Opening the manway of a vessel and pouring powder through it has always been an obvious way of charging bags and drums in the past, but it has never been convenient and certainly not safe. The operator was forced to wear protective clothing, ranging from a dust mask to a full air suit. The process had to be at ambient temperature and pressure before the manway could be opened.
There was always a risk of spillage, and cleaning the area was a long and laborious job. Alongside this there were hazards present due to the exchange of gas between the inside of the vessel and the air entrained with the powder as it falls through the manway, displacing the atmosphere inside, and potentially leading to a dust cloud, ripe for ignition. Nitrogen blanketing was possible but certainly could not be considered safe in such an open application due to its own associated health risks.
In some cases gloveboxes were used, but this led to other problems mainly being one of time as it can take up to 15 minutes to charge a 25Kg bag of powder, acceptable for 2 or 3 bags but not ideal for more. Frequently it can be important to charge the whole batch of powder quickly for the sake of the process, or even dose the powder in a controlled addition, not easy with a glovebox. Also there were issues of blockages within the chute and control of the low negative pressures required within the glovebox. Whenever you connect two vessels there is always an equalisation of pressure which could lead to the gloves being pulled in, or worse still, blown out along with the powder.
The ATEX Directives (94/9/EC, 1994 and 1999/92/EC, 1999) clearly state that the employer shall take technical and/or organisational measures appropriate to the nature of the operation, in order of priority and in accordance with the following basic principles:
• The prevention of the formation of explosive atmospheres, or where the nature of the activity does not allow that.
• The avoidance of the ignition of explosive atmospheres.
• The mitigation of the detrimental effects of an explosion so as to ensure the health and safety of workers.
There is a solution to these issues which has been developed by the DEC Group. It is possible to charge multiple bags, sacks and kegs of powder to a mixing vessel or reactor both safely and quickly.
The Power behind the Powder
The PTS (Powder Transfer System) has been satisfying these ATEX regulations for more than 20 years being used primarily for charging powders safely to reaction vessels. The PTS (installed directly on the receiving vessel) uses a source of vacuum and pressure to move powder within the dense phase conveying region. Powder can be transferred from any receptacle over distances up to 40m thus allowing for more flexibility when designing plant layouts.
The operating principle is simple and effective for all powders, even lumpy, sticky and wet. Powder is sucked by vacuum into the PTS chamber, where the air in the powder is removed by a filter disc of 1micron PTFE to the vacuum pump. When the chamber is full, the cycle is reversed and the chamber is emptied into the receiver by a short blast of compressed gas. At the same time the PTFE membrane (FDA approved) is pulse cleaned ensuring optimum performance.
The blast of compressed gas also ensures that the complete charge exits the PTS chamber and also prevents any vapours within the vessel entering the PTS chamber. The latter being a major problem with other technologies that rely on gravity and suffer from a sticky mess at point of vessel entry. In addition, the dense phase conveying that the PTS employs is both safe and effective for the transfer of powders with difficult flow characteristics.
The PTS – Powder Transfer System – “The Power behind the Powder”
The Multiple Bag/Drum Solution – Contained Bag/Drum Station
As discussed, tipping bags of powder into vessels through the manway is not a good idea but there has not really been an ideal alternative. Until now however, for this duty, a new development is available, the Contained Bag/Drum Station from the Dec Group.
Contained Bag Station for Bags of Powder (optional drum module not shown)
It is best described as a hybrid design, crossing the open aspects of a laminar flow booth with the containment of a glove box isolator. The Bag Station incorporates an open ended stainless steel chamber, enabling the easy loading of bagged powder either by hand or simple lifting device.
This is the key feature of the design which gives the ability for powder to be loaded into the chamber and charged safely with the chamber door open. An optional drum discharge module is also available for bags within drums or kegs. Apart from the obvious ‘house keeping’advantages that this brings, if there are high hazard powders to deal with, the containment level reached with this design is < 1microg/m3, a remarkable achievement considering this station is completely open at all times (except for clean down with the optional integrated CIP system).
The exact geometry of the internal structure remains confidential for the moment, but in essence it is the ‘bullet’ shape of the chamber combined with the unique airflow control regime by pressure regulated modulating damper that provides the required airflow characteristics.
The chamber cross section steadily increases along its length to ensure that the air velocity steadily decreases until it levels out at approx. 0.3-0.4 m/s at the point of opening the bag of powder. This ensures that there is a one way only air transfer from the room, through the chamber, exhaust plenum, filters, fan and finally out to atmosphere. CFD and Smoke tests showed that eddies are created as the cross sectional area of the chamber levels out but as soon as they start to move back along the slope of the chamber the increase in air velocity quickly pulls them back to the centre and through the plenum. Trials have proved that containment is not improved any further by closing the chamber door.
Inside the Main Chamber – A hybrid of Isolator and Laminar Flow Booth
When using the PTS, the Bag Station can be located in a material storage area up to 40m and several floors away from the receiving vessel. Also, by using a simple automated manifold it is possible to charge many vessels from the same Bag Station saving space and more importantly money.
The Contained Bag Station can be used to charge numerous vessels
The chamber base has an integrated discharge transition with a removable grid for mounting a suction hopper. Available as a simple hopper for free flowing powders or with a combination of fluidised wall for sticky wet powders and/or a lump breaker agitator for lumpy powders and bridge breaking.
Suction Cone with De-Lumper Suction Cone with De-Lumper & Fluidised Wall
When a large quantity of bagged powder awaits the operator, this equipment will certainly make his life easier and also safer. But this technology will also appeal from a commercial prospective due to the significant amount of time saved. But when you also consider that process batch times can be reduced because it is no longer a necessity to prepare the vessel for manway opening, (the PTS can safely charge powder directly into a hot solvent mix), then this equipment starts to look like a necessity.