5. SEPARATOR SIZING PRINCIPLES
5.2 Sizing Demister Pads
Mist extractors of the mesh-type are typically constructed of a pad of compact stainless steel wire designed to capture entrained liquid droplets or those droplets too small to settle by gravity. The liquid droplets impinge on the mesh pad, coalesce and fall downward as larger droplets, back through the rising vapour. Demister pads are generally aligned horizontally in both vertical and horizontal
separators, with the vapour and entrained liquid passing vertically upward through the pad. In fact, the pad performance is found to be adversely affected if tilted at an angle of greater than 30 degrees from the horizontal.
As reported by GPSA (SI) volume 1, most installations use a 150 mm thick pad with the minimum recommended thickness being 100 mm. In order to size the diameter of a demister pad, the following correlation may be used:
2 1
. .
4 −
−
=
v v l
K d Q
ρ ρ ρ
π (Equation 5.4)
Where: Uv = Superficial velocity of vapour through the demister pad (m/s).
K = Empirical constant, provided by vendor, but usually in the range of 0.1 – 0.12 m/s.
Q = Vapour flowrate (m3/s)
ρl = Density of liquid phase (kg/m3) ρv = Density of vapour phase (kg/m3)
The diameter of the mist extractor is usually substantially less than the diameter of the separator. In the case of vertical separators with diameters less than 1000 mm, a mesh diameter equivalent to the vessel diameter should be used. In addition, the demister pads are typically located 300 mm below the vapour outlet nozzle of a horizontal separator.
For any variety of service, the material of construction for the mesh pad should be at least 304 ss. For application in corrosive service, either 316 ss or Monel mesh pads should be used.
Appendix A – Separator Sizing Spreadsheet Instructions
In order to allow for designs of separators to be generated rapidly, an Excel ’97 spreadsheet file (Separator Sizing.xls) has been developed. This spreadsheet allows for the following types of separator to be sized:
Horizontal Separators Vertical Separators
2-phase, vapour-liquid 2-phase, vapour-liquid
2-phase, liquid-liquid 2-phase, liquid-liquid
3-phase, liquid-liquid-vapour (with weir, no boot) 3-phase, liquid-liquid-vapour (with boot, no weir)
In the sections that follow, a brief outline is given for each type of separator on each of the following topics:
• Inputs required by the spreadsheet,
• Final outputs provided by the spreadsheet,
• Warnings provided by the spreadsheet, and
• A chronological procedure for using the spreadsheet.
A.1 Vertical Liquid-Vapour Separator
A.1.1 Inputs RequiredThe following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:
1. Physical properties of both phases -
• Maximum flowrates (kg/hr)
• Actual Densities (kg/m3)
• Viscosities (cP)
2. Separator vessel dimensions –
• Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable vapour velocity and specified liquid
residence times are not exceeded.
• Liquid level heights (mm) – heights of LLSD, LLL, NLL, HLL and HLSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are encountered.
In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:
“Standard” Inputs Required Typical / Suggested Values (ρmvm2)max – inlet mixture feed nozzle 1000 kg/ms2 – where no inlet device present
1500 kg/ms2 – where half-open pipe inlet present) (ρmvm2)max – vapour outlet nozzle 3750 kg/ms2
vmax – liquid outlet nozzle 1 m/s
Droplet size (liquid in gas) 150 µm – without demister pad installed 500 mm – with demister pad installed
Liquid Residence Times See Appendix-B.
LLSD height above bottom tangent line 150 mm (minimum)
HLSD height above bottom tangent line A value such that the HLSD is no less than 150 mm below the inlet feed nozzle.
Thickness of Demister Pad 100 mm (minimum), but usually 150 mm.
K-value 1. May be supplied by vendor and manually
inputted into spreadsheet.
2. If not supplied by vendor, may be obtained using information from Fig 7-9, GPSA (SI) Volume 1, Section 7, Page 7-7.
A.1.2 Final Outputs Provided
Once all required input values have been entered, the spreadsheet calculates values for the following:
1. Fluid Properties
• Feed mixture density
• Maximum allowable vapour velocity
• Actual maximum vapour velocity
• Liquid droplet settling velocity & the settling law used in the calculation
• Liquid residence times between specified liquid levels.
2. Vessel Dimensions
• Minimum vessel diameter
• Vessel height (T/T) – taken as a minimum of 2500 mm.
• Minimum inlet nozzle ID∅
• Minimum vapour outlet nozzle ID∅
• Minimum liquid outlet nozzle ID∅
• Clearance between inlet feed nozzle and bottom of demister pad (if fitted) – shall be taken as the greater of 0.7D or 750 mm.
• Clearance between top of demister pad (if fitted) and upper tangent line – taken as the greater of 0.1D or 300 mm.
A.1.3 Warnings Provided
The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:
1. The calculated vapour velocity exceeds the maximum allowable vapour velocity calculated by the K-factor method.
2. The specified liquid level heights are inadequate to provide the required liquid residence times.
3. The vessel height / diameter ratio does not fall in the range of 2 – 5.
An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an
“OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.
A.1.4 Procedure for Spreadsheet Use.
The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows:
1. Enter all the required input data listed in A.1.1, except for the vessel ID and liquid level elevations.
2. Set the vessel diameter to the calculated value for the “minimum vessel diameter” as a starting point.
The minimum value for any nozzle diameter is considered to be 50 mm.
3. Enter arbitrary values for the various liquid level elevations and vary them to meet the required liquid residence times.
4. Examine the vessel height / diameter ratio to ensure that it lies between 2 – 5.
(i) If the ratio is greater than 5, a more economical ratio can be achieved by gradually increasing the vessel ID from the value selected in step 2, while decreasing one or all of the LLSD, LLL, NLL, HLL and HLSD elevations specified in step 3. If the latter approach is taken, it must be ensured that all required liquid residence times remain satisfied and that the LLSD elevation is a minimum of 150 mm above the bottom tangent line.
(ii) If the ratio is less than 2, on the other hand, the elevations of the LLSD, LLL, NLL, HLSD may be increased to extend the height of the vessel and increase the ratio. The vessel diameter may not be decreased from the value selected in step 2. In step 2, the minimum vessel diameter was selected, which cannot be decreased in order to increase the H/D ratio without the maximum allowable vapour velocity being exceeded.
A.2 Vertical Liquid-Liquid Separator
A.2.1 Inputs RequiredThe following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:
1. Physical properties of both phases -
• Maximum flowrates (kg/hr)
• Actual Densities (kg/m3)
• Viscosities (cP)
2. Separator vessel dimensions –
• Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable light liquid velocity and specified liquid residence times are not exceeded.
• Liquid level heights (mm) – heights of LILSD, LIL, NIL, HIL, HILSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are encountered.
In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:
“Standard” Inputs Required Typical / Suggested Values (ρmvm2)max – inlet mixture feed nozzle 1000 kg/ms2 – where no inlet device present
1500 kg/ms2 – where half-open pipe inlet present vmax – light liquid outlet nozzle 1 m/s
vmax – heavy liquid outlet nozzle 1 m/s Droplet size (heavy liquid in light liquid) 600 µm Droplet size (light liquid in heavy liquid) 1000 µm
Liquid Residence Times See Appendix-B
LILSD height above bottom tangent line 150 mm (minimum)
HILSD height above bottom tangent line A value such that the HILSD is no less than 150 mm below the inlet feed nozzle.
A.2.2 Final Outputs Provided
Once all required input values have been entered, the spreadsheet calculates values for the following:
1. Fluid Properties
• Feed mixture density
• Maximum allowable light liquid velocity (assumed equal to 0.85 * settling velocity of heavy liquid droplets in the light liquid phase)
• Actual maximum light liquid velocity
• Light liquid droplet settling velocity & the settling law used in the calculation
• Heavy liquid droplet settling velocity & the settling law used in the calculation
• Liquid residence times between specified liquid levels.
2. Vessel Dimensions
• Minimum vessel diameter
• Vessel height (T/T) – taken as a minimum of 2500 mm.
• Minimum inlet nozzle ID∅
• Minimum light liquid outlet nozzle ID∅
• Minimum heavy liquid outlet nozzle ID∅ A.2.3 Warnings Provided
The minimum value for any nozzle diameter is considered to be 50 mm.
The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:
1. The calculated light liquid velocity exceeds the maximum allowable light liquid velocity.
2. The residence time of the heavy liquid between the vessel bottom and the NIL is less than the light liquid droplet “fall” time in the heavy liquid medium.
3. The specified liquid level heights are inadequate to provide the required liquid residence times.
4. The vessel height / diameter ratio does not fall in the range of 2 – 5.
An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an
“OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.
A.2.4 Procedure for Spreadsheet Use.
The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows:
1. Enter all the required input data listed in A.2.1, except for the vessel ID and elevations of LILSD, LIL, NIL, HIL and HILSD.
2. Set the vessel diameter to the calculated value for the “minimum vessel diameter” as a starting point.
3. Enter arbitrary values for the various liquid level elevations (i.e. LILSD, LIL, NIL, HIL and HILSD) and vary them to meet the required liquid residence times. Also ensure in specifying these levels that the residence time of the heavy liquid between the vessel bottom and the NIL is less than the light liquid droplet “fall” time in the heavy liquid medium.
4. Examine the vessel height / diameter ratio to ensure that it lies between 2 – 5.
(i) If the ratio is greater than 5, a more economical ratio can be achieved by gradually increasing the vessel ID from the value selected in step 2, while decreasing one or all of the LILSD, LIL, NIL, HIL and HILSD elevations specified in step 3. If this approach is taken, it must be ensured that all required liquid residence times remain satisfied and that the LILSD elevation is a minimum of 150 mm above the bottom tangent line.
(ii) If the ratio is less than 2, on the other hand, the elevations of the LILSD, LIL, NIL, HILSD may be increased to extend the height of the vessel and increase the ratio. In doing so, it is again vital to ensure that the light liquid droplet fall time is less than the heavy liquid residence time between the vessel bottom and the NIL. The vessel diameter may not be decreased from the value selected in step 2. In step 2, the
minimum vessel diameter was selected, which cannot be decreased in order to increase the H/D ratio without the maximum allowable light liquid velocity being exceeded.
A.3 Horizontal Vapour-Liquid Separator
A.3.1 Inputs RequiredThe following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:
1. Physical properties of both phases -
• Maximum flowrates (kg/hr)
• Actual Densities (kg/m3)
• Viscosities (cP)
2. Separator vessel dimensions –
• Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable vapour velocity and specified vapour and liquid residence times are not exceeded.
• Vessel length (T/T) (mm) – adjusted such that the minimum length is 2500 mm and that the length / diameter ratio is between 2 – 4 for the final design. The vessel length must also be adjusted such that the required vapour and liquid residence times are not exceeded. Of particular importance in this regard is ensuring that the vapour residence time provided is sufficient to allow a liquid droplet to fall from the top of the vessel to the NLL.
• Liquid level heights (mm) – heights of LLSD, LLL, NLL, HLL and HLSD are varied by spreadsheet user until no warnings regarding insufficient liquid residence times are
encountered. The NLL is in fact one of the first inputs required as it is needed to calculate the minimum vapour flow area and subsequently the vapour residence time (once vessel length is specified).
In addition to the required inputs described above, there are also inputs whose values are more or less standard for the vast majority of vertical vapour-liquid designs, including:
“Standard” Inputs Required Typical / Suggested Values (ρmvm2)max – inlet mixture feed nozzle 1000 kg/ms2 – where no inlet device present
1500 kg/ms2 – where half-open pipe inlet present (ρmvm2)max – vapour outlet nozzle 3750 kg/ms2
vmax – liquid outlet nozzle 1 m/s
Droplet size (liquid in gas) 150 µm – regardless of whether or not a demister pad is installed
Liquid Residence Times See Appendix-B
LLSD height above vessel bottom 150 mm (minimum)
HLSD height above vessel bottom A value such that the HLSD is no less than 150 mm below the inlet feed nozzle.
Thickness of Demister Pad 100 mm (minimum), but usually 150 mm.
K-value 1. May be supplied by vendor and manually
inputted into spreadsheet.
2. If not supplied by vendor, may be obtained using information from figure 7-9, GPSA (SI) Volume 1, Section 7, Page 7-7..
A.3.2 Final Outputs Provided
Once all required input values have been entered, the spreadsheet calculates values for the following:
1. Fluid Properties
• Feed mixture density
• Maximum allowable vapour velocity
• Actual maximum vapour velocity
• Liquid droplet settling velocity & the settling law used in the calculation
• Liquid droplet fall time (from top of vessel to NLL)
• Vapour residence time (from top of vessel to NLL)
• Liquid residence times between specified liquid levels.
2. Vessel Dimensions
• Minimum vessel diameter
• Minimum inlet nozzle ID∅
• Minimum vapour outlet nozzle ID∅
• Minimum liquid outlet nozzle ID∅
• Clearance between inlet feed nozzle and bottom of demister pad (if fitted) – shall be taken as the greater of 0.7D or 750 mm.
• Clearance between top of demister pad (if fitted) and upper tangent line – taken as the greater of 0.1D or 300 mm.
The minimum value for any nozzle diameter is considered to be 50 mm.
A.3.3 Warnings Provided
The spreadsheet issues various warnings to help ensure that the design of the separator is adequate for its desired purpose. These warnings are issued if the following conditions result:
1. The calculated vapour velocity exceeds the maximum allowable vapour velocity calculated by the K-factor method.
2. The liquid droplet fall time (from top of vessel to NLL) exceeds the vapour residence time (also from top of vessel to NLL).
3. The specified liquid level heights are inadequate to provide the required liquid residence times.
4. The vessel length / diameter ratio does not fall in the range of 2 – 4.
An overall status box that is viewable at the top of the spreadsheet at all times is included which displays warning messages if any of the above conditions occur. When this status box displays an
“OK, No Warnings Present” message, all spreadsheet calculations have been completed without any warnings.
A.3.4 Procedure for Spreadsheet Use.
The warnings provided as outlined in the previous section represent one of the most important and user-friendly tools possessed by the excel spreadsheet file to assist the user in sequentially entering all data required for a successful design. A suggested procedure is as follows:
1. Enter all the required input data listed in A.3.1, except for the vessel ID, vessel length and elevations of LLSD, LLL, NLL, HLL and HLSD.
2. As a starting point, set the vessel diameter to an arbitrary value and the vessel length initially as 3 times this quantity.
3. Enter arbitrary values for the various liquid level elevations (i.e. LLSD, LLL, NLL, HLL and HLSD).
(i) If the liquid droplet fall time exceeds the vapour residence time (warning number 2), the vapour residence time may be increased by:
• Increasing vessel diameter as needed – the most successful measure if the vapour residence time is significantly lower than the liquid droplet fall time (i.e. >25%
lower).
• Increasing the length of the vessel – the most successful measure if the vapour residence time is only slightly lower than the liquid droplet fall time (i.e. <25%
lower)
(ii) If the specified liquid level heights are inadequate to provide the required liquid residence times, the residence times may be increased by employing either of the two above measures or alternatively increasing the height difference between the liquid levels where residence time is calculated to be insufficient.
4. If warning 1 occurs, the available cross-sectional area available to vapour flow may be increased by an increase in vessel ID, or by a decrease in liquid level elevations.
5. Examine the vessel length / diameter ratio to ensure that it lies between 2 – 4.
(iii) If the ratio is greater than 4, a more economical ratio can be achieved by gradually increasing the vessel ID from the value selected in step 2, while decreasing the vessel length. If this approach is taken, it must be ensured that none of the warnings listed in A.3.3 occur and that the length of the vessel does not become lower than 2500 mm.
(iv) If the ratio is less than 2, on the other hand, the vessel length may be increased as a remedy, while decreasing the vessel diameter from the value selected in step 2.
A.4 Horizontal Liquid-Liquid Separator
A.4.1 Inputs RequiredThe following lists outline the input data required by the spreadsheet. Many such inputs are highly sensitive on the precise process requirements of the separator, and include:
1. Physical properties of both phases -
• Maximum flowrates (kg/hr)
• Actual Densities (kg/m3)
• Viscosities (cP)
3. Separator vessel dimensions –
• Vessel diameter (mm) – varied by the spreadsheet user until the vessel cross-sectional area is sufficient to ensure that the maximum allowable vapour velocity and specified vapour and liquid residence times are not exceeded.
• Vessel length (T/T) (mm) – adjusted such that the minimum length is 2500 mm and that the length / diameter ratio is between 2 – 4 for the final design. The vessel length must also be adjusted such that the required vapour and liquid residence times are not exceeded. Of particular importance in this regard is ensuring that the vapour residence time provided is sufficient to allow a liquid droplet to fall from the top of the vessel to the NLL.
• Vessel length (T/T) (mm) – adjusted such that the minimum length is 2500 mm and that the length / diameter ratio is between 2 – 4 for the final design. The vessel length must also be adjusted such that the required vapour and liquid residence times are not exceeded. Of particular importance in this regard is ensuring that the vapour residence time provided is sufficient to allow a liquid droplet to fall from the top of the vessel to the NLL.