MEK 생산 공정 설계 단계의 경제성 고려

박민환, 박선원

한국과학기술원 화공과

Consideration of the Economics for the Design of MEK process

Minhwan Park, Sunwon Park

Department of Chemical Engineering, KAIST

Abstracts

The purpose of chemical processes is not to make chemicals: The purpose is to make money. The chemical process which products specific chemicals has lots of process structures. Although there are alternative process, the most profitable process must be existed. With economic aspect, this may be determined by criteria of cash flow and economic evaluation. The best methods of assessing the profitability of alternatives are based on projections of the cash flows during the project life. In order to find the most alternatives, in this paper, MEK (Methyl Ethyl Ketone) process design problem is considered. MEK, or 2-butanone is an important commercial chemical that is produced in large quantities for a wide variety of processes and products. The economic evaluation for these two type processes has been applied.

Introduction

There are three basic methods for the manufacture of MEK. These are synthesis from refinery gases, dehydrogenation from 2-butanol and selective oxidation of 2-butanol. Of the different processes, it has been found that dehydrogenation of 2-butanol is the most economical and this process has been chosen for design in this paper. But, in this process there are two type combination of units. First one has H2 scrubber, reactor, flash, dehydration column and purification column and the other one has units of first one and TCE separator which recovers TCE from  solvent and extractor, in addition. Each one has recover flow from bottom product of MEK purification column. For this economic estimation of these process, cashflow and cost calculation is needed.

Background Knowledge

It must be known that cash-flow plays an important role in design of process and how the capital cost and the working capital is calculated. Firstly, the cash-flow of project is considered.

Figure 1 shows the cash-flow pattern for a typical project. The cash flow is a cumulative cash flow. Consider curve 1 in figure 1. From the start of the project at A, cash is spent without any immediate return. The early stages of the project consist of development, design, and other preliminary work, which causes the cumulative curve to dip to B. This is followed by the main phase of capital investment and the curve drops more steeply to C. Working capital is spent to commission the process between C and D. Production starts at D, where revenue from sales begins. Initially, the rate of production is likely to be below design conditions until full production is achieved at E. At F, the cumulative cash flow is again zero. This is the project break-even point. Toward the end of the project life, at G, the net rate of cash flow may decrease due to, for example, increasing maintenance costs, a fall in the market price for the product, etc.

Figure  Cash-flow pattern for a typical project.

Ultimately, the process might be permanently shut down or given a major revamp. This marks the end of the project, H. If the process is shut down, working capital is recovered, and there may be salvage value, which would create a final cash inflow at the end of the project.

Payback time: Payback time is the time that elapses from the start of the project, A, to the break-even point, F. The shorter the payback time, the more attractive is the project.

Net present value(NPV): Since money can be invested to earn interest, money received now has a greater present value than money received at some time in the future. The net present value of a project is the sum of the present values of each individual cash flow. Time is taken into account by discounting the annual cash flow ACF with the rate of interest to obtain the annual discounted cash flow ADCF. Thus, at the end of year n,

[eqn: rm A_DCFn ``=`` A_CFn OVER {(`1`+`iti`)^n}]

The sum of the annual discounted cash flows over n years[eqn: rm Sigma ``A_DCF] is known as the net present value (NPV) of the project. The value of NPV is, of course, directly dependent on the choice of the fractional interest rate i.

Discounted cash-flow rate of return(DCFRR) is defined as the discount rate i which makes the NPV of a project zero. The value of i given by this equation may be found graphically or by trial and error. The higher the value of the DCFRR, the more attractive it is. The minimum acceptable value of the DCFRR is the market interest rate. If the DCFRR is lower than market interest rate, it would be better to put money in the bank. For a DCFRR value greater than this, the project will show a profit; for a lesser value, it will show a loss.

Net present value measures profit but does not indicate how efficiently capital is being used. DCFRR measures how efficiently capital is being used but gives no indication of how large the profits will be. If the goal is to maximize profit, NPV is used. If the supply of capital is restricted (which is usual), DCFRR is used to decide which projects will use the capital most efficiently.

Second, the Guthrie's method is used to calculate the capital cost. This method considers the bare module cost and the base cost. The bare module cost indicates update factor and the base cost consists of one of pressure vessel, process equipment and module factor.

Process description

In this paper, MEK process is considered in order to evaluate the economic criteria. Firstly, MEK is a low-boiling, chemically stable compound also known as 2-butanone. MEK is a flammable, colorless liquid possessing a typical ketonic odor. It has very good solvent properties, a fast evaporation rate, and is miscible with most organic solvents. Commercially, MEK is predominantly produced by the catalytic dehydrogenation of SBA in the vapor phase over zinc oxide or brass. The two types of process is dealt.

Figure 2 shows that the SBA feed containing 0.1 mol% water is mixed at a fixed feedrate with a recycle stream from the purification column. The combined liquid stream is fed to a hydrogen scrubber where the SBA scrubs residual MEK from the hydrogen byproduct stream subject to the maximum MEK flowrate. The SBA feed is then pumped up to the reaction pressure and heated to the reaction temperature via a heat exchanger. Once at the reaction temperature, the highly endothermic reaction takes place in the isothermal catalytic reactor. The mixture of MEK, H2 and unconverted SBA is cooled further and then sent to a separator where the hydrogen is flashed off. Hydrogen is further scrubbed, and the liquid phase of the separator is fed to a dehydration column where water over 99 mol% of feed water into the column is removed as the phase of H2O-MEK azeotrope. MEK of at least 99 mol% purity is produced as the top product of the purification column. The bottom product is completely recycled.

Figure  Flowsheet of the MEK process A

Figure  Flowsheet of the MEK process B

Figure 3 shows other units, in addition to figure 2., which are explained as followed. The effluent liquor discharged from the gas absorption column will be treated with TCE to extract the MEK and 2-butanol. The extract phase containing the MEK and alcohol will be distilled to recover the products and the solvent will be recycled to the extraction column. All the MEK cannot be extracted from the aqueous solution, so TCE is required to extract MEK in the extractor.

Result

The process for the manufacture of MEK consists of dehydrogenating 2-butanol continuously in a reactor at 400℃ to 500℃ at atmospheric pressure. 90.0% of the butyl alcohol can bee converted to MEK and the feasibility of separating the MEK from the reaction products and using the exhaust gases as a fuel, or separating the MEK, recovering and recycling the unconverted alcohol, drying the hydrogen and using this as fuel must be considered. It is suffice to state that separation of the unchanged alcohol for recycle is the most economical. The feedrate of 1.4×103 kg/hr 2-butanol is considered. The plant life is 20 years.

Table  Result of the index of estimation

	Process A	Process B
Payback time	4.26 year	5.12 year
NPV	（8,710,000	（9,458,000
DCFRR	0.31	0.26

Conclusions

For the estimation of the alternatives between different project, payback time, NPV and DCFRR is considered. Table 1 shows that the process A has faster payback time than one of the process B. It means that the process A recovers the capital cost earlier than the process B does. But, the Process B may make more money at the end of the project than the process B with NPV. The Process uses the money efficiently, in view of the DCFRR.

Predicting future cash flows for a project is extremely difficult with many uncertainties, including the project life. However, providing that consistent assumptions are made, projections of cash flows can be used to choose between competing projects.

Acknowledgment

This work was partially supported by the Brain Korea 21 project.

References

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McKetta, J.J., "Methyl Ethyl Ketone", Encyclopedia of Chemical Processing and Design, Vol. 30, pp. 32-50, 1989

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