Dense Phase Conveying in Chemical Production

A chemical plant conveying a powder has three problems at once: the product may be harmful, it may be reactive with oxygen or moisture, and the line is shared between campaigns.

Dilute phase conveying makes all three harder because of the air. A large air volume needs a large filter, a large vent and a large extraction, and every one of those is a place where product can escape and air can enter. Inerting a dilute phase line with nitrogen is possible and expensive.

Dense phase changes the arithmetic. The air volume is a fraction of the dilute phase figure, which makes a closed loop with an inert gas practical, shrinks the receiving filter and reduces the vent.

What it does not solve is compatibility and carryover. The pipeline, the bends, the valves, the vessel and the seals all see the product and the cleaning agent, and a line that carries three campaign products has to be emptied and cleaned between them in a way somebody can state as a number.

A conveying system sized so the air or inert gas volume is small enough to make containment, filtration and inerting proportionate.

Compatibility verified along the whole line, including bends, valves, flexible sections and seals, against every product and cleaning agent in the campaign plan.

A line that can be emptied and cleaned between campaigns, with the residual quantity measured rather than assumed.

A feed and grain plant measures the result rather than the route: pellet durability taken at the press and never after the conveying line hides what the bends did.

The material is characterised first: permeability, de aeration behaviour and the pressure needed for the duty. For chemical powders this often has to be done per product, because a campaign plan may include materials with very different conveying behaviour on the same line.

Where the product is oxygen or moisture sensitive, a closed loop is designed rather than an open one: the conveying gas is recirculated, cooled and filtered, with make up and oxygen monitoring. Dense phase is what makes the gas inventory affordable.

Containment is then designed at the ends. The feeding vessel and the receiver are where product meets the plant, so those are the containment boundaries; the pipeline itself is usually the easy part. Filter change, vessel entry and the drain at a low point are the operations to design for.

Changeover is designed in. Drain points at low spots, an emptying sequence, an accessible route for cleaning and a measured residual quantity give a carryover figure that a quality question can be answered with.

• Each campaign product characterised separately for conveying behaviour.
• Closed loop with inert gas where oxygen or moisture sensitivity requires it, with oxygen monitoring.
• Containment designed at the feeding vessel and the receiver, which are the real boundaries.
• Compatibility verified for pipeline, bends, valves, flexible sections and seals, including cleaning agents.
• Drain points and an emptying sequence for changeover between campaigns.
• Residual quantity measured so the carryover figure is a number.

• Exposure limit of the products, which decides how much the reduced air volume is worth.
• Oxygen or moisture sensitivity, which decides between an open system and a closed inert loop.
• Conveying behaviour per product, measured, since a campaign plan can span very different materials.
• Compatibility of every wetted component with products and cleaning agents.
• Carryover permitted between campaigns and how it will be measured.
• Area classification, which applies to the vessel, the valves and the instruments.