Life cycle assessment of solvent extraction as a low-energy alternative to distillation for recovery of N-methyl-2-pyrrolidone from process waste

2.1 Base case: incineration

The resin precursor process manufactures liquid polyimide (PI) and polybenzoxazole (PBO) precursors that are cured by DuPont’s customers to form PI and PBO resins. PI and PBO resins can be used in microelectronics, microelectromechanical systems and other applications [37]. The Parlin plant currently handles the waste by disposing it off-site for hazardous waste incineration. This approach is expensive, and even with heat recovery, generates significant life cycle emissions. The waste generated equates to 1,077,000 kg/year of waste composed of 81.5% water, 17% NMP, 0.5% HEMA, 0.5% TFA and 0.5% HCl. Additionally, 1,209,000 kg/year of total life cycle emissions are generated, 89% of which are CO₂ emissions. NMP manufacture and hazardous waste disposal account for 64% and 19% of the total life cycle emissions, respectively. The life cycle operating cost of the base case is 1,466,000 $/year. NMP manufacture and hazardous waste disposal account for 54% and 39% of the life cycle operating cost, respectively. More details regarding the base case can be found in Pastore et al. [2].

2.2 NMP recovery: distillation

After investigating several NMP recovery options, Pastore et al. [2] determined that distillation would serve best as the recovery method for this application. The proposed distillation recovery operation utilizes a single distillation column for two batch distillation steps. The first removes most of the water and acids from the base case’s hazardous waste, which is then neutralized and disposed of as nonhazardous wastewater. The second step then removes the HEMA from the remaining waste. The HEMA is disposed of as hazardous waste, and the remainder is 99.97% pure NMP, ready to be reused in the resin precursor manufacturing process. The distillation system recovers 94.7% of the NMP from the hazardous waste. Relevant flowrates and compositions for this recovery system can be found in Table 3.

Recovery of NMP using distillation reduces total life cycle emissions by 44% and reduces life cycle CO₂ emissions by 40%. The ReCiPe endpoint single scores for recovery using distillation in the human health, ecosystems and resources categories were reduced by 46%, 42% and 27%, respectively, when compared to the base case. The life cycle operating cost of the base case is reduced by 1,217,000 $/year or 83% when NMP is recovered using distillation. The installed capital cost of the equipment for this recovery method is $1,498,000, according to the Aspen Capital Cost Estimator. Using the economic analysis described by Pastore et al. [2], distillation saves $1,750,000 over the first 5 years after instillation and $3,128,000 over the first 10 years (Table 2). Other alternatives that were considered as well as a more detailed description of the distillation process can be found in Pastore et al. [2].

2.3 NMP recovery: on-site solvent extraction

As stated in the previous case study, distillation can be energy intensive, which can negatively affect operating costs and environmental impact [14], [21], [23], [31], [38]. This concern is particularly applicable in this case due to NMP and HEMA being high-boiling organic compounds, which indicates that large amounts of energy will be required to separate the two compounds. Similarly, the major component of the waste is water, which is a lower boiling compound than both NMP and HEMA. As a result, a large amount of energy in the form of steam is required to remove the water from the waste in the first distillation step of the proposed recovery method. In fact, 87% of the steam used in the distillation recovery scheme goes towards removing the water.

Extraction as a low-energy alternative to distillation has been explored for use in biofuels, for separating high-boiling organic compounds from fermentation broths with a high water content [39], [40]. Again, the purification of aqueous mixtures with low mass fractions of high-boiling organic material is both energy and cost intensive. In this case, a high-boiling organic, namely NMP, is to be removed from waste with a high water content. Research performed in the field has proven the efficacy of extraction for similar aqueous-organic separations, but at the time of the previous study [2], investigation of extraction was not considered, due to limited data for NMP extraction from aqueous mixtures [23], [38]. A laboratory study by Yang and Zhu [41] tested several extracting solvents and showed chloroform performed best for recovering NMP from solvent waste. These claims were independently evaluated using their Hansen Solubility Parameters [42] and process simulation in Aspen Plus. The results of the study were confirmed, and chloroform was chosen as the extracting solvent for future process simulations. Chloroform not only showed high NMP recovery, but was also easy to separate and recycle from the extract stream using distillation. Selecting a solvent with such properties is crucial for designing an energy efficient extraction-based solvent recovery process. Additionally, DuPont uses chloroform at another facility to recover large quantities of NMP from solvent waste, further supporting the choice of extracting solvent.

3.1 NMP recovery: on-site solvent extraction

The extraction process for recovering NMP from the hazardous solvent waste is shown in Figure 1. As in the distillation design, the waste collects in a holding tank, prior to treatment. Once enough waste is collected, it is preheated before being fed to an extraction column along with recycled extracting solvent and make-up extracting solvent. The feed is preheated because the recycled extracting solvent is at a high temperature after leaving the first distillation column. Process simulations performed in Aspen Plus showed that a feed at room temperature resulted in a higher water content in the extract. Other alternative methods for heating and cooling particular streams resulted in higher operating costs and emissions.

Figure 1:

Extraction recovery system flow diagram.

The raffinate, which is composed of most of the water and acidic components of the waste, is sent to a neutralization tank where it is treated before being disposed of as nonhazardous wastewater. The extract contains the extracting solvent, NMP and HEMA. The extract is sent to a distillation column to recover the extracting solvent. The distillate is mostly chloroform, with a small amount of water, which is removed using a decanter. The water removed is sent to the previously mentioned neutralization tank, and the recovered extracting solvent is reused in the extraction process. The bottom product of the first distillation column is a mixture of NMP and HEMA. This mixture is sent to a second distillation column where these components are separated. The HEMA is treated as a hazardous waste and sent for incineration. The recovered NMP is stored in a holding tank, ready to be reused in the resin precursor manufacturing process. The extraction system shown in Figure 1 recovers 96.2% of the NMP from the hazardous waste at a purity of 99.98%. The remainder consists of 0.014% HEMA as well as trace amounts of water, TFA, HCl and chloroform. Relevant flowrates and compositions for this recovery system can be found in Table 3.

Like the proposed distillation recovery system, this design also requires high-pressure steam to operate the two distillation columns. However, the necessary amount of steam is reduced by about 48%. This reduction in steam usage is responsible for reductions in life cycle emissions; damage to human health, ecosystems, and resources; and operating costs. The life cycle emissions associated with the base case, distillation recovery option and on-site extraction recovery option are shown in Figure 2. The on-site extraction recovery option generates 463,000 kg/year of total life cycle emissions and 436,000 kg/year of CO₂ life cycle emissions. These values represent a 62%, and 59% reduction in total life cycle emissions and CO₂ life cycle emissions, respectively, compared to the base case. Compared to the previously proposed distillation design, total life cycle emissions are reduced by 32% and CO₂ life cycle emissions are reduced by 33%. The NMP recovery using extraction results in life cycle emissions reductions because of reduced virgin NMP use, reduced hazardous waste disposal and reduced steam usage. The chloroform used in the extraction process does not have a significant impact on the LCA; the design incorporates a chloroform recovery system and only therefore the make-up chloroform is accounted for in LCA calculations.

Figure 2:

Life cycle emissions for base case and alternative cases.

Total, total emissions; CO₂, carbon dioxide emissions.

The ReCiPe endpoint single scores for the base case and recovery cases are displayed in Figure 3. Recovery of NMP using on-site extraction produces single scores for human health, ecosystems, and resources of 18,200, 9100 and 16,100 Pt/year, respectively. The NMP recovery using on-site extraction reduces the scores by 54%, 58% and 54% for human health, ecosystems, and resources, respectively. Compared to recovery using distillation, the scores are reduced by an additional 16%, 27% and 37% for human health, ecosystems, and resources, respectively. Most of the further reductions are caused by the decrease in steam usage compared to the distillation recovery option.

Figure 3:

ReCiPe endpoint single scores for base case and alternative cases.

HH, human health; E, environment; R, resources.

The life cycle operating costs for the base case and alternative cases are displayed in Figure 4. On-site recovery using extraction has a life cycle operating cost of 224,000 $/year, resulting in a reduction of 1,242,000 $/year or 85%. These reductions are similar to those produced by recovery using distillation, which were 1,217,000 $/year or 83%. Again, the reduction in steam usage reduces operating costs, but a greater number of pieces of equipment increase maintenance costs. Only the make-up chloroform costs are included in the operating expenses as the boundaries include the chloroform recovery system. To better compare the economic feasibility of recovery options, a more detailed economic analysis was required. This analysis takes both operating and installed capital cost of the alternative into consideration. The details of the analysis method are discussed in detail by Pastore et al. [2]. Ultimately, the capital cost of this alternative was determined to be $2,062,000. This, along with the savings of 1,242,000 $/year, results in 5-year net present value (NPV) of $1,334,000 and a 10-year NPV of $2,762,000 (Table 2). The distillation recovery method results in a 5-year NPV of $1,750,000 and a 10-year NPV of $3,128,000 (Table 2). In the long run, this alternative reduces emissions, and saves money even with the purchase of equipment. It reduces emissions more than the distillation recovery option, but does not save as much money over the course of 5 or 10 years.

Figure 4:

Life cycle operating costs for base case and alternative cases.

3.2 NMP recovery: solvent extraction and reuse at the sister plant

Another aspect of the study was to determine if there were other DuPont facilities that used/disposed NMP and would be interested in this recovery project. One thought was that multiple locations with similar waste streams might have a desire to “pool” resources for a capital improvement project for a joint solvent recovery facility or the other plants might already have underutilized capacity to recover the solvent with existing extraction equipment. The TRI data for all DuPont facilities was analyzed to determine facilities of interest based on NMP disposal.

One plant (referred to as “the sister plant” from this point forward) was identified and investigated as a candidate for off-site solvent recovery and reuse. This DuPont facility produces other specialty chemicals, generates a significant amount of aqueous NMP waste, and recovers NMP for reuse on-site with an extraction process. Based on the analysis of TRI data, despite having an NMP recovery system, this plant still disposes highly contaminated NMP waste. Therefore, it was assumed that this plant has a need for more purified NMP waste, and would benefit from recovering and reusing the NMP in Parlin’s waste. The amount of waste processed at this facility is very large compared to the waste generated by the Parlin facility, which is equivalent to <1% of the other plant’s waste. This indicates that processing of the Parlin waste should not have a significant impact on the other plant’s day-to-day operations.

This plant has a large-scale extraction column that uses chloroform as the extracting solvent for NMP recovery. To provide a uniform method of LCA analysis, the process at the other plant was modeled as the proposed on-site design without the distillation column that further purifies the recovered NMP by removing the HEMA. The other plant uses NMP in the manufacture of specialty chemicals that do not require electronics-grade NMP, allowing for the removal of the second distillation column that separates the NMP and HEMA. The HEMA left in the recovered waste, when mixed with the waste the other plant already recovers, results in a HEMA concentration of approximately 12 ppm. Due to the scale of the other plant’s operation, as well as an unknown composition of their waste, determining the recovery and purity of the waste using this alternative poses a challenge. However, processing additional waste would reduce virgin NMP usage at the other plant by 180,000 kg/year. Relevant flowrates and compositions for this recovery system can be found in Table 3.

The life cycle emissions associated with the base case and all three recovery options are shown in Figure 2. Solvent extraction at the sister plant generates 464,000 and 435,000 kg/year of total and CO₂ life cycle emissions, respectively. Compared to the base case, this results in a 62% reduction in total life cycle emissions and a 60% reduction in CO₂ life cycle emissions. Compared to the distillation recovery alternative, extraction at the sister plant results in reductions of 32%, and 33% for total and life cycle CO₂ emissions, respectively. These numbers are very similar to those for the on-site recovery option. Where emissions are avoided by removing the second distillation column, new emissions are introduced due to transportation of the waste from Parlin to the other plant.

The ReCiPe endpoint single scores (Figure 3) show that extraction and reuse of NMP at the sister plant results in single scores of 21,700, 10,300 and 19,100 Pt/year for human health, ecosystems, and resources, respectively. This results in reductions of 45%, 52%, and 45% for human health, ecosystems, and resources, respectively, compared to the base case, and reductions of 0%, 18%, and 25% for human health, ecosystems, and resources, respectively, compared to the distillation recovery design. Significant reductions in steam usage are slightly offset by production of the make-up chloroform and transportation of the waste from one plant to the other.

The life cycle operating costs of the base case and three alternatives can be found in Figure 4. Solvent extraction and reuse of NMP at the sister plant has a life cycle operating cost of 289,000 $/year. Compared to the base case, operating costs are reduced by 1,177,000 $/year or 80%. Extraction and reuse at the sister plant is slightly more expensive than on-site extraction and on-site reuse due to shipping costs, but a complete economic analysis shows that recovery and reuse at the sister plant is much more favorable in the long run; the main reason for this being the absence of any capital costs. Additionally, recovery and reuse at the sister plant has a higher operating cost for the Parlin facility, because they are still required to purchase virgin NMP for use in the resin precursor manufacture process. Also, it is unknown whether Parlin with pay the other plant (for hazardous waste “disposal”) or vice versa (purchase of recoverable NMP); regardless of any transaction between the two plants, the overall operating cost within the DuPont Corporation is reduced by the NMP recovered and reused at the other plant as transactions between plants are within the system boundaries. Similarly, reductions in life cycle emissions and damage scores are for the DuPont Corporation, not for the Parlin facility.

As stated earlier, the sister plant already has the means to recover NMP using a solvent extraction system. Therefore, no equipment needs to be purchased or installed. Using the same methods as those used by Pastore et al. [2], the 5-year and 10-year NPV were calculated. The NPV after 5 years for solvent recovery and reuse at the sister plant was $2,923,000 and $4,198,000 after 10 years (Table 2). Over the course of 5 years, this alternative economically outperforms the distillation recovery option by $1,173,000 and over the course of 10 years by $1,070,000. By eliminating capital expenses, the NPV of the recovery of NMP dramatically improves.