Front End Engineering Design 1 for Vinyl Acetate Monomer (VAM) Plant Development

CHEN 426 - 502

Group 10

Samir Sandarusi Ricci Seguban Cody Shoop

Zachary Stamm Craig Tilley

Bradley Tomes

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Table of Contents

Abstract……………………………………………………………………………………. 2

Introduction………………………………………………………………………………... 2

Problem Statement……………………………………………………………………….. 2

Acetaldehyde……………………………………………………………………………... 3

Acetic Anhydride………………………………………………………………………….. 5

Acetic Acid……………………………………………………………………………….... 9

Acetylene………………………………………………………………………………….. 14

Ethane……………………………………………………………………………………... 18

Ethylene…………………………………………………………………………………… 20

Conclusion………………………………………………………………………………… 23

References………………………………………………………………………………... 24

Chemical Price References……………………………………………………………… 26

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Abstract The team has been tasked with designing a 1000 tonne/day vinyl acetate monomer

chemical plant. To develop the most cost effective and profitable design, six different feedstocks were evaluated for cost. The six feedstocks evaluated are acetaldehyde, acetic anhydride, acetic acid, acetylene, ethane, and ethylene. For each feedstock, the metric for inspecting sales and reactants (MISR), annual net (after tax) profit, and return on investment (ROI) were estimated and then compared to each other. Almost all feedstocks had MISRs of greater than one, which indicates that their process will be profitable. Feedstocks such as acetylene, acetaldehyde, and acetic anhydride, only had MISRs greater than one for high recoveries or in certain geographical locations. Ethane and ethylene had, by far, the greatest MISRs, reporting values of 4.12 and 1.68 respectively. Only four of the chemical pathways provided positive after tax profit estimates, and these were: acetic acid reacting with acetaldehyde, ethane, ethylene, and acetic acid reacting with hydrogen. However the acetic acid reacting with acetic anhydride pathway is only profitable if the facility is built in China. These four pathways were also the only pathways to produce positive ROIs; all between the values of 0.1 and 0.3. Introduction

The overarching goal of this project is to research, design, and build a vinyl acetate monomer (VAM) chemical plant that produces a minimum of 1000 tonnes of 99.9% purity product. The goal of this specific paper is to determine the most economically favorable avenues to pursue for this project. To do this, the economics of a total of six different feedstocks, ethane, ethylene, acetic acid, acetaldehyde, acetic anhydride, and acetylene, along with each of their specific pathways, were researched. Specifically, the metric for inspecting sales and reactants (MISR), capital expenditures (CAPEX), operating expenditures (OPEX), return on investment (ROI), as well as a process description and block flow diagrams (BFD) were researched. The results of our research are listed below, and are organized by feedstock. Problem Statement

The goal of this project is to research and design a 1000 tonnes per day VAM plant. The VAM produced needs to be at 99.9 wt% purity and have an annual capacity of 330,000 tonnes. Through simulation and optimization, we hope to design a plant that’s not only technically sound but also economically feasible.

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Approach

Acetaldehyde

Acetaldehyde (C2H4O) is an important feedstock and intermediate in the production of VAM. It is known to produce VAM by reacting with acetic anhydride to make ethylidene diacetate.1 Syngas-based production of VAM is a potential candidate because of its lower material cost compared to processes like the ethylene oxidative acetoxylation route. The main issue with these pathways are that they involve the expensive recycle of acetic acid through a carbonylation reactor. One way to mitigate this issue was to instead hydrogenate acetic acid to provide the acetaldehyde needed in another step.2 The chemical pathway is as follows:

Dimethyl Ether + 2 Carbon Monoxide → Acetic Anhydride Acetic Anhydride + Acetaldehyde → VAM + Acetic Acid

Acetic Acid + Hydrogen → Acetaldehyde + Water

Dimethyl ether (C2H6O) is reacted with carbon monoxide (CO) to form acetic anhydride (C4H6O3). Acetic anhydride and acetaldehyde, through reactive distillation, produce VAM and acetic acid (CH3COOH). Reactive distillation is beneficial in this reaction because it drives equilibrium toward VAM and acetic acid, and uses one distillation column. Recycled catalyst solution and acetic anhydride is fed to the middle of the column, while acetaldehyde is fed at the bottom of the column. Since the catalyst causes the acetic anhydride stream to be increasingly rich, when the acetaldehyde rises in the column, the formation of products will be favored.2

The hydrogenation of acetic acid generates the acetaldehyde that can be used in the

distillation. This reaction is endothermic, but can proceed directly or by a ketene route. The direct hydrogenation of acetic acid can reasonably work using a catalyst containing Fe2O3 and Pd. Conditions such as temperature, pressure, and the hydrogen amount greatly affect the performance of this catalyst. In addition, the reaction will need to compete with the reaction of acetic acid to acetone. Studies found that the highest acetaldehyde selectivity of the catalyst was at a 7:1 ratio of hydrogen and acetic acid, a pressure of 18 atm, and a temperature of 300℃. 2 The block flow diagram of this pathway can be seen in Figure 1.

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Figure 1: Block flow diagram of acetaldehyde pathway.

The mass balance of this process is based on the production of 1000 tonnes of VAM per

day. The flows for VAM, dimethyl ether, carbon monoxide, and acetaldehyde were calculated using reaction stoichiometry. They were calculated for ideal recovery (100%), 90% recovery/theoretical yield, and 70% recovery/theoretical yield (Table 1).

Table 1: Flow rates of acetaldehyde pathway

Flows (kilotonnes/year) for a Specific Recovery/Yield

Compound 100% 90% 70%

Dimethyl Ether 177 196 252

Carbon Monoxide 215 239 307

Acetaldehyde 169 188 241

As seen in Table 2, the process is not economically viable since no MISR values are

greater than 1 and in the ideal case, it breaks even.

Table 2: MISR calculation for acetaldehyde pathway

MISR Value for a Specific Recovery/Yield

100% 90% 70%

1.00 0.900 0.700

The fixed capital investment (FCI), also known as the capital cost (CAPEX), was

calculated assuming a turnover ratio of one. These CAPEX values are very rough estimates

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since they are equal to the annual sales for that specific recovery/yield. The operating cost (OPEX) was calculated assuming raw materials equal 70% of the total OPEX. These calculations are shown in Table 3.

Table 3: CAPEX and OPEX calculations for acetaldehyde pathway

Economic Analysis ($MM) for a Specific Recovery/Yield

Parameter 100% 90% 70%

CAPEX (FCI) 413 413 413

OPEX 589 655 842

The return on investment (ROI) of this particular pathway was also investigated. The

annual depreciation was assumed to be 10% of the annual income, since the recovery period of a property is 10 years on average. The total cost investment was also assumed to be 85% of the fixed capital investment. Lastly, the tax rate was assumed to be 20%. These assumptions resulted in a ROI of approximately -0.274 (Table 4).

Table 4: ROI evaluation of acetaldehyde pathway

Annual Income ($MM)

Annual Depreciation

($MM)

Total Cost Investment

($MM)

Tax Rate Annual Net Profit ($MM)

ROI

413 41.3 485 20% -133 -0.274

Overall, the negative ROI of this pathway and the lack of applicability to industry rules

this process out as a candidate for further investigation. Acetic Anhydride

Acetic anhydride ((CH3CO)2O) was a common feedstock for the production of VAM in the

United States until the late 1960’s, and may still be used in smaller VAM plants in countries such as China, India, and Mexico.3 The reason for this is that acetic anhydride has been either banned or become a controlled substance in many countries due to the fact that acetic anhydride is used as the major precursor for heroin and improvised explosive devices. Because of this, acetic anhydride plants have become a rare commodity, and the purchase price of acetic anhydride varies greatly between countries. In October 2013, acetic anhydride in the United States cost approximately $1885/tonne.4 However, in China, the purchase price of acetic anhydride was only $800/tonne.4

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There are two viable chemical pathways to produce VAM from acetic anhydride. The first one (Pathway A) reacts acetaldehyde (CH3CHO) with acetic anhydride in a liquid phase reactor using a iron (III) chloride (FeCl3) catalyst at approximately 120°C - 140°C. This reaction produces an ethylidene diacetate (CH3CH(OCOCH3)2) intermediate that undergoes thermal decomposition around 120°C. The reaction uses an acid catalyst to produce VAM, the desired product, as well as acetic acid, a byproduct.5 The chemical reactions for Pathway A can be seen below.

Acetic Anhydride + Acetaldehyde → Ethylidene Diacetate Ethylidene Diacetate → Vinyl Acetate Monomer + Acetic Acid

The second pathway (Pathway B) starts one step before Pathway A. Pathway B

produces the acetaldehyde for the first step of Pathway A by hydrogenating acetic acid. The hydrogenation will usually take place somewhere between 250°C to 350°C, but ideally within a range of 290°C. to about 310°C.6 Furthermore, the hydrogenation may react best under pressure, and possibly with an inert gas, such as nitrogen, fed into the reactor.6 The exact pressure/gasses/concentrations, however, depend on the type of feed that is used. After the hydrogenation, Pathway B is essentially the same as Pathway A. The overall chemical reactions can be seen below.

Acetic Acid + Hydrogen → Acetaldehyde + Water Acetic Anhydride + Acetaldehyde → Ethylidene Diacetate

Ethylidene Diacetate → Vinyl Acetate Monomer + Acetic Acid

For both Pathway A and Pathway B, the acetic acid byproduct that is produced can be separated out and then heated in order to form ketene (C2H2O) and water vapor. If the ketene produced is further reacted with acetaldehyde in the vapor phase with a suitable acid catalyst (such as a zeolite catalyst), then more VAM will be synthesized.6 The described chemical reactions can be seen below.

Acetic Acid + Heat → Ketene + Water Ketene + Acetaldehyde → Vinyl Acetate Monomer

The addition of these steps eliminates all byproducts formed in both pathways (except

for water), and maximizes the amount of VAM that is produced. In an ideal world, Pathway A would result in two moles of VAM for every mole of acetic anhydride and every two moles of acetaldehyde consumed. Pathway B would result in two moles of VAM for every mole of acetic anhydride and every two moles of acetic acid consumed. Block flow diagrams for Pathways A and B can be seen in Figure 2 and Figure 3, respectfully.

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Figure 2: Block flow diagram of Pathway A.

Figure 3: Block flow diagram of Pathway B.

In order to meet the demand of 1000 tonnes of VAM produced per day, it becomes

necessary to determine the rate at which the feed will enter the plant. Since this changes with each chemical compound that both Pathways A and B use due to their differences in molecular weights, a different feed flow will need to be established for acetic anhydride, acetaldehyde, and acetic acid. The ideal feed flows for each specific compound, as well as the required feed flows assuming different process efficiencies, are listed in Table 5. With the feed flows now determined, it is possible to obtain a rough estimate of the total cost of raw materials per year for both Pathways A and B. With this value, we can estimate a rough numerical value for the VAM plant’s overall OPEX. Both the total cost of materials per year and the OPEX using acetic anhydride as a feedstock, can be found in Table 6

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Table 5: Feed flows for different compounds and efficiencies (kilotonne/yr)

100% 90% 70%

Acetic Anhydride 169 188 241

Acetaldehyde 196 217 280

Acetic Acid 115 128 164

Table 6: Total cost per year and OPEX

On average, Table 6 shows that Pathway B will spend at least $80 MM less on raw materials per year than Pathway A. The same trend is seen in the OPEX results. With Pathway B having a lower cost of raw materials, it appears to be more economically viable than Pathway A, but other economic features of both pathways should be investigated. The MISR for both Pathways A and B are shown in Table 7, while the roughly estimated CAPEX, or FCI, for both pathways are shown in Table 8. The CAPEX numerical values have been determined by assuming a turnover ratio of 1.

Table 7: MISR calculation for Pathway A and Pathway B

Pathway A Pathway B

Efficiency 100% 90% 70% 100% 90% 70%

US 0.776 0.689 0.536 0.767 0.690 0.537

China 1.264 1.138 0.885 1.266 1.140 0.886

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Table 8: Estimated CAPEX, or FCI, for Pathway A and Pathway B in million USD

Pathway A Pathway B

413 413

While the actual numerical differences between the MISR calculations for Pathways A

and B are not that significant, the regional prices of acetic anhydride makes a huge impact. The idea of developing a VAM chemical plant in the US using acetic anhydride as a feedstock is now out of the question, as the calculated MISR values of < 1 prove that it is just not profitable for either of the pathways. From the turnover ratio correlation, both of the CAPEX for Pathways A and B sit at around $413 MM. However, as this value is only a rough estimation, the actual values may differ greatly.

The final piece of economics to cover for the acetic anhydride processes is the ROI. For simplicity, the ROI for this process was calculated assuming no salvage value, a recovery period of 10 years, and a 20% tax rate. The ROI for each process can be found below in Table 9. As Table 9 shows, Pathway B possesses the larger ROI. However, in order to be profitable, the chemical plant needs to be built in China.

Table 9: ROI calculation

Pathway A Pathway B

US -0.571 -0.371

China -0.071 0.129

If acetic anhydride is used as a feedstock in order to produce VAM, Pathway B is the

better option economically. It is much more cost effective to produce acetaldehyde in house from acetic acid than to purchase it as another feedstock. Pathway B, however, is not without its drawbacks. Should this pathway be chosen, the VAM plant would have to be built in China, where the price of acetic anhydride is much cheaper. Also, while the ROI for Pathway A in China may be negative, the value is actually very close to zero. Further analysis of this process may prove that the process is, in fact, profitable.

Acetic Acid

Another method for producing VAM involves only acetic acid as feedstock. The process occurs in three distinct reaction steps. First, the two molecules of acetic acid are reacted with hydrogen to form ethyl acetate. The reaction is shown below.

2 Acetic Acid + 2 Hydrogen → Ethyl Acetate + 2 Water

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The ethyl acetate from the first reaction step must be concentrated to above 50 weight percent using a stripping column. The enriched ethyl acetate stream is then sent to a cracking reactor. This can be done either thermally or catalytically. Catalyst used for cracking of ethyl acetate are usually sulfonic acid resins or perfluorosulfonic acid resins. Thermal and catalytic cracking follow the same pathway:

Ethyl Acetate → Acetic Acid + Ethylene

The products from the cracking reactor are fed into the third reactor. In this reactor,

ethylene and acetic acid react with oxygen in the presence of a catalyst to form the desired VAM product. The reaction is shown below.

Acetic Acid + ½ Oxygen → Vinyl Acetate Monomer + Water

The oxygen can either come from a pure oxygen stream or from a compressed air

stream. The compressed air would be the more economical option. The overall reaction is shown below.

2 Acetic Acid + 2 Hydrogen + ½ Oxygen → Vinyl Acetate Monomer + 3 Water

The VAM product is separated from the rest of the stream leaving the third reactor in

three steps. First, the volatiles such as nitrogen, carbon dioxide, and ethylene are removed in a stripping column. The ethylene can be separated from this stream and recycled back to the third reactor if desired. The bottoms from this column containing VAM, water, and acetic acid are sent to an azeotropic distillation column where the acetic acid is removed from the stream. This acetic acid can be recycled back to the first or third reactor. The water is removed from the VAM stream using a settling column to yield a pure VAM product.7

When analyzing the economics of this process, the first step was to calculate the MISR

for a few different cases. The flows for each of the compounds are shown in Table 10, while the MISR for each case is shown in Table 11.

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Table 10: Mass flows for the feed and product (tonnes/day)

Efficiency

100% 90%

Vinyl Acetate Monomer 1000 1000

Acetic Acid 1395.1 1550.11

Hydrogen 46.46 51.63

Oxygen (if using pure oxygen) 185.85 206.50

Air (if using compressed air) 889.24 988.5

Table 11: MISR for each of the considered cases

Case MISR

Ideal Using Pure Oxygen 1.10

Ideal Using Air 1.15

90% Recovery Using Pure Oxygen 1.03

90% Recovery Using Air 1.03

For each of the cases, the MISR is greater than 1. Therefore, a further economic analysis can be performed on the process.

The next step is to calculate the fixed capital investment and the total capital investment for the plant. The fixed capital investment was calculated in two ways. The first method was using an empirical correlation for gas phase plants. Using this method, the calculated FCI was approximately $459 million. The second method was using the turnover ratio. It was assumed that the turnover ratio was one. The FCI was calculated by dividing the annual sales by the turnover ratio and was determined to be approximately $413 million. The total capital investment was calculated by assuming that the FCI was 85% of the TCI. The resulting TCI was $485 million. After calculating the FCI and TCI for the process, the annual net profit and the return on investment can be calculated. The annual net profit for the process was calculated to be -$119 million with an ROI of -24%. Based on this analysis, the process is not economically feasible and no further investigation needs to be done.

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Another process involving acetic acid is the formation of ketene to react with acetaldehyde and produce VAM. The reactivity of ketene made the pathway a viable candidate since it directly produces VAM. The reaction pathway is as follows:

Acetic Acid → Ketene + Water Ketene + Hydrogen → Acetaldehyde + Water

Acetaldehyde + Ketene → VAM

First, acetic acid is heated to produce ketene. Ketene is then hydrogenated to form acetaldehyde. The reaction is exothermic, but there are no references in literature that show this reaction happening continuously. No free acetaldehyde is produced in this reaction. The reaction has been found to readily happen under very mild conditions, with palladium the most efficient catalyst for the reaction. In a preliminary kinetic study it was found that at a low ketene conversion, hydrogen was approximately 0.6 order and kenete was approximately 0.3 order. Carbon monoxide is a poison for this reaction, so if the ketene feed contains 2.5% CO, the efficiency of the reaction drops to about 50%.8

The reaction of ketene with acetaldehyde is most effective using an arenesulfonic acid

as the catalyst in a gas stripped reactor. The reaction temperature is preferably between 120℃ - 160℃.8 The block flow diagram of this pathway can be seen below in Figure 4.

Figure 4: Block flow diagram of acetic acid - ketene - acetaldehyde pathway.

The mass balance of this process is based on the production of 1000 tonnes of VAM per

day. The flows for VAM, acetic acid, and hydrogen were calculated assuming ideal recovery (100%), 90% recovery/theoretical yield, and 70% recovery/theoretical yield (Table 12). The flow of hydrogen is much lower than the flow of acetic acid due to their great difference in molecular weight.

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Table 12: Flow rates of acetic acid - ketene - acetaldehyde pathway

Flows (kilo tonnes/year) for a Specific Recovery/Yield

Compound 100% 90% 70%

Acetic Acid 230 256 329

Hydrogen 27.73 8.59 11.0

As seen in Table 13, the process is considered economically viable because all three

MISR values are greater than 1. This means this pathway could be considered for future analysis.

Table 13: MISR calculation for acetaldehyde pathway

MISR Value for a Specific Recovery/Yield

100% 90% 70%

2.28 2.05 1.60

The CAPEX and OPEX calculations for this pathway are show in Table 14. The CAPEX

values are greater than the OPEX values, meaning that the total annual sales of this pathway is greater than its operating costs. Even though hydrogen costs $250 more per tonne than VAM, since less hydrogen is consumed than VAM produced, there is profit that can be made.9

Table 14: CAPEX and OPEX calculations for acetaldehyde pathway

Economic Analysis ($MM) for a Specific Recovery/Yield

Parameter Ideal (100%) 90% 70%

CAPEX (FCI) 413 371 289

OPEX 258 232 181

Compared to the previous acetic acid feedstock pathway described, this pathway is

more economically viable since a profit is made. The ROI of 0.271 means that within three years, the money spent on this project will break-even.

Table 15: ROI evaluation of acetaldehyde pathway

Annual Income ($MM)

Annual Depreciation

($MM)

Total Cost Investment

($MM)

Tax Rate Annual Net Profit ($MM)

ROI

413 41.3 485 0.2 132 0.271

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Although this process is economically viable, it may not be pursued because there is not

substantial research done to show how it can reasonably be used in industry.

Acetylene

The first industrial practice of producing VAM was produced through the acetoxylation of acetylene over a catalyst.10 This reaction has a long history of use in industry, and the 1 to 1 mole ratio reaction is as follows:

Acetylene + Acetic Acid → Vinyl Acetate Monomer

Over decades of research have shown that numerous metals have provided a wide

range of increased activity of the the reaction. The following are some different metals in the order of their activity increase:11

Hg(II) > Bi > Cd > Zn > Ni > Mg > Co > Fe > Ca > Ba

The mercury, bismuth and cadmium catalysts have the best catalytic properties for the

acetoxylation of acetylene. However, all three are also expensive, and highly toxic to people and to the environment.11 Because of that, zinc catalysts are the most widely used in industry for their cost effectiveness and their effective catalytic properties. The most common variation of the zinc based catalysts is zinc acetate (Zn(OAc)2). Other variations such as PCS-Zn (porous carbon sphere), Zn-Ni, and Zn-Ni catalysts have proven to be slightly better, but due to the higher costs, they are not implemented on the industrial scale.11 The zinc acetate catalyst has also been impregnated on various forms of charcoal to improve conversion and selectivity.12

Today the most common practice for the acetoxylation of acetylene is to run the reaction

in the gas phase over a zinc acetate catalyst imbedded on charcoal. The reaction is performed in the temperature range of 170-250ºC at atmospheric pressures.10 From this process, the single pass acetylene conversion is within the range of 60-70%, and the VAM selectivity can reach 93% based on acetylene and 99% based on acetic acid.10 With the lower conversion rates, a chemical plant producing vinyl acetate from acetylene and acetic acid will also have recycle streams for both acetylene and acetic acid to the reactor. Du Pont has a patent for a specialized reactor for the acetoxylation of acetylene using a zinc acetate with charcoal formed from coconut shells.13 Acetylene and acetic acid are fed into the reactor, in the gaseous phase, at a ratio range between 3 to 1 or 5 to 1, respectively.13 The specific reactor is able to achieve a 50% conversion of acetic acid.11 A higher conversion, potentially up to greater than 90%, will be possible by exchanging the coconut charcoal with bituminous charcoal.12 The catalyst consumption for the reactor is approximately one pound of catalyst for every 23 pounds of VAM produced.13 The following two figures are a drawing of Du Pont’s acetoxylation reactor, as well as a graph showing the conversions of different charcoals.

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Figure 5: Du Pont’s acetoxylation reactor.13

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Figure 6: Relative activities for various charcoals.12

To be able to produce VAM from acetylene and acetic acid, the main process equipment

and units needed are as follows: a preheater to head the feeds to the reactor to the required temperature, a vaporizer to convert acetic acid to the gaseous phase for reaction, a reactor to convert acetylene and acetic acid to VAM, a condenser to condense the VAM formed and separate it from any unreacted acetylene, and separation unit consisting of potentially multiple distillation towers to separate out VAM to be put into storage and then shipped, and recycle streams to recycle unreacted acetylene and acetic acid. The following figure is a simple block flow diagram of the process.

Figure 7: BFD for VAM production from acetoxylation of acetylene.

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In the United States, the price of acetylene has been steadily increasing over the past

few decades. Currently, the acetylene price is approximately $0.73/lb, which equates to about $1606/tonne.14 Acetic acid is much less expensive at around $735/tonne. With these prices the MISR can be calculated and evaluated to give a first pass evaluation of the feasibility of the process. Table 16 displays the MISR for this process at perfect recovery, 90% recovery, and 70% recovery.

Table 16: MISR for acetoxylation

With the current price estimates, the CAPEX and OPEX for a chemical plant that uses acetylene and acetic acid can be estimated and are shown in Table 17. Under the assumptions of a 10 year operating period, a linear depreciation model, no salvage value, a tax rate of 20%,and ideal recovery, the annual net (after tax) income and ROI were calculated to evaluate the economic feasibility of this process. The annual net (after tax) income and ROI and displayed in Table 18 and Table 19.

Table 17: CAPEX and OPEX for acetoxylation process

Table 18: Annual net (after tax) income acetoxylation process

Table 19: ROI for acetoxylation process

The base economic evaluation shows that while the MISR has a value above one, the process leads to a negative annual net (after tax) profit for a plant based in the United States.

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The ROI is also negative, indicating that a company will never see any return on an investment on such a process. However, some sources say that due to the cheap price of coal in China, the acetoxylation process is still used to produce VAM in East Asia.10 This process would need more rigorous evaluation to determine the feasibility of building the facility in East Asia to take advantage of these lower prices.

Ethane

Another method of producing VAM is to only use ethane and oxygen as feedstock. This pathway is similar to the one with ethylene and acetic acid being the feedstocks. The main difference is that in this method, ethane is partially catalytically oxidized to form ethylene and acetic acid, which are then used as a feed to another reactor to produce VAM. This method of producing VAM has many benefits over other methods. For example, this process does not produce any carbon monoxide, and the process is economically viable due to the low price of ethane.

The first reactor that is used in this pathway involves the partial oxidation of ethane over a catalyst of MoVNbPd which has the following reactions:

Ethane + 0.5 Oxygen Ethylene + Water→ Ethane + 1.5 Oxygen Acetic Acid + Water→

Ethane + 3.5 Oxygen 2 Carbon Dioxide + 3 Water →

It is only a partial oxidation, because no carbon monoxide is formed which would normally form if ethane were to be fully oxidized. The catalytic reactor for this step is to be operated at 286°C and 200 psi The products of the partial oxidation reactor are then.15 separated by phase. The gas stream proceeds to an absorption unit which is used to remove carbon dioxide. The remaining ethane, ethylene, and oxygen goes to the VAM reactor. The liquid stream, which contains acetic acid and water, is first separated in a distillation column. The water is recycled, and the acetic acid proceeds to the VAM reactor. The second reactor has the following reactions:

Ethylene + Acetic Acid + 0.5 Oxygen Vinyl Acetate Monomer + Water→ Ethylene + 3 Oxygen 2 Carbon Dioxide + 2 Water→

The reactions take place over a palladium or gold catalyst, as describe in the ethylene

and acetic acid feed section. This reactor is to be operated between 140°C to 180°C, and 1 to 20 bar The stream exiting the VAM reactor is separated into a gas and liquid stream. The.16 gas stream consists of carbon dioxide and the unreacted ethylene and acetic acid, which are recycled back to the first oxidation reactor. The liquid stream contains water, acetic acid, and VAM. The stream enters a distillation column where the components are separated. The water

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and acetic acid can be recycled, and the VAM can be recovered at this point. The block flow diagram for this pathway is shown in Figure 8.

Figure 8: BFD for the ethane process.

We have determined this method of producing VAM one of the more cost effective

pathways, since the feedstock is only ethane, which is relatively cheap. In order to produce the stipulated 1000 metric tons of VAM per day, a mass balance shows that the flow rate of ethane must be 20.2 kg/s based on a conservative estimated conversion rate from existing technology of 20% The MISR for both ideal and expected conversions is shown in Table 20, and the.15 OPEX and CAPEX for this method are shown in Table 21.

Table 20: MISR values for ideal and actual conversion

Table 21: CAPEX and OPEX for ethane process

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Since the MISR for this process is well over 1 even at the realistic conversion, it can be economically viable. An economic analysis for the process was done to determine the annual net (after tax) income and the ROI which are shown in Table 22. The calculations were made for the conservative conversion rate on the assumptions of 20% tax rate, turnover ratio of 1, and a linear depreciation model with a 10 year operating period and no salvage value.

Table 22: ROI evaluation of ethane pathway

Annual Income ($MM)

Annual Depreciation

($MM)

Total Cost Investment

($MM)

Tax Rate Annual Net Profit ($MM)

ROI

413 41.3 485 0.2 53 0.109

Although the ROI is slightly less than preferred, we believe that it will increase as more

research is done into optimizing the process. Due to the high MISR and decent ROI, this method is economically viable and should be further investigated in FEL 2.

Ethylene

The most common industrial process used to make VAM uses ethylene, acetic acid, and oxygen as feedstocks.17 Ethylene, acetic acid, and oxygen are reacted in gas form in a reactor with a palladium or palladium/gold catalyst to form VAM and water.18 This reaction pathway also has an unwanted side reaction, the combustion of ethylene and oxygen to form carbon dioxide and water.18 The reactions can be seen below.

Ethylene + Acetic Acid + ½ Oxygen VAM + Water→ (A) Ethylene + 3 Oxygen 2 Carbon Dioxide + 2 Water→ (B)

This process starts by taking liquid acetic acid and gaseous ethylene and vaporizing the

mixture in a vaporizer. This is a necessary step because the reactor is a gas phase reactor and the acetic acid must be vaporized before the reaction can begin. The vapor mixture leaves the vaporizer and is mixed with an oxygen stream before entering into the reactor.18

The reactor is a catalytic packed bed reactor using a palladium catalyst to improve the

rate of the reaction for the desired VAM producing reaction.20 There has been significant research done into improving the catalyst used in this process and a palladium/gold catalyst has seen an increase in use. It is believed that a small addition of gold to the catalyst (atomic ratio of palladium to gold of 4:1) enhances the palladium by reducing the binding energy of the palladium active sites.19 The reactor conversion and selectivity is highly dependent on the operating temperature and pressures. It is important that the oxygen composition in the feed to the reactor remains below 7 mol %.18 The flammability limit for this mixture is approximately 8 mol % oxygen and safe operating conditions must always been observed when mixing

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hydrocarbons and oxygen.18 Another reason to maintain a low concentration of oxygen is because as oxygen concentration increases, the selectivity for the desired reaction decreases. Research has shown that as reactor temperature increases, the overall conversion of ethylene increases, but the selectivity for the desired reaction decreases.19 This is due to the fact that the desired reaction A has a lower activation energy than the undesired reaction B, and as temperature increases more of the ethylene is being combusted.19 Temperatures higher than 182°C (455 K) cause irreversible catalyst degradation, but temperatures must remain high enough as to not reduce the rate of reaction or form liquids in the reactor. Normal operating temperatures and pressures for the reactor should range from 393K - 433K and 5 - 8 bar.18

After leaving the reactor the mixture is cooled and sent to a flash tank where the lighter

components flash to vapor due to a pressure drop. The vapor from the flash tank is sent to an absorber where the vapor is washed with acetic acid to recover VAM. The liquid from the absorber joins with the liquid from the flash tank and is sent to the first distillation column. The vapor from the absorber is sent to a second absorber where it is treated with monoethanolamine (MEA) to remove carbon dioxide from the vapor. The vapor is then compressed and recycled to mix with the ethylene stream at the inlet of the vaporizer.18

The liquid from the flash tank and the absorber are sent to the first distillation column.

The first distillation column, the de-ethanizer, is meant to remove any unreacted ethylene still in the liquid and recycle it back to the front of the process. Ethylene comes off this column in the overhead and the bottoms contain a mixture of VAM, acetic acid and water. This bottom stream is sent to a second distillation column where the distillate is a mixture of water and VAM, and the bottoms are acetic acid. The acetic acid is pumped back to the front of the process to join the feed to the vaporizer. The water and VAM mixture is decanted, the water is purged and the VAM is piped sent to storage tanks for sales.18 An overview of the process can be seen in Figure 9, while Table 23 shows the overall mass balance for the process on a yearly basis.

Figure 9: BFD for VAM production using ethylene, acetic acid, and oxygen.

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Table 23: Plant mass balance (metric tonnes/year)

100% VAM Recovery 90% VAM Recovery

Ethylene 107,521 119,467

Acetic Acid 230,191 225,767

Oxygen 61,331 109,033

Water 68,997 82,797

VAM 330,000 330,000

Carbon Dioxide 0 33,723

The MISR calculation for this process are very promising, the results can be seen in

Table 24. The one calculation was performed using an ideal stoichiometric conversion of ethylene, acetic acid, and oxygen. The other MISR calculation was done using an assumed 90% recovery of VAM product, taking into account the side reaction and product loss. A sale price of $1250/metric tonne21 was used for VAM and purchase prices of $595/metric tonne22, $735/ metric tonne23, and $200/metric tonne24 were used for ethylene, acetic acid, and oxygen respectively. Based on these assumptions, the MISR values indicate that this process is a viable option.

Table 24: MISR calculations for ethylene process

Ideal MISR 1.68

90% conversion 1.51

An exploratory estimate of the CAPEX (FCI), OPEX, ROI, and Annual Net After Tax

Profit (ANATP) was conducted for this process. An assumed value of 1 for the turnover ratio was used to calculate the estimated CAPEX for this process.

Table 25: CAPEX, OPEX, ROI

CAPEX OPEX ROI ANATP

100% VAM Recovery

$412,500,000 $304,578,725 0.195 $94,587,020

90% VAM Recovery

$412,500,000 $350,102,932 0.120 $58,167,654

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Based on these calculations this process appears viable and is worth further investigation in FEL 2. Conclusion

Based on economic analysis of each VAM production pathway, there are 3 viable

options that will be investigated with more detail in FEL 2. The processes with ethane and ethylene as the feedstock have the highest ROI and MISR values which indicate that they are the most economically viable. Both of these methods have the potential to be profitable and viable, so they will be further investigated and simulated in FEL 2 to provide a more detailed analysis in determining which method will be incorporated in the final design.

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