ANALYTICAL STUDY OF LIQUID/VAPOUR SEPARATION EFFICIENCYBY Dr. W.D. Monnery Chem-Pet Process Technology Ltd. 335 Ranchridge Bay NW Calgary, AB

Dr. W.Y. Svrcek Department of Chemical & Petroleum Engineering University of Calgary Calgary, AB T2N 1N4

September 5, 2000

2 SUMMARY The purpose of this work was to establish the separation efficiency of flare knock-out drums and determine the expected entrained liquid droplet diameter that is carried over to the flare. This was accomplished by using a field pilot plant skid at the Prime West East Crossfield gas plant. The skid consisted of gas and liquid inlets test separators and entrained liquid collection in a filter/coalescer. The raw test data was entrained liquid carryover amount as a function of gas velocity data. Experimental results provide incipient entrained liquid carryover velocities. The data show that carryover rises sharply after the incipient carryover velocity and separation efficiency drops below 99.9%. Experimental results indicate that entrained liquid carryover average droplet diameters are 200 to 600 microns for flare knock-out drums at 10 to 100 psig. Calculations show that the maximum stable droplet size can be very large at low velocities and the calculated liquid droplet size distribution indicates that there can be substantial variance in the droplet size and that the latter may not be very uniform. In order to verify the estimated droplet sizes and distributions, further experimental work must include the addition of online droplet size and distribution measurement equipment. Experimental results provide quantitative data for the relationship between horizontal and vertical K factors and allowable velocities, which has to date been empirical and subjective. These results show that the factor between horizontal and vertical K factors and allowable velocities vary from about 1.33 to 1.67 as L/D varies from 3.5 to 6.5. Modelling results based on using the experimental data give entrained liquid average droplet diameters that are consistent with API 521 for flare knock-out drums (300-600 microns) as well as other open literature. To avoid carryover, flare knock-out drums should be designed using a droplet size of 300 microns.

3

1.0 INTRODUCTION/BACKGROUNDThis study is part of the Alternative Flaring Technologies program sponsored by Environment Canada, CAPP and PTAC. This study focuses on the efficiency of gravity separation as it relates to flare knockout drum design and operation. One of the critical issues in facilities process design and operation is vapour/liquid separation. This is also an important issue for the improvement of existing flaring systems. The problem for flaring systems is that with the uncertainty of design and operating conditions, liquid carryover droplets may be of such a size and composition that they are incompletely combusted. This results in the emission of many undesirable compounds to the atmosphere, as has been outlined in previous studies and of the current Government and Industry study aimed at mitigating emissions in flares. There is an abundance of literature available on vapour/liquid separation and equipment design, yet there has never been a systematic, comprehensive study to verify the accepted design methodology. Liquid vapour separator design is described in several engineering and operating company guidelines, the GPSA Engineering Data Book and recent publications such as Svrcek and Monnery (1993) and Monnery and Svrcek (1994). Other publications of note are Watkins (1967) and Talavera (1990). The present design philosophy is to simply attempt to be conservative enough so that separation equipment will work. Unfortunately, the definition of how conservative designs are remains in question. Furthermore, equipment that does function properly at design rates may need to be re-rated for increased rates or at off-design operating conditions and the above mentioned problem appears again (how conservative?). Although general design methodology is well accepted, it is the subjectivity of some of the separation parameters used in the models that are in question. As such, the purpose of the research is to determine the efficiency of gravity separation. Specifically, it is to determine the velocity at which carryover occurs and to estimate the liquid particle size going to flare. This data can also be used to check current design criteria and estimate liquid carryover at operating conditions.

4

2.0 THEORY OF GRAVITY SEPARATION AND SIMPLIFIED COALESCENCE MODELLINGIn a liquid-vapour separation vessel, there are typically three stages of separation. The first stage, primary separation, uses an inlet diverter to cause the largest droplets to impinge by momentum and then drop by gravity. The next stage is gravity separation of smaller droplets as the gas flows through the vapour disengagement section of the separator. The final stage is mist elimination, where the smallest droplets are coalesced on an impingement device, such as a mist pad or vane pack, followed by gravity settling of the larger formed droplets. In vessels like flare knockout drums, we are primarily concerned with gravity separation since they typically have no coalescing internals, such as mist pads. For gravity separation, the allowable velocity is determined so that the required disengagement area can be determined. For a vertical vessel, performing a force balance on the liquid droplet settling out provides the necessary relationship. When the net gravity force, given by Eq.1,FG = M P ( L V ) g L gc

(1) Balances the drag force, given by Eq. 2,2 2 ( / 8)C D D P U V V FD = gc

(2)

the liquid droplets will settle at a constant terminal velocity, UT. Equating Eqs. 1 and 2 results in:UT = 4 g DP ( L V ) 3C D V

(3)

Hence, as long as the vapour velocity, UV, is less than UT, the liquid droplets will settle out. Eq. 3 can be rewritten as Eq. 4, in the well-known Souders-Brown form:UT = K ( L V ) V

(4)

whereK= 4 g DP 3C D

(5)

5 The drag coefficient can be calculated from a curve fit of Fig 7-3 in the GPSA Engineering Data Book as follows:X =3 0.95 10 8 D P V ( L V ) 2

(6)

where DP is in ft (microns 3.2808 10-6), densities are in lb/ft3 and viscosity is in cP.CD = 5.0074 / ln( X ) + 40 .927 / X + 44 .07 / X

(7)

The K factor from Eq. 5 is the theoretical value for vertical gravity settling. It requires a known liquid droplet diameter, DP, and determination of the drag coefficient, CD. For coalescing devices such as mist eliminators, the droplet diameter changes as coalescence occurs and cannot be predicted with any accuracy. As such, the K factor for coalescing devices is usually an empirical value, determined from experiments. A well-known source of empirical K factors for mist pads is the GPSA Engineering Data Book. Typical values are given below in Table 1. In addition, values can be obtained from vendors for their particular coalescing devices. Table 1 GPSA K Factors Separator Type Horiz with Vert pad Vert/Horiz with Horiz Pad Pressure (psig) Atmospheric 300 600 900 1500 Vacuum K Factor (ft/s) 0.40 0.50 0.35 0.33 0.30 0.27 0.21 0.20

Notes: 1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig 2. For glycol or amine solutions, multiply above K values by 0.6 0.8. 3. Typically use one-half of the above K values for approximate sizing of vertical separators without mist eliminators. 4. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 0.8. For horizontal vessels, the forces of gravity and drag no longer oppose each other and a simple vector analysis is not possible (Talavera, 1990). However, experience has shown that horizontal velocities can be greater than the vertical terminal values, as shown in the above table. As such, several literature publications apply multipliers to correct the vertical terminal velocity or vertical K factor as shown in Eq. 8:

6K H = F K V

(8) where subscript H indicates horizontal and subscript V indicates vertical. The factors are either empirical or based on the fact that the time the liquid droplet takes to drop vertically through the vapour flow area must be less than the time it takes to travel horizontally between the inlet and outlet nozzles. This results in the correction factor F stated as follows, Eq. 9:F = LE / H V

(9)

where LE is the effective horizontal length of travel of the liquid droplet and H V is the vertical distance from the inlet to the liquid surface. As such, there is considerable subjectivity in determining horizontal K factors.

3.0 METHODOLOGYThe research program, as outlined originally by Environment Canada, CAPP and PTAC was as follows: 1. 2. Identify current liquid removal technologies and practices and develop a standardized testing methodology for knockout systems. Identify acceptable knockout performance: Knockout efficiency will be the measure of performance. The definition of acceptable knockout efficiency must be based upon what is attainable with the technology under field conditions. [Is 99% efficiency attainable?] Lab testing of the current technology: The identified liquid removal system(s) must be tested to confirm that they will meet proposed regulations and under what operating conditions. The effect of such parameters as pressure, flow rate, compositions, ambient temperature, water content and hydrocarbon liquids content on knockout efficiency must be determined. Field Pilot Testing: Confirm the successfully lab tested liquid removal systems handle the rigours of field operations and deliver the rated knockout efficiency. 5. Commercialization

3.

4.

7 3.1 EXPERIMENTAL APPARATUS AND PROCEDURE

To make the results as realistic as possible, all testing was done in the field at the Prime West (previously Amoco) East Crossfield Gas Plant, with no lab testing done. The apparatus used for the experiments was a pilot plant scale skid, which is shown in Figures 1 - 9. The apparatus consists of gas inlet piping, liquid pumping and injection, test separators and a high efficiency filter/coalescer to collect entrained liquid from the test separators. There are three horizontal test separators, each a 10 inch nominal outside diameter (9.13ID) and lengths of 26, 36 and 56, used in order to study the effect of vessel length on allowable velocity. The test separators each had external cage throttling level control with the float custom made to work for fine control within the separator dimensions and for the butane liquid. A manual globe valve located on the gas outlet piping controls the skid pressure. The gas flow is monitored downstream using a Haliburton flow indicator and controlled by a manual globe valve on the gas inlet piping. Gas inlet temperature was that delivered to the skid from the gas plant. The liquid injection pump is a JAC metering pump with a maximum flow of 37.5 gph at 750 psi. Experiments proceeded as follows (refer to Figure 1). For each horizontal vessel experiment at a given pressure, a gas flow rate was calculated, a priori, to give the desired velocity at the set liquid level for that particular experiment. At the beginning of the experiment, the gas pressure was set using the outlet manual globe valve and flow was adjusted using the inlet globe valve thus providing the desired flow at the desired pressure. Once the gas flow stabilized, the liquid flow was started in an amount known to over-saturate the gas. At this time, the skid was left until the flows and temperatures reached a steady operating condition. During the experiment, the gas-liquid mixture flowed to the selected test separator and the overhead vapour stream leaving the selected test separator along with any entrained liquid flowed to the filter/coalescer vessel, where the entrained liquid was collected in the boot. The liquid level in the boot was recorded at the beginning and at the end of the test period, with the difference being the collected entrained liquid. The gas leaving the filter/coalescer was metered such that this value along with the amount of liquid collected provided a bucket and stopwatch type experiment. In order to ensure that entrained liquid was not in the gas downstream of the filter/coalescer, composition and hydrocarbon dewpoint measurements of the inlet and outlet gases were taken. In addition, collected liquid was analyzed. The experimental data (liquid level, gas flow, liquid (carryover) collected) was then used to determine the gas velocity, corresponding separation efficiency and entrained liquid droplet diameter.

Horizontal Separator A273mm O.D. x 1676mm S/S D.P. 9930 KPaG @ 38'C c/w 3.2mm C.A.

Horizontal Separator B273mm O.D. x 1067mm S/S D.P. 9930 KPaG @ 38'C c/w 3.2mm C.A.

Horizontal Separator C273mm O.D. x 762mm S/S D.P. 9930 KPaG @ 38'C c/w 3.2mm C.A.

Vertical Separator114mm O.D. x 1829mm S/S D.P. 9930 KPaG @ 38'C c/w 3.2mm C.A.

Filter / Coalescer Separator273mm O.D. x 762mm S/S D.P. 9930 KPaG @ 38'C c/w 3.2mm C.A.

VAJ Metering Pu

Max 37.5 GPH @ 5172 609mm NPSHA

Gas Inlet

HC(Gas)-2-80 FCV

PI TI

Mixing ChamberBy Pass Valve

HC-2-80By Pass Valve

--

HC-2-80

Separator ALIC HC(Liquid)-1-80 FI PI TI

Vertical Separator

Liquid Inlet

PI

TI PD Pump HC-2-80

Separator CLICPI TI

HC-2-80

LI

Separator BLIC PI TI

FI

Drain

HC (Liquid)-1-80 HC-2-80

LCV

Figure 1 Process Flow Schematic For Separator Skid

FI

8Filter / CoalescerPI TI

PCV

9 Figure 2 Skid Front Side Showing Inlets and Outlets Figure 3 Skid Side View

Figure 4 Inlet Manifold/Mixing Chamber

Figure 5 Liquid Injection/Metering

10

Figure 6 Separator A

Figure 7 Separators C and B

Figure 8 Vertical Separator

Figure 9 Filter/Coalescer

11 For vertical separator, tests were performed using a 4 nominal outside diameter (3.826 ID) by 30 height vessel fitted with manual level control. The experimental procedure was the same as for the horizontal separator case. The separator gas velocity was again determined from the gas rate and vessel diameter. Again, the recorded experimental data, the gas flow and liquid carryover, were used to determine the entrained liquid droplet diameter. 3.2 EXPERIMENTAL FEED COMPOSITIONS

Originally, it was proposed to study various liquids to mimic both lean and rich solution gases, however due to project delays only one set of compositions was used. The composition was established by adding sales gas and liquids from the de-butanizer overhead such that the gas was over-saturated and carried free liquid. Since the amount of gas for each run was different to obtain a different superficial velocity in the test separators but the liquid amount to over-saturate the gas was constant, the soluble versus free liquid would be different for each experiment. However, the composition of the saturated gas was only dependent on the pressure and temperature and would be the same for all experiments at the given conditions. These saturated gas compositions are given in Table 2. It can be seen that the 10 psig composition representing the solution gas going to flare is hydrocarbon liquid rich. This can be considered a worst case situation from the point of view of a flare knock out drum.

12 Table 2 Saturated Gas To Test Separators Component 10 psig Nitrogen Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane n-C11 n-C12 n-C13 n-C14 n-C15 n-C16 Methylcyclopentane Cyclohexane Methylcyclohexane Benzene Ethyl-Benzene 124-Trimethylbenzene Toluene p-Xylene o-Xylene 0.0377 0.7573 0.0183 0.0055 0.0231 0.0766 0.0298 0.0271 0.0157 0.0029 0.0005 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0018 0.0014 0.0008 0.0009 0.0000 0.0000 0.0005 0.0001 0.0000 Mole Fraction 100 psig 0.0446 0.8958 0.0217 0.0065 0.0041 0.0132 0.0051 0.0046 0.0028 0.0005 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0003 0.0003 0.0001 0.0002 0.0000 0.0000 0.0001 0.0000 0.0000 400 psig 0.0454 0.9113 0.0221 0.0066 0.0020 0.0060 0.0022 0.0021 0.0014 0.0003 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0001 0.0001 0.0001 0.0001 0.0000 0.0000 0.0001 0.0000 0.0000

13 3.3 DATA PROCESSING

For a given experiment, the experimental data consisted of pressure, temperature, actual gas flow and the amount of liquid collected/carried over. From the temperature and pressure and the gas and liquid compositions, density and viscosity values were obtained from the process simulator (HYSYSTM). The following describes how data was processed to determine parameters: Separation Efficiency (%) = [Liquid In (m3/d) Liquid Collected (L)/Experimental Time (hr)* 24 hr/d / 1000 L/m3] / Liquid In (m3/d) * 100 Velocity (ft/s) = Actual Gas Flow (ft3/hr) / Flow Area At Set Liquid Level (ft2) / 3600(hr/sec) Experimental K Factor (ft/s) = Velocity (ft/s) / [(L - V)/V]0.5 Droplet diameter was determined by iterative calculations as follows: 1. Estimate droplet diameter 2. Calculate X from Eq. 6 3. Calculate CD from Eq. 7 4. Calculate K from Eq. 5 5. If Calculated K = Experimental K, done; if not, adjust DP and go to step 2 Note, since the force balance equations are applicable to only the vertical case, vertical data at incipient carryover was first used to determine the droplet diameters. To determine droplet diameters from horizontal vessel data, the horizontal experimental K factor data was related to vertical data at incipient carryover for 100 psig case, Table 3. In order to further describe the entrained liquid droplets, a statistical analysis was done. Assuming a normal distribution, to determine the standard deviation, the required parameters were the maximum and the average liquid droplet sizes. Then using a translated normal probability table, the standard deviation could be calculated, Eq. 10:3.30 = D P , MAX D P , AVG

(10)

where 3.30 is the translated Z value corresponding to 99.95% of the liquid droplets being of smaller diameter than DP,MAXES and is the standard deviation. The average droplet diameter was obtained from the experiments and the maximum value was estimated by the theory shown in Jepson et al. (1989) and provided as Eqs. 11, 12 and 13:

14D P , MAX DPIPE = 1.91 Re 0.1 V We 0.6 L 0.6

+ 0.4

G LE LU V

(11) where Re is the Reynolds Number and We is the Weber Number (characterizes the maximum stable liquid droplet size in two phase flow):Re =

V U V DPIPE V2 V U V DPIPE gc

(12)

We =

(13)

and GLE is the entrained liquid mass flux in lb/s ft2, is the liquid surface tension in lbf/ft and other variables and units as previously defined.

4.0 RESULTS AND DISCUSSIONIt should be noted that two sets of data were taken. The first set taken in October to November 1999 indicated that there was no carryover until velocities were high enough that all liquid entering the test separator was carried over. This was not deemed correct and was attributed to an extra separation effect from a gas dome on the test separator exit nozzle and the level control not being sensitive enough. As such, the skid was modified so that no extra separation effect occurred at the outlet nozzle, liquid level control was made more sensitive and entrained liquid measurement in the filter/coalescer was improved with additional valving. A summary of the raw experimental data along with processed results is presented in Appendix I. The incipient carryover data and results are summarized below in Table 3. Note each experiment was repeated at least three times. The raw data is available on request.

15 Table 3 Experimental Data at Incipient Liquid Carryover Separator Horiz (L/D: 3.0 - 4.2) Horiz (L/D: 6.5) Vert Press (psig) Velocity (ft/s) 10 100 400 100 400 10 100 400 8 2-4 (3) 1 4-5 2 5-6 3-5 2 K Factor (ft/s) 0.417 0.200-0.398 (0.297) 0.188 0.397-0.498 0.378 0.272-0.319 0.300-0.496 0.376 Droplet Diameter (microns) 761 278-615 (430) 219 615-830 509 460-555 439-837 509

As would be expected from field measurements there is some scatter in the data but the trends are correct. For example, the allowable velocity decreases as pressure increases, as expected from the Souders-Brown equation with a higher vapour density and the accepted trend of a lower K factor. Both the horizontal and vertical data give this expected trend. However, the horizontal data appears to be more consistent when the resulting K factors are examined because they also decrease as pressure increases, as they should, whereas the vertical values do not. It should be noted that some of the actual values of the K factors in Table 3 seem somewhat high as values for 10 to 100 psig are expected to be about 0.175 for vertical separators and slightly higher, about 0.20 to 0.25 ft/s, for horizontal separators. The horizontal vessel data also show the correct trend that a longer length or higher L/D ratio provides a higher allowable velocity. Overall, experimental incipient liquid carryover velocity data correspond to theoretically calculated average liquid droplet sizes that range from about 300 to 800 microns at 10 to 100 psig. These droplet sizes are not unreasonable when compared to literature. For example, as stated by Capps (1994), gravity separation is only efficient for droplets of about 375 microns or larger. In comparing the vertical and horizontal data, we would expect that the vertical incipient carryover velocities would be lower than the horizontal ones and this is the case for the 10 psig data but not for all the 100 and 400 psig data. Strictly speaking, the force balance applies to vertical separation and so an adjustment should be made to the horizontal droplet sizes. In order to compare data from all the separators, the 100 psig data had to be used as it was the most complete data set. We selected 4 and 5 ft/s data for the horizontal data versus 3 ft/s for the vertical data. This results in a K factor adjustment of 1.33 for the 3-4 L/D horizontal data and 1.67 for the 6.5 L/D horizontal data. This adjustment along with assuming an adjustment of unity for the lowest typical L/D in horizontal vessels, 1.5, results in the following fit of the data for the F factor in Eq. 8:F = 0.8644 ( L / D ) 0.350

(14)

16 These adjustments compare to those previously stated in the literature. For example, Watkins (1967) stated that the horizontal adjustment should be about 1.25 for horizontal vessels, commonly designed with an L/D of 3.0. Adjusting the horizontal K factors to vertical ones using Eqs. 8 and 14 and then applying the force balance equations gives the entrained liquid droplet diameter results shown in Table 4. Table 4 Horizontal Incipient Liquid Carryover Equivalent Vertical Droplet Size Separator Press (psig) Velocity (ft/s) 10 100 400 100 400 8 2-4 1 4-5 2 Equivalent Vertical Droplet Diameter (microns) 553 219-457 170 334-431 270

Horiz (L/D: 3.0 - 4.2) Horiz (L/D: 6.5)

The data in Table 4 show that the average droplet sizes at incipient carryover for horizontal vessels at 10 to 100 psig may be more like 200 to 600 microns. Strictly speaking, applying the vertical force balance to the horizontal case over estimates the droplet diameter. The logic for this effect, shown in Table 4, is as follows. For a given velocity, a smaller diameter droplet will settle in the horizontal direction compared to the vertical direction. This is the same effect that allows a given droplet size to settle in a higher horizontal velocity then vertical, resulting in higher horizontal K factors. In addition to average entrained liquid droplet diameter, the maximum stable droplet diameter and standard deviation of the droplet diameters for the 10 and 100 psig cases were calculated. The calculation spreadsheet for both unadjusted and adjusted droplet horizontal diameters is in Appendix II, with the results summarized in Table 5. Table 5 Horizontal Incipient Liquid Carryover Droplet Distribution Separator Horiz (L/D: 3.0 - 4.2) Horiz (L/D: 6.5) Press (psig) Velocity (ft/s) 10 100 100 8 3 4.5 Avg Droplet Dia (microns) 761/553 430/338 722/383 Max Droplet Dia (microns) 1512 4448 2847 Std Dev (microns) 228/291 1218/1245 644/747

The data in Table 5 shows that there can be substantial variance in the droplet size and

17 that it may not be very uniform. In addition to the droplet size and distribution, the separation efficiency was calculated as a function of the gas velocity. The results are given below in Appendix I. It can be seen that carryover rises sharply after incipient carryover velocity is reached and the separation efficiency drops below 99.9%. Capps (1994) quotes typical carryover values for gravity separation of 0.1 vol%, which is equivalent to about 200 lbs/MMscf of the butane liquid used in this study. Talavera (1990) quotes Souders-Brown gravity separation carryover values of 75 to 150 USgal/MMscf. As shown in Appendix I, these values correspond to 90 95% separation efficiency. Finally, the data can be used for modelling. However, for modelling, the experimental data with the correct qualitative trends and adjusted for the scatter was used. An anchor data point was chosen to be a horizontal K factor of 0.225 ft/s at 100 psig, based on the experimental data and expected values of 0.20 0.25 ft/s. The K factor was taken to be the same at 10 psig and adjusted with pressure as 0.005 ft/s for each 100 psi pressure change above 100 psig, based on the GPSA rule of taking 1/2 of the values for mist eliminators. Calculations are given in Appendix III and are summarized in Table 6 for an L/D = 3 horizontal vessel. Table 6 Modelling Droplet Sizes Press (psig) 10 100 400 700 1000 K Factor (ft/s) 0.225 0.225 0.210 0.195 0.180 Calcd Dp (microns) 332 253 193 166 147 Calcd Velocity (ft/s) 5.2 2.4 1.1 0.8 0.6 Exp Velocity (1) (ft/s) 8 2-4 1

Notes: 1. Experimental incipient carryover velocity. The calculated liquid droplet diameters are based on calculating the K factor to match the recommended values described above, with the horizontal factor applied as per Eqs. 8 and 14. The droplet diameters in Table 5 at 10 to 100 psig compare relatively well with API 521 recommended values for flare knock-out drums (300 to 600 microns). In addition, the values at higher pressures compare well to values recommended by Arnold and Sikes (1986), who analyzed industrial fabricators guidelines based on field experience and recommended 140 to 150 microns for liquid-vapour separators. The values in Table 6 are somewhat lower than the recommended value of 700 microns for a carryover of 7 lb/MMscf from an unpublished study by an engineering company (Monnery, 1995).

18 It should be noted that although some experiments were run with mist eliminator pads, because they are not used in flare knock-out drums, these results are not discussed in detail. However, the data show that for the same velocities, mist eliminator pads decreased the entrained liquid carryover by 20 33% of the values for gravity separation alone for the test separators with an L/D of 3 4. For the test separator with an L/D of 6.5, mist eliminator pads decreased the carryover by 33 50% of the values for gravity separation alone.

5.0 CONCLUSIONS AND RECOMMENDATIONS5.1 Conclusions

1. Although there is some scatter in the data, experimental data verify the accepted qualitative trends of allowable velocity decreasing as pressure increases and larger allowable velocity for longer (higher L/D) horizontal vessels. 2. Experimental results indicate that entrained liquid carryover average droplet diameters are in the range of 200 to 600 microns for flare knock-out drums at 10 to 100 psig. 3. Calculations suggest that the maximum stable droplet size can be very large at low velocities. 4. The droplet size distribution indicates that there can be substantial variance in the droplet size and that it may not be very uniform. 5. Experimental results provide quantitative data on the relationship between horizontal and vertical K factors and allowable velocities, which has to date been empirical and subjective. These results show that the factor between horizontal and vertical K factors and allowable velocities vary from about 1.33 to 1.67 as L/D varies from 3.5 to 6.5. 6. Experimental results show that carryover rises sharply after the incipient carryover velocity and separation efficiency drops below 99.9%. 7. Modelling results based on using the experimental data give entrained liquid average droplet diameters that are consistent with literature.

19 5.2 Recommendations

1. To avoid carryover, flare knock out drums should be designed using a droplet size of 300 microns. This is an allowable vapour velocity below the Incipient Carryover velocity determined in this study. 2. A continuous online droplet size and distribution measurement system must be installed before any further experimental data is collected. 3. Testing of other separation and coalescing devices should be undertaken.

6.0 NomenclatureCD, D, DP, DPipe, F, FD , FG , g, gc, GLE, HV, K, KH, KV, L, L E, MP, Re, UT, UV, We, X, Z, , , L, V, , , Drag Coefficient Diameter, in or ft Droplet Diameter, microns Pipe Diameter, in Factor in Eq. 8 Drag Force, Gravity Force Gravity Acceleration, ft/s2 Dimension Proportionality Constant, (lbf/lbm)(ft/s2) Entrained Liquid Mass Flux, lb/s-ft2 Vertical Height, ft K Factor (Eq. 5), ft/s Horizontal K Factor, ft/s Vertical K Factor, ft/s Length, ft Effective Length, ft Droplet Mass, lb Reynolds Number Terminal Velocity, ft/s Vertical Velocity, ft/s Weber Number Parameter Defined by Eq. 6 Translation Variable in Normal Distribution Viscosity, cP Pi Number Liquid Density, lb/ft3 Vapor Density, lb/ft3 Surface Tension, dyne/cm Standard Deviation (Eq. 10)

20

7.0 REFERENCESArnold, K.E. and C.T. Sikes, Droplet settling theory key to understanding separatorsizing correlations, Oil & Gas J., July 21, 1986, p. 60. Capps, R.W., Properly Specify Wire-Mesh Mist Eliminators, Chem Eng Prog, December 1994, p. 49. Gas Processors Suppliers Association, Engineering Data Book, 10th Edition, Vol. 1, Ch. 7 (1987), Tulsa, Oklahoma. HYSYSTM AEA/Hyprotech Ltd., Calgary, Alberta Jepson, D.M., B.J. Azzopardi and P.B. Whalley, The Effect of Gas Properties on Drops in Annular Flow, Int. J. Multiphase Flow, Vol. 15. No. 3, 1989, p. 327. Monnery, W.D. (1995), private communication. Monnery, W.D. and W.Y. Svrcek, Successfully Specify Three-Phase Separators, Chem Eng Prog, September, 1994, p. 29. Svrcek, W.Y. and W.D. Monnery, Design Two-Phase Separators Within the Right Limits, Chem Eng Prog, October 1993, p. 53. Talavera, P.G., Selecting Gas-Liquid Separators, Hydrocarbon Processing, June 1990, p. 81. Watkins, R.N., Sizing Separators and Accumulators, Hydrocarbon Processing, November, 1967, p. 253.

Appendix I Experimental Data Summaries

Appendix II Droplet Distribution Calculations

Appendix III Modelling Calculations