Autoclave Design and Cost Model

Lynton Gormely Ken Lamb

1

Introduction

• Ken Lamb maintains and uses the model • I interpret testwork and provide assumptions on chemistry and stoichiometry • The model has been developed and used on more than 20 projects and studies

The last time we wore suits (c. 1997)

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Why Another Program?

• Provides control of calculations and understanding of results • Enables quick analysis of autoclave circuits including heat balances, reagent consumption, equipment sizes and costs • Assesses impact of changes in basic assumptions and design criteria, such as: – reduced O2 purity requires higher total pressure to maintain O2 partial pressure, and affects oxygen utilization – estimated steam saving and corresponding equipment costs by using two pre-heat towers instead of one – cost of a vessel in Titanium Gr 2 in comparison with, say, Duplex alloy 2205

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Excel Spreadsheet

• • • •

Mass balance Heat balance Autoclave sizing Flash and preheater sizing • Major equipment costing • Tabulation of results • Dimensioned design sketches

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Mass Balance

• Chemical balances based on input stoichiometry, slurry flow • • • • • •

design criteria, and test data Mineralogical makeup explains chemical assay data Known or assumed mineralogy Substitute minerals with known thermochemical data when obscure minerals are identified Material balance determined by required oxygen overpressure, steam vapour pressure at selected temperature, sources of inerts Extent of reaction, by compartment Densities of stream components (solids and liquid) are calculated, providing volumetric flows

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Heat Balance

• Heat content for each process stream determined and based on: – published heats of formation as function of temperature for the various feed and product components – heat capacity data provided as function of temperature – material balance quantifying solid, liquid, and gaseous flows, especially steam production

• Pre-heater approach temperatures are set and any limiting data, eg, temperature of slurry fed to the PD pumps • Flash vessel pressures adjusted until vent flows from preheaters are suitable for minor slurry or steam flow variances

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Autoclave Sizing

• Flows in each compartment obtained from balance calculations • Vessel diameter for given residence time and number of compartments: – need test results to fix residence time corresponding to assumed extents of reaction (adjust compartment volume until required residence time is obtained) – adjust vessel diameter in 2nd from last compartment; target 1:1 compartment length to diameter for best agitation for gas mixing; from compartment volume and flow cross-sectional area, compartment length is determined – headspace allowance for gas disentrainment – weir calculations for dynamic volume

• Sum compartment lengths to get overall vessel length

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Autoclave Sizing (cont’d)

• Select lining material and determine lining thickness to achieve • • • •

required membrane temperature Determine allowable stress from built-in ASME code stress data and use it, with vessel diameter (adjusted for lining thickness) to determine required vessel shell and head thickness. From mass flow rates and velocity criteria, determine required nozzle size; from ASME code data, determine flange rating Use above results to calculate total vessel weight Size agitator from either a gas mixing correlation or blending correlation based on vessel diameter

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Flash Vessel Sizing

• Gas and liquid flows from balance calculations • Select design – brick-lined or brick and alloy vessels • Calculate vessel diameter to meet gas entrainment requirements: – use Souders-Brown equations – flow factors based on experience

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Flash Vessel Sizing (cont’d)

• Vessel height based on criteria developed over many years, and based on: – separation height to provide time for gas liquid separation – jet dissipation length – liquid height for kinetic energy dissipation

• Above data, in conjunction with nozzle weights, allow calculation of overall vessel weight • Let-down design is a separate program

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Preheater Sizing

• Diameter selected to control entrainment like a flash vessel (apply to conditions at the base of the vessel) • For steam water mixing (eg, for gold slurries), 10’ contact height is used • For gas flows with inerts, height is based on volumetric heat transfer data published by Fair, modified for viscous fluids • Lining and shell thickness determined by same considerations as autoclave

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Vessel Costing

• • • •

Based on unit cost database Vessel shell based on weight and material Lining based on area and material Nozzles based on weight of standard integral long-weld neck nozzle at required rating • Density adjusted for specific material

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Summary of Results

• Heat and mass balance with required reagents (eg, oxygen or acid consumption) and utility requirements (steam and water) • Tabulation of balance quantities • Costs for major equipment estimated to ±30%

• Dimensioned vessel data sheets • Tabulated cost estimates • Sketches and data sheets issued for quotation when more accurate costs are required

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Autoclave Data Sheet

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Autoclave Details

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Preheater Data Sheet

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Flash Vessel Data Sheet

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Summary

• The program is not a “black box”: – – – –

must be maintained and used by a knowledgeable professional outputs must be reviewed and adjusted as required costs continuously updated by regular contacts with vendors technical content under continuous revision, based on changes in industrial standards and practice

• The program is not offered for sale