Perform a preliminary design evaluation for a plant to produce 30,000 te/yr of Maleic Anhydride (MA), C4H2O3, from a liquid feedstock containing 95% n-butane.

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CET PART IIA EXERCISE 6

Process Synthesis

25th February 2003


Introduction

The objective of the exercise is to perform a preliminary design evaluation for a plant to produce 30,000 te/yr of Maleic Anhydride (MA), C4H2O3, from a liquid feedstock containing 95% n-butane. The remaining 5% of the feed is iso-butane, which is taken to form carbon dioxide instantaneously under the reactor conditions. The reactor must be kept above 100 ºC everywhere to prevent condensation on the catalyst. The reactor operates around 400 ºC and 2.5 bara.

Reaction Scheme

The following reactions will take place in the reactor:

        C4H10 + 3.5O2  C4H2O3 + 4H2O

        C4H10 + 6.5O2  4CO2 + 5H2O

The selectivity of generating Maleic Anhydride, is given by

        S = 0.70 – 0.3X2                                                           [1]

where X is the conversion of n-butane

Figure 1 below shows the distribution of products.

        

Figure 1: Diagram showing distribution of products

Using the information in Figure 1, the required flowrate of n-butane in the feed can be found, together with the compositions and flowrates of the product stream. The minimum air flowrate is defined by the fact that the concentration of n-butane in air must be below its lower explosion limit of 1.7 mol %.


Basic Flowsheet

The basic flowsheet to be considered is as follows:

Figure 2: Basic Flowsheet Design

Equation [1] shows that selectivity decreases with conversion, which suggests that the optimal reactor may operate at a low conversion. A low conversion would mean there is a fair amount of n-butane in the product stream, which is why the recycle stream is added.

Since one of the main design choices is the conversion in the reactor, an estimation of how costs vary with conversion will be calculated to help in making this decision. The costs will be of the raw materials (butane feed) and utility costs. Utilities include steam, cooling water and electricity. The income for the process will come from the sale of the maleic anhydride product and steam, which is generated from excess heat.

Balance over Reactor

The product and reactant streams will depend on the required production rate of maleic anhydride and the conversion in the reactor. Sample calculations for a conversion of 0.5, together with summaries for different conversions can be found in Appendix A.

Reactor Cooling

Both the main reaction and the side reactions are highly exothermic, which means that the reactor needs significant cooling to maintain the reactor at a temperature of 400 ºC.

Since the reactants enter the reactor at just over 100 ºC, energy will be required to heat up the reactants to 400 ºC. The enthalpy of reaction for the main reaction is given as –1230 kJ/mol. The enthalpy of reaction for the side combustion reactions can be found from the enthalpies of formation for the reactants and products. Unfortunately, these are usually only given as 298 K (25 ºC), which means that the calculated enthalpy of reaction will be that at 298 K. This means that enthalpy changes for the cooling for reactants from 400 ºC to 25 ºC need to be considered, as well the heating of products to 400 ºC. This is done according to Hess’ cycle.

        

The calculations for the duty required in cooling the reactor can be found in Appendix B. A summary of the calculations is shown in Table 1.

Table 1: Cooling/Heating Duty for the reactor

The table above shows that at conversion lower than 0.4, the reactor requires a net heat input to maintain a constant temperature. This occurs because at low conversions, the recycle stream and reactor feed stream are much larger. This has the effect of increasing the amount of air required to keep the butane below its lower explosion limit.

Separation System

Since the behaviour of the separation system is unknown, its heating/cooling requirements cannot be calculated. However, since the composition and flowrates of the stream entering the separation is known, the flows and compositions out of the separation can be estimated after making some assumptions. The main assumption is that the separation is perfect, and the various fractions are removed as pure components. This is a fairly valid assumption to make after looking at the boiling points of the components. These are shown in Table 2.  

Table 2: Boiling Points at 2.5 bara

As Table 2 shows, there is at least a temperature of 102 ºC between adjacent components, which would support the assumption of perfect separations. A more accurate analysis would have to consider that the water might react with the maleic anhydride, meaning a perfect separation would not be achieved using only a sequence of condensers or distillation columns.

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For the butane to be separated from the other waste gases, it will have to be liquefied. This will mean it will have to be cooled to below 26 ºC. This would then mean it would then require vaporising before feeding back to the reactor inlet.

Heaters and Compressors

Since the reactor is operating at 2.5 bara, the air taken from the atmosphere at 1 bara will need to be compressed, which would require the need for a compressor. The recycled butane stream would require heating and vaporising as discussed above, which means a heater would also ...

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