Primary Reformer Energy Balance

The primary reformer is overall endothermic. The pre-processed gas entering the reformer is considered to be at 673 К (400°C), and as has been stated, the exit temperature is fixed at 1123 К (850°C).

An energy balance based on these temperatures is carried out to find the amount of fuel that will supply the energy requirements of the reactor. The analysis carried out here requires enthalpy values. These can be calculated in a number of ways with the outcomes for the fuel requirement being (as they should be!) within reasonable agreement.

In theory, we can carry out an appropriate energy balance over the primary reformer based on the mass balance figures obtained for the reformer.

It is self-evident that to do the energy balance, values of data relevant to energy transfer are required. Appendix 3 outlines the data that were available to carry out the energy balance. These data were enthalpy values for each of the species in the primary reformer from different sources. As has been previously pointed out and is repeated, the purpose of Appendix 3 is not to give a detailed presentation of the enthalpy property, but some basics are presented. The various data sources are reported. It was not considered relevant that differences in the data should be highlighted. The important element was that the data from different sources were in reasonable agreement and could be presented as giving the energies involved in the processes described. As will be seen in the energy balance carried out, the data given by Graham M Hampson (Private Communication) were used and the figures calculated are used as the basis in subsequent calculations. The relevant polynomial constants are given in Table A3.3 in Appendix 3.

To carry out a calculation for the relevant enthalpies, the input and output operational temperatures are required. Normally, if these are not quoted in the text, the values fixed can be found on the block diagram.

Once the appropriate values of enthalpy were available from the Hampson polynomials, the simple energy balance, based on the assumption that the reactor could be considered adiabatic, was carried out (Table 1.5):

TABLE 1.5

Primary Reformer Energy Balance

Input

(kmol)

Output

(kmol)

Input Specific Enthalpy (kj/kmol)

Input

Enthalpy (kJ)

Output Specific Enthalpy (k)/kmol)

Output Enthalpy (kJ)

CH4

94.68

21.6

-3.9715 + 04

-3.7602E + 06

-9.4027E + 03

-2.0310E + 05

c2H„

3

0.000

-4.4367 + 04

1.331E + 05

-

-

c,Hs

0.5

0.000

-4.8422 + 04

-2.421 IE+ 04

-

-

C-jEfio

0.4

0.000

-5.1880 + 04

-2.0752E + 04

-

-

N,

1.4

1.400

-2.0738 + 04

2.9033E + 04

3.3563E + 04

4.6988E + 04

H,0

363.3

251.1

-2.1600E + 05

-7.8473E + 07

-1.9799 E +05

-4.9715E + 07

CO

0

41.51

-

-

-7.9390E + 04

-3.2955E + 06

H,

0

266.52

-

-

3.4231E04

9.1233e + 06

о

p

0

35.5

-

-

-3.5034E05

-1.2437E + 07

Total input

-8.2508E + 07

Total output

-5.6716E + 07

Output-input

2.5792E + 07

For the purposes of calculating the fuel requirements, the value of 2.5792E + 07 kJ will be used.

Energy Supply for Primary Reformer by burning Natural Gas

Energy is provided by burning a suitable fuel in a furnace box through which the catalyst tubes pass. For the purposes of this study, the same natural gas as used for the feedstock will be employed. This will be assumed to enter the burner at 25°C. Combustion air is usually heated and will be supplied at 200°C, and flue gases will necessarily leave at temperatures higher than the reformer gas temperature, usually at about 950°C-1000°C. For the purposes of this calculation, we will use 950°C.

Combustion Mass Balance

In considering the combustion, the combustion stoichiometry needs to be considered. Certain estimates are included in this calculation. We will assume that the air is supplied with an excess of 5%. The furnace will be under vacuum so that air leakage of 10% into the furnace will be assumed; this air will be at a temperature of10°C.

The following stoichiometric equations will be assumed (the gas used will not have been desulphurised):

In analysing the combustion of the natural gas as an energy source, there are a number of calculated figures quoted. These figures are not difficult to obtain in themselves, but it is vital that each number is checked and understood so that the mass and energy balances make sense. The learning process depends on the reader doing this; otherwise, the labour involved in the calculations is wasted.

We can draw up the following table (Table 1.6) based on 100 kmol of natural gas and the stoichiometry of the combustion equations based on 100% burning:

TABLE 1.6

Product Amounts from Combustion of Natural Gas assuming 100% Conversion

Species

kmol in

Gas

o2

Required

co2

Produced

H20 Produced

so2

Produced

CH4

94.68

189.36

94.68

94.68x2=189.36

C2H6

3.0

10.50

2x3 = 6.00

3x3 = 9.00

C,HS

0.5

2.50

3x0.5=1.50

4x0.5 = 2.00

c4H10

0.4

2.60

4x0.4= 1.60

5x0.4 = 2.00

H2S

0.02

0.03

0.02

0.02

n2

1.4

0

Total

100.00

205.0

103.78

202.38

0.02

If we supply the air at 5% excess, then the extra amount of 02 supplied will be:

Thus, the О, supplied to the burner should be:

If we also take a leakage of air into the furnace based on the original 02 calculation and using the 10% leakage already stated, then we get a leakage of 02 into the furnace of:

We can take the total 02 based on 100 kmol of fuel as:

Taking the composition of the air as 21% v/v O, and 79% v/v N2, we can calculate the amount of N, as:

 
Source
< Prev   CONTENTS   Source   Next >