Three separate flowsheets were created for each of the waste sources being analyzed. While the feed streams and compositions may vary between them, the units and connections between them are the same in order to maximize the flexibility of the system when switching between types of industrial waste. An example of the process design can be seen below. Fust, the raw industrial waste is processed through a crushing unit in order to get the particles down to the designated size distribution. The refined waste is then sent to the leaclier where it is mixed with water, brine, extraction salt if applicable to the system, and an aqueous recycle stream. After the particles have been leached of the calcium ions, the solution is sent through a filter to remove any remaining solid particles, as detailed by the two-step approach. The filtered solution is then passed into the carbouation reactor, where CO, is bubbled in to be mineralized with the alkaline solution. The stream, now present with precipitated carbonate material, is then passed through another filter, represented as a separator, in order to remove the PCC product. Finally, the remaining electrolytic solution is sent to a splitter in order to recycle a specified fraction. Figure 2 is a schematic representation of the flowsheet.
The scale for this project will be based on a case scenario of a 100 MMscfd shale gas processing plant. This equates to an approximate feed flow of 94200 kg/hr worth of shale gas. The mass composition of CO, in shale gas varies from well to well, usually in the range of 1 to 9 wt%. Taking a median value of 5 wt% CO, and assuming the acid gas removal process produces a pure CO, stream, this equates to a flow rate of about 4700 kg/hr. Based on this reasoning, the flowsheets in the project will utilize a 5000 kg/lir feed of CO,.
In order to accurately simulate the intended processes, the chemistry needs to be properly defined in the program. The ELECNRTL property method was used in Aspen in order to describe the highly electrolytic solutions being modelled and to accoimt for the numerous dissociation and precipitation reaction equilibriums. While some equilibrium data is available in Aspen for the present species, missing parameters must be filled in with external data. The process can be divided into two major reaction components: The leaching unit and the carbouation unit. Because the leaching unit only contains sets of dissociating and equilibrating species, a GIBBS reactor block was used to model this process. The carbonator, however, is the specified unit for containing the mineralization reaction and is modelled using a STOICH reactor block. Details on these two units are listed in the sections below.
The GIBBS reactor block in Aspen takes the defined chemistry, thermodynamic, and equilibrium data to minimize the Gibbs free energy of the mixture input in order to calculate the thermodynamic equilibrium.
Data from literature indicates that, for leaching systems using waste particles of sizes smaller than 150 microns, steady-state is reached in well imder two horn s. This steady-state system can be approximated as an equilibrium and justifies the use of a GIBBS reactor unit. The specific components present in the reactor depends on the waste being utilized.
As specified in the defined efficiencies section, flow sheets involving the use of ammonium salts will utilize the extraction efficiencies reported in literature as a constraint in the Gibbs equilibrium calculations. This is done by placing restricted equilibrium definitions in the unit description parameters. By modifying the molar extent of the specific calcium leaching reaction in question, the proper extraction efficiency can be replicated and the remaining components in the system can reach equilibrium based on this constraint.
The carbonation vessel
The two input streams to this unit are the pure CO, coming from the theoretical shale gas plant and the filtered aqueous stream containing the leached ions. Because we are considering the calcium to be the only reacting component from this aqueous stream and we have the available carbonation efficiency data, we can model this reaction vessel using a STOICH reaction block. Here, the defined carbonation reaction mechanisms, as mentioned in the previous chapter, can be defined and restrained with a fractional conversion of calcium equivalent to the efficiency data.
The first unit in the process design is the crusher, wherein unrefined ash is pulverized down to a predetermined size and fed into the leaching unit. While the cited literature typically uses particle sizes in the region of less than 150 microns, for this project, particles will be comminuted down to a distribution around 75 microns in order to assure proper equilibrium is reached in the leaching unit. Indeed, it was determined that between variations of particle size, vessel temperatures, and leaching times, the size distribution of the waste particles had the largest impact on how quickly the system reached steady-state (Hall et al., 2014).
The utility of the crushing unit is a function of the particle size distribution being input into the system. In order to evaluate and take into consideration the varying possible particle sizes of imported wastes, a sensitivity analysis will be performed to determine the utility usage and performance across a range of plausible waste sizes.
Filters and recycle
Two filters are implemented in the process design, one after the leaching unit and another after the carbonation step. The first filter unit is set to separate all the solid components of the suspension stream, which contain both inert components originally present in the waste particles as well as any additional precipitated materials, like gypsum and calcium hydroxide, depending on the specific flow sheet. The second filter is represented as a separations unit, where the precipitated calcium carbonate is separated as its own stream as well a stream of uureacted CO,. This CO, in reality would be a product of the carbonation vessel, but was represented in the separations unit due to the limitations of the STOICH block.
A splitting block is used to represent the recycle of solvent back to the leaching unit and the remaining discarded waste stream. The recycle flow is described using two separated streams, a recycle- out of the splitter and a recycle-in to the leacher. While it is possible to represent a recycle stream in Aspen using a single connection, convergence issues can quickly arise if error tolerance standards are not met. This disconnected stream approach uses iteration as a means of approximate convergence. The recycle-out stream composition is copied into the recycle-iu stream and the simulation is executed once more. This procedure is then repeated several times until mass compositions are in agreement with mass discrepancies well imder 1%.
In this project, the influence of brine as both a solvent and a calcium source is explored alongside the use of waste particles. However, because brine is naturally acidic and the carbonation process requires basic conditions, the system needs to be supplemented with an additional water stream. In total there will be three streams acting a source of water: The brine stream, the water stream, and the recycled stream from the end of the process. A liquid to solid ratio of 50:1 was chosen as a compromise between efficiency and feed costs. As a consequence, the solid waste input will feed at a rate of 1000 kg/hr with the combined liquid streams equating to a rate 50,000 kg/hr.
The carbonation process naturally depletes the hydroxide concentration in the system and the output stream is highly concentrated with ions. Careful considerations must be taken to ensure that recycling solvent doesn't impact the pH levels of the next iteration of input streams. A recycle flow of 1000 kg/hr was chosen after calculations showed that regardless of the waste composition input the pH level of the aqueous stream was maintained above 8.5, as calcium carbonate begins to dissolve back into solution at pH levels below 8.3.
Results and Discussion
Across each of the three flowsheets, sensitivity analyses were performed by varying calcium weight fractions of the wastes. Subsequently, profit rates were determined, given the rate of product produced through each variation based on feed costs as well as waste disposal costs. Afterwards, the built-in economic analysis tools in ASPEN Plus were used to map and size equipment in order to calculate a rough estimate of the capital cost and evaluate rates of returns based on the varying production capacities. We will explore each waste individually and then will make recommendations on how to proceed forward based on the calculated results.
One of the main interests going into this project was determining how comminution of particles would impact the economics of the carbonation process. While the final particle size was set to be 75 microns across every simulation and waste type, a range of plausible input waste sizes was examined to see its effect on the grinding utility. A key reason for choosing industrial waste as an alkaline mineral source was the fact that the particles generally are already at viable reaction sizes. After evaluation at a few interval ranges, it is clear that the impact on grinding utility is near identical across the whole considered region. Particles in the size interval 1000-1050 microns resulted in a grinder electrical usage of 3.54 kW, while particles in the interval of 1550-1600 drew a usage of 3.80 kW. This already pushes the boundaries of typical of waste particle sizes and. for our purpose, can be assumed as the theoretical limit. ASPEN Plus Process Economic Analyzer also gives a theoretical axmual utility usage and estimates a value of approximately 470 MWh over an 8000-hour working year. This is equivalent to an approximate rate of 58.7 kW of power, and at an assumed electrical cost of 12 cents per kWh results in a utility rate of
7.04 dollars per hour.
Fly ash process evaluation
Fly ash is a special case in this analysis as there are two major calcium components typically found in this waste. Both calcium oxide and calcium sulfate are major leacliable components that come at a variety of mass fractions. To account for this, a sensitivity analysis was performed individually for both components. A calcium composition range of 20-55 percent was analyzed as this represented a plausible range for this waste type. One of the resulting process flow sheets is shown by Figure 3 with labeled flow rates for each stream. Figure 4a-d represent the PCC production rate and the resulting profit flow incorporating all of the costs (e.g., utilities, feed, waste) and assuming the selling price of $400 per ton of product.
Figure 4a. PCC production rate vs. CaS04 wt%.
Figure 4b. PPC production rate vs. CaO wt%
Figure 4c. Hourly profit vs. CaO wt%.
Figure 4d. Hourly profit vs. CaS04 wt%.
Waste cement evaluation
Similar to the fly ash evaluation, a sensitivity analysis was performed by varying the calcium oxide content from a weight percent range of 20-60. The process was performed with the addition of NH4NOs in an amount that would result in a 0.5 M concentration in the leacher (2000 kg/hr). This feed cost was considered in the profit rate analysis, with ammonium nitrate assumed to be valued at S300/ton. An example flowsheet from this simulation is shown by Figure 5, with stream flow rates displayed. Results of the analysis are shown by Figures 6a and b.
Steelmaking slag evaluation
As with the other wastes, the composition of the calcium source was varied; in this case, the source is dicalcium silicate. A weight percent range of 20-55 was evaluated and the process included the use of the ammonium salt NH4C1. In order to give a concentration of 2.0 M, a flow rate of 5349 kg/hr of salt is needed. This feed cost is considered in the profit analysis, where ammonium chloride is valued at $130/ton. An example flow sheet is presented by Figure 7, with stream flows. The results are shown by Figures 8a and b.
Fixed capital and investment return
ASPEN Plus Process Economic Analyzer was used to produce an estimated cost for the capital investment of the theoretical process after specifying the type of equipment each unit represented. For example, the crushing unit was specified as a ball mill grinder used for real world mineral pulverizing purposes and the two reactor vessels were representing enclosed agitation units. After allowing ASPEN Plus to automatically size the units based on flow rates, the estimate for a fixed capital cost was approximately $1.49 million. A sensitivity analysis was earned out in order to assess how the fluctuation in PCC pricing affects the profitability of the designed process. As by Figure 9, the range of PCC pricing from 200 to the original 400 dollars per ton is evaluated for all of the absolute minimum calcium mass percentages considered in the previous evaluation sections beforehand.
It is worth noting that the aforementioned case study is intended to illustrate the potential for mineralization as a strategy for monetizing CO, into value-added chemicals. Analogies can be made with other sources of CO, and other products. A key aspect to understand about shale gas processing is that CO,
Figures 6a and b. Results for configuration #2.
is not the predominant GHG emission. Methane is emitted during multiple steps of the treatment of shale gas and is present in higher concentrations than conventional gas. More CO, is produced from combustion of the gas rather than its processing. What makes shale gas appealing for this type of integration process, as opposed to other facilities, is the purity of the CO, stream that occurs as a by-product of treatment. However, the applications of this mineralization technology are not limited to this specific instance of industrial development. Other common industrial sources of CO„ like flue gas emissions, show promise of acting as a feed source for mineralization. A previous pilot scale study investigated the liability of mineralizing CO, emitted from a coal-fired power plant by reacting fly ash with flue gas emissions in a fluidized bed reactor. It was determined that an appreciable amount of CO,, as well as SO, and Hg, was captured and mineralized in the process (Reddy et al., 2011). The exact purity of the calcium carbonate is unknown and additional separation steps would be necessary, but further research into this technology would be a worthwhile endeavor. While the economic liability of such a process would have to be evaluated on a case-by-case basis, the possibility of adapting this technology towards major industrial emitters of CO, is appealing.
Another important consideration is the demand and market size for carbonate products. A report from 2018 estimated that the global demand for calcium carbonate in 2016 was 113.7 million tons and is expected to grow to 180.1 million tons by 2025 (Market Research Reports & Consulting). Alkaline particle wastes are available in large enough quantities to supplement this carbonate market with transportation being the main obstacle. While this market growth is encouraging for mineralization technology, it has also been estimated that the amount of anthropogenic CO, produced is orders of magnitude greater than
Figures 8a and b. Results for configuration #3.
Figure 9. Sensitivity analysis of hourly profit as a function of selling prices.
the market size for carbonate products. Further investigation into the economics of other mineralization pathways, like the production of magnesium carbonate, could be one option to diversify and promote mineralization technology. More likely though, the solution to alleviating the global CO, burden will come from a combination of sequestration and utilization technologies.
Mineralization is a relatively new consideration for carbon dioxide sequestration. While the benefits of long-term storage and value-added product potential have made it a compelling pathway for utilization, hurdles that have kept it from becoming a major solution to the challenges of CO, reduction still remain. It has been said that the current knowledge of mineralization is insufficient for us to be able to conclude if the process is energetically and economically feasible. However, no industrial scale process has been implemented or thoroughly investigated and this project aims at helping to determine its potential.
Multiple layers of sustainable opportunities have been merged together in order to create a unique and practical operation. Carbon dioxide utilization, hazardous waste fixation, and brine water application are independent processes that, with proper integration, could lead to possibilities for creating a greener but still profitable industrial system.
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