BM-011 | DESIGN OF LARGE CAPACITY DIGESTERS FOR ALUMINA REFINERY

Bayer Process Masterclass | Process Design, Calculations for Sizing & Agitation

Author: Rajendra Kunwar, Bayer Process Expert

A modern alumina refinery must achieve more than design production capacity. Its digestion system must provide high alumina extraction, stable heat transfer, uniform slurry flow, low maintenance, reliable pressure containment and long operating campaigns. The design of a large digester is therefore an integrated process, mechanical, thermal, hydraulic and mixing-engineering exercise. Increasing vessel volume alone does not guarantee higher productivity or lower operating costs.

This article develops a preliminary engineering methodology for a large capacity digester handling gibbsitic bauxite, sizing calculations, pressure-shell design methodology and agitator power estimation to establish a design with utmost accuracy.

1. Design philosophy

The proposed digestion installation comprises six identical digesters, with five operating in series and one maintained as standby. The total slurry flow, including additives, is estimated at 2,800 m³/h and the total nominal residence time across the five operating vessels has been considered as one hour.

The design must provide the required dissolution performance within this residence time while ensuring adequate agitation, heat transfer, pressure containment, erosion resistance and mechanical availability.

2. Digester Sizing Calculations           

Considering five identical operating vessels with equal working volumes.

Hence working volume of each digester = 2800 m³/5 = 560 m³

If working volume is provisionally taken as 80% of gross geometric volume, then

Design volume of each digester = 560/0.80 =700 m³

The standby digester will have the same nominal capacity as it is intended to replace any one of the operating digesters for preventive maintenance. The six (6) installed vessels would then have a combined gross geometric volume of approximately 4,200 m³.

3. Preliminary vessel dimensions

For vertical cylindrical vessel, the ratio a straight-shell height-to-diameter ratio is generally considered 2:1. It would be prudent to mention here that a taller, narrower vessel and a shorter, wider vessel of the same volume will not perform identically. The selection affects the following:

•   Vertical suspension of bauxite and residue particles.
• Axial circulation and temperature uniformity.
• Agitator shaft length, bending and critical speed.
• Heat-transfer surface area and heating-coil arrangement.
• Foundation loads, erection logistics and transportation.

For a large vessel, the optimum diameter-to-height ratio must be established by process modelling and mechanical assessment rather than a universal rule of thumb.

Assuming 2:1 as H/D ratio, the digesters having design capacity of 700 m3 each, the diameter and height work out to following figures:

Diameter ≈7.64 m

Height =2D≈15.28 m. 

These are preliminary geometric dimensions, not a final vessel drawing. Head volume, bottom configuration, freeboard and the actual operating liquid level must be included in the detailed design.

 

4. Shell thickness of digester

The specified design pressure is 10 kg/cm²(g), equivalent to approximately 0.981 MPa(g).

The design pressure must be established separately, taking account of operating conditions, static liquid head, pressure-control scenarios and the governing pressure-vessel code. For design shell thickness calculation, let us assume design pressure of 1.20 MPa(g), slurry density of 1,200 kg/m³, allowable stress of 120 MPa, weld-joint efficiency of 0.85, corrosion allowance of 3 mm and approximate working liquid depth of 12.2 m, the actual thickness need to be calculated using the selected steel grade and its allowable stress at design temperature, confirmed design pressure, actual liquid depth, code requirements, plate tolerances and all applicable loads. Detailed wall thickness calculations have already been covered in earlier published article.

A large digester may need different thicknesses in different shell courses. The final design may be governed by additional conditions as listed below: 

• Static liquid head, which increases pressure toward the bottom.
• Local stresses around large nozzles, manways and agitator-support structures.
• Wind and seismic loads on a tall vessel.
• External pressure or vacuum during draining, cooling or steam condensation.
• Fatigue from pressure and temperature cycles.
• Slurry erosion, corrosion, thermal gradients and local heating-coil loads.
• Agitator torque, shaft-bearing loads and vibration.The heads, bottom closure, nozzle reinforcement, support skirt, anchor bolts and foundation must all be designed separately as applicable.

5. Agitator design

Each operating vessel will handle the full slurry flow of 2,800 m³/h, but only contain approximately 560 m³ of working slurry. The agitator must be designed for the vessel's actual mixing duty.

 A top-entry, multi-impeller arrangement can be evaluated, with the following design checks:

• Adequate axial circulation and solids suspension.
• Uniform temperature and concentration.
• Acceptable impeller power consumption.
• Shaft bending, torsion, deflection and critical speed.
• Gearbox torque, starting loads, vibration and seal reliability.
• Safe operation during start-up, shutdown and standby changeover.

The power calculation to be done based on type of selected impeller geometry and validated power number using following equation: 

P_i ​= N_p​. ρ. N³. D_i⁵​

 Where:

• P_i ​ = power absorbed by one impeller, W
• N_p = impeller power number, dependent on geometry and flow regime
• ρ = slurry density, kg/m³
• N = rotational speed, revolutions per second
• D_i = impeller diameter, m

The impeller diameter, speed, number of impellers and motor rating cannot be reliably finalised without slurry rheology, solids loading and vendor mixing data. Accordingly, no definitive motor rating should be assigned at this stage. 

A successful design to establish agitator adequacy demonstrates the following: 

• Adequate suspension of the relevant solid particles.
• Sufficient axial circulation throughout the working volume.
• Acceptable temperature and concentration uniformity.
• No unacceptable stagnant regions or persistent solids deposition.
• Acceptable shaft deflection, vibration and mechanical-seal performance.
• Reliable operation during start-up, shutdown and credible upset conditions.

 6. Consolidated design basis

Parameter	Preliminary basis
Refinery capacity	2.0 MTPA
Total slurry flow	2,800 m³/h
Operating vessels	5 in series
Standby vessels	1
Total nominal residence time	1 hour
Working volume per operating vessel	560 m³
Total working volume in operating train	2,800 m³
Gross volume per vessel	700 m³
Illustrative internal diameter	7.64 m
Straight-shell height	15.28 m
Operating pressure	10 kg/cm²(g)
Top design pressure	1.20 MPa(g)
Bottom-shell thickness	53.8 mm
Agitator power	To be established.

7. Final recommendation

The revised sizing now correctly reflects one-hour total residence time across five digesters in series, giving 2,800 m³ total working volume and 560 m³ working volume per vessel for an equal-volume arrangement.

8. CONCLUSIONS:

The largest practical digester is not necessarily the vessel with the greatest geometric volume. It is the design that delivers the required alumina extraction and throughput while maintaining effective mixing, uniform heating, pressure integrity, acceptable erosion and corrosion rates, and high mechanical availability. 

9. RECOMMENDED ENGINEERING WORKFLOW FOR LARGE DIGESTERS

•  Freeze complete process design basis.
• Complete the process and thermal calculations.
• Optimize the vessel geometry
• Validate mixing and suspension.
• Perform code-compliant mechanical design.
• Complete safety and reliability reviews.
• Obtain independent design verification from qualified pressure-vessel designer and agitator supplier.

 For a large-capacity Bayer digestion system, process modelling, mechanical code, detailed calculations, mixing validation and independent safety review must be completed together. The final wall thickness and agitator specification must emerge from these calculations with adequate safety margin for the pressure vessel. 

Process and plant designers and Bayer process engineers are welcome to share their opinions, feedback and technical remarks on this article for further improvement in design of digesters.