Hilliard Emission Controls, Inc. 3100 Edloe Street Suite 350 Houston, TX 77027 888-621-3132       hank.hilliard@purgit.com

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Discussion of mobile equipment used for tank vapor control.

 
The object of vapor control is to reduce the volatile organic compounds (VOC) emitted during tank cleaning. Any improvement beyond that is a benefit. PURGIT's improvement is that we condense and our benefit is recovery. We are proponents of using refrigeration and condensers for controlling VOC from tank degassing. Our condensers change tank vapors back to liquid suitable for recycling. When we condense and recover 1 pound of VOC, we haven't just recovered 1 pound of VOC, we stopped 1 pound of greenhouse gases from going into the atmosphere. The fact is that we recovered over 85 tons of VOC condensate in just 18 months. We are proud of our development of the mobile condenser system. This paper shows how our proprietary refrigeration system is different from combustion systems.

States regulate the release of VOC vapors from sources like tank cleaning. The tanks could be barges, ships, rail cars, tank trucks, floating and fixed roof tanks, pipelines, etc. Tank farms require mobile equipment because of the remote tank locations. Shipyards may call for specialized control equipment from time to time for unusual cargoes. Many facilities use contractors for vapor control. PURGIT is a contractor that does vapor control with mobile equipment in Houston TX and other places. We usually work for the tank owners, operators or other contractors.
 
We had a choice of any type of vapor control device when the business started in 1993. After owning internal combustion engine devices and inspecting the mini combustors, we decided they had too many flaws and started developing a refrigerated vapor condenser. The condenser we built in December of 2006 is fast and effective. We believe we now have the finest vapor control device available. It can handle a wide range of cargoes.
 
The competitors use mini-combustors and internal combustion engine devices called ICE. The claims made regarding flow rates for these devices are overstated and the devices have drawbacks. Some of the competitors use the IC Engines, the very same Ford 460 engines that PURGIT owned and found to be woefully inadequate for the job. However they seem to start and complete tank vapor control jobs in time frames that we could never achieve when we had the engines. Some competitors use mini combustors that are pulled behind 1 ton pick-up trucks. We question the capacity of those units because we have rented an enclosed combustor that is much larger, but carries a smaller flow rating. See note 1.
 
Condensers are a better idea. But how do you know a vapor condenser is working? The proof is in the measurable reduction in the tank vapor and in the recovered liquid condensate. See note 2. PURGIT routinely recovers more energy in the form of condensate than we use! No combustor can say that. Proof that a combustor or ICE engine is performing at the rated design can only be confirmed by continuous exhaust stack testing. Exhaust stack testing is problematic and expensive. And even if they test the exhaust stack the only thing that matters is the concentration in the tank, and combustor operators do not have any effective way to measure that.
 
The flow rates claimed for the IC Engines and the mini combustors need some basis in fact. That is, there should be agreement between the flow rate determined by the capacity of the unit and the amount of time it takes them to complete a given job.

The following is my understanding of how the flow rates for these devices should be calculated and general information about devices used for tank vapor control. First the advantages of the refrigeration system followed by combustors, engines, etc. The flow rate is very important because low flow rate contributes to channeling in the tank vapors. That results in the heavy tank vapor not coming out of the tank. Therefore the tank is not degassed.

The PURGIT Refrigerated Vapor Condenser System has these important advantages.
* Liquid nitrogen is used for the refrigerant. Nitrogen is not a pollutant and it is not flammable.
* Refrigerated systems can handle concentrated vapors and can ramp up quickly. The condensers perform very well with rich input vapor.
* They do not have hot surfaces.
* It actually processes the rated flow. That means a 650 cfm unit will process 650 cfm from the storage tank from minute #1 to the completion.
* PURGIT uses a turbine flow meter to accurately measure flow and volume, not pitot tubes. The flow meter register is proof of flow.
* Temperature indicators send signals to control valves which adjust the temperature of the coils to as low as -200oF. We set the temperature to match the cargo.
* Condensers do not make NOx or CO or other pollutant. They do not have exhaust stacks.
* The diesel engine on the blower can be replaced with an electric motor for additional emission reduction.
* Credits may be available for the NOx reduction.
* Detonation arresters protect the storage tank.
* The Hilliard Closed Loop configuration can add nitrogen gas to prevent an explosive mixture forming inside the storage tank by drawing in air.
* When degassing a storage tank, the tank vapors can be re-circulated. In that configuration, no emissions are emitted until the tank vapor space meets the rules for open venting. Even if the refrigeration system fails, vapors are circulated in the closed loop until the condition is fixed. There are no emissions.
* It recovers cargo, it does not destroy it. The recovered cargo can be recycled. Humidity already in the tank is the only possible contaminant. It handles chlorinated solvents without scrubbing and returns the condensate liquid for recycling as cargo.
* An empty 150' dia. tank with a floating roof at 6' has 106,000 cu/ft volume. At 650 cfm a PURGIT dual condenser will take 11 hours for 4 volumes.
* An empty 40' dia. tank with a floating roof at 7' has 8,800 cu/ft volume. At 650 cfm a PURGIT dual condenser will take 1 hour for 4 volumes. Set up and knockdown not included (add 2 hours or so).
* Tanks can be degassed without opening. With the refrigerated condenser unit, the cargo tank does not open for additional work or venting until the vapor space meets the owners or regulatory specification.
 
Enclosed Mini Combustors:
Mini combustors are simply enclosed flares. The contractors who use these mini-combustor units claim they are rated at 1,000 cfm or more.  But a mini-combustor can only handle 1,000 cfm with a very lean input concentration of tank vapor. The volume of the fire box in the mini-combustors limits their capacity.  The industry standard nominal heat load rating in a combustor firebox is 500 btu per cubic foot per minute. So if the mini-combustor firebox is 4ft x 4ft x 6ft, then the capacity is 48,000 btu/min. Gasoline has 1,389 btu/cu/ft, so in reality the mini-combustor can only do a paltry 35 cfm, not the claimed 1,000 cfm, from the storage tank. Contractors who claim they have a rate of 1,000 cfm should tell the other half of the truth which is that the vapors must be very lean, perhaps in the LEL range. High concentration will result in fire and smoke coming out of the stack. The volume of an empty 150' dia. storage tank with a floating roof at 6' is 106,000 scf. A mini combustor with a 4' x 4' x 6' firebox will take 202 hours to complete 4 volumes at 35 cfm.
 
The informed tank owner will ask the contractor for a data sheet from the combustor manufacturer that shows the design BTU rating and the combustion chamber volume. Then insist on a gas flow meter between the tank and the control device to accurately measure the inlet flow and volume. Since the volume is one of the regulation requirements for vapor control, then the flow should be measured by a meter during the tank cleaning at actual flow conditions.
 
Other disadvantages of the enclosed flares (mini-combustors):
* They are a source of ignition in the tank farm; they generate heat and sparks can come out of the exhaust stack. If you have ever seen one operating at night, you may have seen sparks coming out of the exhaust stack.
* They always create an explosive condition inside the storage tank by drawing in fresh air.
* They make NOx, CO and other pollutants. Every pound of VOC they burn becomes a product of combustion which makes greenhouse gas.
* They cannot handle chlorinated solvents without scrubbing the exhaust gas. Scrubbing makes hazardous waste.
* The state of Texas does not allow a flare to claim more than 98% efficiency without performance testing. Therefore they cannot claim more than 98% control in these combustors. Therefore 2% of the VOC may not burn and would be exhausted as a pollutant into the air.
* They require a source of supplemental fuel (propane) to start the combustor and when tank vapors are very lean it has to be added again to keep the firebox up to operating temperature. If the firebox isn't 1,200oF, all the VOC may not burn in the firebox and instead travel out of the stack.
 
Internal Combustion Engines:
ICE = internal combustion engine. PURGIT had 3 of these units in about 1997 which we tried to use for tank vapor control on the large tanks here in Texas. They operate with 2 Ford 460 cu/in gasoline engines. The engines start on propane and then once they are running, the carburetor suction is switched over to the storage tank. When the tank vapors get lean, propane is added to the carburetor to keep them going.
 
Operators claim that hundreds of cubic feet per minute are controlled with a dual engine system. This infers that the flow rate from the storage tank is in the hundreds of cu/ft/min. Flow through the engines is limited to the displacement in the cylinders. The engines are 460 cubic inches, so the actual flow is: 460 * the RPM (use 3,000) divided by 2 (four cycle engine) divided by 1,728 (to get to cubic feet) = 400 cfm total gross displacement. A stoichiometric fuel ratio that will pass an emission test is about 14.7 to 1. That is a 6.8% flammable mixture for gasoline. Therefore 400 cfm flow * 6.8% = 27.2 scfm tank vapors per engine. Yes, they may be able to 'make' the engine run richer, but fuel will be coming out of the exhaust stack.

The volume of an empty 150' dia. storage tank, with a floating roof height at 6', is 106,000 scf.  At 54.4 cfm (dual engine unit flow rate), it will take 129 hours for 4 volumes when the tank has 10% concentration by volume, which is not unusual. This is in line with meter readings from a flow meter PURGIT used on the units it operated. The operators will say they avoid the terrible performance by 'loading' the engine. What parameter do they change? Do they increase the RPMs or do they put more than 6.8% fuel in the engine?
 
The informed tank owner will ask for a detailed data sheet from the manufacturer, but since the manufacturers data seems to be confusing or unavailable, he will simply insist on emission test and a flow meter between the tank and the ICE. Why is a flow meter so important? Low flow rates are hard to measure by pitot tubes that seem to be on some devices. It takes a positive displacement or turbine flow meter to have real accuracy. Only those types of meters have registers that prove flow and volume. It is possible to ask the ICE owner to provide a Wide Band - Air Fuel Ratio Meter on the manifold of the engine. None of them have that now because it would show the puny efficiency.
The following is based on actual experience with the units:
* The ICE units cannot handle rapid changes in concentration. When the concentration changes rapidly, the engines flood or go lean with fuel and they shut off. So they are useless at truck loading racks or on pipelines. ICE units are complicated.
* The control system and engine is maintenance intensive.
* The units have numerous sources of ignition. Red hot exhaust manifolds, distributors, etc. No gasoline engine is approved to work in a classified area.
* Do they even have spark arresting mufflers?
* They will always create an explosive condition inside the storage tank by drawing in fresh air.
* They require a source of supplemental fuel (propane) to start the engine and when tank vapors are very lean it has to be added again to keep the engines running. 
* They cannot be used to generate electricity as a by-product because of the limited time in operation and low horsepower.
* They do make NOx, CO and other pollutants. In fact, if they are operated at stoichiometric fuel ratios other than 14.7 to 1 they make excess NOx and CO.
* Combustion devices like the ICE cannot handle chlorinated solvents without scrubbing the exhaust gas. Scrubbing makes hazardous waste.
 
Activated Carbon:
Activated carbon is a technique that is used occasionally. The handling of activated carbon is a nuisance. Carbon is delivered in large boxes and has to be transferred to the carbon/plenum container. The cost of carbon is between $2 to $5 per pound considering disposal, handling, containers, etc. It may take anywhere from 2 to 5 pounds of carbon to control 1 pound of VOC. PURGIT often recovers 2,000 pounds of chlorinated or hydrocarbon liquids from a tank. So the carbon cost may be $8,000 or more for one tank.
* Carbon works best with high velocity vapor streams with low concentration of VOC (under 2,000 ppm) according to the Texas Natural Resource Conservation Commission Chemical Section Engineers, December, 1994  They also say it works best with high molecular weight compounds rather than low molecular weight compounds. And some carbon systems should not be used to control reactive compounds such as ketones and alcohols. Moisture affects the adsorptive capacity of carbon. They also point out that the adsorptive wave front through the carbon bed is curved and breakthrough VOC concentrations may occur before the bed has reached its theoretical capacity.
* Carbon can develop bed fires, especially when multiple cargoes are used with one carbon bed. Detonation arresters are a necessity.
* There should be at least 2 carbon containers, a primary and a secondary. The primary filters the vapor and the secondary is the backup. The vapor stream between the containers is monitored and when a sensor indicates breakthrough, the first box is shut off and the second becomes primary and a new box is brought in, or the process is shut down and the first box is changed out.
* There is always the chance that some time in the future the discarded carbon will become a pollutant if it is put in a landfill.
* Regenerating carbon only transfers the problem since the VOC is stripped out by vacuum or heat. The collected liquid may be contaminated with other products in the carbon which may make it hazardous waste. If the stripped cargo is burned then NOx and CO are created.
 
Lean Oil Vapor Recovery System:
Lean oil is put in a tank and the cargo vapors are bubbled through it. The cargo tank vapors are absorbed into the lean oil and after a certain point, the lean oil will no longer absorb the cargo tank vapors. That makes rich oil, which must then, itself, have a vapor control system. 
* The issue with lean oil is that it takes a simple problem and compounds it, because now there are many hundreds of gallons of contaminated oil. The operators have the original vapor control problem and many hundreds of gallons of contaminated oil.
* The rich oil must be heated or vacuumed to drive out the VOC.
* If the lean oil is used for more than one cargo, then the problem is magnified by having multiple compounds to recover.
 
There is a place for each of these vapor control devices. No one device is perfect for every application. In our experience, the refrigerated system is superior in speed and efficiency for MOST storage tank degassing applications.

It is too bad this information is so complicated. If you have gotten this far, you are one of the few. The reason vapor control is so complicated is that the regulators make rules and then the regulators disappear. There seems to be no enforcement of the rules that are on the books or investigation to determine if a device is really controlling emissions or just adding to the air pollution problem.
 
Hank Hilliard
PURGIT is a registered trademark
copyright 2008

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Note 1: The rental unit we used is on a 40', 18 wheeler trailer and has a 50 Hp electric combustion air motor. The firebox on the rental unit isabout 1,500 cubic feet and the verified flow is 600 scfm.
Note 2: Gasoline has about 80 cubic feet of vapor in a gallon, so when we recover 500 gallons, we have reduced 40,000 cubic feet of tank vapor to liquid.