In a vacuum steam stripping plant with mechanical vapour recompression (MVR), the most important layout rule is to keep the vapour circuit short. The strippers, vapour separator, compressor and condenser/reboiler should sit together as one tight group, because the large, low-pressure vapour lines between them are what the whole design depends on. This was the key instruction in a design brief for the 3D general arrangement of an electrically driven ammonia recovery plant treating up to about 160 m³/h of wastewater.
The challenge: a large plant that must stay serviceable
The drafting team was asked to build a credible 3D model of a proposed plant. The model had to support client presentation, confirm the footprint, check access and maintenance, develop pipe routes, and coordinate civil and structural work. It was explicitly not to be a conceptual rendering.
The plant uses a different process from a conventional air-stripping ammonia plant. The process sequence in the brief was:
- Feed passes through feed/effluent heat recovery.
- Vacuum steam stripping removes the ammonia.
- A vapour separator removes liquid carry-over.
- An MVR compressor raises the vapour’s pressure and temperature.
- The compressed vapour condenses in a condenser/reboiler, giving up its heat to generate more stripping steam.
- Ammonia is harvested under vacuum, cooled and hydrated to produce aqueous ammonia directly.
Compared with the earlier conventional arrangement, there is no process air blower, no air-stripping system, no cold-water ammonia scrubber, and no separate ammonia concentration column. Heat is recycled by the compressor, with only start-up and trim heating from outside. That is why the plant is described as electrically driven.
The approach: a preliminary design basis for the drawing office
Main equipment
| Item | Preliminary size | Why it matters for layout |
|---|---|---|
| Vacuum steam strippers | 3 columns, about 3.1 m diameter and 14.6 m vessel height | The tallest, heaviest items, about 87 tonnes in total |
| Vapour separator | About 1.8 m diameter by 4.5 m | Must sit next to the strippers |
| MVR compressor package | About 6.0 x 2.8 x 3.2 m, about 15 tonnes | At the centre of the vapour circuit |
| Condenser/reboiler | About 7.0 x 3.0 x 3.5 m, about 24 tonnes | Next to the compressor |
| Hydration column | About 1.5 m diameter by 7 m | Part of the ammonia recovery zone |
The overall process footprint was about 20 m by 15 m, within a reserved site of about 35 m by 20 m and an initial height allowance of around 12 m. The brief estimated 190 to 210 tonnes dry installed mass and 240 to 270 tonnes in normal operation. Those are order-of-magnitude figures for layout and civil loading, and the brief says they are not vendor-certified.
Zones
The preferred arrangement divided the plant into zones:
- High process zone: the three strippers, vapour separator and hydration column.
- MVR zone: compressor and condenser/reboiler, close to the strippers.
- Feed and heat recovery zone: feed pumps, strainers and the heat recovery skid, forming the lower-level area.
- Ammonia recovery zone: vacuum package, ammonia gas cooler and hydration package.
- Utility zone: trim heat pump and start-up heater.
- Electrical zone: the motor control centre and control room, placed at the outer edge and preferably away from the ammonia-handling equipment.
- Product interface: product receiver and export pumps, with space left for future bulk storage.
Why it works: maintenance comes before footprint
The brief states that the layout should not be compressed just to reduce footprint if that harms maintenance. Its working rules, until a hazard and operability study is done, were:
- About 1.0 to 1.2 m clear pedestrian access around process equipment.
- At least about 1.5 m next to large pumps and the maintenance side of packaged equipment.
- 2 to 3 m of withdrawal or lifting space where exchanger bundles, motors or compressor internals may need to be removed.
- Access to stripper manways, lifting access above major rotating equipment and unobstructed escape routes from elevated platforms.
- No major pipe racks directly over equipment that needs crane access.
Access structure
A shared structural steel access system was proposed around the three strippers, with platforms at about +3 to +4 m for intermediate manways, +7 to +8 m for packing, distributor and manway access, and a top level for nozzles and relief valves. Stairs rather than ladders were specified for routine access, with platforms at least about 1 m wide, handrails, toe plates, intermediate landings on stairs, lifting beams and removable handrail sections.
Piping
The large-diameter vapour lines between the strippers, separator, compressor and condenser/reboiler operate at low pressure or under vacuum, so they must be as short as possible, smoothly routed and with few fittings. Liquid lines can use the main pipe rack. The model had to distinguish visually between feed, treated liquid, steam and vapour, ammonia-rich vapour, condensate, aqueous ammonia product, cooling water and drains.
Staged model development
The brief set four revisions: A (spatial model with equipment envelopes), B (process arrangement with major piping), C (design review model with maintenance spaces, valves and instruments) and D (coordinated general arrangement with vendor data and a confirmed P&ID). Revision A was to be reviewed before any detailed piping, so the basic arrangement could be approved first. The model was also built so individual items can later be swapped for vendor models.
What to consider for your own project
- Treat the MVR circuit as one unit. Draw it first, then arrange everything else around it.
- Reserve the full site envelope early, as heights and loads are significant: three heavy columns act as concentrated loads on the foundations.
- Put the control room away from ammonia.
- Leave space for storage. The model identified a battery-limit connection for later bulk ammonia tanks.
- Review at revision A. Changing a spatial model is cheap, whereas changing a piped model is not.
For the conventional thermal approach, see our worked example of thermal ammonia stripping using waste heat.
FAQ
What is mechanical vapour recompression?
A compressor raises the pressure and temperature of process vapour so it can be condensed to supply heat for more stripping. It recycles heat instead of buying steam.
Why keep vapour lines short?
The lines are large in diameter and run at low pressure or vacuum, so long or tortuous routes would add pressure loss and cost.
How does this differ from air stripping?
The brief’s design has no air blower, no air-stripping system, no cold-water scrubber and no separate concentration column. Ammonia is harvested under vacuum and hydrated to aqueous ammonia.
Are the dimensions final?
No. They are preliminary and for design development, not construction.
Planning an ammonia recovery plant?
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