Anaesthesia, 1976, Volume 31, pages 30-U)

Adsorption of anaesthetic vapours on charcoal beds

F. A . P. M A G G S

AND

M . E. S M I T H

Introduction The hazards of working in badly ventilated operating theatres were reported as far back as 1893 by Frederick Hewitt,' but concern about the dangers of prolonged exposure to low concentrations of anaesthetic vapours has only recently been expressed. The numerous conditions, both pathological and teratogenetic, which have been to the effect of persistent exposure, have been reviewed by Hawkins.' It is also obvious that the narcotic effects of exposure to anaesthetic vapours can only lead to impaired efficiency in the operating theatre. It therefore seems desirable that theatre staff should, whenever possible, be protected from anaesthetic vapour concentrations. A reduction of the anaesthetic vapour concentration in the theatre can be effected by the installation of vapour-adsorbing filters in the air filtration plant but a simpler and more effective alternative would be the removal of the vapour at source, that is, from the patient's exhaled air. If, moreover, the local filter can be regenerated, its initially reasonable cost effectiveness is considerably enhanced. Hawkins' has examined several commercially available canisters for this purpose, and Capon has reported on repeated regenerations of an adsorptive ~ a n i s t e r . ~ It is well-known that the adsorption of vapours on a bed of charcoal is influenced by factors such as the air velocity through the charcoal bed, bed depth, grain size, evenness of packing, vapour concentration, and humidity as well as by the nature of the charcoal. The other parameter of particular interest to the user, the air flow resistance, will also show a dependance upon the first four factors. It would appear, however, that the marked variations in the performance of adsorbent filters which can occur with changes in the conditions of use have been appreciated by only few of those working with anaesthetics. The present paper reports the results of experiments carried out under controlled conditions with the object both of providing basic data in this field, and of illustrating that several-fold variations in performance may result from variations of design and use of adsorbers. Particular F. A. P. Maggs, BSc, PhD, FRIC, and M. E. Smith, Chemistry Division, Chemical Defence Establishment, Porton Down, Salisbury, Wilts, SP4 OJQ.

30

Adsorption of anaesthetic vapours on charcoal beds

31

attention has been paid to the humidity conditions. An unsealed canister of charcoal adsorbs atmospheric water vapour and effective use depends on the ability of the anaesthetic vapour to displace the adsorbed water. Experiments were therefore made after first equilibrating the charcoal bed with air at 80% RH. Since regenerated (and therefore dry) charcoal may also be used, beds of dried charcoal were also tested; in one series of experiments a dry air stream was employed, whilst in other experiments air at 80% RH was used, since the patient’s exhaled air will certainly be moist. The properties of the charcoal beds have been assessed in the usual way by measuring the penetration time for the anaesthetic vapour: that is, the time interval between the start of addition of the vapour to the air stream, and the point at which the vapour is first detected at a given low concentration in the effluent air. The majority of the work reported in this paper was carried out on the adsorption of halothane onto medium activity nutshell charcoal. The behaviour of several charcoals has been compared, and data relating to other anaesthetics was also obtained. Materials Charcoals The following four charcoals were used ; all were good quality respirator charcoals : A. Medium activity nutshell charcoal; B. High activity nutshell charcoal; C. Coal charcoal ; D. Impregnated coal charcoal. Table 1. Properties of the charcoals Heats of wetting (cal/g) Trichloro- MethoxyCharcoal Chloroform ethylene flurane Halothane A B C D

27.7 33.9 20.0 22.1

28.8 37.5 22.0 24.6

29.6 35.3 15.5 18.6

24.0 30.8 15.0 17.6

Bulk density (g/ml) 0.49 0.40

0.44 047

Air flow resistivity, R, (cmH’0 per cm/s per cm bed depth) 5.1 x 4.1 x 7.1 x 6.1 x

lo-’ lo-’ lo-’ lo-’

All charcoals were graded 14x25 BSS. The bulk density (Table 1) of the dried material was measured after packing the charcoal by means of the Porton filling machine, which gives a reproducibly close-packed bed. The beds of charcoal to be subjected to vapour penetration tests were similarly packed. Heats of wetting” of the charcoals (Table 1) were determined to provide a guide to the activity of the samples with respect to the anaesthetics to be used. The air flow resistances of the close-packed charcoal beds were measured at several flow rates. The air flow resistivity (R) (given in Table 1) is defined as the pressure drop (cmH,O) across a 1 cm thick bed when the air velocity is 1 cm/second. The definition of R in terms of the superficial air velocity, rather than the volume flow rate, gives a simple and general characterising constant. In this work the air velocity is defined by :

L (cm/s) =

(Volume flow rate (ml/s) Cross-sectional area of the bed container (cm2)

32

I;: A . P. Maggs and M. E. Smith

Values recorded in Table 1 for each charcoal refer to flow velocities less than 8.6 cm/s. Anaesthetics

The anaesthetics used, and their physical properties are listed in Table 2. Table 2. Properties of adsorbates

Anaesthetic

Formula

Halothane Methoxyflurane Trichloroethylene Chloroform

Molecular weight

CF3CHCIBr CHCl 2CF20CH3 CHCl :CCll CHClj

Boiling point (“C)

197 165 131 119.4

Molar volume (ml/mol)

Density Wml)

56 104.7 87 61.3

1.86 1.43 I .46 1.48

107 115.5 89.7 79.6

Experimental method The penetration time of the vapour through the charcoal beds was measured by use of the apparatus shown diagrammatically in Fig. 1 . Air at the desired humidity was drawn at a measured rate through the bed of charcoal held in a refillable container. The vapour concentration was obtained by use of a calibrated, driven syringe ejecting the anaesthetic on to a low wattage evaporator in the air stream. The penetration of vapour through the charcoal bed was detected with a calibra-

--

r

__-----

Charcoal bed

-+A

9’ -

I I

-

C

Pump

Mechanically driven syringe injection

1 1

I’

3

C.D.E halogen detector

,:k

Humidity controller

Fig. 1. Vapour penetration test apparatus.

Adsorption of anaesthetic vapours on charcoal beh

33

ted CDE halogen detector” coupled to a recorder. The penetration time was recorded when the effluent concentration reached 1% of the influent test concentration. Penetration times (T) when the effluent concentration had reached only 10 pg/l (1-2 ppm depending on the test vapour) were also noted for reference; a linear relationship was observed between the two times: T (10 pg) = T (1%) x (0.9410.3). A measured volume of charcoal was filled into a refillable container (cross-sectional area 58.2 cm’), using the Porton filling machine. For design purposes it may be useful to note that a volume flow of 1 litre/minute through the container gives a superficial linear velocity of 0.29 cm/s through this bed. Three humidity conditions were examined : (i) Dry charcoal: dry air; (ii) Dry charcoal: air at 80% RH;(iii) Charcoal equilibrated with air a t 80% RH: air at 80% RH. For brevity, the terms ‘moist air’ and ‘moist charcoal’ will be used where appropriate; it is to be noted that this usage, ‘moist charcoal’, does not imply that the charcoal contains liquid water. The conditions of flow and concentration were varied to cover the range of interest; the conditions used are recorded with the experimental results.

Results The measured penetration times are recorded in the following Tables. The weight of vapour adsorbed at penetration per 100 g charcoal (dry basis) has also been calculated to allow a ready assessment of the effect of change in conditions. Table 3 refers to the halothane/charcoal A system; Table 4 compares the behaviour of the four charcoals towards each anaesthetic vapour under a given set of conditions. Table 3. Halothane/charcoal A Halothane concentration Volume Flow charcoal rate (%v/v) (mg/l) (ml) (I/min) 1.34 1.34 1.34 1.34 1.34 1.34 0.67 2.68 1.34

110 110 110 110 110 110 55 220 110

100 200 100 200 200 200 100 100 100

4 4 8 8 16 12 16 4 16

Moist charcoal/moist air

Dry charcoal/dry air

FT

W%

FT

W%

15 40 8 16 7 11 5 10

14 18 14 15 13 16 9 18 -

61 150 34 72 32 49 29 15

62 68 62 65 58 66 52 54

-

Discussion At the point of initial penetration of an adsorbent bed by a vapour the amount delivered by an air stream of flow rate F, (F x C x T) equals the amount adsorbed (Wg), and a mass balance equation can be formulated (V = volume of charcoal (ml), C = concentration (g/litre) and T = time (minutes)). The amount adsorbed by the bed will clearly be less than the equilibrium adsorption and it is convenient to consider

I;: A . P . Maggs and M . E. Smith

34

Table 4. Comparison of charcoals

Methoxy- TrichloroHalothane flurane ethylene Chloroform Test vapour concentration: 100 mg/l 64 mg/l 49.6 mg/l 44 mg/l (1.34% V/V) (0.94% v/v) (0.91% v/v) (0.88% v/v) Test conditions Charcoal

PT

W%

PT

W%

PT

W%

PT

W%

A A A

150 143 40

68 67 18

184 164 129

83 73 25

237 250 208

50 41 29 74 67 53

43 39 13

151 131 45

248 197 145 299 270 217

242 218 75

B

48 43 34 76 80 66

274 259 92

60 56 20

92 77 29

46 38 20

181 158 129

53 46 37

197 188 105

171 150 62

34 30 12

99 99 43

47 47 20

166 172 162

45 47 44

210 206 161

44 37 24 44 44 34

172 178 89

32 33 16

Charcoal

B B C C C D D D

Air

All tests carried out on 200 ml charcoal at a flow rate of 4 litreslminute.

two arbitrary parts of the bed: that which has not been fully utilised (V,, the critical volume), and a portion (V- V,) which adsorbs to its equilibrium capacity (N g vapour/ ml charcoal). The mass balance equation thus becomes W = CFT = (V-V,) The inter-relationships between C, F, N, and V, are complex but the interpretation of experimental data in terms of this simple equation affords a useful guide to the behaviour of a system, even when insufficient data are available for an accurate evaluation of N and V,. If the case of halothane adsorption on dry charcoal is taken as an example, an average value of 340 mg/ml(630 mg/g) for N and 10-15 ml for V, may be estimated from the data at a halothane concentration of 1.34% v/v; as might be expected from Equation 1 the larger the charcoal bed, the less is the influence of the critical bed (i.e. percentage uptake, W%, approaches the value of N). Change in flow rate was observed to have only a secondary effect on the uptake in this system.

Influence of moisture If an adsorbent bed, already containing moisture adsorbed from the atmosphere, is to be effective the contaminant vapour must be able to displace the moisture from the surface. This ability will depend on the water/carbon and the vapour/carbon interactions, and on the respective vapour pressures in the system under consideration. Variations in behaviour between charcoals and between different adsorbates may be expected. This is illustrated by the present data. In all cases the dry charcoal/dry air system adsorbs more vapour than does the

Adsorption of anaesthetic vapours on charcoal beds

35

moist charcoal/moist air system. The ratios of the amounts of vapour adsorbed at penetration (taken from Table 4) for the two systems are presented in Table 5. Table 5. Ratio of halothane adsorption on moist charcoal to that on dry charcoal Charcoal Halothane Chloroform

Trichloroethylene

Methoxyflurane

A B C D

0.26 0.30 0.43 0.43

0.30 0.30 0.35 0.50

0.57 0.73 0.53 0.77

0.71 0.87 0.70 0.98

Boiling point "C

56

61

87

105

The order of decreasing interference by adsorbed water (with one minor exception) is halothane, chloroform, trichloroethylene, methoxyflurane. Pre-adsorbed moisture can reduce the useful life of charcoal against halothane and chloroform by a factor of approximately three. The ability of a vapour to displace adsorbed water might be expected to rise with increasing adsorption interaction, other factors being equal; for the systems examined here, the above order is, in fact, also that of increasing boiling point of the liquids, often taken as a rough guide to intermolecular forces operative in adsorption. The charcoals themselves also differ in their response to the displacement of adsorbed water, although less consistency of behaviour can be seen in this respect. Coal charcoals appear less susceptible to previously adsorbed moisture than nutshell charcoals. In the context of the mass balance equation, the more extensive data for the halothane and charcoal A system allow evaluation of the effect of previously adsorbed moisture on the parameters V, and N. It is found that at the three flow rates examined, the poorer performance of the moist charcoal is attributable to an increase in V, (roughly two-fold) and a decrease in N (roughly three-fold). When an anaesthetic of low boiling point is used in conjunction with moist charcoal, it is necessary to avoid the use of small beds of charcoal (where V, is important); when larger beds are examined (as for the data of Table 5 ) the influence of N predominates. When both the anaesthetic vapour and water vapour compete for a dry charcoal surface (Table 4, dry charcoal, moist air), adsorption of the anaesthetic predominates and, even in the worst case, the penetration time is only reduced to 80% of the 'dry/ dry' penetration time. The experimentally difficult condition of water at saturation vapour pressure has not been examined but it is unlikely that very different results would be obtained. The use of dry charcoal is recommended particularly when vapours of low boiling points are to be adsorbed: a three-fold improvement in uptake may be obtained. Type of charcoal

The four charcoals show a two-fold range of adsorptions at penetration: choice of the best charcoal is therefore important. The extent of the adsorbing surface of a charcoal with respect to a given vapour will be related to the heat of wetting of the

36

F. A . P . Maggs and M.E. Smith

Heot of wetting (col/gl

X Halothane;

0 Chloroform;

0 Trichloroethylene;

A Methoxyflurone.

Fig. 2. Dependence on amount adsorbed on heat of wetting.

charcoal in the same liquid. The amounts adsorbed on dry charcoal at penetration are plotted in Fig. 2 as a function of the heats of wetting for each adsorbate; the expected correlation is observed in three cases. The reason for the divergence shown by methoxyflurane is not immediately apparent. The performance of a given volume of charcoal, rather than its weight, is relevant in many applications. The data from Table 4 are expressed in Table 6 in terms of weight adsorbed per 100 ml charcoal, by use of the recorded bulk densities. The same order of merit is preserved although the difference in capacity between charcoals A and B is lessened. Table 6. Adsorptive capacities of the charcoals (at penetration) expressed as: I, g/100 g charcoal; 11, m1/100 g charcoal; 111, m1/100 ml charcoal

Chloroform

Halothane

Trichloroethylene

Methoxyflurane

Charcoal A B C D

I

I1

I11

I

I1

I11

I

I1

I11

I

I1

I11

68 83 44 47

37 48 27 27

18 19 12 13

43 60 34 32

29 41 23 22

14 16 10 10

51 74 44 44

35 51 31 31

18 20 13 14

48 76 53 45

34 53 36 30

16 20 16 14

Relative vapour pressure P/Po

0.042

0.042

0.081

0.34

Variation of adsorbate

The form of the adsorption isotherm and the relative vapour pressures at which the

Adsorption of anaesthetic vapours on charcoal beds

37

experiments were conducted could influence the amount adsorbed since the adsorption at equilibrium plays a significant part in the performance of canisters; l 2 the relative pressures corresponding to the test concentrations are therefore included in Table 6. If the simple hypothesis is accepted that the adsorption process entails micro-pore filling, the amount of vapour adsorbed is best expressed, for comparative purposes, in terms of the liquid volume adsorbed per 100 g of charcoal (Table 6). The volumes of halothane, trichloroethylene and methoxyflurane are substantially independent of the relative pressure for each charcoal. This suggests that the flat part of the adsorption isotherm is operative.” The lower value for the uptake of chloroform, even though the relative vapour pressure is nearly the same as that of halothane, is related to the weaker intermolecular forces operating in this liquid, as expressed by a parameter such as electronic polarisability.’

Airflow resistance All the charcoals used were nominally of 14 x 25 BSS grading (sieve apertures 1.3-0.7 mm), and a similarity in air flow resistance would be expected. The variation recorded (Table 1) is very probably related to different distributions of sizes within this grading range. In order to place the resistances in perspective, the pressure drop across a 200 ml bed of charcoal held in a 10cm diameter bed has been calculated for flow rates of 4 and 30 litres/minute in Table 7. It should be noted that Ap = 103RFV/60A2 cmH,O gauge (where R = flow resistivity (see Table l), F = volume flow rate (litres/minute) V = volume of charcoal (ml) and A = cross-sectional area of bed, (cm2). Table 7. Pressure drop across 10 cm diameter beds of granular charcoal Pressure drop, cmHIO Charcoal A B C D

4 l/min

30 l/min

0.11 0.09 0.15 0.13

0.83 0.66 1.15 0.99

It is clear that effectiveadsorbers having low breathing resistance (below that quoted in Table 1) may be designed. If the maximum flow resistance to be tolerated, and the minimum time of use required, are specified, the data provided in this paper would readily allow a filter meeting these requirements to be designed. The limitations on design are, in fact, of a practical nature: the packing of shallow and wide granular beds having low resistance is easily disturbed by movement and is therefore impracticable; large charcoal beds are cumbersome, and may not be easily regenerated; a well-designed filter may be expensive to manufacture, particularly if the precautions taken to preserve the granule packing against rough usage are elaborate. One solution to the problem may be found in the use or adaption of a well-established filter, as described in the next section.

38

F. A . P . Maggs and M . E. Smith

New developments A simple adsorber, derived from available components, was constructed in the course of this work for an extension of a trial conducted by Capong of regenerable adsorbers attached to an anaesthetic machine. Two metal canisters acting in parallel, and each holding 200 ml of nutshell charcoal A are threaded into a plastic holder (Figs. 3 and 4). The air flow resistance of the ensemble is 0.1 cmH,O gauge at 4 litres/ minute flow and 0.6 cm/H,O gauge a t 30 litres/minute flow. The dry canisters were tested for adsorption against 1.34% v/v halothane, in a 4 litres/minute flow of 80% RH air. A penetration time of 250 minutes was recorded (60% w/w adsorbed). The regeneration of charcoal in this type of canister has been shown to be effective and simple, using equipment available in many hospitals,* and the value of the twin adsorber is being assessed in a hospital trial.

Fig. 3. Twin canister holder. C l a n air

Clean air

1

I

1 L -

r

Contaminated air from anaesthetic machine

Fig. 4. Schematic diagram of CDE anaesthetic adsorber.

*A

cannister has been regenerated 260 times, without impairment (private communication, J. Capon).

Adsorption of anaesthetic vapours on charcoal beds

39

A new material, charcoal cloth, produced at the Chemical Defence Establishment, makes the design of a high capacity, low resistance filter possible, and avoids the problems associated with a granular material. A prototype filter, 10 cm diameter, 10 cm long, was constructed and tested. The air flow resistance was 0.07 cm/H,O gauge at 4 litres/minute flow and 0.51 cm HzOgauge at 30 litres/minute flow. At 4 litres/minute flow, 1.34% v/v concentration, 80% w/w halothane was adsorbed (penetration time, 200 minutes) under dry charcoal/dry air conditions; with moist air, moist charcoal, 50% w/w was adsorbed (penetration time, 125 minutes). Hospital trials of this filter have shown that it is effective in practice as a halothane adsorber; some modifications of the materials of construction of the prototype are required to allow more than about 20 regenerations of the filter.’ It should be noted that the flow resistance can readily be reduced further by dimensional alterations. The influence of moisture causes far less deterioration in performance than was found with granular charcoal (Table 3).

Conclusions Given the air flow and concentration conditions encountered at the exhaled air exit from an anaesthetic machine, suitable charcoals, when dry, adsorb between 30 and 50 ml (expressed as liquid) of the four anaesthetic vapours examined, per 100 g charcoal, before penetration occurs. The amount adsorbed (and thus the useful life) is roughly proportional to the volume of charcoal and varies inversely as the concentration (at a given air-flow rate); for example, 200 ml of charcoal will be effective for a period of several hours under average conditions. If the charcoal is not dry initially, a reduction in performance occurs; the extent is dependent upon the vapour and the charcoal. The choice of charcoal is important: even when charcoals which are likely to be suitable are selected (as in the present work) considerable differences in performance may be observed.

Summary

Tke adsorptive properties of four charcoals have been examined in relation to their value as adsorbents in filters attached to an anaesthetic trolley. Data for four anaesthetics have been obtained. The test conditions (flow rate, concentration, humidity) were those relevant to the application, and the effect of variation of these conditions was also studied. The air flow resistance of the charcoals was measured. Basic data are thus provided for the design of suitable adsorbers. The performance of two prototype anaesthetic adsorbers has been measured.

References 1. HEWITT, F.W. (1893) Anaesthetics and their administration. Charles Griffin, London. 2. BRUCE, D.L., EIDE,K.A., LINDE,H.W. & ECKENHOFF, J.E. (1968) Causes of death among anesthesiologists: a 20-year survey. Anesthesiology, 29, 565. 3. LASSEN,H.C.A., HENRIKSEN, E., NEUKIRCH, F. & KRISTENSEN, H.S.(1956) Treatment of tetanus: severe bone-marrow depression after prolonged nitrous oxide anaesthesia. Lancet, i, 527. 4. BRUCE,D.L. & KOEPKE, J.A. (1966) Changes in granulopoiesis in the rat associated with prolonged halothane anesthesia. Anesthesiology, 27, 81 1.

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F. A . P. Maggs and M.E. Smith

5. VAISMAN, A.I. (1967) Work in surgical theatres and its influence on the health of anaesthesiologists. Eksperimental’naya Khirurgiya i Anesteziologya, 3, 44. 6. CORBETT, T.H. &BALL,G.L. (1971) Chronic exposure to methoxyflurane: a possible occupational hazard to anesthesiologists. Anesthesiology, 34, 532. 7. VAN DYKE, R.A. & CHENOWETH, M.B. (1965) Metabolism of volatile anesthetics. Anesthesiology, 26, 348. 8. HAWKINS, T.J. (1973) Atmospheric pollution in operating theatres. A review and a report on the use of re-useable activated charcoal canisters. Anaesthesia, 28,490. 9. CAPON, J.H. (1974) A method of regenerating activated charcoal anaesthetic adsorbers by autoclaving. Anaesthesia, 29, 61 1. 10. MAGGS,F.A.P. & SCHWABE, P.H. (1962) A recording calorimeter for the rapid determination of heats of wetting. Journal of Scientific Instruments, 39, 364. 11. MAGGS,F.A.P. (1972) A non-destructive testing of vapour filters. Annals of Occupational Hygiene, 15, 35. 12. MAGGS, F.A.P. (1973) Adsorbents in air filtration. Filtration and separation, 10, 413. M.M. (1960) Chemical Reoiews, 60,235. 13. DUBININ,

Adsorption of anaesthetic vapours on charcoal beds.

The adsorptive properties of four charcoals have been examined in relation to their value as adsorbents in filters attached to an anaesthetic trolley...
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