Pneumatic Conveying Q&A
Clay in small particle form is very fluidizable and retains its fluid nature well. In a high velocity pneumatic convey system (dilute phase), the turbulence of the air keeps the material in a totally fluid state and will arrive at the destination this way. A low velocity pneumatic convey system (dense phase) will have significantly less turbulence and will fluidize the material less (but still quite a bit). Depending on which of these you have, you will be closer or further from your goal.
You likely want to pursue de-aeration or densification once it arrives at the destination. Depending on the available equipment and orientation you may be able to utilize one of the following:
1) Resonance Time – if the material can be captured in the receiver for a period of time before being discharged then it will naturally de-aerate over time. Adding vibration to the cone of the bin will help the process along or even densify the material somewhat. This becomes a batch process.
2) Vacuum densification – Material received at the destination can be exposed to a vacuum source that will actually draw the air out of the material and densify it. This vacuum may be separate from the convey air movement device. This becomes a batch process.
3) High vacuum conveying – vacuum can be used to convey the material. Doing so at low velocities will make it a vacuum dense phase system. The vacuum can be increased during conveying so that the resulting air is so rarified that it arrive in the receiver with very little air entrained. This can be operated as either batch or continuous.
Hopefully one of these suits your application.
As it seems you understand already, the risk you are facing is the small particle dust in your material which could be found in concentrations sufficient to support ignition if combined with an O2 rich environment and an ignition source. Low MIE materials require less of a “spark” so static discharge is the most likely concern. The convey line should grounded whereever possible and avoid using non-conductive materials like acrylic sight-glasses because these can act like capacitors and build charge.
Creating an inert environment (N2 convey gas) is the ideal solution because you have removed (1) of the (3) critical requirements for ignition. A closed loop system can be created to minimize the consumption of N2 but it will still require an N2 supply and monitoring to insure O2 levels are below thresholds. Having an existing N2 supply available in the plant will lessen the cost and difficulty or implementing this solution.
If N2 conveying is not viable, the more common method in the industry to mitigate explosion risk is by explosion protection equipment. Equipment with considerable volumes (usually filters and storage bins) can be outfitted with one of the following devices:
Rupture Panel – a disk or square panel is built into the design so that if ignition occurred, the weak seam would rupture and exhaust the flame. These would need to be located outdoors or adjacent to an outside wall
Flameless vents – Similar to the above but the exhaust is passed through a media that will not propagate a flame. Can be used on indoor devices.
Chemical isolation – A suppression chemical is mounted to the unit and a sensor monitors the pressure in the bin. If a pressure rise consistent with deflagration is registered the canister releases and quenches the flame.
Mechanical isolation – Any filter or bin with piping or duct connections larger the 4 in. require an isolation to keep the flame from propagating to other places. Additional chemical isolation can be used here or a quick acting gate to act as a block.
There are pros and cons to each of these so you will need do some research and figure out which one suits your situation best.
The exact arrangement that best suits you may be influenced by how quickly the truck needs to be emptied (convey rate) but here are some general approaches:
Pressure Only: A pressure pneumatic system (dense or dilute) requires the material be funneled to a common point and then injected into the convey line. This usually means a pit is involved and the conveying equipment is located in the pit. The material is then delivered directly into the bin and a filter is placed on the bin itself or down on the ground. These are best for large tonnages and quick discharge of the truck.
Vacuum Only: With some custom connections to the truck or a shallow hopper below the surface, the material can be vacuumed straight to the top of the storage device. A vacuum filter and airlock will need to be placed on top of the storage device (think maintenance) and then the air source can be on the ground. Usually requires large equipment because the vacuum system is relatively inefficient.
Vacuum-Pressure Combo (Recommended): Vacuum works better with the truck (no pit), but pressure works better with the silo (little or no equipment on top) so you utilize the advantages of each. The material is pulled off the truck a short distance and then deposited into a pressure system which pumps it up to the silo. The location of this equipment is flexible but ground based (above ground) where it is easily maintained.
An eductor is an attractive solution to applications with short distances (<50 ft) and low material rates (<1000 PPH) but the throat of the eductor is susceptible to scaling and build-up.
Hydrated lime has been shown to build up easily in the presence of CO2. The chemical reaction of hydrated lime produces water and it is believed this moisture makes the resulting solid sticky and then dries once stuck. This effect is more significant in convey lines that are lightly loaded (high CO2 to material ratio). Eductors actually draw in additional ambient air so the effect is worse. In lab experiments, hydrated lime through an eductor powered by air choked off in a matter of hours. The same arrangement powered by N2 showed no build-up in the same time frame. Unless a convey air void of CO2 can be used to power the eductor, it is not recommended due to the scaling effect.
Trona and Bicarb are not known to have such issues as the hydrated lime. For this reason, those products have not been studied as closely. If these products were exposed to high moisture content air I believe there would be moisture adsorption and potentially create the same effect. Use of a dehumidifier on the air source inlet would be recommended or pulling the air from a conditioned environment. I believe eductors are acceptable for these products.
There are several pneumatic conveying supply companies which will design the systems for you and guarantee their performance. However, you are likely referring to a fan-driven vacuum system (negative air lift) which is relatively simple to calculate and can be done yourself if desired. Since these systems are relatively lightly loaded, they resemble a single branch dust collection system where air alone losses are a large contribution to the total fan static.
The calculation can be found in several reference manuals (ex. Chemical Engineers Handbook) and likely on the Internet (although I don’t have a particular site). I have also seen it called the “Energy Method.”
Essentially a line size, distance, material rate (PPH), and velocity are used to calculate pressure drops for both the airflow and material contributions. To this should be added component losses like the cyclone/filter and any clean air losses. At this point the fan airflow and static pressure should be known and therefore a selection can be made.
Reference material could of course mean different things to different people but I will attempt to cover a range of needs. To address the fundamentals of pneumatic conveying, one should attempt to view a range of products that spans both particle size and bulk density. To select examples across the range you could end up with (9) references: weights and sizes in the table above are examples only.
Since identifying (9) products that are consistently available in these ranges would be difficult, you might choose (3) or (4) that offer a decent cross-section such as #2,4,6,8 or the extremes #1,3,7,9. When we went looking for reference material we wanted items that were readily available in bulk at local stores and decided on flour, sugar, popcorn, playground sand. Having an exact particle size (such as calibration materials) is not necessary.
Design guides are little less defined. There are several rules-of-thumb that mostly apply to proper piping layout (straight pipe away from the feedpoint, no inclines, no back-to-back elbows). Most other design information (unless a published sizing technique) is highly material dependent and therefore based on experience.
There are basic pneumatic systems that operate in the pharma industry but I cannot say whether they operate in the Class 100 environment or not. I will however describe what I believe to be the relevant factors in determining whether it would be suitable.
Systems can be operated as vacuum or pressure depending on how the system is configured. Vacuum would be preferred in the case you describe because pressure systems tend to release air to the environment which would obviously present a room contamination issue. The vacuum system should draw air from a clean source outside the room (as opposed to free air inside the room). Depending on the rate of product feed this may necessitate an airtight feed device near the operator. The convey line would take the air and material to the destination (at a velocity that will not damage the product) and then deposit the product through another airtight feed device. The convey air (and any particles generated during convey) will be carried out of the room and filtered before returning the air to the environment.
An airtight feed device in pneumatic conveying generally means a rotary valve but these are not truly airtight in a strict sense. A rotary valve made of alternative materials to metal so that the pieces actually contact would create a better seal. There may be other devices for this service that I am not familiar with as well.
Build-up or scaling in a convey line or receiver generally occurs when there is sufficient velocity to impinge a particle onto a surface but insufficient velocity to remove it. Then, depending on the material characteristics, the particles adhere, dry, or absorb their way to becoming more rigid deposits. In your case, the convey line may stay clean because the velocities are high enough remove any deposits but the larger volume of the receiver means any velocities incurred are lower. Turbulent flows may promote deposits which will not be automatically removed.
There is not much that can be done for the current piece of equipment. You might try an externally mounted vibrator (running all the time) to encourage the small deposits to fall down the surfaces before they accumulate. This may decrease the amount of build-up. Further modifications may include surface treatments of the filter internals to reduce friction and lessen the deposition. Epoxy finishes and polishing are popular for this.
If you are seeking a more effective means to clean the surfaces quickly, I was introduced to “dry ice blasting” that was very effective. Essentially dry ice is used as a sand blaster, however the blasting material disappears (sublimates) and all that remains is the product removed from the surface. A local supplier would likely be willing to do a free demo on your equipment.
If you are willing to entertain an equipment change, cross contamination in similar processes is often managed using cyclones as receivers because they much higher internal velocities and no filtration media. Cyclones are not 100% efficient in their separation so the carryover must be managed and is generally done with a filter. The filter can return the material to the convey line (yes, this reintroduces the cross-contamination issue but with significantly less material) or be brought off-line (carryover is a waste stream). If the amount of carryover is a concern, a second in-line cyclone can be implemented to boost the capture efficiency. This is more of an engineered solution.
Corn (maize) starch is one of many common dry materials that is flammable and when placed in the wrong set of circumstances can create an explosion risk. These circumstances are:
1) small particle size – high surface area
2) presence of oxygen – conveying air stream or dust cloud
3) ignition source – spark, flame, or other heat source
Sugar dust and wheat flour are two other common material where the same thing can happen. Using nitrogen as the convey gas will remove oxygen from the equation and remove the risk of explosion, however this is very expensive to operate. The more common and cost effective approach is to place explosion protection devices on the filter housings and storage tanks. The most common is the rupture vent which becomes the weak point in the vessel and directs the pressure from a potential explosion out the vent to safe area. Local regulations generally provide direction for acceptable explosion protection requirements, but a qualified system supplier should also be able to devise an acceptable explosion protection plan and equipment to meet that plan.
In addition to the equipment design, when handling corn starch also use good housekeeping practices. Make sure there are no dust leaks in the system that will create a dust cloud. Clean up material that has spilled or been emitted. Ensure there are no sources of spark or heat on the equipment where starch will be handled.