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This behavior is shown in Figure 6. Thus, all of the acid, additive, and hydrocarbon become an aerosol upon release. The difference is in the form of the release.

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The total H 2 SO 4 acid inventory is 36, gallons. This release description will allow the H 2 SO 4 units to be evaluated on the same basis as the HF units; what is the hazard associated with an acid release from a two-inch hole? In addition, the emulsion has a high viscosity that retards the hydrocarbon vapor bubbles from shattering the liquid stream. These phenomena result in very little of the H 2 SO 4 acid remaining airborne.

From Figure 8, it can be seen that it takes about twenty-seven minutes for the emulsion in thecontactor to be released. These results are documented in work performed for the Petroleum Energy Research Foundation [Johnson, ]. According to the test data, a small amount of H 2 SO 4 was not captured in the immediate vicinity of the release. The primary reason for this was due to the capture array being too small. During some of the tests, it was observed that some of the drops missed the capture pans either on the side or over the end of the array.

The 1. The acid would then remain suspended on the hydrocarbon bubble until the bubble burst. Upon bursting, most of the liquid H 2 SO 4 drops would fall to the ground.

When the emulsion in the contactor has been driven out of the two-inch hole, the pure acid layer from the settler is then released. During this period of time, about fifty-three minutes in this example, all of the H 2 SO 4 released falls to the ground.

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This results in a local hazard due to the liquid acid, but not a significant vapor cloud hazard. Once the pure acid layer has been released out of the contactor, the emulsion that was pumped to the settler now is driven from the hole. For this example, the composition of the emulsion was assumed to be the same as that in the contactor. This behavior is shown in Figure 8. A summary of the acid release rates for the eight scenarios described above is given in Table 1 and presented graphically in Figure 9. As can be seen from Figure 9, the release of pure HF results in the greatest mass flow rate of acid that remains airborne The two active mitigation systems, rapid deinventory andwater spray, yield the same high flow rate until the systems are activated.

Upon activation, the acid rate remaining airborne is lowered water spray or the duration of the release is shortened rapid deinventory. In all cases where the acid is diluted, either with additive or hydrocarbon, the rate of acid remaining airborne is lowered. The definitions of the airborne mass rates for the acid, hydrocarbon, and additive for each of the releases evaluated above provide the initial conditions for dispersion calculations. In order to compare the hazards associated with each of the releases, dispersion calculations were performed until the acid concentration diluted below Emergency Response Planning Guideline Level 3 ERPG For this example, we are concerned with the acid cloud that is immediately formed following the release.

In some cases, this is an aerosol cloud; in other cases, the acid is released as a vapor. In all cases, the acid is released with some velocity associated with its storage under pressure. This model has been modified to incorporate aerosol thermodynamics and has been reviewed by independent third parties [Hanna, Strimaitis, and Chang, ; TRC, ].

Dispersion calculations for any acid evolving off a liquid pool resulting from a release were not made for this study. A summary of the results of the dispersion analysis is presented in Table 2 and Figure The worst-case conditions were defined as low winds, 1. None of the H 2 SO 4 releases produced downwind dispersion distances greater than about ft 95 m from the release point.

This is due primarily to the inability of the release to form a significant cloud with H 2 SO 4 in it. The benefits and limitations of three different HF acid mitigation systems were reviewed. In order to compare the effectiveness of the systems under real world conditions, calculations were made for a release from a two-inch hole located at the bottom of the alkylation unit acid cooler.

The mitigated and unmitigated results for releases containing HF were then compared to a similar release from an H 2 SO 4 alkylation unit. The addition of a rapid deinventory system shortens the duration of the overall release. In particular, the duration of a pure acid release is shortened. For the example two-inch hole, the pure acid release duration was reduced from twenty-one to four minutes.

The dispersion analysis to the ERPG-3 endpoint would not show a significant difference between a twenty-one and four minute release. However, if these results were incorporated into a risk analysis, the use of a probit relation to quantify the dose received at a downwind location would show a significant reduction in risk due to the addition of a rapid deinventory system. The effectiveness of any water spray mitigation system is subject to many parameters.

Unlike the rapid deinventory system, the effectiveness of a water curtain is determined by whether the water contacts the HF. The use of an additive to the HF has two effects on the overall behavior of an acid leak. First, the additive reduces the volatility of the acid phase; second, the additive dilutes the acid. The result of these two effects is to release acid at a lower rate than a pure acid release dilution and for the acid that is released, a greater percentage falls to the ground reduced volatility and does not enter the atmosphere immediately.

The dilution rate is determined by how much additive is added to the acid phase.


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Thus, when the dilution effects and reduced volatility effects are combined, the result is that, for equal sized holes e. When the releases with and without additive are compared, the airborne HF rate is reduced from In this case, the reduction is due purely to dilution. The use of any HF acid mitigation system has the potential to reduce the impact of an acid release from an alkylation unit. This study has shown how the different mitigation systems currently suggested for use rapid deinventory, water spray curtains, and acid additives can affect the airborne acid release rates and thus the extent of an HF vapor cloud.

In all cases, the rate of HF that would remain airborne following a release would be larger than the rate of H 2 SO 4 remaining airborne. Johnson and John B. Cornwell Presented At First Risk Control Engineering Seminar Maracaibo, Venezuela October , Abstract Hydrogen fluoride HF is used in the petroleum industry as a catalyst in the alkylation process for producing high octane gasoline components.

The three mitigation systems to be evaluated are: Rapid acid deinventory system Water spray system Acid additive Several studies [Technica, ; Maher, Kaiser, and Alderman, ] have attempted to compare the risk of using HF as the alkylation acid to the risk associated with the use of sulfuric acid H 2 SO 4 as the alkylation acid. Introduction Download a PDF this paper will be divided into three sections: Advantages and disadvantages of each mitigation system.

Effect of each mitigation system on the release rate and airborne fraction of acid.

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Effect of each mitigation system on the downwind dispersion distance of airborne acid. Advantages and Disadvantages of Mitigation Systems Each mitigation system listed above has the potential to reduce the potential hazard zone associated with an HF release. Rapid Deinventory System The primary aim of a rapid deinventory system is to reduce the amount of HF that might be released during an accident.

The potential advantages of a deinventory system are: reduces the duration of a potential release by removing inventory. The potential disadvantages of a deinventory system are: does not reduce the rate at which HF is released. Water Spray System There are two principal benefits to a water spray system.

The potential advantages of a fixed water spray curtain are: absorption of HF by water, thus removing the acid from the vapor cloud.

Guidelines for Vapor Release Mitigation | Wiley Online Books

The potential disadvantages of a fixed water spray curtain are: does not reduce the rate at which HF is released. In general, a desirable additive would have the following characteristics: the additive increases the ratio of HF that falls to the ground versus the HF that remains airborne during a release. The potential advantages of using an additive are: the additive reduces the amount of HF that remains airborne following a release when compared to the no-additive release.

There is no response time; the mitigation is built into the system.

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Effect of Each Mitigation System on the Release Rate and Airborne Fraction of Acid In order to compare the effect of a mitigation system on the amount of acid that remains airborne or falls to the ground, a consistent set of initial conditions must be defined. Figure1 Hf Alkylation This release description will allow the mitigation systems to be evaluated on the same basis; what is the hazard associated with an acid release from a two-inch hole?


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  • Release Number 8 — H 2 SO 4 Release When the emulsion in the contactor has been driven out of the two-inch hole, the pure acid layer from the settler is then released.