser which was operated in series as far as the oxygen flow was concerned . the basic elements are shown in fig 4 . a small purge was led away from the additional condenser and rejected . the net result is a relatively large purge from the main plant condenser , and an appreciable accumulation of impurities in the additional condenser . this , however , is small and suitably protected , so that if , in fact , an explosion should occur , relatively little damage is done . whilst the above arrangement together with correct condenser design has been largely used in the past , the tendency today is undoubtedly towards the use of adsorption of the impurities from one or more of the process streams . silica gel is the adsorbent commonly used . there are a number of places at which one can apply such a clean-up system and they will be briefly mentioned . fig 5 shows a hypothetical and simplified plant flow diagram in which the various positions in which such adsorbers can be used is indicated . for illustration , the plant cycle shown is a low pressure plant using regenerators and producing gaseous oxygen . adsorption from the gas phase at or near the saturation temperature has attractions and silica gel adsorbers placed after the regenerators on low pressure plants provide a very effective clean up . the adsorbers are , however , large and relatively costly . the effect when a number of impurities are present on their individual adsorptive capacities under dynamic conditions , must be allowed for , i.e the occurrence of sorption displacement has to be considered . for example , Karwat showed that in solution in liquid oxygen trace impurities reached their break point in a silica gel adsorber in the order of ethane &amp;sol; propane &amp;sol; nitrous oxide + ethylene &amp;sol; carbon dioxide &amp;sol; propylene and acetylene , whereas in gas phase adsorbers the order of break through was ethane &amp;sol; ethylene and nitrous oxide &amp;sol; propane , and then acetylene and propylene . 