Collection of Emissions from Vessel Cleaning Operations

COLLECTION OF EMISSIONS

FROM VESSEL CLEANING OPERATIONS

by: Gary N. Lawrence, P.E.
Introduction:
For a variety of reasons, the emissions generated from the cleaning of marine vessels have long been ignored by environmental and other regulatory agencies, even though they represent many tons of volatile organic compound (VOC) emissions each year. With tighter regulations, such as the Clean Air Act of 1990 and closer work place scrutiny by the Occupational Safety and Health Administration (OSHA), uncontrolled marine emissions are becoming a more visible problem. Further, many companies have put into practice programs, such as the Chemical Manufacturers Association's "Responsible Care" program, which are designed to increase environmental considerations throughout refining and transportation of their products.
With this outlook in mind, Hank Hilliard then with a large barge cleaning operation in Houston, Texas, decided to pursue technologies which would allow them to capture and reduce emissions from the cleaning operation. The facility contracted my engineering firm to study the problem and develop recommendations.
The goal of the study was to develop a method to remove volatile organic compounds from barges during cleaning operations, while meeting the following parameters:
1. Minimize any purge gas costs
2. Utilize existing nozzles and other appurtenances of barges
3. Minimize dilution of the VOCs collected by the purge gas
4. Minimize additional time required by the process.
The attached information outlines the results of this study, and provides information concerning a purging system which safely, effectively and efficiently achieves all the project goals for which a patent has been applied.

Current Barge Cleaning Operations
Marine ships and barges are floating tanks utilized for the bulk transfer of gasoline, chemicals and many other products. Often these vessels continuously transfer a single, or related, product such as gasoline and are considered to be in "dedicated" service. Dedicated vessels are generally cleaned only when it is necessary, such as for inspections or repairs. Nondedicated vessels might carry any number of different products over a year's time. Often the chemicals from a previous load are not compatible with the next product, or the previous load would represent a contamination of the new load. In these cases, the vessel must be cleaned between product movements.
Vessels which are to be cleaned typically arrive with some residual liquid product (heal) which must be removed before the cleaning can begin. As identified by the American Waterways Shipyard Conference, the common procedure for cleaning barges is briefly outlined as follows.

Strip Liquid Free
During this procedure, cargo valves are opened and any bulk liquid is removed from the tank. Pipelines and deep wells are stripped if the vessel is outfitted with stripping lines. After stripping, the tanks may still have residual liquid, and the sumps may refill depending on the following factors:
1. The amount of rust in the tank, since rust traps and holds liquid
2. The amount of time between stripping and loading
3. The flatness of the cargo tank bottom plating.
When completed, the cargo pump, check valve, valve seats and stripping lines may still have liquid from the last cargo. Drip pans and attached cargo hoses should be stripped and/or drained.

Strip and Blow
Just as in stripping, the cargo valves are opened and any bulk liquid is removed from the tank. In addition, the cargo tanks, attached hoses, pump, check valve, main cargo pipeline, and deep well are ventilated by forcing air through each component. Drip pans are stripped, and the cargo tank is entered to inspect for any remaining liquid from the previous cargo.

Clean for a Marine Chemist's Certificate
This cleaning procedure is specifically for the purpose of obtaining a Marine Chemist's Certificate on the vessel. It does not necessarily clean the vessel for a change of cargo, and will vary according to the last cargo and the nature of the work to be done on the vessel. The Marine Chemist inspects the final work.

Cold Water Hand Wash
This procedure flushes water through the pipeline system, then force ventilates the cargo tanks before they are washed with a hand hose. After washing, the tanks will be stripped of any visible water. The pump and check valve will be inspected for cargo and water and will then be ventilated with forced air. Loose rust will be removed from the cargo tank. Attached cargo hoses will be rinsed, drained and ventilated, and drip pans will be stripped.

Cold Water Pressure Wash
The cargo compartments will be pressure washed with cold water by a mechanical tank washing machine, and then force ventilated with air. Cargo lines and pumps will be flushed. The tanks will then be stripped of any visible water. Cargo hoses will also be rinsed and drained, and drip pans will be stripped. Pipelines and any attached cargo hoses will be ventilated with forced air.

Hot Water Pressure Wash
The cargo compartments will be pressure washed with hot water by a mechanical tank washing machine, and then force ventilated with air. Cargo lines and pumps will be flushed. The tanks will then be stripped of any visible water. Cargo hoses will also be rinsed and drained, and drip pans will be stripped. Pipelines and any attached cargo hoses will be ventilated with forced air.

All the above procedures which use ventilation will normally emit the ventilating air directly to the atmosphere along with any volatile organic compounds (VOC) it may contain. These emissions may represent a health hazard to the workers and the surrounding general public, as well as contribute to growing atmospheric pollution problems.

Emissions From Barge Cleaning Operations
Emissions are created during the cleaning of barges when ventilating air is forced into the storage compartments and allowed to vent through open cargo hatches. The purpose of the ventilation process is to remove all the vapor existing in the storage compartment.
The exact amount of VOC in the compartment vapor space is open to some judgment. One segment reasons that, as the barge is unloaded, air or nitrogen is allowed to fill the space in the barge previously filled with liquid. It is believed this air does not completely saturate. This viewpoint is fairly accurate when considering gasoline. The residual liquid left after a gasoline barge is unloaded would vaporize the lighter gasoline components, but the heavier components of the gasoline would not vaporize. Experience with gasoline tanker trucks shows upper limits of 60% to 75% saturation is all that is to be expected.
However, when dealing with pure components such as benzene, the only limit to the amount of vaporization which can occur is the amount of liquid present and the amount of time allowed for the vaporization to occur. Since barges are subjected to considerable motion during transfer, the saturation process is improved. When barges are removed from service and sent for cleaning, they often have at least one day before they are cleaned. In general, the conditions are in place to allow a barge in chemical service to saturate the vapor space from the time it is unloaded until the time it is either next loaded or cleaned.
Data is available for two facilities operating in Houston, Texas. These facilities cleaned 436 and 1,434 barges respectively during the calendar year 1990. Most of these barges last hauled VOC generating compounds before the cleaning process. A wide variety of VOCs were emitted, ranging from typical gasoline to benzene and chlorinated compounds. Total emissions from the two facilities for 1990 were 146 tons of VOC from ventilation of barges only.
Chart One shows emissions from the ventilating of a single 10,000 barrel barge for benzene, acrylonitrile and gasoline. Each shows emissions based on winter and summer temperatures. It is interesting to note the emissions from one 10,000 barrel barge can be more than 1 ton.

Consequences of Emissions:
As previously stated, barges transport a wide variety of commodities. The great majority of these commodities represent VOC emissions to the atmosphere during the cleaning process. VOCs, when emitted to the atmosphere, mix with another atmospheric pollutant referred to as NOx, or nitrogen oxides. When these two compounds are mixed with oxygen and exposed to sunlight, the resulting reaction forms a complex mixture called SMOG.
Environmental Protection Agency studies have shown a direct link between the levels of smog in the atmosphere and the occurrence of bronchial problems.
Other chemicals such as acrylonitrile and benzene represent serious exposure hazards to personnel working around vapors generated by either chemical. If cleaning facilities are located in areas near public access or other businesses, emissions from acrylonitrile/benzene type compounds represent a health hazard to off site personnel.
Emissions from barge cleaning facilities have often been treated leniently by environmental agencies. This leniency was not out of favoritism but was due to the inability to safely collect and control the vapors. However, liquid loading facilities are safely recovering vapors now, and the technology utilized by these facilities can easily be modified to work with barge cleaning facilities. With safe collection now a reality, barge cleaning emissions are coming under closer scrutiny. In the State of Texas, for instance, the regulatory agencies can take action any time the "Health and Welfare" of its citizens are endangered.

Collection of vapors:
A key part of the evaluation was to determine if the collection of vapors could be done in a safe manner. Phoenix has installed and has in operation over a dozen systems which collect vapors from the loading of a variety of marine vessels. Their technology for the safe collection of barge cleaning emissions was adapted to provide a safe barge cleaning system.
The Phoenix modifications require specific detail to prevent damage to the barge from two major areas of concern. The first is over or under pressurization, and the second is prevention of detonation in the vapor collection system.
In the first case, pressure sensors are installed in the vapor collection header to alarm and shutdown the operation if pressures in the barge are approaching unsafe conditions. In addition, the purge gas injection system is designed to always maintain injection pressures within safe levels for barges and other marine vessels.
To prevent detonation, the vapors from the vessel are monitored as they are collected and enriched as required to maintain them outside the flammable limit of the chemical being recovered. In addition, United States Coast Guard accepted detonation arresters are used in the vapor collection header to stop any detonation which might occur.

Methods of Vapor Collection:
Several methods for either collection or combustion of the vapors were evaluated. Some methods, such as lean oil absorption, were ruled out initially since a lean oil which would work for a wide variety of chemicals is not available. The potential systems evaluated are as follows:
1. Carbon Adsorption
2. Refrigeration
3. Combustion.

Carbon Adsorption
Schematic One shows a brief outline of a typical carbon adsorption system. These systems have been used successfully for a number of years to collect vapors generated from gasoline and distillate loading. These systems are in successful use in marine loading applications.
The basic premise of the system is to pass a VOC laden vapor stream across a carbon bed. The carbon adsorbs the VOC from the vapor stream, and a relatively clean air stream is emitted from the top of the carbon bed. These systems typically utilize two carbon beds, so that as one is collecting VOC, the second can be regenerated.
During regeneration, the VOC is removed from the carbon, so the carbon can be used again to collect VOC. Most often the regeneration process utilizes vacuum to regenerate the carbon bed. A vacuum pump lowers the pressure inside the carbon bed and pulls the removed VOC out of the carbon bed. The vacuum pump discharges the VOC through a tower which contacts the vapor with an absorption medium. The VOCs are then collected into the liquid absorbent. Any unabsorbed VOC is recycled into the carbon bed which is processing vapors. In the evaluation, several areas of concern with carbon adsorption systems were identified.
Poor Efficiency at Low Concentration. If the barge is ventilated by air, the VOC reaching the carbon bed is diluted. As concentration lowers, the carbon bed efficiency drops and, in worst cases, an air stream could actually remove adsorbed VOC and emit it to the atmosphere.
Variety of Chemicals. Carbon does not work well with all VOCs. Some oxygenated compounds can create problems such as hot carbon. Also, it is just as difficult to find an absorbing medium for the regeneration tower as it is for lean oil absorption systems. Polymerizing chemicals would pose a plugging problem for the system.
Eventual Carbon Disposal. Even though the carbon may last many years, it must eventually be disposed of as a hazardous waste. Carbon adsorption systems may be a prime candidate for handling chlorinated products despite its draw backs.

Refrigeration Systems
Schematic Two shows a brief outline of a refrigeration system. Typically, the VOC laden air is brought into contact with cold cooling coils inside the refrigeration exchanger. Here, the VOC is cooled below its dew point and condenses. The liquid can then be collected and stored for later disposal. In the evaluation, several areas of concern with refrigeration systems were identified.
Poor Efficiency at Low Concentrations. Refrigeration systems are affected similarly to carbon adsorption systems. As the VOC becomes leaner, the temperature required for condensation becomes lower. This increases the operating cost or decreases the efficiency of the system.
Water Freezing Problems. Since the air in the vessel will be from ambient sources, it carries some amount of water. The water will freeze the cooling coils of the refrigeration system.
Variety of Chemicals. A wide variety of chemicals complicates operation of a refrigeration system. Materials of construction can be affected drastically by acidic or basic chemicals. Operating set points vary widely for different compounds. Polymerizing chemicals would also pose a plugging problem for the system.
Disposal of Recovered Liquid. The recovered chemical will most likely be contaminated with water. If a market does not exist for the condensate, it would most likely have to be disposed of as a hazardous waste. Storage before transfer may have to be controlled as a hazardous waste.

Combustion
Schematic Three shows a brief outline of a combustion system. Combustion systems can have either open or enclosed flames. The open system is less expensive generally, but the enclosed flame was determined more desirable for a system used in barge cleaning. The enclosed systems can be sampled and controlled to a slightly greater degree of efficiency.
Combustion is the most forgiving system when a wide variety of chemicals is to be collected. However, in order for combustion to be efficient, sufficient heat content must be maintained in the vapor stream to allow for complete burning. Therefore, the combustion system is also impacted by the dilution effect of the air used to ventilate the barge. As VOC decreases in the stream, a fuel source such as natural gas must be added to the vapor stream.
Each system has its merits, depending on several factors to be analyzed at each cleaning facility. However, a common problem exists for each. Typically, 10 volumes of air are required to ventilate a barge. This dilution effect reduces the VOC content drastically and results in reduced operating efficiency for any type VOC collection or destruction system.

Solving the Dilution Problem:
After analyzing the vapor collection systems which can be reasonably used with barge cleaning, it became apparent that a method of reducing the effects from dilution would have to be developed. Without the ability to maintain the VOCs in as rich a state as possible, the operating economics were too expensive and recovery efficiencies were too low. Several methods of reducing the dilution were studied.
Before the evaluation, a method of injection compatible with existing openings in barges had to be developed. A typical barge will have several openings of various sizes in the top of the deck. In addition, there will be a liquid loading line which will include a pipe which extends near the bottom of the vessel compartment. This drop pipe is designed to minimize splashing when loading liquid on the barge. Some barges are outfitted with vapor collection headers which are mounted at the top of the vessel compartment.
Based on the layout, it was decided the best method for injection was to introduce purge gas into the liquid header, introducing the purge into the bottom of the barge. The collected VOC would be pushed out the top of the barge compartment into a vapor header if the barge is outfitted for vapor collection. For barges not outfitted with a vapor header, the VOC would be collected from one of the existing manways or other openings in the top of the compartment.

Nitrogen Purging
Nitrogen purging has been used for many years in the gas freeing of marine vessels in high vapor pressure service such as propylene. Nitrogen purging is a practical alternative which can lower the dilution factor to approximately 2 to 1 in the worst case. Nitrogen purging would initially be -injected into a barge compartment, and the early emissions from the compartment would be only the vapors which were in the compartment. Since nitrogen is relatively light, it will mix as it rises into the VOC, and a mixed interface of nitrogen and VOC is created. Even though nitrogen reduces the dilution of vapors, it does not completely eliminate dilution. After the purge, the nitrogen would be blown to the atmosphere using the same ventilating procedure and air blowers currently used for removing the vapors. The use of nitrogen injected to the top of the vessel where a heavy gas, such as chlorinated solvent, exists is a very likely scenario. The heavy gas would be removed by the fixed loading lines.

Water Filling
Water would be a good medium for pushing the VOC out of the barge. As it is introduced in the bottom, it forms a level plug which pushes only the VOC laden vapors out of the compartment. Very little mixing between water and vapors would occur, and the vapors flowing to the collection device would be undiluted. Problems with water begin after the vapors have been removed. The water must be pumped out of the barge, which could take several hours. It then must be stored and treated for future reuse, all the while being treated as a hazardous waste.

Carbon Dioxide Purging
By using carbon dioxide (CO2) rather than nitrogen or water, some of the benefits of both are achieved. The carbon dioxide, because of its heavy molecular weight, should layer into the bottom of the vessel much the same as water. This should allow the use of a single tank volume to complete a compartment purge. After purging is finished, the CO2 can be blown to the atmosphere in a relatively short period of time. CO2 is also approximately 50% less expensive per cubic foot than nitrogen.
Blowing of CO2 may cause some concern to people who are interested in the "green house effect", which is considered to be affected by CO2 build-up in the atmosphere. However, the CO2 used in this process is basically recycled. The CO2 which is purchased is removed from plant emissions which would have been emitted to the air. By using the recycled CO2 in this manner, it is still emitted to the air, but it is put to a positive use first.
After analyzing the alternatives, a decision was made to pursue the use of carbon dioxide in the purging of barges before cleaning. Nitrogen purging is an alternative means in the ease of incompatibility problems with CO2 or heavy gasses in the cargo tanks.

Developing the Carbon Dioxide Purge system:
With the selection of carbon dioxide (CO₂) as the primary purge medium of choice, the next step was to determine the effectiveness of the system. The desired goals of the test were:
1. To remove the VOC without dilution
2. To use only one compartment volume to achieve an effective purge
The design of the system utilized the CO2 at low pressure to avoid damaging the vessel from overpressure. Since CO2 tends to form solid dry ice as pressure is dropped, a means of preheating the CO2 had to be developed in order to avoid solids plugging problems. An advantage of CO2 is that it is dense and therefore heavy and would tend to settle to the bottom of the vessel. Therefore, the preheating needed to be controlled to maintain the CO2 as cold as possible without causing any structural damage to the vessel.
The final injection method was a two step process which would drop the carbon dioxide from liquid storage pressures to an intermediate pressure. The CO2 was then heated to a point where it could be dropped to approximately 2 psig and a temperature of 0o F. (See Schematic Four.) At this point it would be introduced into the barge at a controlled flow rate. The rate of flow was designed to avoid turbulent flow in the liquid piping downcomer.
The initial test of the system was done using a barge which had been cleaned and was completely filled with air. A barge filled with air was selected for safety reasons, since the air could be safely discharged to the atmosphere without impacting the personnel involved in the test or creating pollution. In addition, the reliability and availability of oxygen analyzers is greater than for hydrocarbon analyzers.
The CO2 was introduced into the barge while two oxygen analyzers were monitoring the oxygen concentration in the barge. One oxygen analyzer monitored continuously, while the second was a batch sampler. The two analyzers were placed on opposing sides of a single compartment. The barge used for the test had a 10,000 barrel capacity roughly divided between three tanks. Each tank was purged separately in three independent tests. The first two tests injected CO2 at the same rate through the liquid fill line. To provide a better picture of the effect of mixing from one side of the vessel to the other, the position of the oxygen analyzers was switched in the two tests. The third test injected CO2 through a large plastic pipe inserted through a 24 inch manway. In each test, the equivalent volume of the compartment was to be the maximum amount of CO2 injected. In actuality, slightly less than one compartment volume was required to lower the oxygen concentration to the levels desired for the test. In each test, the first oxygen samples were taken two feet off the bottom of the tank, and then the probes were raised two feet. The probes remained in place until the analyzers showed a 2% to 3% oxygen concentration.
If the injection method and CO2 worked, it was expected to see a sharp drop from 20.9% oxygen to a lower value. As the probes were raised, the concentration was expected to increase and then decrease as the vapor was pushed upward. The sharp drop of the wave front is seen in Tests One and Two. However, Test Three did not show the same sharp drop. The Test Three injection method was deleted from further consideration.
Based on the results of the air tests, it was decided the CO2 injection method was valid, and further tests with VOC laden vapors would be conducted.

Test Results with Acrylonitrile Vapors
Over a period of two weeks, three separate barges which had been in acrylonitrile service were purged using CO2 injection through the liquid loading lines. The vapors were collected from the vapor header of the barges and sent to an enclosed combustor. The client who provided the barges for cleaning wanted the carbon dioxide to be used to lower the oxygen concentration to 50% of the lower flammability limit. The injection system was able to do this, but it took longer than simply removing the bulk of the vapors and then ventilating to atmosphere to reduce the mixture below combustible limits or to safe levels for personnel entry. The testing was set up in the same manner as the tests on the air filled barge. However, the only hydrocarbon analyzer used was a combustibles meter set for flammability limits read-outs. The initial concentration of acrylonitrile in the barge was saturated.
As stated, the injection method was able to lower the oxygen concentration to less than 10 ppm by volume. However, some system problems were uncovered.

Stripped Liquids
The barge must be stripped of free liquids as much as possible. Residual liquid left in the barge can re-saturate the vapors. This is only a problem during applications where very low ppm levels. of hydrocarbon are desired during the purge. A normal system which ventilates after the CO2 purge can drive off the remaining hydrocarbons. Good liquid stripping does enhance the operation.

Distribution
The barges in the acrylonitrile tests were built with side by side compartments. The initial system was not sized to be able to purge all compartments simultaneously. Therefore, only two compartments at a time were purged. As the concentration was monitored in the two compartments, it became obvious the distribution was not equal. Use of liquid line block valves allowed for the distribution to be equalized.

The PURGIT system:
As a result of the testing, a joint patent application held by Henry T. Hilliard, Jr., Jerry Roach, PE. and Gary N. Lawrence, PE. has been submitted to the United States Patent Office.
The concept of the PURGIT system is to use CO2 or other gasses when appropriate to force vapors from vessel compartments for collection. The PURGIT system has the unique ability to provide these vapors to a vapor collection or destruction system undiluted by the purge gas. The PURGIT system can be used in the purge of a wide variety of vessels, from marine barges to shore tanks, trucks and rail cars.
Depending on the length of purge, the VOC level can be reduced to any level desired. The most effective approach is to use a single tank volume of the PURGIT system which will remove 95% to 98% of the vapor contained in the tank. After the PURGIT system purge, the carbon dioxide and any remaining quantity of vapor would be ventilated using forced air. After this step, the vessel would be safe for personnel entry. Lower levels of emissions can be achieved effectively and efficiently with the PURGIT system when compared to existing methods of purging, but are not as cost effective as the two step approach. In another option, the barges would be washed before the CO2 is ventilated to the atmosphere. They could then be purged again to further reduce emissions.

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