Process and Cost Modeling of Saturated Branched-Chain Fatty Acid

Sep 10, 2012 - The development of new heterogeneous chemocatalytic processes for the conversion of vegetable oils and animal fats into high-value biob...
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Process and Cost Modeling of Saturated Branched-Chain Fatty Acid Isomer Production Helen L. Ngo,* Winnie C. Yee, Andrew J. McAloon, and Michael J. Haas U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States S Supporting Information *

ABSTRACT: For decades, lubricants and hydraulic fluids were almost entirely based on petroleum. In recent years, the potential health risks of these materials as a result of their poor biodegradability have stimulated public awareness and concerns. It is therefore becoming increasingly important to implement environmentally friendly biobased fluids for the chemical industries. The development of new heterogeneous chemocatalytic processes for the conversion of vegetable oils and animal fats into highvalue biobased industrial products can also have important positive impacts on the U.S. agriculture industry. Saturated branchedchain fatty acid isomers (sbc-FAs) such as isostearic acid, which are produced from renewable materials, are of interest because of their excellent lubricity and potentially good biodegradability. These unique features make them attractive in many important applications. Currently, sbc-FAs are produced as a byproduct of industrial dimer acid production, are synthesized in small quantities, and are costly to produce. In this paper, an efficient and effective isomerization process that produces predominantly the sbc-FA materials is presented as a case study to evaluate the potential of the technology to be implemented on the industrial scale. The case study was simulated using SuperPro Designer software to estimate the capital and process costs for producing sbc-FAs at an annual production of 4.5 × 106 kg (10 × 106 lb). The studies show that the process is cost-effective, with an estimated production cost of U.S. $2.53 kg−1 ($1.15 lb−1). global annual consumption of sbc-FAs is 4.5 × 107 kg (10 × 107 lb). The bulk of these sbc-FAs is presently obtained as minor coproducts of dimer acid production,5 which makes them costly to produce and of limited production capacity. Thus, there exists a need to develop efficient and cost-effective processes for the production of sbc-FAs, as such materials can potentially serve as replacements for petroleum-based materials in the lubricant industry. A very promising approach to chain branching relies on the skeletal isomerization of unsaturated linear-chain fatty acids (ulc-FAs) such as oleic acid (OA) that is catalyzed by solid acids (e.g., zeolite materials). Zeolite catalysts can isomerize ulc-FAs to unsaturated branched-chain fatty acids (ubc-FAs) at much higher selectivity and conversions than the traditional clay or silica−alumina type catalysts.6−9 The ubc-FAs resulting from the skeletal isomerization reactions can be further converted to sbc-FAs via nickel- or palladium-catalyzed hydrogenation reactions.6−9 Reports and patent disclosures illustrate the potential of solid acid catalysts for the skeletal isomerization of ulc-FAs.6−9 Our group has also carried out the skeletal isomerization process with a highly active solid catalyst, modified H-Ferrierite zeolite, which resulted in predominantly sbc-FA products and small amounts of dimer coproducts.10,11 The sbc-FA materials obtained were substantially purer than comparable materials produced by industry.10,11 Their lowtemperature and thermo-oxidative stability indicated that the

1. INTRODUCTION The advent of inexpensive imported petroleum has made petroleum-based materials ubiquitous in the market today. According to the 2009 British Petroleum (BP) statistical survey of world energy, in 2008 the global production of crude oils was 4.0 × 1012 kg (8.7 × 1012 lb) and Americans consumed 8.6 × 1011 kg (1.9 × 1011 lb).1 Mineral oils are common petroleumbased materials widely used in many applications, ranging from body care to lubrication. However, the nonbiodegradable nature of such petroleum products is a potential issue as they place a significant burden on the environment.2 In contrast to petroleum-based lubricants and hydraulic fluids, fatty acids and/or their ester derivatives produced from renewable materials are nontoxic and environmentally benign since they exhibit better biodegradability. For this reason, renewable resources such as vegetable oils and animal fats have attracted significant attention as replacements for petroleum-based lubricants and hydraulic fluids. Vegetable oils, for example, exhibit excellent lubricity properties and have good biodegradability and are already being used in lubrication technology.3,4 Although these oils have commercial value, their use is limited to applications that do not require good low temperature properties or high thermo-oxidative stability. On the other hand, isostearic acid materials (IA), the term typically applied to a mixture of saturated branched-chain fatty acid isomers (sbc-FAs) containing 18 carbons, do not exhibit these limitations. They are liquid at room temperature because of their low melting points and have good thermo-oxidative stability and lubricity properties. Commercial IA materials are used in applications including cosmetics, lubricant and fuel additives, surfactants, coatings, soaps, and body washes. The © XXXX American Chemical Society

Received: February 20, 2012 Revised: August 15, 2012 Accepted: August 17, 2012

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Figure 1. Process flow diagram for the production of branched-chain isomers from unsaturated fatty acids (ulc-FAs). H2O, water; HCl, hydrochloric acid; CFUGE1, centrifuge; CCW, cocurrent wash; DR-101, drier; VAP, water vapor; Ulc-FAs, unsaturated fatty acid; N2, nitrogen; H2, hydrogen; NiSiO2, nickel on silica. Solid lines indicate the flow of streams.

2. PROCESS MODEL DESCRIPTION OF THE SBC-FA PRODUCTION FACILITY 2.1. Process Model Based on Experimental Procedure. A process flow diagram for the production of sbc-FAs was created and entered into the simulation program. The process model was developed based on data and information gathered through experimentation results and from various technical sources including the literature, industrial experts, and equipment suppliers.15,16 The flow diagram served as a basis for specific equipment sizing and technical features and capabilities and ultimately resulted in capital and operating cost estimates of the process. For this study, a facility operated for 24 h a day, 330 days a year with a plant capacity of 4.5 × 106 kg year−1 (10 × 106 lb year−1) of sbc-FAs was selected. Figure 1 shows the flow diagram of the model with three processing sections: (1) zeolite catalyst treatment, (2) isomerization of ulc-FAs, and (3) product recovery. The model was developed in continuous mode and was based on results from our batch scale experiments whose results are listed in Table S1 of the Supporting Information. The catalyst retained substantial activity following reaction and could be reused (Table S1 of the Supporting Information). Five reuses were incorporated into the model, since data indicated that product yield at that stage remained high while purity was also acceptable. 2.2. Zeolite Catalyst Treatment. A commercially available solid Ferrierite zeolite containing potassium (K+) cations (TOSOH Corporation, Shiba Minato-Ku Tokyo, Japan) was chosen for this study because it had been shown to work well in the application.10 The catalyst (usage: 28 144 kg year−1 (61 917 lb year−1)) was fed into a continuously stirred tank reactor (CSTR) followed by the addition of concentrated HCl (Figure 1, HCl) diluted to 1 N by the deionized water stream (Figure 1, WATER). This ion-exchange treatment served to convert the K+ form of the catalyst to its protonated (H+) form since the untreated Ferrierite zeolite solids do not catalyze the isomerization reactions.10 The CSTR was heated at 55 °C for 24 h, and the treated zeolite solids were separated from the acid solution by centrifugation (Figure 1, CFUGE1). They were then passed through the cocurrent washing unit (Figure 1, CCW) containing deionized water (Figure 1, WATER1) to

materials are excellent candidates for replacements of petroleum-based lubricant materials. We also evaluated the lubricity properties of the materials, and the results further confirm the high value of the materials.12 These encouraging results have led us to examine the estimated industrial-scale capital and process costs of the developed technology for sbcFA production. A process simulation program, SuperPro Designer, version 8.5 (Intelligen Inc., Scotch Plains, NJ), has been used to estimate the production cost of the process on an industrial scale and to assess economic and environmental impacts of the technology. This software is attractive for this application because models developed using this software have been successfully used to examine existing processes and evaluate the feasibility of proposed processes. A demonstration version allowing one to examine the output is available at no charge (www.Intelligen.com). In this modeling study, chemical engineering models of each process were first developed and then integrated with cost models to define the physical operations of a process and the associated capital and production costs. Such modeling studies are very useful for this purpose. For instance, Kwiatkowski et al. evaluated the economic feasibility of the corn dry-grind process for fuel ethanol production using this type of software.13 Lima et al. recently reported a process model for the production of activated carbons from broiler litters using the SuperPro Designer software.14 In this paper, a base model has been constructed to estimate the capital and production costs of the isomerized sbc-FA products using SuperPro Designer. A cost analysis, using technical modeling to estimate the economics of the developed zeolitic skeletal isomerization process, is also presented. The process model is based on a facility producing 4.5 × 106 kg year−1 (10 × 106 lb year−1) of sbc-FA materials, which is about 10% of the global annual consumption. Such environmentally benign and sustainable processes can lead to new cost-competitive outlets for domestic oils and fats. The development of this heterogeneous chemocatalytic process for the conversion of oleic acid into high-value biobased industrial products (i.e., sbc-FAs) can thus have important impacts on the U.S. agriculture industry. B

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column operating at 0.25 mmHg to remove the oligomeric fatty acids (i.e., dimer fatty acids), another coproduct of the process (Figure 1, dimer fatty acids). In line with laboratory results, the model shows the dimer fatty acids at the bottom of the column as being >99% pure. The distillate at the top of the column, which contains the major product (Figure 1, Sbc-FAs (isostearic)), is a mixture of branched-chain isomers with 88% purity, also containing branched and linear-chain lactones.

remove the excess acid solution, as the residual HCl solution may also catalyze the isomerization reaction but with less specificity and activity. The washing was done in a 10:1 ratio of volume of water to the initial weight of the zeolite. The water from these steps, which contained potassium chloride (KCl) and excess HCl, was disposed of as waste streams (Figure 1, WASTE1 and WASTE2). The H+-exchanged zeolite solids were then placed in a dryer (Figure 1, DR-101) to give a final moisture concentration of ∼30% (H-Ferr-K). 2.3. Isomerization of Oleic Acid (91.2 wt %). Oleic acid (91.2 wt %) containing small amounts of linoleic (6.1 wt %) and stearic (2.7 wt %) acids is commercially available in industrial amounts and was the substrate in the isomerization reaction (usage: 5 628 796 kg year−1 (1 238 335 lb year−1)) along with 2.5 w/w% H-Ferr-K catalyst (capacity consumption: 28 144 kg year−1 (61 917 lb year−1)) and 2% (w/w on the basis of catalyst) water (Figure 1, WATER2) were modeled as a continuous reaction in a closed, high-pressure stainless-steel reaction vessel (Figure 1, isomerization). The vessel was purged with nitrogen gas (Figure 1, N2) at 100 psi to remove oxygen and then was heated to 250 °C for 6 h. The crude unsaturated branched-chain fatty acid isomer products (ubc-FAs) with weight percent compositions listed in Table S1 were removed from the reactor, and the H-Ferr-K solids were separated from the crude ubc-FAs by filtration (Figure 1, FILTER). An isomerization efficiency of 97.5% was used in this model, which was the average of the first five reuse experiment results obtained from the laboratory (Table S1 of the Supporting Information). The reacted zeolite catalysts were then recycled by retreating them with acid solution as stated in section 2.2 (Figure 1, recycle). Following isomerization, the crude ubc-FA products were transferred to a hydrogenation reactor, also modeled as a CSTR, to convert them to crude saturated branched-chain FA materials (sbc-FAs) (Figure 1, hydrogenation). The materials were pressurized with hydrogen gas (Figure 1, H2−IN) and 1% (w/w) Pricat 9910 nickel on silica solid catalyst (Johnson Matthey Inc., West Chester, PA; Figure 1, NICKEL) at 170 °C for 3 h. The reactor was depressurized (Figure 1, H2−OUT), and the nickel catalysts were separated from the saturated crude products by filtration (Figure 1, FILTER2). The resulting crude sbc-FA materials were a mixture of branched-chain isomers, linear-chain fatty acid (slc-FA), C18-γ-branched and linear-chain lactones, and oligomeric fatty acids (primarily C36-dimer fatty acids), as shown in Table S1 of the Supporting Information, and were transferred to the next step for purification. The used nickel catalysts were transferred to the disposal stream (Figure 1, NiSiO2). 2.4. Product Recovery. The components in the crude sbcFA materials were separated by two sequences, namely, recrystallization and distillation. The crude sbc-FA products were recrystallized in acetone (2:1 ratio of acetone to crude products) at −3 °C for 24 h and then filtered (Figure 1, FILTER3) to remove the slc-FA material. A yield of 94% slc-FA was assumed in this model because in general the recrystallization process does not completely remove 100% slc-FA material. The purity of the slc-FA was >90%, and these are classified as one of the coproducts (Figure 1, slc-FA (stearic)). The liquid stream containing a solution of the purified sbc-FAs in acetone was passed through the flash tank (Figure 1, FLASH) to recover the solvent. According to the model, ∼2% of the used acetone was lost during recycling. The solvent-free purified sbc-FA materials were fed into a distillation

3. ANALYSIS AND DISCUSSION The flow diagram in Figure 1 was used to estimate the capital and operating costs for the synthesis of sbc-FA materials from oleic acid (91.2 wt % purity). On the basis of the model, the isomerization process produces three products with the following outputs: (1) 4 529 317 kg (9 982 614 lb) of sbcFAs, as the primary product, (2) 476 253 kg (1 049 661 lb) of slc-FA, and (3) 752 059 kg (1 657 538 lb) of dimer fatty acid as coproducts (Table S2 of the Supporting Information). The model was used to estimate the production cost of the primary product. The calculated unit production cost is U.S. $2.53 kg−1 ($1.15 lb−1). 3.1. Capital Costs. The facility cost for a processing facility consists of the capital cost of the facility, depreciated over its economic life, and other facility-related charges such as maintenance expenses, insurance, and property taxes. The projected capital cost of this facility is based on an engineering factored estimate developed from the costs of the individual equipment items necessary for the process (Table S3 of the Supporting Information). An additional allowance of ∼15% was added to these equipment costs to cover those processing equipment items that could not be defined at this time but would be required for the process. The total capital cost was then generated from these equipment prices by using a capital cost to equipment ratio of 3 to cover all other costs of construction materials and installation, engineering, and other charges associated with the construction of an industrial facility. In those cases where the capacities of the equipment in the model varied from the equipment for which quotations were available, the quoted costs were adjusted through the use of equipment cost scaling factors. A discussion of the technique of adjusting equipment costs to compensate for changes in capacities can be found in various publications.17 Equipment costs were obtained from in house files such as Superpro Designer. Annual maintenance expenses were estimated at 3.0% of the capital cost, insurance at 0.80% of the capital costs, and general factory expenses at 0.75% of the capital costs. We have not included an allowance for property taxes. 3.2. Operating Costs. Operating costs are based on the following expenses: material costs, utilities costs, labor costs, and facility expenses (Table S4 of the Supporting Information). Interestingly, material costs constitute 84% of the total operating costs. The primary feedstock for this facility is an oleic acid mixture. The cost of material was estimated at U.S. $1.66 kg−1 ($0.75 lb−1) based on a bleached fancy tallow cost of U.S. $0.99 kg−1 ($0.45 lb−1) plus a surcharge of U.S. $0.66 kg−1 ($0.30 lb−1) for conversion charges and losses.18 The oleic acid costs are 64% of the total production cost. The zeolite catalysts are consumed in the process at a rate of 0.005 kg of catalyst per kg of oleic feedstock. At a purchase price of U.S. $55 kg−1 ($25 lb−1), zeolite is the second largest material cost for the process and makes up slightly more then 10% of the total production cost. A nickel−silicon oxide catalyst is also consumed in the process at a rate of 0.0093 kg of catalyst per kg of oleic acid C

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feedstock. At an estimated cost of U.S. $15 kg−1 ($6.80 lb−1) of catalyst, this contributes 6% to the production cost. Acetone, used as a solvent for the product separation stage, contributes 3.0% to the production costs. Water, hydrochloric acid, nitrogen, and hydrogen, the remaining materials required for the process, contribute 1.0% of the production cost. Sbc-FAs are the main product, but two coproducts, slc-FA and dimer fatty acids, are also produced. The production of sbcFAs is maximized in this process and constitutes over 78.6% by weight of the total product mix (Table S2 of the Supporting Information). Dimer fatty acid coproduct is estimated to have an annual production rate of 752 059 kg (1 657 538 lb), or 13% by weight of the total production (Table S2 of the Supporting Information). Slc-FA makes up the remaining 8% of the total product weight (Table S2 of the Supporting Information). Electricity, steam, and two cooling agents, chilled water at 5 °C and a refrigerant at −15 °C, are the utilities required in the process. Utility requirements of the various equipment operations are calculated and totaled within the program. The utilities are treated as purchased utilities, and unit costs were assigned to each utility based on estimates of current market conditions. An electric rate of U.S. $0.028 per megajoule ($0.10 kWh) was used in the calculations. The price of steam was set at U.S. $25.00 per 1000 kg based on a natural gas price of U.S. $360.00 per 1000 kg of natural gas (U.S. $7.70/million BTUs).19 Chilled water charges were estimated at U.S. $0.40 per 1000 kg of chilled water used and −15 °C refrigerant at U.S. $0.60 per 1000 kg of refrigerant used. Because the utilities were treated as purchased items, the capital costs for their generation are not included in the capital cost estimate but are reflected in their unit prices. Facility labor costs have been included for two plant operators per shift at an all inclusive rate of U.S. $50.00 per plant operator hour. Maintenance charges, labor, and materials have been included at 3% of the capital costs in the facility cost section. No provision has been included for laboratory or general administrative personnel. Annual production costs of the sbc-FAs and coproducts (slcFA and dimer) were summarized by summing the raw material and utility costs, charges for the facilities plant operators, maintenance and supervisory labor, operating and maintenance supplies, allowances for insurance and local taxes, and an allowance for depreciation and dividing this by the total amount of isostearic acid, stearate, and dimer produced. A breakdown of the sbc-FAs annual production cost and the percentage of the production cost by category are shown in Table S4 of the Supporting Information. The model estimates that the unit average production cost of the three products is U.S. $2.53 kg−1 ($1.15 lb−1). The capital and operating costs of items outside the production area are not included in these estimates. Office buildings, laboratories, utilities to the site, or railroad tracks to the facility would not be included. The cost of any water or air pollution control equipment to comply with environmental regulations is not included. Working capital, the cost of money, income taxes, and tax credits or return on the investor income is not included.

impact of further catalyst reuse on the production cost of sbcFAs, the model was modified to include up to 10 reuses. Figure 2 shows that the estimated unit production cost was U.S.

Figure 2. Impact of isomerization catalyst reuse experiments on the unit production cost.

$3.61/kg ($1.64/lb) if the catalyst was used only once, a 50% increase in cost relative to the base (five reuses) model. This shows the importance of catalyst reuse. Also, interestingly, based on the sensitivity studies, the impact seems to level off after six catalyst reuses, indicating that at that point the cost of the catalyst becomes insignificant compared to the total operating cost of the process.

5. CONCLUSIONS Process modeling provides important information on process economics and allows evaluation of the commercial viability of a technology. It also gives researchers insight into where to optimize the process so that the overall process becomes affordable on the industrial scale. Several parameters could be optimized to further improve the technology based on our process model: first, screening and development of less expensive new solid catalysts for the process; second, reduction of the relatively high temperature (250 °C) required for this reaction which could facilitate large-scale production; third, the recrystallization step to remove the majority of slc-FA; and finally, the distillation step to remove the dimeric byproducts. The identification of more efficient and selective zeolite catalysts, allowing the reaction temperature to be lowered and the recrystallization and distillation steps to be eliminated, would be of great importance in making this process commercially viable.

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ASSOCIATED CONTENT

* Supporting Information S

Tables on isomerization of unsaturated linear-chain fatty acids, product yields derived from the process model, and predicted isomerization facility capital costs and annual operating costs. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. SENSITIVITY STUDIES As mentioned previously, the treated zeolite catalyst (H-FerrK) can be reused at least seven times (Table S1 of the Supporting Information). The base model was designed with the zeolite catalyst reused only five times. To evaluate the

AUTHOR INFORMATION

Corresponding Author

*Tel.: 215 233-6643. Fax: 215 233-6559. E-mail: helen.ngo@ ars.usda.gov. D

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Notes

Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.

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REFERENCES

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