Volatile Organic Compounds (VOCs) are organic compounds that have high vapour pressure and low solubility in water.
Volatile Organic Compounds are emitted as gases from many solids/liquids organic chemicals such as household chemicals like paints, varnishes, pesticides, lacquers, building and furnishing materials, and wax.
All these contain organic solvents as do many cleaning, disinfecting, cosmetic, degreasing, hobby products, hydraulic fluids, glues, adhesives and petroleum fuels.
Storage and use of these materials release VOCs which enlist a number of chemicals that may have short- and long-term adverse impact on the health of humans.
What are the definitions of VOC as per different countries?
VOCs have diverse definitions and many countries have classified them accordingly to regulate air pollution and entry of foreign chemicals in the atmosphere.
Classification of VOC according to these countries:
Canada: It classifies VOCs as organic chemicals having boiling points in the range between 50 to 250-degree centigrade. (122 to 482degree Fahrenheit)
European Union: Here, VOCs are chemicals with an initial boiling point up to 250 degrees measured at a pressure of 1 bar (101.325 kPa).
China: China defines VOC as “originated from automobiles, industrial production and civilian use, burning of all types of fuels, storage and transportation of oils, fitment finish, coating for furniture and machines, cooking oil fume and fine particles (PM 2.5),” and similar sources.
India: In 1981 and amended in 1987, the Central Pollution Control Board of India released the Air Prevention and Control Pollution Act which does not differentiate between VOCs and other pollutants which constitutes NOx, SO2, PM10 and Suspended Particulate Matter (SPM).
How VOC is generated and what are the impacts?
In the maritime industry, VOCs are mainly generated in oil and chemical tankers, which are involved in loading, transporting and unloading organic chemical substances through ports.
Volatile Organic Compounds are generated when cargo splashes in the piping system of the ships from the source to the cargo tanks, and from evaporation from the surface of oil or chemicals stored in oil cargo tanks during and after loading.
VOCs are generated by a build-up of positive pressure, through vaporization, evaporation and boiling.
During loading of crude oil and chemical tankers and the successive transit, VOCs may be generated and released to the atmosphere with the inert gases.
The amount of VOCs that have evolved into the inert gas tank atmosphere is associated with the oil’s volatility; the tendency of a substance to vaporize.
The cargo tank atmosphere contains traces of VOCs from the previous cargo and during loading the rising cargo level results in pressure increase of the cargo tank and these compounds are lastly vented into the atmosphere through specially allocated vent pipes onboard.
VOCs generation during transit increases complexities in maintaining constant pressure in the tankers and the discharge of VOCs leads to environmental pollution.
VOCs emission also leads to loss of a great amount of energy which can be utilized as fuel for propelling the ship. The environmental impact of these emissions is hazardous and disastrous as the emitted gas consists of a range of hydrocarbons from methane to higher (typically C6+).
The non-methane fractions, known as NMVOC, react in the presence of sunlight with nitrous oxides and create a toxic ground-level ozone and smog layer, which has detrimental environmental effects on vegetation, and on human health, particularly on the eyes and lungs.
Regulations by the International Maritime Organization for Volatile Organic Compounds:
As regulating VOC emissions are necessary both for environmental protection and to mitigate light end product losses, IMO published MEPC.1/Circ.860 (27 July 2009) which is a supplement to resolution MEPC.185(89) providing technical and operational guidelines to assist in the development of VOC Management Plans that come into effect on 1 July 2010.
The circular states, “The purpose of VOC management plan is to ensure that the operation of a tanker, to which regulation 15 of MARPOL Annex VI applies, prevents or minimizes VOC emissions to the extent possible.”
Vapour Emission Control Systems (VECS) MSC/ Circular585 dated 16 April 1992- MARPOL Annexure VI/15 states the standards and requirements for the design, construction and operation of vapour collection systems on tankers with vapour processing unit on board to meet the vapour collection and processing design requirements for a shoreside terminal as per the satisfaction of the country/ Flag state of the ship’s registry.
Emission control methods of VOC:
There are two generic methods of VOC recovery, known as active and passive methods of recovery.
Considering VOC active recovery, the emission systems typically include a compression step followed by condensation, absorption and adsorption while passive VOC emission systems use vapour balanced loading/unloading with VOC as blanket gas.
The following are the methods, technologies for emission control of VOC:
Reduction in Volatility– The volatility of the cargo in the tankers can be reduced by removing volatile components from the crude oil at the offshore platform before loading. This is not feasible in most of the cases due to the cost and availability of such equipment.
Thermal Oxidation: It is the most common method of controlling VOCs during offshore loading. VOC laden air is combusted using enclosed flares to catalytic oxidisers with heat recovery. The risk of safety and combustion emissions are mitigated by flame/denotation arrestors, inerting, enrichment or dilution.
Absorption: Absorption by chilled or cryogenic liquid is a common method and the most popular, wherein the chilled liquid absorbent is fed concurrently to the flow of hydrocarbon vapours through a packed column, thereby dissolving hydrocarbons in the absorbent and removing it from the air/vapour mixture.
Adsorption: Organic Molecules are absorbed on to carbon and permanent gases such as air or CO2 pass through the carbon bed and are vented to the atmosphere. Two carbon beds with an automatic switch are recommended to maintain uninterrupted operation. Depending on the carbon beds sizes, the nature of adsorbent material and the degree of regeneration; the efficiency of this method can be up to 99%.
Membrane Separation: This technique uses a liquid compressor and a semi-permeable membrane to separate organic molecules from air/vapour mixtures, with the membrane more permeable to organic compounds than to inorganic gases. Organic molecules after selective migration through the membrane are removed by a vacuum pump and fed into the inlet side of the compressor.
Cryogenic Condensation: It involves passing the vented gas/vapour through a nitrogen-cooled condenser and this method can reach over 99% efficiency. Lowering the temperature results in fewer concentrations of VOC that exits and drains into the collection tank from which it can be recycled and recovered for use.
Cargo Pipeline Pressure control: It is designed to prevent change in properties by balancing the pressure during crude oil transfer, reducing the pressure build-up and ultimately reducing the VOC generation. The technology used to prevent the change in properties doesn’t use power and thereby no production of harmful gases such as CO2 and NOx.
Sequential Transfer of Tank Atmosphere: This procedure moves cargo tank atmosphere (VOC and inert gas) sequentially between the cargo tanks before venting into the atmosphere by displacement of gases during loading cargo. This process is achieved by the installation of a special gas piping arrangement through the series of cargo tanks designed specifically for ease of opening and closing at different points to account for different loading programs. This process is used to avoid the excessive venting of VOC gases to the atmosphere by venting clean inert gases from the empty cargo tank several tanks away from the loading tanks.
VOCON Procedure: With the help of both constant pressure valves and an automated release valve, this procedure allows tank pressures of the cargo tanks to be maintained at higher mean level with smaller, more accurate automated tank venting releases. This ensures reduced cargo loss.
VOC Recovery Systems by different Companies:
Wärtsilä’s VOC recovery system patented GasReformer unit, typically used onboard FSUs and shuttle tankers, in combination with our dual-fuel engine technology, gives the highest efficiency and flexibility providing environmental benefits with reduced VOC emissions, and economic benefits with fuel cost reductions of up to 40%.
The recovery of VOC is achieved by introducing tank vent gas into the VOC recovery system, and then the heavier hydrocarbon fractions of the gas are separated using the condensation process.
The lighter hydrocarbon fractions being non-condensable are fed to a power generating module; meaning that the VOC recovery is 100% and VOC emissions are entirely eliminated. The separated heavier fractions are liquefied from the VOC unit and stored in a pressurized storage tank.
LVOC is a light hydrocarbon fuel that can be utilized as a clean fuel in power-generating modules and inert gas generator units.
The Wärtsilä VOC recovery system has been successfully utilized onboard several shuttle tankers and floating storage units (FSU) in the North Sea.
Commercial, Chemical & Development Company
CDCC has developed a VOC recovery system with technical inputs from Clariant, Germany aimed for low concentration vapour recovery using adsorption principles, where recovery of solvent can be fetched between 95% to 99.9%
The economics of such a technology is dependent on the vapour stream composition as well as the desired percentage recovery. This VOC recovery system can be used to recover recovery and removal of SO2, H2S, Mercaptans, Oil field VRU, Production of CNG from biogas, Flue gas desulphurization, Clause tail gas treatment.
It can also be used as an extractive system for breaking of azeotropes such as Ethyl alcohol – water, Isopropyl alcohol – water, Toluene-methanol – water, Ethanol – Ethyl Acetate – Water.
How crew Manage VOC during transit and Crude oil washing operation?
In the VOC Management Plan, a person is designated to be responsible for implementing the plan and the person in charge may assign an appropriate person to carry relevant tasks.
Procedures followed during VOC management:
Ship-specific procedures are provided in the plan to address the VOC emissions during the following operations:
- Carriage of the relevant cargo (transit)
- Crude oil washing.
VOC EMISSION CONTROL DURING TRANSIT
Cargo Tank Pressure Control (VOCON Procedure):
In order to minimize the excessive pressure build-up, atmospheric pollution and cargo losses, both venting and vapour loss have to be monitored. The designated person should ensure the following activities during laden passage:
Conditioning Monitoring and Maintenance of P/V valves- The cargo tank P/V valves should be manually inspected for correct operation prior to any loading by using the check lifts facility. The P/V valve flame screens should be visually inspected at least once every month and checked/cleaned every three months. Prior to this, P/V beaker liquid level should be checked too within the set limits.
Condition monitoring of Gaskets for Hatches and Piping- The operation of loading should be executed in “closed” mode with all cargo tank hatches and openings shut. Local gauging and sampling should also be done through the vapour locks or the fixed-ullaging equipment.
Inert Gas Topping up procedures- The procedures listed in vessel’s IGS manual should be followed while topping up the pressure with inert gas of not more than 5% oxygen content and the cargo pressure should be monitor and recorded on an hourly basis.
Partially Filled Tanks- Partially filled tanks should be kept at a minimum during and after loading operations.
Loading Sequence and Rate: The assigned person should ensure monitoring and controlling of the loading rate and cargo tank pressure in accordance with the relevant loading plan.
VOC EMISSION CONTROL DURING CRUDE OIL WASHING
The procedures of the vessel’s approved COW manual should be complied with during all COW operations and the relevant records should be kept in accordance with ISM procedures.
Duration of washing: VOC emissions can be reduced by shortening the duration of washing. Hence during the crude oil washing operation, the designated person ensures the proper monitoring of the operations and decrease the time spent to the possible limit.
Closed Cycle Crude Oil Washing should be done.
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