From: amirza@bronze.ucs.indiana.edu (Anmar Caves)
Subject: Re: reheatable handwarmers
Organization: Indiana University
Date: Wed, 16 Dec 1992 22:23:43 GMT

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 THERMAL COMPARISON OF ACETATE AND IRON "HEAT PACKS"
 
 by Rane_Curl@um.cc.umich.edu
 
 INTRODUCTION
 
     The following is a largely theoretical analysis of the
 thermal properties of two type of "heat packs", referred to
 hereafter as the "Acetate" pack, and the "Iron" pack. It is hoped
 that this information, combined with practical experience in
 their uses, will provide bases for choosing the most appropriate
 device for providing heat to patients in a cave rescue situation.
 
 THE ACETATE PACK
 
     An Acetate pack is a plastic pouch filled with sodium acetate
 trihydrate, Na(C2H3O2)-3H2O. This is a crystalline solid that
 melts at 58 C. The melt has the property of sustaining
 considerable subcooling: the melt can be cooled to room temp-
 perature and below, without crystallization occurring. However if
 crystallization is initiated (in the practical unit, by bending
 a metal disc included in the pouch), crystallization proceeds
 with the release of the heat of fusion, at least until the
 temperature increases to 58 C. The Acetate pack is reusable, by
 remelting the contents in boiling water or a microwave oven. It
 is reported, however, that spontaneous initiation
 of crystallization occurs if the pack is lowered sufficiently in
 temperature.
 
     How much heat is available from an Acetate pack depends upon
 the initial and final temperatures between which the pack is
 used, and the thermal properties of sodium acetate trihydrate.
 The device must heat itself from its initial temperature, so the
 lower that is, the less useful heat is available. Also, there is
 a final temperature below which no significant heat is provided
 to a patient. Both of these temperatures depend upon the
 conditions of use and the state of the patient, and therefore no
 fixed "available heat" can be stated. However the available heat
 can be estimated for several values of initial and final
 temperatures, which provides at least comparisons with the
 Iron pack.
 
     The heat of fusion of sodium acetate trihydrate was estimated
 from heats of formation for sodium acetate (solid) and sodium
 acetate (3H20), from NBS Circ. 500 (USGPO, Wash. D.C., 1952),
 and that for water from NBS Tech. Note 270-3 (USGPO, Wash. D.C.,
 1968). The heat capacities of the solid and molten salts were
 estimated by use of Kopp's Rule, modified by estimating the
 contribution of water of crystallization as Cp = 9 cal/gmolK, and
 water in the melt as Cp = 19 cal/gmoleK. The resulting heat
 capacities are Cp = 51.7 cal/gmolK (0.38 cal/gramK) for the
 solid, and Cp = 92.5 cal/gmolK (0.68 cal/gramK) for the melt.
 (The utility of sodium acetate trihydrate in this application
 comes from its heat of fusion, and not from an unusually
 high heat capacity.)
 
     If an Acetate Pack initially at temperature Ti is
 "triggered", the release of heat of fusion will raise the
 temperature until it reaches the melting point, 58C. Further
 crystallization will only occur if heat is removed by transfer to
 the surroundings. Based upon the thermochemical properties given
 above, the following Table shows the fraction of the acetate that
 will still be molten at 58 C, starting at the given Ti,
 assuming that the phase transition is done adiabatically:
 
                      Ti     Fraction still molten
 
                     10 C         0.49
                     20 C         0.60
                     30 C         0.70
 
     Thereafter, as heat is removed, the pack will stay at 58 C
 until the contents are totally frozen, and then the pack
 temperature will fall. At how low a temperature is the heat from
 the pack still useful? This will depend upon the cirumstances,
 and therefore the total heat available from an Acetate pack was
 calculated assuming different initial and final temperatures. In
 the following table, this heat is given as calories/gram sodium
 acetate trihydrate:
 
                          Final Temperature, Tf
 
                  Ti     10C       20C       30C
 
                 10 C    49.5      45.6      41.8
                 20 C    56.2      52.5      48.6
                 30 C    63.0      59.2      55.4
 
     For comparison, just plain water, provided at 30 C, and used
 until it reaches 10 C (probably not physiologically useful,
 however), would provide 20 calories/gram. In fact, a hot water
 bottle provided at 100 C (if it could be sufficiently insulated
 during transport, say in a Thermos (TM) bottle) would provide 70
 calories/gram, cooling from 100 C to 30 C, and therefore
 outperform an Acetate pack. The advantage of the Acetate pack, of
 course, is that it will retain its thermal potential for a long
 time if not triggered, and is therefore an immediately available
 source of low-level heat.
 
 THE IRON PACK
 
     An Iron pack is a pouch containing a mixture of finely
 powdered iron along with water, salt, activated carbon, and wood
 pulp (or vermiculite, etc). When the pouch is exposed to air,
 oxygen diffuses into the pack, and it and water react with the
 iron to form, literally, "rust". This reaction produces
 considerable heat. One version is sold with the name Handiwarmer
 (TM). One filled pouch weighs about 19 grams.
 
     The salt serves to depassivate the iron in an oxidizing
 atmosphere, thereby acting as a catalyst for the reaction. The
 wood pulp is apparently just an absorbent for the water, to keep
 the contents moist, but prevent leaking. The purpose of the
 activated carbon is less certain: it may be to help disperse the
 very fine iron particles (ca. a few micrometers) and prevent
 them from forming agglomerates of iron oxide, which could slow
 the access of oxygen to the remaining unreacted iron. There is a
 nearly two-fold expansion in the oxidation of iron, which could
 contribute to such agglomeration.
 
     The reaction that takes place is assumed to be
 
                Fe + 0.75 O2 + 0.50 H2O = FeO(OH)
 
 The product of iron oxidation in the presence of water can be
 Fe2O3, FeO(OH), or Fe(OH)3, depending upon the circumstances.
 Any of these products still require 0.75 moles O2 per mole iron,
 and differ only slightly in their heats of reaction. The above
 reaction has a heat of reaction of DH = -99.4 kcal per gram
 mole, at 25 C (thermochemical data from NBS Tech. Note 270-4,
 USGPO, 1969).
 
     A used and dried Iron pack was opened and the contents
 removed, and found to weigh 19.9 grams. It showed no
 efflorescence of dried salt, and therefore the mass of salt was
 assumed small (little is required for depassivation).  The
 reddish-brown powder was reacted with hydrochloric acid to
 dissolve the iron oxide. The residue, consisting of carbon and
 wood-pulp, was dried, and found to weigh 5.3 grams. Therefore
 the iron oxide weighed 14.6 grams and, if assumed to have been
 FeO(OH), formed from 9.2 grams of iron. From the heat of
 reaction given above for iron oxidation, each Iron pack can
 therefore provide 16.3 kilocalories. Since the pack itself
 weighed 19 grams, this represents about 860 calories per gram of
 Iron pack, an order of magnitude more than the heat available
 per unit weight from an Acetate pack.
 
     The Iron pack has, also by comparison to the Acetate pack,
 negligible mass, and therefore its initial and final temp-
 eratures makes little difference in the available heat. A direct
 comparison is that an Iron Pack is comparable to an Acetate pack
 weighing 360 grams and working between Ti = 10 C and Tf = 20C.
 However the Iron pack is not reusable, as is the Acetate pack.
 
     The Iron pack requires oxygen for its operation. In fact,
 even after starting an Iron pack, it can be stopped by depriving
 it of oxygen, say be closing it in a Ziploc (TM) bag. If allowed
 to run to exhaustion, the 9.2 grams of iron in the Iron pack will
 consume 0.123 gmoles oxygen, corresponding to a volume of 2.8
 liters pure oxygen, or the oxygen in 13.1 liters air (both at
 STP). This oxygen consumption occurs over a considerable time,
 but one should still take into consideration the possible
 depletion of oxygen in the vicinity of an Iron pack.
 
     The operation of an Iron pack is controlled by the
 construction of its pouch, and by the diffusion of oxygen through
 the contents. The pouch is a felted synthetic fabric, lined on
 the inside with a plastic film (apparently a polyhydrocarbon).
 One one side this film is punctured with a grid of small holes,
 which permit oxygen to diffuse into the pouch. No study has been
 made of the maximum rate of reaction possible by continuous
 exposure to air. The rate is also limited by internal diffusion,
 which can be partially hastened by mixing the contents, during
 use, by kneading the pouch. The package containing the Handwarmer
 (TM) says "7 HOURS", but this will clearly depend upon how the
 device is used.
 
     It was observed that an unused Handiwarmer (TM) can be picked
 up with a moderately powerful hand magnet. The pouch contents,
 after reaction, would not be magnetic, so this provides a test of
 whether the device is still moderately unreacted. This test could
 be made quantitative, given the magnet that one wishes to use.
 
 DISCUSSION
 
     Both the Acetate Pack and the Iron Pack appear to have use as
 heat sources in cave rescue situations. The Iron Pack has a
 considerable weight advantage (ca. 300 to 1) over the Acetate
 Pack, but the Acetate Pack is reusable. Heat release from the
 Acetate pack can only be controlled by the degree of its
 insulation, while the heat release from the Iron pack can be
 controlled both by insulation and by the degree of isolating
 it from its oxygen supply. How to manipulate these controls
 will have to be learned from experience, but it can be stated
 that, in either case, the side away from the patient should be
 very well insulated, if the available heat is not to be wasted.
 
 PERMISSION TO REPRODUCE THIS ARTICLE IN ENTIRETY IS GRANTED

-- 
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