Bulk solids handling - from silos to containers

Paul Boughton
Modern solids handling systems, from silos via conveyors to bags or containers, have to be carefully engineered, installed and maintained. Eugene McCarthy reports.

Dhunseri Petrochem & Tea Ltd. (DPTL) is a leading global manufacturer of bottle-grade polyethylene terephthalate (PET) resin. The company's plant is located in the port city of Haldia in West Bengal, India. The facility is a continuous chemical process plant that operates non-stop, year-round. It produces 600t/d of PET resin.

PET manufacturers around the globe typically employ pneumatic conveyors to transport purified terephthalic acid (PTA) from storage silos and hoppers to downstream processing vessels. The Haldia plant was no exception. The facility receives PTA in 15 tonne bulk truckloads. From the massive hopper at ground level, 25m3 of PTA powder is conveyed each hour up to a staging silo 35 metres above.

"Our original process relied upon pneumatic conveyors to move PTA powder," noted Subrata Mazumdar, senior general manager of engineering for DPTL. "The pneumatic conveying system - mainly nitrogen compressors - consumed an enormous amount of energy and nitrogen."[Page Break]

With the cost of energy and nitrogen contributing to higher cost of production, an alternative conveying method was sought.

"After much research we realised tubular drag conveying would be the most efficient and effective means for conveying PTA powder," said Mazumdar. "We chose Hapman because they provided a better design and robust construction of drag conveyor."

The tubular drag conveyor consists of a stationary outer casing through which a chain is pulled by a sprocket drive. Flights are attached to the chain at regular intervals. As this endless chain and flight assembly moves through the casing, bulk material is pulled from the in-feed points to the discharge ports.

"While similar in some ways to cable- and aero-mechanical-style conveyors, tubular drag technology is superior to these systems because it utilises a heavy-duty chain to move material at a low velocity," says Naresh Gandhi, md of Hapman India. "The result is a conveying method that is rugged yet gentle for the widest array of materials with virtually no maintenance, is quiet and consumes little power."

Gandhi also noted the slow-moving, positive-displacement action of the chain assembly is ideal for handling friable and/or blended materials without separation or degradation. Because the fully enclosed system does not introduce air, a constant supply of nitrogen is not required for explosion suppression. The system does employ a purging system, which requires a little nitrogen.

The Hapman conveyor's 200mm casing is constructed of stainless steel and employs three inlets and one outlet, as well as a discharge vibrator to ensure 100 per cent discharge of the powder. A low-horsepower (25hp) motor moves a stainless steel chain with polyethylene flights throughout the conveyor's 99 metre circuit to a final discharge height of 35 metres at a 79° incline.

"We've achieved our goal of reducing energy and nitrogen consumption when conveying PTA by replacing the 320 kWh pneumatic conveyor with a 25hp tubular drag conveyor," noted Mazumdar. "We are satisfied with this system."

Hapman continues to expand the capability of its innovative Solidquid solid/liquid delivery system with numerous options to improve process efficiency and operator safety when dealing with materials that agglomerate.

The custom-engineered system uses frame-mounted pneumatic bag agitators to maintain an uninterrupted flow of sluggish materials from bulk bags. Hapman's LumpMaster effectively breaks up agglomerated material prior to entering the PosiPortion feeder. This assures a more consistent feed, eases mixing into the motive liquid stream and prevents obstructions in the throat of the eductor.

Flexicon has also been busy, adding the new Single-Trip Bulk Bag Unloader to its portfolio. The allows rapid, low cost discharging in low headroom areas, and conveying of the material at any angle to downstream processing, storage or packaging equipment.

The unloader relies on the user's forklift or plant hoist to lower single-use bulk bags onto a four-bladed bag piercing knife that cuts the bag bottom from seam-to-seam, promoting complete discharge without the use of flow promotion devices otherwise needed to move material through narrow bag spouts.

The Single-Trip Unloader relies on the bag bottom to self-seal against a thick gasket affixed to the rim of the hopper and to discharge through a wide diameter hole in the centre of the gasket, eliminating the cost and additional headroom required for bag spout interfaces used on conventional unloaders of reusable bags. An optional dust plenum, consisting of a hollow hopper rim with interior slots and exterior port vented to a plant dust collection system, is offered as a secondary dust containment measure.[Page Break]

Single-use bags are simply lowered onto the piercing knife, ready to discharge many times faster, and with less labour, than possible when connecting reusable bags to unloaders.

Compared with reusable bags, many of which are discarded after one use, single-use bags can be less costly to purchase, and reduce the amount of material to be discarded or recycled.

The hopper, which is equipped with a hinged door to access the interior safely, charges the intake adapter of an integral flexible screw conveyor that handles free- and non-free-flowing bulk materials ranging from large pellets to sub-micron powders, including products that pack, cake, seize, smear, fluidise, break apart or separate, with no separation of blended products.

The hopper is also available equipped with intake adapters and rotary airlocks for pneumatic conveying systems, as well as with universal flanged outlets for connection to the user's downstream equipment.

Construction is of stainless steel finished to industrial, food, dairy or pharmaceutical standards, or of carbon steel with a durable industrial finish.

The unloader joins Flexicon's other products including its PNEUMATI-CON dilute phase pneumatic conveying systems. These range from single-point 'up-and-in' installations to cross-plant systems with multiple pick-up and discharge points and automated controls, satisfying an exceptionally wide range of bulk conveying applications. All are custom-engineered and fully integrated with other Flexicon equipment and systems.

PNEUMATI-CON dilute phase pneumatic conveying systems move bulk materials that are suspended in an air stream that is introduced by a positive pressure blower upstream of material intake points, or by a vacuum pump that removes air from the system downstream of material discharge points. Material is separated from the conveying air at the use point, then discharged on a batch basis via butterfly or slide gate valves, or continuously via rotary airlock valves.

Meanwhile dense-phase conveying specialist Air-Tec is rising to the challenges of product abrasiveness caused by the powders that many of its customers need to move, including malic acid, urea, citric acid, sodium carbonate, tartaric acid, alumina, percarbonates for detergents, copper sulphate, soda, caustic potash, sodium monoclorate and other materials.

In response, the company has developed three main proprietary technologies: Bart for short trajectories and flows up to 1t/h; Bart 1 for up to 20t/h transportation for medium distances and TPA (Air-Tec Pneumatic Transport) for trajectories of over 500 metres and capacities of over 100 t/h. TPA is the company's most advanced solution, enabling greater flexibility and control of transport cycles.

The company also offers a wide range of accessories and components needed to deliver customised solutions.

The advantages of dense phase pneumatic transport, particularly its low velocity of transport, simple installation, flexibility and reduced maintenance costs, are also finding favour in the waste-to-energy business (Fig. 1).