Part 2: Maximizing Isobutylene
Production in the FCC Unit
By J.B. McLean, W.S. Winkler, R.G. McClung, M. Feldman*
This article is the second in a two-part series dealing with maximizing iso-butylene yield from an FCC unit for the production of oxygenates. In the previous article, the various catalyst properties affecting iso-olefin yields were discussed and the IsoPlus-1000 and IsoPlus-2000 series catalysts were introduced. This article will focus on the commercial applications of these catalysts using BASF's FCC yield correlation model and evaluate the overall economic impact on a hypothetical refinery using the REFINE model of Wright-Killen. The results will show that both IsoPlus-1000 and IsoPlus-2000 options have good payouts when investment is required to maximize isobutylene production. In addition, all cases have a lower capital cost per barrel of reformulated gasoline than the next investment alternative, dehydrogenation of isobutane.
*[Mike Feldman is Vice President of Wright-Killen Information Technology other authors are BASF personnel.]
Discussion FCC Yields
The results from the laboratory studies on various catalysts given in Part 1 of this series were used as input to BASF's commercial projection model to estimate the yield effects on a commercial FCC unit. Five cases corresponding to laboratory tested catalysts were chosen as alternatives for maximizing isobutylene yield from the FCC unit.
These five cases are as follows:
The "Base Case" uses a typical RE-USY catalyst. In this base case, we are assuming 10% spare capacity in wet gas compression and handling facilities. We are also assuming 10% excess capacity in the alkylation unit and a 50% excess capacity in the existing MTBE unit. This latter assumption is based on a U.S. survey of actual operating MTBE units.
This "ZSM-5" case uses 3% addition of ZSM-5 additive at a constant make-up for the "Base Case" RE-USY catalyst. The reactor temperature is adjusted to meet the wet gas compression limit.
This "Iso-1000" case uses BASF's IsoPlus-1000 catalyst at constant make-up rate compared to the "Base Case" and uses varying reactor temperature to meet the wet gas compression limit 10% above the base case.
This "Iso-1000-1" case is the same as Case 3, except the wet gas compression and handling facilities are expanded by 30% over the "Base Case".
This "Iso-2000-2" case uses BASF's IsoPlus-2000 catalyst. The make-up rate is increased over the "Base Case", the preheat is reduced by 300°F and catalyst coolers are installed. The addition of catalyst coolers allows use of higher cat/oil ratios to achieve higher conversion at lower reactor temperature.
The yields and operating conditions for each of these five cases are listed in Table 1. Several points are noteworthy regarding these projections.
- Cases 1 thru 3 require no additional capital investment based on the assumptions used. Use of IsoPlus-1000 definitely provides the largest increase in isobutylene among these cases, 52% over the "base case".
- Case 4 & 5 both require some investment, the specifics of which will be discussed in a succeeding section of this article. However, both IsoPlus-1000 and IsoPlus-2000 do provide significant additional isobutylene for manufacture of MTBE, 74% and 121%, respectively over the base case. Which catalytic option is chosen will depend on a variety of factors including such primary issues as the cost of MTBE purchased on the open market and the volume of reformulated gasoline required by the refinery.
Each one of these catalytic options for maximizing isobutylene has a different overall yield structure. In addition, several other unit operations in the refinery contribute toward gasoline volume and are either directly or indirectly impacted by the FCC unit operation and the manufacture of MTBE. The large contributors toward additional gasoline volume include the catalytic reformer operating at lower severity, the added volume of MTBE and the alkylation unit processing additional propylenes and butylenes. The overall impact of these catalytic options for increasing isobutylene yield on gasoline volume for a typical refinery flow scheme will be discussed in the subsequent section.
Overall Refinery Impact
There are a number of assumptions required in order to determine the overall gasoline volume impact of increased isobutylene production for MTBE and its associated effects on the FCC unit. These assumptions are as follows:
1. 2.7 Wt.% oxygenates are required corresponding to approximately 15 Vol.% MTBE.
2. A regular grade at 87 (R+M)/2 will be the single reformulated gasoline product, the volume of which is set by isobutylene availability.
3. Three unleaded grades of gasoline will be made.
1. The refinery process schematic is illustrated in Figure 1.
2. The refinery crude throughput is 100,000 BPSD using Arabian Light (45%) and West Texas Crudes (55%).
3. Spare capacity is available in cases 1 thru 3 to alkylate additional C3 and C4 olefins. Cases 4 and 5 require investment in additional alkylation capacity.
4. All the increased gasoline volume can be sold.
Gasoline Volume Impact
FCC, Alkylation and MTBE Units
Since this article focuses on the FCC unit as a source of feedstock for gasoline manufacture, an interesting approach would be to look at gasoline volume generated by the FCC and directly associated unit operations, the MTBE unit and alkylation unit. The total gasoline volumes and individual contributions of the FCC, alkylation and MTBE units are illustrated in Figure 2. Note that the volume contributed by FCC gasoline varies with each catalytic option. However, the overall gasoline yield from the FCC and directly associated units actually increases because of additional alkylate and MTBE generation .
Total Gasoline Volume Impact
In view of the increased gasoline volume from the FCC and directly associated units, the overall gasoline yield should also rise. This data is illustrated in Figure 3. Note also that the proportion of reformulated gasoline also increases with each catalytic change to increase isobutylene yield.
Table 2 lists the reformer octane, reformate volume, hydrogen yield and net hydrogen yield for the 5 cases studied. As expected, the effect of adding MTBE to the gasoline pool is to reduce reformer severity. In this case the reformate octane decrease is approximately 3 numbers, but the net hydrogen is still in excess of the process hydrogen requirements for the assumed refinery flow scheme. A higher proportion of reformulated gasoline requiring importing of MTBE would have the effect of reducing the reformer octane and hydrogen yield even further. The amount of excess hydrogen is also naphtha feedstock dependent. The naphtha chosen for this study assures an excess of hydrogen. On further analysis if hydrogen generation were marginal, the reformer severity could be increased and the C5/C6 isomerization could be idled to avoid octane giveaway and increase hydrogen make.
Wright-Killen's REFINE model was used to evaluate each of the five cases studied for its profitability. REFINE uses a representative set of raw materials and product values along with unit operation's fixed and variable cost to calculate an overall refinery profit. The additional profit made over the "Base Case" operation is illustrated along with reformulated gasoline volume in Figure 4. This additional profit is used to offset any additional investment needed to manufacture more MTBE, alkylate or in revamping the FCC unit. The investment figures for each of these cases is listed in Table 3.
1. Base Case capacity is assumed to be 1125 BPSD equivalent to 50% excess capacity.
2. Alkylation capacity is assumed to be 6500 BPSD or 10% over the Base Case thruput of 5916 BPSD.
3. Investment estimates are based on building grass roots units of the same size as shown under "Necessary Incremental Capacity". Estimate include MTBE plant and FCC revamp.
4. Investment per barrel of reformulated gasoline. Compare to an isobutane dehydrogenation process at $3000/B.
In each case, in order to simplify analysis, the investment estimate is based on building a grass roots unit capable of making the incremental MTBE or alkylate. The MTBE investment also includes the cost of the cat. cracker additional facilities. This method of estimating does give an investment figure which is higher per barrel of finished product than investing in debottlenecking steps. Despite this simplified approach, good payouts are shown for all cases except the ZSM-5 case. The poorer payout is credited to additonal investment in alkylation capacity to accommodate the additional propylene preferentially produced by ZSM-5 over butylenes.
Another interesting case for economic comparison is to compare the investment cost per barrel of reformulated gasoline for each case to that for a grass roots isobutane dehydrogenation scheme (includes butane isomerization, dehydro unit and etherification process). The investment cost per barrel of reformulated gasoline are also shown in Table 3. For the cases which require investment, the alkylation unit capital is the most significant portion. However, all cases compare favorably at $1400 to $2050/BPD with a dehydrogenation unit based MTBE production scheme. A 12,500 BPSD dehydro unit based scheme is estimated to cost $250MM or $3000/BPSD of reformulated gasoline.
1. Estimated as 1.25 Barrels of TAME/Barrel of converted Isoamylenes equals 60% of total Isoamylenes available.
2. RFGReformulated Gasoline, 17 Vol% TAME Gives Approximately 2.7 Wt% Oxygen
3. Includes that produced from MTBE in Tables 5&6
Additional Gasoline From Amylenes
Recognizing that isobutylene yield alone may not be sufficient to provide needed oxygenates, the isoamylenes yield can also be used to make oxygenates. Using several educated assumptions derived from commercial and pilot plant data, we can make a reasonable estimate of isoamylenes convertible to tertiary amyl ether (TAME)
1. Assume total convertible isoamylenes yields equals the isobutylene yield in volume percent for all cases.
2. Assume that the etherification process is once through and 60% of the available isomers are converted to TAME.
These assumptions allow calculation of approximate TAME yields and potential additional reformulated gasoline. These numbers are presented in Table 4. The percentage of the gasoline pool that can be provided as reformulated gasoline using the FCCU as the sole source of iso-olefins, now varies from approximately 13% to 27% including the MTBE oxygenated portions.
Summary and Conclusions
A number of FCC catalysts and corresponding isobutylene yields have been examined for a specific crude slate and refinery flow scheme. The total oxygenates available from FCC isobutylene alone allows making from 9.2% to 19.2% of the total gasoline production as reformulated regular gasoline. The optimum case the ones presented that allows maximization of profit and reformulated gasoline simultaneously with minimum investment, uses BASF's IsoPlus-1000 catalyst. For this case, the maximum percent of reformulated regular gasoline using MTBE only is approximately 16% and investment associated with additional wet gas compression and processing, air blower and other equipment related to the FCC unit is necessary. The payout on this investment is calculated at approximately 1 year.
On the other hand, if a larger production of reformulated gasoline is required, IsoPlus-2000 provides about the same profit over the base case as the IsoPlus-1000. This operation produces reformulated gasoline at approximately 19% of the total gasoline production, at higher capital cost than IsoPlus-1000, but with a less than 2 year payout. For either IsoPlus-1000 or IsoPlus-2000, the investment required per barrel of reformulated gasoline compares favorably with the alternative investment in dehydrogenation of isobutane.
Isoamylenes also represent a source of oxygenates for reformulated gasoline manufacture using TAME. The amount of reformulated gasoline made using this raw material is approximately 60% of that made from MTBE. Supplemented by the isoamylenes, the maximum production of reformulated gasoline using the FCC unit alone as a feedstock source varies from approximately 13% to 27% for the catalyst options studied.