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Analysis of degassing silos

Removal of residual monomers during LDPE production
Analysis of degassing silos

During the production of LDPE the plastic pellets contain up to several thou-sand ppm of ethylene after underwater pelletisation, depending on the production process. In the past damage has repeatedly occurred to degassing silos as a result of small silo fires and even explosions. For this reason Coperion Waeschle has collaborated with several customers in analysing the operation of degassing silos and determining the causes of such damage.

Dr. Bernhard Stark, Dr. Hans Hoppe

In order to remove residual monomers from the plastic pellets, the pellets are pneumatically conveyed downstream of the underwater pelletisation section from the centrifugal drier to degassing silos. Purging air is supplied into the degassing silos at one or more application points on the silo cone in order to dilute and wash out residual monomers, such as ethylene, which are discharged from the plastic pellets due to the production processes involved.
The aim of gas purging is to prevent the formation of highly inflammable or explosive gas compounds in the silo, i.e. silo operation is well below the lower explosion limit (LEL). Melted plastic pellet agglomerates are frequently found in the product. These agglomerates can be the first indication of critical operation of a degassing silo.
Diffusion of residual monomers
The diffusion of residual monomers (e.g. ethylene) from pellets depends on the following parameters:
  • The residual monomer content in the pellets: The higher the residual monomer content of the pellets, the greater the quantity of residual monomers which is emitted at the beginning of the diffusion process. The decisive parameters influencing the solution balance of ethylene in LDPE are the pressure and the temperature of the melt; if errors occur during the process, the residual monomer content in the pellets can increase considerably.
  • The temperature of the pellets: An increase in temperature causes the quantity of degassed residual monomers to increase considerably (Arrhenius equation).
  • The pellet diameter: The smaller the pellet (shorter diffusion path and greater specific surface), the greater the quantity of residual monomers which is degassed at the beginning of the diffusion process.
The greater the initial quantity of degassed residual monomers, the greater the danger that explosive gas compounds will form locally in the pellets which have been filled into the silo. The diffusion of residual monomers from plastic pellets can be approximately reconstructed by examining the diffusion from a sphere with an equi-valent diameter. Unsteady diffusion for a sphere is derived in the book „The Mathematics of Diffusion“ by J. Crank (1975). Figure 1 shows, as an example, the decrease in the ethylene content of LDPE pellets with a diameter of 3.5 mm at a product temperature of 60 °C over a period of time. The ethylene initially degasses from the plastic pellets extremely quickly. Since the capacities of production plants, and therefore the bulk flows requiring conveying and the corresponding degassed residual mono-mer quantities, are steadily increasing, the correct design and calculation of the degassing silos requires special attention.
Lower explosion limit (LEL)
During combustion or explosion of ethylene the follow chemical reaction occurs in case of complete combustion:
C2H4 + 3 O2 Ý 2 CO2 +2 H2O
The stoichiometric ratio is therefore 1 mol ethylene to 3 mol oxygen. Air contains approx. 21% oxygen, i.e. the stoichiometric content of ethylene in air is approximately 6.5%. The lower explosion limit (LEL) of ethylene in air is 2.3%, i.e. 100% LEL corresponds to 2.3% of ethylene in air. Of this lower explosion limit, only an additional volume of approximately 20 to 30% LEL is permitted for the safe operating range of a degassing silo, i.e. a safety factor of 3 to 5 is provided, for example in order to take irregular gas distribution of the purging air in the silo into account. However, a regular flow of gas through the tipped bulk solid is not ensured in all cases, depending on the filling level of the silo, as the purging gas always follows the path of least resistance and therefore tends to flow along the silo wall as it takes the shortest route through the bulk solid pellets.
The exact operating limits and emergency measures, for example if gas purging fails, are normally stated by the licenser.
Minimum ignition energy
The supposedly wide safety span to the lower explosion limit does, however, have a second origin – the minimum ignition energy of ethylene in air is only 60 to 180 µJ. The electrostatic discharge from tipped bulk solids is in this case completely sufficient as an ignition source for explosive gas compounds. Electrostatic discharge from the tipped bulk solid occurs when plastic pellets are pneumatically conveyed into a silo. The electrostatic charge cannot be discharged quickly enough and accumulates. The electrical discharge takes place through the bulk solid surface. However, the purging gas also exits at the bulk solid surface with the maximum ethylene content and can ignite if the lower explosion level is reached.
The maximum equivalent energy of the electrostatic discharge of tipped bulk solids according to M. Glor and B. Maurer is:
WAE [mJ] = 5.22 * D3.36 [m] * d1.462 [mm]
This estimation applies to silo diameters of D = 0.5 to 3 m and particle sizes of d = 0.1 to 3 mm. With a silo diameter of D = 3 m and a particle size of d = 3 mm, the equivalent energy is 36 mJ during electrostatic discharge of tipped bulk solids. This energy is fully adequate to ignite an inflammable mixture of ethylene and air, as the ignition energy is almost 2 to 3 powers of ten higher than the minimum ignition energy.
Degassing process in the silo
It is not sufficient to only calculate the required purging air quantity on the basis of the overall data of the degassing silo. The calculation of the purging air quantity is basically correct if the above-mentioned parameters influencing degassing of ethylene and the bulk solid flow with which the silo is filled are taken into account; however, excessive ethylene concentrations can occur locally, especially in the silo cone, as the purging gas flow is normally supplied through several application points at varying heights on the silo cone.
At the beginning of the filling operation of the degassing silo, only part of the entire purging gas flow is consequently available and this part flow is blown into the gravity pipe below the silo cone. The purging gas volume which is normally blown into the gravity pipe is relatively low due to the small cross-sectional area.
The main volume of purging gas which is blown into the silo through distribution rings has no diluting effect on the ethylene which degasses in the tipped bulk solid; the maximum permissible ethylene limit, therefore, can be exceeded locally very quickly.
Coperion Waeschle has developed its own calculation program for post-calculation of existing degassing silos and to permit optimum calculation and design of degassing silos in new plants with regard to explosion prevention.
Example of an existing plant
The calculation method is described below taking an existing plant as an example. During operation of this plant local fires and explosions occurred in the cone of the degassing silos.
The ethylene content of the vented air is determined in the degassing silos and the LEL shown in percent (Fig. 3). When the conveying system is switched over to another degassing silo, the measured LEL value increases sharply. The reason for this is deactivation of the conveying air. The conveying air lowers the LEL value measured with the analyser (dilution effect).
The ethylene concentration (LEL in %) measured in the vented air was re-calculated during filling and during degassing: the dilution effect of the conveying air was not included in calculation. The initial ethylene content in the plastics pellets was taken as 1200 ppm in order to obtain the maximum ethylene content of 18% LEL at the end of the filling operation. A particle diameter of 3.0 mm was assumed as the spherical diameter of the plastic pellets.
The calculation reveals somewhat more rapid degassing during the time after filling. This can be due to various sources of error, for example
  • The filling capacity is incorrect
  • The purging gas volume is incorrect
  • The assumed plastic pellet diameter is too small
  • The assumed diffusion law is incorrect (see above)
Furthermore, the pellets start to degas during conveying to the degassing silo. This means that the diffusion process in the de-gassing silo is slightly slower, as the ethylene has already been discharged from the outer layers of the pellet during conveying. It also signifies, however, that the result obtained by calculation produces a faster de-gassing rate than that which actually occurs at the beginning of the filling operation of the degassing silo. The above calculation is therefore on the safe side.
The degassing silo was run with two application points for purging air. Part of the purging air was blown into the gravity pipe between the rotary valve and the silo outlet; considerably more purging gas was blown into the silo cone through two circular gas lines in the silo cone.
This example demonstrates how quickly the lower explosion limit LEL is reached during the initial silo filling phase. The assumed filling capacity is 25 t/h. Approximately 750 kg of product are filled in 110 s. After 110 s the product filling level in the silo cone reaches the first ring for purging air distribution. Subsequently, during silo filling, the purging air which is blown through the first distribution ring contributes to degassing. The ethylene contained in the product between the gravity pipe and the first distribution ring is therefore diluted (Fig. 5). At the beginning of the silo filling operation the LEL is exceeded below the first distribution ring, as indicated in Fig. 5, which shows the progression of the ethylene concentration throughout the filling time of the silo. This example illustrates very clearly that the gas distribution in the silo cone is also an essential factor in preventing the lower explosion limit being exceeded both locally and in terms of time. If fans are used for pressure generation, it can be difficult to calculate the design of the purging air distribution correctly, as the characteristic curve of the fan is sensitive to pressure fluctuations. The pressures in the gas distribution pipes are, however, also dependent on the filling level of the plastic pellets in the silo. The degassing concept with fans is therefore particularly endangered, especially if the fans are calculated in the upper range of the pressure rise. If incorrect assumptions are made for the calculation (e.g. underestimation of the bulk density in the silo), this can have fatal consequences for the supposedly safe operation of the degassing silo.
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