Methanol is one of the most important raw materials in the manufacturing of several valuable chemicals such as formaldehyde, acetic acid, methyl tert-butyl ether (MTBE), dimethyl terephthalate, methyl chloride, methyl amines and many other chemicals. Methanol can also be used as fuel in modern Polymer Electrolyte Membrane Fuel Cells to generate power for portable devices, such as laptops, as a replacement of batteries.
In the most common current manufacturing process for methanol synthesis a copper-zinc-oxide catalyst is used to convert synthesis gas (a mixture of CO, CO2, and H2) at temperatures and pressures in the range of 200 – 300°C and 5 – 10 MPa, respectively. Typically the synthesis gas is produced from steam reforming of natural gas (mainly CH4) followed by water gas shift reaction. The product gas leaving the reformer contains water vapour which must be removed to reduce the amount gas that must be compressed (for high pressure methanol synthesis reaction) and minimise the impact of water on the catalyst for the subsequent conversion of CO to methanol. The dried synthesis gas then compressed and cooled before introducing it into the “Converter Loop” where synthesis gas is converted to methanol.
In this assignment you will be simulating the Converter Loop with a view to understand how control the temperature in a highly exothermic reaction and to analyse a process with recycle and bypass streams to estimate the conversion of synthesis gas and the yield of methanol.
Converter Loop Process Description
The synthesis gas coming out of the reformer unit followed by heat recovery and compression is available at 100°C and 7.5 MPa. This gas enters the converter loop, where it is mixed with a recycle gas at 35°C and 7.5 MPa. The converter loop consists of a recycle compressor, whose primary purpose is to provide the pressure head required for the gas to flow through the system– the methanol synthesis reactor (MSR), heat exchangers, a methanol condenser, and a gas-liquid separator (flash drum). The mixture that is to become the feed to the MSR consists of recycle gas and fresh synthesis gas. After the recycle gas and fresh synthesis gas are blended, the mixture flows through the recycle compressor and then is heated to 130°C by a partially cooled product stream leaving the MSR (the partial cooling is described later). The recycle compressor is sized to circulate the recycle stream at a rate that is 7.8 times the rate at which fresh synthesis gas is fed to the converter loop. The blended recycle-fresh feed mixture leaving the heat exchanger following the compressor is split into two streams: one, containing 30% of the mixture, is sent to another heat exchanger where its temperature is raised to 220°C by a fraction of the product stream from MSR and injected into the first stage of MSR; the remaining 70%, which is still at 130°C, is injected at various location along the MSR.
The key reactions occurring in the MSR are –
The product gas leaving the MSR is partially cooled by being split into two streams, each of which passes through a heat exchanger before being recombined; one is used to heat the feed stream to the fresh stage of the MSR to 220°C, and the other passes through a waste-heat recovery unit. The recombination product stream is cooled further in an air-cooled exchanger before being brought to 35°C, a liquid consisting of the condensed methanol purification unit. The uncondensed gases are split, with a portion being purged from the system and the remainder forming the recycle gas that is blended with fresh synthesis gas to form the feed to the recycle compressor. The condensed crude methanol is sent to a recovery unit which typically contains two distillation columns (out of battery limit). The process described above is illustrated in Figure 1.
Simulate the process illustrated for the converter loop, taking a basis of 100.0 kmol/h of fresh feed and perform analyses as described in the Part A and B below.
Figure 1: Flowchart for methanol synthesis reactor (Converter Loop).
Perform an analysis of the converter loop by determining
- The composition and flow rate of the purge stream,
- The composition and flow rate of liquid methanol stream, and
- Single pass and overall conversion of CO and CO2.
Use a basis of 100 kmol/h of fresh feed into the converter loop and consider the two scenarios given below to answer the questions (i – iii) in this part.
- (a) Assume that the feed is 5 mole% CH4, 25% CO, 5% CO2 and the remainder H2.
- (b) To determine the impact of water vapour in the fresh feed on the methanol synthesis consider a scenario where the heat recovery and compression cycle before the converter loop is not able to remove all the water in the synthesis gas exiting the reformer unit. Revise the feed composition to 2.87 mole% CH4, 19.71% CO, 0.35% H2O, 3.59% CO2 and the remainder H2.
RESULTS & DISCUSSION (5-10pages):
– report and discuss the specific features requested and answer all questions. Include graphs or tables which are relevant in the text – do not include superfluous detail. Focus on interpretation. You can put detailed information in the appendix.
REFERENCES & NOMENCLATURE:
– correct referencing style in text and in the list
– nomenclature list
QUALITY OF FIGURES:
– labelling and scale of axes
– data points and trend lines
– clear informative captions and appropriate presentation
– appropriate presentation
– title page
– table of contents
– headings and subheadings
– numbering of equations, figures, tables, graphs
– general organisation