DE VILLARREAL
5. LA INTE DE TEODORO
DI-ISOCYANATES WORKS
1 INTRODUCTION
(i) Di-isocyanates works are defined as “works in which di-isocyanates (DIC) are made, or partly polymerised, or used in the manufacture of expanded or rigid plastics”. DIC works are preferably, not recommended for PCFC.
(ii) The air pollution control regulations require that the Owner of a works shall use the best practicable means (a) to prevent the emission of noxious or offensive substances, either directly or indirectly;
and (b) to render harmless and inoffensive such substances as are necessarily discharged. Smoke, grit, dust, vapour, aerosol and gases are included in the list of noxious or offensive substances.
(iii) In di-isocyanates works there is often a wide range of operations where di-isocyanates in admixture with other substances are used, for example, in two-pack and non-drip paint, for making expanded plastics for bed mattresses, pillows, cushions, etc., for direct spraying onto construction work as a means of protection and insulation, for making rigid articles including panels for construction work, etc. The most commonly used di-isocyanates are TDI = toluene di-isocyanate (2.4 and 2.6 isomers), and MDI = 4.4’ - diphenyl methane di-isocyanate.
(iv) The characteristic hazard associated with TDI is not just the primary chest irritation following exposure to high vapour concentration, where “high” relates to the Maximum Exposure Limit (MEL) of 0.02 mg/m3 as - NCO (isocyanate) in air, but the liability to sensitisation. It is well established that a proportion of people working with TDI can become sensitised to it and thereafter be unable to tolerate exposure to unmeasurably low concentrations. The symptoms exhibited are similar to acute asthma. Medical opinion is that sensitisation can be caused either by brief exposure to vapour concentrations above the MEL, or by repeated exposure to sub-MEL Concentrations. Industrial sensitisation seems to affect only a small proportion of workers. Within the factories, careful medical screening and supervision, and high standards of hygiene control by ventilation are needed. With this medical picture, a wide margin of safety for the general public is essential.
The problems of MDI and the remaining di-isocyanates in industrial use, excepting :TDI, arise from dust or droplet mists and no significant public health hazards are normally encountered.
2 SAMPLING, ASSESSMENT OF EMISSIONS AND MONITORING
(i) The reference test method for TDI and MDI shall be based on the Marcali method, for testing occupational atmospheres, but modified to take account of the higher concentrations and different circumstances when sampling chimney emissions. An introduction to this method is given below.
Other methods may be agreed with the Control Authority for control purposes if shown to give results comparable with the reference method.
3 INTRODCUTION TO METHOD OF TEST FOR ISOCYANATES
(i) In 1983, a new MEL control limit was introduced which applies to all substances containing the free isocyanate group, - NCO. The control limit as -NCO is 0.02 mg/m3 for an 8-hour time weighted average and 0.07 mg/m3 for a 10-minute time weighted average. The analytical methods to assess potential exposure by these two standards differ. To date, only one method has been developed that is capable of measuring the concentration of all isocyanate-containing species in the atmosphere to meet the new control limit. This method employs high performance liquid chromatography (HPLC) with dual electrochemical (EC) and ultraviolet (UV) detection, which is very sophisticated instrumentation outside the scope of most isocyanates users.
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Also, there are distinct advantages in having the capability of on-site measurement, such as is afforded by the Marcali method in use previously for specific isocyanates. Accordingly, it is proposed to use the modified Marcali method for measuring the concentrations of specific isocyanates in ducts or chimneys discharging process gases to air. If the resulting concentrations at ground-level are to be measured, it is recommended that HPLC be used, as giving a more sensitive and precise assessment of the potential hazard, if any.
4 EMISSION LIMITS
(i) The aim shall be to achieve a concentration of total di-isocyanate in discharges to air not exceeding 0.1 ppm (equivalent to 0.70 mg/m3 for TDI and 1.0 mg/m3 for MDI). The important aspect is to have a good suction at all points of the processing equipment, before discharge to air at a suitable height, and after treatment if necessary.
5 CONTROL OF EMISSIONS
(i) All di-isocyanate processes shall be operated so as efficiently to contain all emissions from the process and to minimize losses of di-isocyanate to air.
(ii) The containment and extraction arrangements for each process emission shall be agreed with the Authority.
(iii) The spraying of articles in the works shall take place under cover in well-ventilated booths and droplets shall be eliminated before discharge to air.
(iv) Droplets and aerosols from the “blowing-out” of injection heads by compressed air or by solvents shall be contained and ventilated to the main extraction system.
(v) Block foam is allowed to mature for 24 hours after production, during which time TDI is released in small amounts as vapour. The foam should be matured either in the open air, or in a well-ventilated storage space under cover.
(vi) In the case of large plants, it may be necessary to install waste gas, alkali scrubbers, depending upon the mass rate of release of total di-isocyanates from the site. “Large” is expressed in terms of the volume of discharge V from all di-isocyanate sources (cubic meters per hour) and the concentration T of total di-isocyanates in the emissions, expressed as cyanate (-NCO)(in units of milligrams per cubic meter), and is defined as works in which the product VT exceeds 50,000.
6 HEIGHTS OF DISCHARGE
(i) Vent heights for the residual di-isocyanates emissions shall be assessed as follows:- A basic height is calculated from the formula
H = (0.0086 VT)1/2
where H = chimney height in metres,
V = volume of emission, cubic meters per hour, T = concentration of total di-isocyanates, expressed
as cyanate, - NCO, milligrams per cubic meter.
The product VT is summed over all di-isocyanate emissions from the works.
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(i) The value of T used in the determination above shall be based upon the highest expected emission for all the process formulations in general use, or anticipated to be in general use.
(ii) The height so obtained shall then be adjusted to allow for the height of neighbouring buildings and for any special local circumstances, and subjected where appropriate to the minimum height requirement.
(iii) The minimum discharge height for all small works shall normally be 5 meters above the roof ridge level, and for all other works shall normally be 15 meters, or 8 meters above roof ridge level, whichever is the higher. In this context “small” means works in which the emission volume (V) is less than 8,500 m3/h and the concentration of total di-isocyanate does not exceed 0.1 mg/m3.
(iv) Vent efflux velocities shall not be less than 10 m/s except in the case of vents following a wet scrubbing system, when final efflux velocities shall not exceed 9 m/s. The emission from any such scrubbing system shall be free from droplets at all times.
(v) When a new plant is being designed, it is important that emissions take place from the minimum number of chimneys or vents. Unless there are sound technical reasons to support it, a multiplicity of discharges shall be avoided.
7 STORAGE AND HANDLING OF DI-ISOCYANATES
(i) Where the bulk import of di-isocyanates is concerned, storage of di-isocyanates should wherever practicable be in fixed tanks fitted with closed circuit, inter-vessel vents covering all tankers, storages and intermediate process vessels. The system should only be allowed to breathe through silica gel traps or be vented to the main process vent stack. The storage room should be air conditioned to maintain a temperature not exceeding 20oC.
(ii) Where storage and handling of di-isocyanates is in drums,
a. Drums shall be stored in an air-conditioned room where the temperature does not exceed 20oC.
b. Drums shall be inspected by a responsible person before storage or movement of drums to ensure lid tightness and absence of leaks.
c. Drums shall be discharged by submerged pump to fixed intermediate tanks vented through silica gel traps. Portable containers shall not be used except for special purposes and by agreement with the Control Authority.
d. Other noxious or offensive substances used in the process, e.g. methylene chloride, shall be treated with
e. the same care and attention as the di-isocyanates
f. Empty drums shall be decontaminated by an agreed process, or otherwise checked for lid tightness by a responsible person. Drums shall be disposed of in a manner, which is harmless and inoffensive.
g. Adequate arrangements to deal with di-isocyanate spillage shall be agreed with the h. authorised representative of the Authority.
8 GENERAL
(i) Best practicable means applies not only to the control of emissions, but also to efficient maintenance, proper use of equipment and the installation of duplicate equipment, where necessary, to maintain continuity of production and avoid stoppages which lead to emissions to air. Operators must be properly trained, instructed and supervised to minimize emissions.
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(ii) Some ancillary processes such as cutting, abrading and disintegration of expanded plastics produce finely divided particulate matter and these processes must be extracted by fans to suitable filters.
(iii) A high standard of housekeeping shall be maintained throughout the works.
(iv) Malfunctioning or breakdowns leading to abnormal emissions shall be dealt with promptly; in serious cases the plant shall be shut down for repairs. The Authority shall be informed when abnormal emissions occur.
For more information please contact Environment, Health and Safety – Free Zone (EHS-FZ) Department on Tel# 8040275, Fax # 8817023 or P.O.Box17000, Dubai.
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