Hydrazine and Aqueous Hydrazine Solutions: Evaluating Safety in

Article pubs.acs.org/OPRD

Hydrazine and Aqueous Hydrazine Solutions: Evaluating Safety in Chemical Processes Jeffry K. Niemeier* and Douglas P. Kjell Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, Indiana 46285, United States S Supporting Information *

ABSTRACT: This contribution provides a summary of the hazards of hydrazine and its aqueous solutions and an understanding of the important role dilution plays in increasing the inherent safety of aqueous hydrazine solutions. Rather than provide an extensive description of the hazardous properties of hydrazine, the intent is to provide enough information to allow the reader to decide if hydrazine may be acceptable for a given application, and if so, what concentration range may provide an acceptable safety and environmental risk. Calculations are used to explore the favorable effect of dilution on the risk of fire, explosions, runaway reactions, and toxic exposure. Examples are provided to illustrate the strong effect catalysts can have on decomposition reactions. An analysis indicates the previously published lower flammability limit (LFL) is nonconservative.

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available to provide information needed to make final decisions on the suitability of hydrazine and the required processing equipment, procedures, and safeguards.2,4

INTRODUCTION In addition to extensive use as a rocket propellant, hydrazine is an important reagent in the fine chemical and pharmaceutical industries. Hydrazine derivatives are used as chemical blowing agents, pesticides, fungicides, and pharmaceuticals of many kinds.1,2 While hydrazine has many potential uses in the synthesis of chemicals, the decision on whether to select it as a reagent requires careful consideration of its properties. As with any reagent selection several factors need to be considered including potential safety and environmental hazards, risk tolerance, effect of reagent concentration, equipment capabilities, accident mitigation potential, plant site location relative to surrounding communities, process economics, supply chain reliability, and product quality. Hydrazine has a number of hazardous properties including high energy content, wide flammability range, the potential to support combustion in the absence of air, and high toxicity. Mitigation of the hazards is possible by using a dilute aqueous form. Aquesous hydrazine is commercially available at several concentrations between 15 wt % and 64 wt % (hydrazine hydrate3). This paper provides information on the flammability, explosion, thermal runaway, reactivity, and toxicity hazards of aqueous hydrazine solutions to assist the reader in assessing risks. A similar risk assessment must also be conducted on process streams that contain hydrazine. Anhydrous hydrazine poses serious risks, including the potential for vapor phase flammability and detonation in the absence of air. Thus, use of anhydrous hydrazine should be left to experts with equipment, facilities, and operating experience appropriate for handling highly flammable and detonable materials. Generally speaking, the more dilute the solution the greater the inherent safety. Because of the significant safety risks of hydrazine, the degree of inherent safety is of high importance. Selection of an appropriate concentration is a balance between safety, cost, and environmental considerations. The reader should obtain expert guidance on interpretation and application of hydrazine data. Excellent monographs are © 2013 American Chemical Society

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OVERVIEW OF HAZARDS Some of the more important hazards are summarized in this section. Hazards are discussed in more detail in sections below, and the Supporting Information (SI) provides tabular summaries of the hazards and physical properties of hydrazine and its aqueous solutions. Hydrazine has unusual flammability properties. Its flammable range is very wide (∼4−100% vapor).5 Its upper flammability limit of 100% indicates no oxygen is required; that is, decomposition of hydrazine is energetic enough to sustain combustion. Fortunately, the flash point of anhydrous hydrazine at atmospheric pressure is above typical ambient temperatures (38 °C closed cup),4,6 and aqueous solutions have even higher flash points. Hydrazine solutions of less than 29 wt % do not have a flash point.7 As with most materials with elevated flash points, it is still possible for combustion at ambient temperature when operating under vacuum. Deflagration or detonation of liquid anhydrous hydrazine can be initiated by rapid compression if air or vapor bubbles are present. Addition of 30% water makes the mixture insensitive to rapid compression.2,8 Hydrazine can form salts (e.g., alkali hydrazides and hydraziniums) with high energy release potential, and some are known to be explosive.2 Aqueous hydrazine solutions have good thermal stability unless contaminated by catalytic material. However the consequences of a runaway thermal decomposition with hydrazine are severe. Even with dilute solutions there is a risk of serious pressure buildup and formation of gases. Elevated temperatures are needed before appreciable decomposition Special Issue: Safety of Chemical Processes 13 Received: May 10, 2013 Published: September 5, 2013 1580

dx.doi.org/10.1021/op400120g | Org. Process Res. Dev. 2013, 17, 1580−1590

Organic Process Research & Development

Article

Table 1. Effect of increasing dilution for aqueous hydrazine

occurs. The critical temperature for runaway reaction is reduced significantly by many catalysts, particularly copper, cobalt, molybdenum, iridium, and their oxides, as well as iron oxide. Dilute solutions are inherently safer from risk of runaway reaction due to their ability to absorb the heat of decomposition. Hydrazine is very reactive with oxidizers, and concentrated solutions may be hypergolic (result in instantaneous ignition) when exposed to oxidizers.4 Hydrazine is highly toxic. The ACGIH threshold limit value (TLV) is only 0.01 ppm.9 This extremely low value is partially offset by the low volatility of hydrazine. Toxic exposure risk can be reduced by using dilute aqueous hydrazine. The distance to the toxic end point for a spill of 25 wt % hydrazine is about half the distance for a spill containing the same amount of hydrazine, but in the form of hydrazine hydrate (64 wt %). The potential to concentrate hydrazine by evaporation or boiling of aqueous solutions should be considered. The trade-off to using dilute hydrazine is the increased liquid volume handled per mole of hydrazine, in addition to the possibility the chemistry may not run well with extra water. Increased volume will result in increased cost, increased material handling, increased environmental (waste disposal) load, and increased potential for a spill. Table 1 provides a summary of the impact of dilution on process parameters. All of these factors should be considered when selecting the hydrazine concentration to be used. Figure 1 shows the effect of hydrazine concentration on some of the inherent safety properties of hydrazine solutions.

Figure 1. Safety properties of aqueous hydrazine solutions.

During process development it is important to collaborate with the potential manufacturing sites to obtain their input on the importance of each of the dependent variables when selecting a hydrazine concentration.

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FLAMMABILITY Flammability at Atmospheric Pressure in Air. Key questions: • Could storage, handling, or processing temperatures exceed the flash point of the aqueous hydrazine mixtures? • Could hydrazine be concentrated such that storage, handling, or processing temperatures exceed the flash point?

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Figure 2. Effect of hydrazine concentration on flash point (at atmospheric pressure) and predicted adiabatic flame temperature.

One of the first major uses of hydrazine was by Germans in their World War II rocket-powered ME-163 fighter jet. Hydrazine is still used as a rocket fuel. With oxygen or fluorine as the oxidizer, hydrazine’s “specific impulse” (kg of thrust per kg of fuel consumed per second) is only exceeded by that of hydrogen. It is also used as a monopropellant (no oxidizer) in engines for attitude and in-orbit control of satellites and spacecraft.1 A hydrazine fire can occur with any form of hydrazine including vapor, liquid, mist, and droplets. Hydrazine vapor has a wide flammability range of 4.7− 100%10 and anhydrous hydrazine has a relatively low closedcup flash point of 38 °C at atmospheric pressure. Literature LFL, flash point, and vapor pressure data are not self-consistent. At the stated flash point of 38 °C hydrazine has a vapor pressure of 0.040 atm (from Aspen Properties11), rather than the 0.047 atm predicted by the measured LFL data. This indicates the LFL is 4.0% rather than 4.7%. On the basis of vapor pressure data, an LFL of 4.7% would result in a flash point of 41 °C rather than 38 °C. The discrepancy may be due to the fact that the LFL of 4.7% was measured at 100 °C. Depending on the materials of construction of the test apparatus12 and how long some of the hydrazine was in the test vessel before ignition at this elevated temperature, some decomposition of the hydrazine to ammonia and nitrogen or oxidation to water and nitrogen may have occurred. The flash point of hydrazine hydrate (64 wt % hydrazine) is more favorable (73 °C by open-cup method).13 At concentrations below 40 wt %, aqueous hydrazine is said to have no flash point.1,4 However, this is based on open-cup flash point testing, and as described below further analysis indicates this may not be a safe limit. Figure 2 illustrates the effect of hydrazine concentration on flash point. At low hydrazine concentrations water dominates the vapor phase and serves as an inerting agent. This is illustrated by the sharp increase in predicted adiabatic flame temperature at about 30 wt % (19.4 mol %) hydrazine.14 Concentrations of less than 29 wt % can be heated to boiling at atmospheric pressure without reaching a hydrazine concen-

tration of 4.0%, the presumed LFL. Note that extended boiling of aqueous hydrazine will serve to concentrate the hydrazine and may lead to a hazardous situation. If lack of combustibility at atmospheric pressure is used as a basis of safety, the conservative approach would be to maintain hydrazine concentrations at several percent below 29 wt % (rather than the 40 wt % which is said to have no flash point). The required safety margin will depend on concentration variability and control. The data indicate hydrazine hydrate could have a flash point as low as 60 °C (rather than the 73 °C measured using the open-cup method). The open-cup flash point method typically gives nonconservative (high) numbers, and this may explain the discrepancy between measured and predicted flash point. Effect of Pressure on Hydrazine Flammability. Key question: • Does the operating pressure increase the flammability risk? Any conditions resulting in a hydrazine−air mixture with 4− 100% hydrazine may pose a risk. A reduction in pressure will result in higher hydrazine concentrations above liquid hydrazine and its solutions. Tests indicate that combustion is not possible at pressures below 0.014 atm.4 As an example of the potential for combustion under vacuum, hydrazine hydrate (64 wt %) is predicted to have a flash point of 60 °C at atmospheric pressure, while it is predicted to be flammable at only 35 °C at 0.2 atm pressure (adiabatic flame temperature of 1205 K using CEA2 program13). The partial pressure of hydrazine at this temperature is estimated to be 0.0107 atm (from Aspen Properties) and the vapor volume percent to be 5.35%. At elevated pressure, hydrazine vapor will constitute a smaller percentage of the gas phase at a given temperature, and if the pressure is high enough, the hydrazine concentration will be too low to support combustion. The important message here is that hydrazine should not be distilled under vacuum without careful consideration of the hazards. 1582



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Table 2. Hydrazine LFLs in various atmospheres5 calculated values mixture

N2H4 concentration at LFL (vol %)

pressure (kPa)

hydrazine−air hydrazine−nitrogen hydrazine−helium hydrazine−water hydrazine−water hydrazine−heptane hydrazine−heptane

4.7 38.0 37.0 30.9 37.4 86.8 86.8

100.9 100.5 100.9 91.8−118.5 28.5−35.3 53.9−110.2 53.9−110.2

test T (°C)

assumed T for calculations (°C)

calcd flame T using NASA-Glenn CEA2 program (K)13

92−102 109−112 105−118 130−135 98−100 104−133 104−133

97 110 111 132 99 104 133

1177 1040 1117 936 981 967 975

• Is there a risk of deflagration or runaway reaction causing equipment rupture? • Could hydrazine form a salt with explosive properties? A deflagration is defined as a reaction propagating at subsonic velocity. A detonation is a more severe event and is characterized by a reaction propagated by a shock wave. Detonations are much more damaging than deflagrations. Therefore, much greater attention to detail is required when handling materials with a potential for detonation, such as hydrazine. Although hydrazine is a high-energy molecule, liquid anhydrous hydrazine is insensitive to shock. However, its vapors can be detonated. Detonations have reportedly occurred during pumping operations due to compression of hydrazine vapors.18 Lab-scale tests on liquid hydrazine indicate that it is not detonable.2 Hydrazine vapors at greater than 50% in air pose a risk of detonation at atmospheric pressure. Lower concentrations are detonable at reduced pressure (e.g., 10% hydrazine in air at 1.5 psia).4 Addition of 30% water makes the mixture insensitive to rapid compression.2,8 Hydrazine can form salts (e.g., alkali hydrazides and hydraziniums) with high energy release potential, and some are known to be explosive. These form under low or high pH conditions.2,19 An explosion can also occur if equipment overpressures and ruptures. With hydrazine this could occur due to a deflagration or thermal decomposition, depending on the circumstances (e.g., depending on the materials of construction, contaminants, emergency pressure relief capacity, equipment pressure rating).

The Potential for Deflagration in the Absence of Air and the Effect of Inerting Agents on Hydrazine Flammability. Key question: • Could vapors from the aqueous hydrazine mixture form a flammable mixture even without air present? Literature values for LFLs of hydrazine in different atmospheres at temperatures of about 100 °C are shown in Table 2. This illustrates the potential for hydrazine explosions in the absence of an oxidizer. The calculated flame temperatures are relatively consistent. Differences between inerting agents can be expected due to differences in mass transfer (diffusivity), heat transfer, and heat capacity. However, the calculated flame temperature for 4.7% hydrazine in air is above the other values. This is another indicator that the LFL is indeed 4.0% instead of 4.7%. The data in Table 2 indicate that a combustion temperature of at least 900 K is required to sustain a hydrazine flame regardless of inerting agent. Therefore, calculation of the adiabatic flame temperature can provide insight into the flammability of a given hydrazine mixture. The data in Table 2 also indicate that, regardless of the inerting agent, hydrazine vapor concentrations exceeding about 30% can create a flammability hazard. Vapor concentrations exceeding 30% can easily be achieved by heating hydrazine hydrate (64 wt % hydrazine) in the absence of air, or by applying vacuum as described in the previous section. In contrast, vapor concentrations exceeding 30% are not possible in equilibrium with aqueous hydrazine of less than 52 wt % (38 mol %).15 This makes aqueous solutions of less than 52 wt % inherently safer when operating near ambient temperature.16 As discussed earlier, hydrazine vapors are flammable at concentrations as low as about 4% in air, whereas 30% or higher concentration is required in most inerting agents. In the former case the hazard is combustion, and in the latter it is decomposition flames. Effect of Catalysts on Hydrazine Flammability. In the presence of air, contact with metallic oxide surfaces may lead to flaming decomposition, even at ambient temperatures. In addition, absorption of hydrazine by rags, cotton waste, sawdust, other organic materials, or porous/fibrous surfaces may result in spontaneous ignition, depending on the hydrazine concentration.17 See the Thermal Stability section for information on the effect of catalysts on stability.

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THERMAL STABILITY AND THE EFFECT OF MATERIALS OF CONSTRUCTION Key questions: • Is there a risk of thermal runaway? • Could materials of construction catalyze decomposition? • Could contaminants affect thermal stability? • Could a thermal runaway lead to autoignition of vapors? Because the stability of hydrazine is dependent on materials of construction, thermal stability and materials of construction will be considered together. Elevated temperatures are needed before appreciable decomposition occurs. However, this temperature is reduced significantly by many materials, particularly copper, cobalt, molybdenum, iridium, and their oxides. Iron oxides also catalyze decomposition.1 Decomposition rates are proportional to the catalyst surface area.4 Information is available on the measured exotherm detection temperature of hydrazine when in contact with various metals

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EXPLOSION RISK Key questions: • Is there a risk of detonation? 1583



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Figure 3. Adiabatic decomposition temperature and pressure for aqueous hydrazine solutions for a vessel initially 50% full at ambient conditions (25 °C and 1 bar).27

heat of reaction is estimated to be −112 kJ/mol of hydrazine (see SI for calculations).

and nonmetals.2,4 Accelerating rate calorimetry (ARC) exotherm detection temperatures as low as 58 °C have been recorded for liquid hydrazine with some metals.4 Autoignition of hydrazine/air mixtures may occur at room temperature in the presence of some catalysts. Absorption of carbon dioxide from the air can be extensive (up to 33% weight gain) and will affect the rate of decomposition.2,20 The rate of decomposition of hydrazine in stainless steel vessels (which is accompanied by corrosion) is directly proportional to carbon dioxide concentration between 20 and 250 ppm.21 Data on decomposition catalysis must be used with caution, considering the phase(s) tested, concentration and purity of the hydrazine, surface area of the catalyst, temperature, pressure, and any coatings (e.g., oxides formed during passivation). Hydrazine should be handled with care using scrupulously clean systems. All concentrations can be handled in 304 (low Mo) or 347 stainless steel at near ambient conditions. One reference says stainless steel 316 is not recommended because of its high molybdenum content.1 Another reference says 316 is suitable to 66 °C for hydrazine of 0−64 wt %.22 Arch Chemicals23 states the following on their Web site:21 The selection of proper materials of construction for use with hydrazine is necessary not only to prevent the hydrazine solution from attacking the materials, but also to avoid decomposition of the hydrazine or contamination of the hydrazine solution with impurities. It has also been found that some materials which have proven satisfactory for use with hydrazine at one concentration may become unsuitable at another concentration. [...] It is recommended that each application be reviewed and tests conducted to ensure proper selection of materials of construction. Schmidt2 provides extensive guidance on selection of materials of construction. The decomposition chemistry will vary depending on temperature. Experimental work has shown that at temperatures up to 320 °C (and perhaps higher) the products are predominately ammonia and nitrogen with a trace of hydrogen (Bennett et al.,19 Wedlich and Davis24). At high temperature or with ample head space, the ammonia will be gaseous, and the

3N2H4(l) → N2(g) + 4NH3(g) ΔHrxn = −112 kJ/mol

Wedlich and Davis measured a heat of decomposition of −123 kJ/mol, but about 15% of the hydrazine was vapor at the onset of the decomposition.25 This will increase the heat of reaction. The heat of reaction for hydrazine vapor reacting to gaseous ammonia and nitrogen is estimated to be −157 kJ/mol (SI). 3N2H4(g) → N2(g) + 4NH3(g) ΔHrxn = −157 kJ/mol

If hydrazine or aqueous hydrazine decomposes in a constantvolume system, high temperatures and pressures will result. With the exception of very dilute solutions, containers with typical fill fractions will become liquid-full during thermal decomposition due to the significant increase in liquid specific volume. The combination of gas evolution plus limited head space results in extremely high pressures. As shown in Figure 3, a 50% full vessel containing aqueous hydrazine at 25 °C subject to catalytic decomposition can reach high pressures. Vessels with higher fill volumes would result in higher pressures. Note that, on the basis of decomposition kinetics with known catalysts (as listed in the AIAA report4), it would be extremely difficult to have a runaway reaction at ambient temperature with typical quantities (e.g., 100 kg; see analysis in the next section). A runaway reaction may lead to autoignition of the vapors. An autoignition temperature (AIT) of 165 °C has been measured for hydrazine vapor (no water vapor present).4,26 Vapor mixtures containing water would be expected to have a higher autoignition temperature. The risk of autoignition makes it especially important to utilize emergency pressure-relief systems to limit the pressure and temperature. Because of the high energy potential of hydrazine, thermal stability data should be obtained on all process streams. 1584



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ANALYSIS OF RUNAWAY REACTION POTENTIAL AS A FUNCTION OF HYDRAZINE CONCENTRATION AND MATERIALS OF CONSTRUCTION Key question: • What is the critical temperature for runaway reaction for my hydrazine mixtures? The potential for runaway reaction can be evaluated by comparing heat generation with heat loss. The AIAA report,4 which provides heat generation and loss equations, physical properties, and kinetic parameters (as a function of material of construction) for neat hydrazine, can be used to evaluate the potential for a runaway reaction. This data was collected in the absence of air. The presence of air will accelerate the decomposition28 and/or lead to oxidation. Physical properties and kinetic parameters specific to the solution in question should be obtained. The potential for runaway reaction can also be evaluated by using the Semenov approach if the surface kinetic parameters are converted into bulk parameters (see Bowes29). As the quantity handled or stored increases, normally so does the potential for runaway reaction. However, since hydrazine decomposition is primarily dependent on catalyst surface area, as is the rate of heat loss, less scale dependence would be expected with hydrazine. Runaway reaction temperatures can be estimated on the basis of heat transfer surface area and catalytic surface area. For the pharmaceutical and fine chemicals industries, 100 kg would represent a reasonable quantity of hydrazine per container.30 Using kinetic parameters from the AIAA report, 100 kg of neat hydrazine in a 120-L 316L stainless steel drum would result in a critical temperature of 169 °C (see Figure 4; critical

relatively small amount of catalyst could cause a runaway reaction. Storage of dilute aqueous hydrazine under the same conditions should be far safer, but this would need confirmation based on actual kinetic data. Figure 5 shows the predicted critical temperature as a function of hydrazine concentration, assuming neat hydrazine decomposition kinetics.

Figure 5. Predicted critical runaway temperature vs hydrazine concentration for 100 kg of hydrazine in a 316L stainless steel drum, assuming neat hydrazine decomposition kinetics.32

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EVALUATION OF PRESSURE GENERATION IN CLOSED VESSELS Key question: • What will the rate of pressure generation be for my hydrazine mixtures? Some hydrazine decomposition will occur even at low temperature with the best materials of construction. Therefore, pressure will build up in a closed system over time. This must be considered when designing equipment and procedures. The AIAA report provides methodology for calculating pressure generated by decomposition of neat hydrazine as a function of temperature, materials of construction, surface area, gas solubility, and head space.33 The same approach can be taken with aqueous hydrazine, but using gas solubility and decomposition kinetics data appropriate for aqueous systems. The accuracy of such predictions can be improved by measuring decomposition kinetics near the temperature of interest.

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EMERGENCY RELIEF SYSTEM CALCULATIONS Key questions: • What is the worst-credible case for emergency relief? • What effluent containment is needed? As always, equipment must have adequate emergency pressure relief to avoid rupture. The most critical part of the relief system design is to accurately define the worst-case credible scenario. A common case to consider is external fire. For hydrazine or aqueous hydrazine under external fire exposure, the required pressure relief capacity will be similar to that for solvents as long as a pressure near ambient is maintained during the event and no catalytic materials are present. Under these conditions the liquid will boil, but hydrazine decomposition rates will be low. The potential to concentrate the hydrazine in aqueous solutions should be considered.

Figure 4. Predicted heat generation vs heat gain for 100 kg of neat hydrazine in a 120-L 316L stainless steel drum.

temperature is the temperature at which the heat-removal line crosses the x axis).31 The critical temperature is the temperature above which runaway reaction is predicted and results in heat loss and heat generation curves that are tangent. A similar analysis for neat hydrazine in a tungsten carbide drum results in a predicted critical temperature of 85 °C. In reality, tungsten carbide would not be used as a material of construction for hydrazine because of its incompatibility, but this analysis shows the strong effect of catalysts on the decomposition of hydrazine. The surface area in this scenario is equivalent to that of 2.9 g of 1 μm tungsten carbide. Clearly, a 1585



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If pressure relief capacity is not sufficient during an external fire and the pressure is allowed to increase significantly, decomposition may become significant. Another pressure relief case to consider with hydrazine is contamination with a catalyst or reactant. Reactivity information should be obtained on any materials that may contact hydrazine, either intentionally or unintentionally. A special concern with hydrazine pressure relief is the high flammability and toxicity of its vapors. Special attention must be paid to the location of the vent exhaust and the need for effluent containment. The design of effluent treatment equipment capable of providing low pressure drop is covered in the DIERS Project Manual.34

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Figure 6. Hydrazine partial pressure as a function of aqueous concentration at 25 °C.27

REACTIVITY Key questions: • What is the reactivity of hydrazine with other process and plant materials? • Could hydrazine come in contact with a strong oxidizer causing a hypergolic reaction? • Could hydrazine come in contact with acids and bases capable of forming explosive salts? Anhydrous hydrazine is a strong reducing agent. Mixtures of hydrazine and strong oxidizers are hypergolic (i.e., they autoignite on contact).35 Hydrazine will react with acids to form hydrazinium salts and strong bases (e.g., sodium and lithium metal) to form hydrazide salts. Many of these salts are explosive.2 Bretherick’s Handbook18 provides information on hazardous reactions between hydrazine and over a dozen materials.

IDLH for all but the lowest concentrations (

Hydrazine and Aqueous Hydrazine Solutions: Evaluating Safety in

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