November 2004

Mark W. Lockhart, V.P. Process Engineering
Joseph T. Ramey, Process Consulting Engineering
G. Lozada, Director of Engineering

Exploring MTBE Alternatives

What are the options for your refinery MTBE Unit with the phasing out of MTBE?

With the current phasing out of MTBE from gasoline pools, refiners are facing a multitude of confusing choices of what to do with existing MTBE units, how to maximize the gasoline pool, and how to meet requirements for oxygenates, octane, aromatics, olefins, and RVP. In this technical brief, we will review these choices and sort through some of the confusion to provide a clearer picture of the decisions to be made and the likely results for the refiner. Refer to Flowchart 1 included with this Technical Brief to enhance your understanding as you read along.

With the phase out of MTBE, the options for using the isobutylene that is currently the feed for a refinery MTBE unit(s) include:

The first option is obvious, but determining exactly why it is not satisfactory may guide the proper decision. The most promising option is usually to redirect available isobutylene to the alklyation unit if the existing unit has sufficient capacity or if it can be cost-effectively revamped to a higher capacity. Isobutylene was typically converted to alkylate prior to its diversion to MTBE production, and this is likely to be the preferred future route. One of the primary reasons for using available isobutylene as feed to the alkylation unit is that each mole of isobutylene reacts with a mole of isobutane to form alkylate. The alkylation reaction of isobutylene and isobutane results in maximizing the volume of gasoline produced per barrel of isobutylene, especially when compared with the production of iso-octane by dimerization and hydrogenation. Consider that for iso-octane production that way, each mole of isobutylene must react with another mole of isobutylene, rather than react with isobutane. Isobutane is not utilized in this iso-octane reaction, which results in nominally half the volume of quality gasoline blending stock per barrel of isobutylene.

If the existing alkylation unit does not have excess capacity and the capacity cannot be increased cost-effectively, converting the MTBE unit to either iso-octane or ETBE continues to be a viable alternative. Converting to ETBE will result in a greater volume of gasoline produced versus iso-octane. This is because ethanol is used to replace the methanol in the etherification reaction. Each ethanol molecule will react with one isobutylene molecule to produce the final blending component, ETBE.

Why not use ethanol directly in the gasoline pool instead of feed to produce ETBE? Ethanol has a very high vapor pressure (18 psia) and attracts dirt and water-soluble impurities into the gasoline pool. ETBE, on the other hand, has high octane (111 (R+M)/2) and a lower RVP (4 psia), a lower solubility in water than MTBE, a narrower distillation range for improved driveability and better VOC control in blending. Producing ETBE comes at a high cost given the high cost of ethanol as a feed stock as well as the processing cost of the converted MTBE unit. As stated above, reacting ethanol with isobutylene does result in a higher production volume since an imported material is utilized to react one for one (mol/mol) with isobutylene.

A note of caution, it is possible that ETBE might share the same governmental legislative fate as MTBE, which would prevent it from being a blending component in the gasoline pool. Therefore, it is best if the conversion to ETBE is a relatively lower capital cost to mitigate this potential legislative risk.

To evaluate the economics of these three scenarios, a comparison must take place, which includes determining the capital cost to debottleneck the current alklyation unit, to convert the MTBE unit to iso-octane production, and to convert the MTBE unit to ETBE production. Next, the costs for feed stocks, additional operating costs, and product values must be determined. The rate of return for each scenario can now be directly compared. This is a good start; however, these economic comparisons cannot be completed without considering the effect on the total refinery framework.

The objective of the economic analysis for the refinery should ultimately be to maximize the gasoline pool and overall profitability while meeting the requirements for octane, aromatics, olefins, RVP, sulfur, gum, oxygenate content and the driveability index.

What other modifications may make sense in your refinery and will also affect the economics?

With the maximum oxygenate level at 2%, MTBE provides almost 11% of the gasoline pool and has represented a significant octane component with a very low contribution to vapor pressure. The use of ethanol as the oxygenate returns 5 to 6% to the gasoline pool, but the higher vapor pressure drives out C4's and C5's for an additional volume loss. MTBE removal results in a reduction to gasoline pool volume, octane and blending capabilities. These factors must be taken into consideration to meet gasoline-blending requirements and maximize refinery profit.

As stated above, butane and butene streams are most likely best utilized in existing alkylation units. Including an expansion of your alkylation unit in your MTBE phase out in your refinery can make the most sense. This has been the case in California where there are limits on the amount of olefins and aromatics in the gasoline pool. This solution is especially important since the oxygenate requirement was met with ethanol, which has the effect of driving out C4's and C5's due to higher vapor pressure. Many California refiners are now utilizing C5 olefins to make alklyate. Also, many use all of their isobutane as alkylation feed and increase the available isobutane by isomerizing normal to isobutane. In many cases, the C5 streams have now become the swing streams. Their usage is often seasonal since there is room for the vapor pressure in the gasoline pool in the winter. Often they have to be sold or stored during the summer.

Octane requirements are primarily met with reformate and to a lesser extent with alkylate. Many area refiners have increased their reformate capacity or improved the performance of their reformers by improved fractionation of the reformer feed and improved catalyst.

The RVP is usually controlled by removing the more volatile components from the gasoline pool. This usually means removing C4's and C5's from the pool.

The removal of MTBE requires that the refiners go back to basics in the evaluation of their gasoline pool and usually requires that LP modeling evaluations be performed. This modeling not only needs to consider the available components that can be reliably and economically purchased to maximize the import value of the available gasoline pool. For example, several refineries in California import alkylate to effectively utilize their blendstocks while meeting the olefin and aromatic limits.

In the ever changing world of gasoline requirements and limitations, refiners should also use this modeling to look past the near term requirements and look to the future when oxygenate requirements could be potentially reduced or eliminated.

The refinery modifications caused by the removal of MTBE require a front-end evaluation and should start long before the compliance date. Unfortunately, many refiners delay too long and have to hurry through the up front evaluations. The correct answer for each refinery should be based upon the economic evaluations derived from the results of the modeling work and from the preliminary capital and operating costs for each alternative.

Interestingly, many California refiners found that after performing a thorough front-end evaluation, the required modifications were less costly than previously anticipated.

At Commonwealth Engineering and Construction (CEC), we have strong process engineering capabilities for assisting you in making these types of technical and economic evaluations. We perform these analyses independently since we do not have specific ties to any of the major licensors. We can accomplish this work cost effectively as well as quickly and efficiently.

Our current backlog includes several MTBE conversion studies and/or actual conversion project(s). We have worked very close with refiners as well as the licensors and have a very good understanding of the amount of revamp required in existing MTBE units for some of the available technologies. Of course, every unit is a little different in size, technology in use for the production of MTBE, and operation requirements. Some MTBE units are still in full production, while others have been out of operation for a while. In some cases, some of the components from non-operating MTBE units have been used by the refinery for other services and thus the unit is not complete. These are some of the factors that will ultimately determine the cost to convert an existing MTBE unit. However, our experience has indicated that the overall capital cost for a conversion is relatively low as previously mentioned in this article.

If you are interested in these types of evaluations for your MTBE conversion project, we would be happy to discuss your particular situation and to provide you with a proposal to suit your specific needs. If you have any questions or would like additional information, please give us a call at 832-251-2246.