Methodological issues

Where caprolactam is produced from benzene, the main process, the benzene is hydrogenated to cyclohexane which is then oxidised to produce cyclohexanone (C6H10O). The classical route (Raschig process) and basic reaction equations for production from cyclohexanone are (Reimschuessel, 1977; p.83: Lowenheim and Moran, 1975; p. 201):

Oxidation of NH3 to NO/NO2 i

NH3 reacted with CO2/H2O to yield ammonium carbonate (NH4)2CO3

(NH4)2CO3 reacted with NO/NO2 (from NH3 oxidation) to yield ammonium nitrite (NH4NO2)

NH3 reacted with SO2/H2O to yield ammonium bisulphite (NH4HSO3)

NH4NO2 and NH4HSO3 reacted to yield hydroxylamine disulphonate (NOH(SO3NH4)2)

NOH(SO3NH4)2 hydrolised to yield hydroxylamine sulphate (NH2OH)2.H2SO4) and ammonium sulphate ((NH4)2SO4) i

Cyclohexanone reaction: C6H10O + 1/2(NH2OH)2.H2SO4 (+NH3 and H2SO4) ^ C6H10NOH + (NH4)2SO4 + H2O

Beckmann rearrangement: C6Hi0NOH (+H2SO4 and SO2) ^ C6HnNO.H2SO4 (+4NH3 and H2O) ^ C6H11NO + 2(NH4)2SO4

Lowenheim and Moran (1975; p. 202) summarise the Raschig production process as follows. Caprolactam is produced via Beckmann rearrangement (conversion of a ketone oxime into an amide, usually using sulphuric acid as a catalyst) by the addition of hydroxylamine sulphate to cyclohexanone. Hydroxylamine sulphate is produced from ammonium nitrate and sulphur dioxide. Ammonia gas and air are fed to a converter where ammonia is converted to hydroxylamine disulphonate by contacting it with ammonium carbonate and sulphur dioxide in series. Ammonium carbonate is produced by dissolving ammonia and carbon dioxide in water, and sulphur dioxide by burning sulphur. The disulphonate is hydrolysed to hydroxylamine sulphate and ammonium sulphate. The addition of hydroxylamine sulphate to cyclohexanone produces cyclohexanone oxime which is converted to caprolactam by the Beckmann rearrangement.

Production of caprolactam can give rise to emissions of nitrous oxide (N2O) from the ammonia oxidation step, emissions of CO2 from the ammonium carbonate step, emissions of sulphur dioxide (SO2) from the ammonium bisulphite step, and emissions of NMVOCs. Emissions of CO2, SO2 and NMVOCs from the conventional process are unlikely to be significant in well-managed plants. The main greenhouse gas to be accounted for from caprolactam production is N2O. Modified caprolactam production processes are primarily concerned with elimination of the high volumes of ammonium sulphate that are produced as a by-product of the conventional process (Reimschuessel, 1977; p.84). NH3 oxidation remains an integral part of all processes to obtain the NO/NO2 required.

CHOICE OF METHOD

Estimation of emissions of N2O from caprolactam production can be treated as analogous to estimation of emissions of N2O from nitric acid production. Both production processes involve an initial step of NH3 oxidation which is the source of N2O formation and emissions.

The choice of good practice method depends on national circumstances. The decision tree in Figure 3.4 describes good practice in adapting the methods to national circumstances. Emissions can be estimated from continuous emissions monitoring (CEM) where emissions are directly measured at all times, periodic emissions monitoring that is undertaken over a period(s) that is reflective of the usual pattern of operation of the plant to derive an emission factor that is multiplied by output to derive emissions, irregular sampling to derive an emission factor that is multiplied by output to derive emissions, or by multiplying a default emission factor by output.

Methods are classified according to the extent of plant-level data that are available. Both Tier 2 and Tier 3 are require plant-level activity data.

Tier 1 method

Emissions are estimated as follows:

Where:

En2o = N2O emissions, kg

EF = N2O emission factor (default), kg N2O/tonne caprolactam produced

CP = caprolactam production, tonnes

When applying the Tier 1 method it is good practice to assume that there is no abatement of N2O emissions and to use the highest default emission factor shown in Table 3.5.

Tier 2 method

Information on emissions arising from caprolactam production and control technologies is limited. Where plant-level information is not available, good practice provides default N2O generation factors as shown in Table 3.5. The default factors are based on N2O emissions from nitric acid plants because there is no information on caprolactam plants and the initial reaction step of oxidation of ammonia is similar for both processes. Good practice encourages the development of factors specific to caprolactam plants.

The number of caprolactam plants is relatively small (approximately 42 plants with around 19 plants using DSM (Stamicarbon) technology). It is unlikely that there are substantial variations in the N2O generation factors between plants. Where default values are used to estimate emissions from caprolactam production, it is good practice to ascertain the extent to which plant emissions vary according to type and to use an appropriate N2O generation factor.

The Tier 2 method uses plant-level production data disaggregated by technology type and default emission factors classified by technology type. Emissions are calculated as follows:

Equation 3.10 N2O emissions from caprolactam production - Tier 2

Where:

En2o = emissions of N2O, kg

EFi = N2O emission factor for technology type i, kg N2O/tonne caprolactam produced

CPi = caprolactam production from technology type i, tonnes

DFj = destruction factor for abatement technology type j, fraction

ASUF_j = abatement system utilisation factor for abatement technology type j, fraction

The basic equation for estimating N2O emissions includes additional terms that recognise current and the potential future use of N2O abatement technologies. The N2O destruction factor has to be multiplied by an abatement system utilisation factor in order to account for any down-time of the emission abatement equipment (i.e. time the equipment is not operating).

Where plant-level information is not available, good practice provides default N2O generation factors as shown in Table 3.5, Default Factors for Caprolactam Production, based on plant types classified by age. To achieve the highest accuracy, good practice is to apply Equation 3.10 at the plant-level taking into account N2O generation and destruction factors developed from plant-specific measurement data. In this case, the national total is equal to the sum of plant totals.

Tier 3 method - direct measurement

The Tier 3 method uses plant level production data and plant-level emission factors obtained from direct measurement of emissions. These may be derived from irregular sampling of emissions of N2O or periodic emissions monitoring of N2O undertaken over a period(s) that reflects the usual pattern of operation of the plant. Emissions can be derived using Equation 3.10.

Alternatively, the Tier 3 method uses the results of continuous emissions monitoring (CEM), although it is noted that most plants are unlikely to employ CEM due to the resource costs. Where CEM is employed, emissions can be estimated based on the sum of measured N2O emissions derived from the concentration of N2O in monitored emissions for each recorded monitoring interval.

CHOICE OF EMISSION FACTORS Tier 1 method

It is good practice to use the emission factor shown in Table 3.5 and to assume that there is no abatement of N2O emissions.

Tier 2 method

If plant-level factors are not available, it is good practice to use default factors. The Tier 2 method uses a default factor. Default values often represent midpoint or mean values of data sets (as determined by expert analysis). The extent to which they represent a specific plant's emission rate is unknown. This is especially true for caprolactam production where the value is based on high pressure nitric acid plants. Default factor in Table 3.5 should be used only in cases where plant-specific measurements are not available.

Tier 3 method

Plant measurements provide the most rigorous data for calculating net emissions (i.e., N2O generation and destruction factors). Monitoring N2O emissions from caprolactam production is practical because these are point sources and there are a finite number of production plants. Given currently available technology, instrumentation for sampling and monitoring emission rates do not limit precision or accuracy of the overall measurement. Usually sampling frequency and timing is sufficient to avoid systematic errors and to achieve the desired level of accuracy.

As a general rule, it is good practice to conduct sampling and analysis whenever a plant makes any significant process changes that would affect the generation rate of N2O, and sufficiently often otherwise to ensure that operating conditions are constant. In addition, plant operators should be consulted annually to determine the specific destruction technologies employed and confirm their use, since technologies may change over time. Precise measurement of the emissions rate and abatement efficiencies requires measurement of both the exit stream and the uncontrolled stream. Where measurement data are available only on the exit stream, good practice is to base emissions on these data. In this case, any available estimates of abatement efficiency should be provided only for information purposes and not used to calculate emissions.

Table 3.5

Default factor for caprolactam production

Production Process

N2O Emission Factor (kg N2O/tonne caprolactam)

Uncertainty

Raschig

9.0a

± 40%

a Based on high pressure plants for nitric acid production.

Source: Default Factors for Nitric Acid Production. (See Table 3.3 in this chapter.)

Figure 3.4 Decision tree for estimation of N2O emissions from caprolactam, glyoxal or glyoxylic acid production

Figure 3.4 Decision tree for estimation of N2O emissions from caprolactam, glyoxal or glyoxylic acid production

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

CHOICE OF ACTIVITY DATA

It is good practice to compile production data at a level of detail that allows the use of Tier 2 or Tier 3 method. Tier 1 method

The Tier 1 method requires data on national production of caprolactam. If national-level activity data are not available, information on production capacity can be used. It is good practice to multiply the total national production capacity by a capacity utilisation factor of 80 percent ± 20 percent (i.e., a range of 60-100 percent).

Tier 2 method

The Tier 2 method requires plant-level production data disaggregated by the age the plant. If additional information on technology type and abatement technology is available, it is good practice to collect this information and disaggregate production data according to the information obtained. That is, it is good practice to gather activity (production) data at a level of detail consistent with that of generation and destruction data. Where plant-level emission factors are used, good practice is to collect plant-level production data. Typical plant-level production data are accurate to ±2 percent due to the economic value of having accurate information.

Tier 3 method

The Tier 3 method require plant-level production data disaggregated by technology type when emissions estimates are derived using data from irregular or periodic sampling of emissions. It is good practice to collect activity (production) data at a level of detail consistent with that of any generation and destruction data. Although production is not used in the estimation of emissions where the estimate is based on CEM, these data should be collected and reported to ensure that changes in variables that influence emissions can be monitored over time. Typical plant-level production data are accurate to ±2 percent due to the economic value of having accurate information.

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment