# Presentation: 279788

## Oxychem, Delaware CityCell Room Mercury Vapor Monitoring and Air Flow Modeling

- Excerpts from a presentation at the
*2005 Chlorine Institute Annual Meeting, 12**th** Mercury Issues Workshop*by
- Michael T. Searfass, P.E., Occidental Chemical Corporation
- March 22, 2005

## Overview

- Emission Rate Calculation
- Mercury Vapor Measurement
- Plant Operations Response
- Convective Air Flow Models (CAFM)

## Mercury Emission from Cell Building

- 	MHg = Mass flow rate of mercury (g/day)
- 	k = Aggregate unit conversion factor
-         =  (1440 min/day)(m3/35.31 ft3)(g/1000 mg) = 0.04078

- _Ventilation Rate (acfm)_
- Direct Mass Balance
- Indirect Energy Balance
- Convective Air Flow Model

- _Hg Concentration (mg/m__3__)_
- Survey by UV Absorption
- Automated UV MMS

## Cell Bldg. Geometry

- _Cell Bldg. Geometry_

**Decomposer **
**Aisle**

**Air **
**Inlets**

**Air **
**Inlets**

**Operating **
**Level**

**Bottom Floor**

**Crane Level**

**Cells**

**Cells**

**Ridge **
**Vent**

**Ridge **
**Vent**

**Temp./Conc.**
**Sensing Locations**

## Reynolds Number

- Where:
- 	De = Equivalent diameter for full-stream, rectangular cross-section (ft)
- 	Q = Volumetric flow rate (ft3/sec)
- 	v = Face velocity of exiting air (ft/sec)
- 	ρ = Air density (lbm/ft3)
- 	μ = Absolute viscosity of air (cp)
- 	      [1 cp = 6.7197 x 10-4 lbm/(ft•sec)]
- 	Aout = Total cross-sectional area of ridge vent	(ft2)
- Turbulent Flow for NRe > 3000

## Ridge Vent Equivalent Diameter1

- Where:
- 	De = Equivalent diameter for full-stream, rectangular cross-section (ft)
- 	A = Area of vent (De average is weighted by area.)
- 	N = Number of ridge vent openings
- 	L = Ridge vent length (ft)
- 	w = Ridge vent width (ft)

## Ridge Vent Flow Regime

|  |  | Winter
Tridge = 65°F
Tatm = 40°F | Summer
Tridge = 90°F
Tatm = 77°F |
| --- | --- | --- | --- |
| Q | v | NRe | NRe |
| acfm/(1x103) | ft/sec | /(1x103) | /(1x103) |
| 25 | 0.1 | 8 | 7 |
| 50 | 0.2 | 15 | 14 |
| 100 | 0.4 | 30 | 28 |
| 200 | 0.8 | 61 | 56 |
| 400 | 1.6 | 121 | 112 |
| 600 | 2.4 | 182 | 169 |
| 1000 | 4.0 | 303 | 281 |

## Reynolds Calculation Conclusions 

- Multiple discrete sample points can approximate broader local concentration of exiting air.
- Need to balance MMS point polling rate against area coverage per point
- 16 points, 4/quadrant, 4-6 cells/point
- Minimum poll rate = 1 min/point, 15 min repeat
- Data indirectly confirm turbulent flow.
- Temperature trends indicate mixing
- Cell Hg vapor emission detected by adjacent sample points (within but not among vents)

## Hg Vapor & Air Temp. Sample Points

- _Hg Vapor & Air Temp. Sample Points_

- 1

- 9

- 5

- 13

- 3

- 11

- 7

- 15

- 2

- 10

- 6

- 14

- 4

- 12

- 8

- 16

- = Hg Vapor (Polled, 1-999 min/point)

- = Temp. (Continuous)

**MMS**

**RH**

**P**

**WS**

**WD**

- N

- C1

- C2

- C

- = Hg Composite (Periodic)

- Electrolyzers

- Decomposers

## MMS Specifications 

- VM-3000 Hg Analyzer:  UV adsorption at 253.7 nm
- Measurement range (μg/m3):  0.1-100, 1-1000, 1-2000
- Automatic zero through vacuum path to detector
- Sample pump (150 L/hr)
- GM-3000 Multiplexer
- Timed controller for sampling and by-pass valves
- Fixed sequence but variable timing
- Sample By-Pass Pump
- 500 L/hr (33 L/hr/inactive channel)
- 2.4 ft/sec/inactive channel purge velocity
- Manufacturer:  Mercury Instruments GmbH

## Concentration Data

- Represent short time intervals
- Daily variation
- Seasonal variation
- Two data types
- MMS analogue output correlated with location
- Individual point trends to be added to DCS
- Single-point data from SW ridge vent

## Interface with Plant Operations

- MMS access
- Field interface:  Photometer settings and status, channel configuration, current measurement values, and alarms
- DCS GUS:  Sample point values, valve status, trends, data historian, and alarms
- Desktop PC’s:  Same as GUS, but no alarm acknowledgement
- DCS board technicians
- Cell Room supervisor and field technicians
- Cell Renewal group
- Plant staff

## Mercury Emission from Cell Building

- _Ventilation Rate (acfm)_
- Direct Mass Balance
- Indirect Energy Balance
- Convective Air Flow Model

- _Hg Concentration (mg/m__3__)_
- Survey by UV
- Automated UV MMS

## Convective Air Flow Model (CAFM)

- Flow model based on principle of natural convection
- Various formulae exist in literature for computing air flow by natural convection
- First principles derivations not available from engineering sources
- Some equations contain errors
- Extensive work done in agriculture (U.S., U.K., Australia) for estimating flow rate through livestock buildings3,4,5

## CAFM(Continued)

- Derivation Basis:  Bernoulli form of Euler’s momentum balance for frictionless flow
- Driving force, ΔT = Tridge - Tatm
- All heat emitted by process inherent in ΔT
- Indirectly accounts for conductive heat losses (walls, surfaces) that do not contribute to natural draft
- Easily measured
- Requires precise measurements of Cell Bldg. geometry (summer and winter)

## CAFM Advantages

- Requires very few measured variables
- Atmospheric and ridge vent temperatures
- Intake and discharge areas
- Stack height
- Validated using indirect energy balance
- Simplified calculation algorithm conducive to
- 	DCS programming

## Generalized Cell Bldg. Geometry

- h1

- h3

- Aout

- Ain,1

- Ain,2

- h2

- Ain,3

- He

- 1

- 2

- ρ1, T1

- ρ2, T2

## CAFM Derivation

- Apply Bernoulli’s theorem (streamlined, frictionless flow):7
- Substitute for Δh = He/[1+(A2/A1)2] and ρatm = ρ1, and rearrange:

- _CAFM Derivation_

- Static Pressure

- Dynamic Pressure

- where F = Buoyancy Force

- 0

## CAFM Derivation(Continued)

- Substitute using  V2 = Q/A2 and rearrange:
- Apply discharge coefficient, CD, for friction losses at inlets and around internal impediments.

- _CAFM Derivation_(Continued)

## Final CAFM

- Where:
- 	Q = Air flow rate (acfm)
- 	CD = Discharge coefficient (0.65 for unidirectional flow)
- 	He = Equivalent stack height (ft)
- 	Tin = Inlet (atmospheric) temperature (°R)
- 	Tout = Outlet (ridge vent) temperature (°R)
- 	Ain = Total inlet area (ft2)
- 	Aout = Total outlet area (ft2)

## Flow Area Considerations 

## CAFM Daily Variation 

## Wind Superposition (Quadrature)9,10

- Each flow affects internal and external pressures around the building and, hence, ventilation rate.
- Exact solution of interactions impossible
- Resultant Q always sub-additive:  Literature average contribution by Qwind is 35% for all building types.
- Input wind speed and direction into DCS.
- Calculate Qwind based on windward Ain (correlate impingement direction to indexed fraction of Ain) .

## Wind Impingement Examples

- 000°

- 270°

- 315°

- 090°

- = 20 mph

- = 45°

**25%**

**40%**

**13%**

**13%**

## Wind Superposition11(Continued)

- Where:
- 	Qwind = Forced draft flow rate (acfm)
- 	k1 = Wind impingement coefficient (0 ≤ k1 ≤ 1)
- 	k2 = Unit conversion factor (88.0)
- 	Cv = Effectiveness of inlet openings
- 		0.50-0.60 for perpendicular winds
- 		0.25-0.35 for diagonal winds
- 	Ain = Total inlet area (ft2)
- 	U = Wind speed (mph)

## Continuing Work

- Input wind speed and direction to DCS
- Install weather vane and anemometer
- Mount in location to reflect local conditions at Cell Bldg.
- Correct CAFM using wind superposition
- Refine CD specifically for Cell Bldg.
- Periodically check DCS model against DMB and IEB results

## References

- 1.  Robinson, Randall N.,  *Chemical Engineering Reference Manual*, 5th Ed., Professional Publications, Inc., Belmont, CA, 1996, p. 5-7.
- 2.  Goodfellow, H., and Tähti, E., *Industrial Ventilation Guidebook*, Academic Press, San Diego, 2001, pp. 589-591.
- 3.  Barre, H. J., and Sammett, L. L., *Farm Structures*,  John Wiley & Sons, New York, 1950, pp. 175-182.
- 4.  Bruce, J. M., “Natural Convection Through Openings and its Application to Cattle Building Ventilation,” *Journal of Agricultural Engineering Research* (1978) **23**, pp. 151-167.
- 5.  Foster, M. P., and Down, M. J., “Ventilation of Livestock Buildings by Natural Convection,” *Journal of Agricultural Engineering Research* (1987) **37**, pp. 1-13.
- 6.  American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc., *2001 ASHRAE Handbook:  Fundamentals*, p. 26.11.

## References(Continued)

- 7.  Fox, R. W., and McDonald, A. T., *Introduction to Fluid Mechanics*, 3rd Ed.,  John Wiley & Sons, New York, 1985, pp. 246-266.
- 8.  Perry, R. H., and Green, D. W., *Perry’s Chemical Engineers’ Handbook*, 7th Ed., McGraw-Hill, New York, 1997, pp. 6-21, 6-22.
- 9.  Sherman, M. H., “Superposition in infiltration modeling,”  *Indoor Air*, **2**:101-14.
- 10.  Wilson, D. J., and I. S. Walker, “Infiltration data from the Alberta Home Heating Research Facility, *Air Infiltration and Ventilation Center Technical Note 41*, AIVC, Coventry, England.
- 11.  ASHRAE, p. 26.10.