Part 1: Comparison of flow sheet software and justification of the chosen software.
Aspen Plus is a process simulation software that applies mathematical models to a given process design with specified thermodynamic models and outputs the expected performance of the process (Karimi, 2009). The advantage of this software includes a robust database that enables the software to handle complex processes including industrial processes. The demerits of the software include a relatively weak graphics and interface when compared to other process simulators, a complicated simulation that requires a definition of kinetic and mathematical models, and Aspen Plus has trouble in convergence. AspenTech developed Hysys, and it shares many features with Aspen Plus. However, Hysys was designed to simulate oil and gas refining processes. The software has features to optimize the feedstock operating parameters. With the support of calibrated models, Hysys allows planning and optimization of refinery processes decision making. The reactor models can be automatically exported to other Aspen PIMS, and other AspenTech. Despite this software being ideal for oil and gas processes, its main limitation is the need for calibrated models for its operation.
ProSim software was developed to aid improve processes in energy, oil, gas, food, beverage, petroleum and other processing applications. The software package is tailored to improve the process design, plant efficiency and reduce hazardous emissions. The usability and functioning of the software have improved over time as the software is increasingly used in research institutions. The focus of the ProSim developers of this software is to enhance mass and energy balance calculations in steady state processes especially in industrial applications (prosim.net, 2019).
Unisom was developed by Honeywell to simulate oil refineries and chemical plants. Key features of the software include heat and material balances, estimation of liquid-vapor phase equilibrium, estimation of physical properties, and simulation of different chemical equipment (Karimi, 2009). The pros of this software are that it comes with inbuilt components that updates to the latest version without any additional cost, has a powerful database and has a user-friendly flowsheet. The cons of the software include it permits impossible behaviors such as water flowing below the freezing point.
SuperPro Designer software enables modeling, analysis and optimization of integrated processes in many industries including pharmaceutical, mineral processing, biotech, water purification, air pollution control among others. The software has tools that enable the design and analysis of the processes during the development phase. In comparison to other process modelers, this software boasts of the ability to model over 140 independent procedures. It has features to support rigorous reactor modules and support both material and energy balance. SuperPro Designer has an extensive database with information related to materials and resources. The software has unique functionality that facilitates equipment sizing and costing, and this enhances process economics. There are tools to analyze throughput and track resources such as raw materials and utilities as a function of time. SuperPro Designer software requires a PC with basic features, most of which are readily available. The primary challenge of the Software is the high acquisition cost.
Considering the plant in the case study is bio plant, SuperPro Designer is selected as the ideal simulator due to its superior features as discussed above, and since it is provided by the university, hence no acquisition cost is incurred
Part 2: methods used to predict physical properties in the system.
Superpro designer uses Raoult’s law for ideal mixtures as a default setting to predict the physical states for each of the chosen components by using pure component data only. When the mixtures deviate from ideal conditions the programme starts to use alternative methods to determine the physical properties of each component. These alternative methods consist of either calculating the compressibility factor (z) or the activity coefficient () depending upon whether the component is predominantly in the gas phase or the liquid phase. For non-ideal gas phase components, the compressibility factor would be calculated from each of the equations of state which are empirically based. The three equations of state (EOS) that the program uses in order to calculate each compressibility factor are the Virial, Peng-Robinson and SRK. If the components are both non-ideal and in the liquid phase, then a relevant activity coefficient is required to be calculated which are determined from one of the inbuilt activity coefficient models within the program. The three default models which can be used are the Wilson and the two variants of the NRTL models.
When using the default settings for superpro designer, there are 3 standard components which are automatically entered from the designer databank to the process file. These three components are Water, Nitrogen and Oxygen which were commonly used throughout the flowsheet design.
There are two main toolboxes contained within the program which can be specified by the user depending upon the level of detail required. These two toolboxes consist of the shortcut and the rigorous. For this flowsheet design, the rigorous toolbox was used as this allowed a more detailed determination of the properties based upon the equilibrium constant, the fugacity coefficient for vapours and the activity coefficient for liquids. An added benefit of this toolbox was that the methods of determination for each of the coefficients could be interchanged based upon the design. On the contrary, the shortcut toolbox determines the physical properties on a component by component basis by using custom built packages which weren’t as suitable for this design.
Due the availability of data for several components, the ideal package was used. This meant that all the individual component properties didn’t have to specified by the user.
For the flowsheet design the default pack for ideal mixtures was used.
Part 3: Description and Justification of Process Units
The first unit operation depicted on the flowsheet diagram which was required for this biogas production process was a vibrating screener. The raw feedstock consisting of animal manure would directly enter the first unit whereby the sand and iron and any other foreign materials would be separated from the manure material itself. Although in the original design a screening grid was selected as opposed to a vibrating screener, these two units use the same method of separating materials by separating materials of different sizes and/or densities in order to collect the desired material. Thus, in this case, the vibrating screen very closely resembles the screening grid which would be required and in terms of process units available within the chosen software package the vibrating screener was the closest match.
The next process unit required for further pre-treatment of the outlet screener material was the pasteuriser. The pasteuriser unit itself contains heating pipes and mixers which are required to homogenously heat the material fed into it. The tank would then begin to pasteurise the material once it had reached a temperature of around 70-85°C and this heating would continue for around 30-60 minutes. This unit is a fundamental pre-treatment process because without this unit, any potentially harmful pathogenic material would remain within the feed material and contamination of the digester culture would occur. Contamination of this kind would significantly inhibit the digestion process and have the effect of reducing the biogas yield further. Pasteurisation of the feedstock also means that valuable fertiliser can be produced if the amount of material fed into the process is too much to begin with and therefore giving other options.
Now that the majority of the contaminants have been removed from the feedstock, the last step in the pre-treatment process was to put the outlet pasteuriser material through a rotary drum mixer. The rotary drum mixer was designed to allow the correct level of dilution of the feedstock to be obtained before entering the digestion process. Therefore, an inlet water stream to the mixer was required so that the dilution rate can be adjusted each time the feedstock comes through. By providing the correct level of dilution to the feedstock, the optimum dry matter or dry solids content can be obtained and therefore enhancing the rate of biogas production. The addition or removal of water from the feedstock was also found to be essential in order to allow the slurry mixture to move through the pipeline system at steady rate.
Once all of the pre-treatment steps have been completed then the feed material can be placed into the next unit on the flowsheet, the digester. The digester, considered as the main process unit, holds the input material to allow the process of anaerobic digestion to take place at a temperature of around 35°C. The process of anaerobic digestion would allow a series of biological processes to take place in order to allow the microorganisms to break down any biodegradable material in the absence of oxygen or air. The flowsheet diagram depicts two digester tanks in series, which was necessary in order to allow the necessary maintenance or cleaning schedule to run meaning there would very occasionally be a one running and one spare type scenario. An additional process outlet stream was placed on the second digester to allow any digestate material to be removed from the digestion process if it had remained within the digester for too long. This material can then be used for other purposes. The digester can be considered as the fundamental unit of this production process and was chosen from the program package as the most generalised form so that different design considerations such as roofing and materials can be swapped around without having to change the units already in place on the process flowsheet.
The mixing unit, after the 2 digester units in series, was required in order to homogenise the biogas compositions from both digesters which consists of mainly methane gas but also a few other components such as carbon dioxide, hydrogen sulphide and water. The water which leaves the mixing unit remains in the liquid phase after leaving the digester due to the digester temperature being set at 35°C. Without the mixing of the biogas product then there would be intermittent streams of biogas going towards the separation units and not a continuous flow of fixed composition biogas. This would therefore significantly decrease the efficiency of the process due to the process almost becoming batch like and wouldn’t enhance biogas yield either.
The first of the separation units for the biogas product is called the absorption column unit. The absorption column was required in order to remove the water from the biogas mixture, working towards the aim of obtaining a stream of methane. The absorption process works by using a gas stream, which in this case was the biogas product stream from the mixing unit, and a liquid stream. The liquid stream would be input into the absorption column and flow in a counter current manner to the biogas product stream in order to absorb the water component from the gas as it flows past it. In order to enhance this process, a liquid stream of ethylene glycol was chosen which has an affinity for the water phase and thus a liquid stream of glycol-water exits from the base of the column. The biogas product stream without the water then exits from the top of the column and proceeds to the next process unit operation. The absorption column unit would also be packed with suitable packing material to enhance the mass transfer of water to the liquid phase. This absorption unit was required because it was the most economical and efficient method of removing water from a gas stream containing other components. Since the process moves at a rapid pace there wouldn’t be sufficient delay in the process to use condensation steam traps to remove the water and thus in order to keep a continuous flow of material an absorption column was the most appropriate choice.
Beyond the absorption column an additional heating unit was required in order to heat the biogas mixture up to a sufficiently high temperature in order to ensure that the entirety of its contents were in the gas phase. This was essential because only gas phase mixtures were permitted within the next unit operation on the flowsheet.
The methane, carbon dioxide and hydrogen sulphide gas mixture were then subjected to the next downstream processing method, as indicated on the flowsheet, which was the adsorption column. As previously mentioned, the inlet stream to the adsorption column must all be within the gas phase and the reason for this was due to the adsorption columns being operated on a pressure swing basis. If any liquid had remained within the gas mixture then the pressure swing would not be sufficiently effective, due to the density of the gas being different to that of the liquid and being much more sensitive to pressure changes. As the inlet stream enters the adsorption column the hydrogen sulphide would begin to adsorb onto the packed adsorbent within the column and the other components would pass through the column with minimal or no adsorption. This occurs because a specific adsorbent material can be selected which would have a high selectivity for the hydrogen sulphide and would not have such a high selectivity for the other two components. This again was a strong reason to choose the adsorption column due to the variety of adsorbents available which can ensure that the maximum amount of hydrogen sulphide can be removed, which is highly beneficial due to its specific hazardous
A heater was placed just before the first adsorption column to let the gas product to reach optimal temperature (175.
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Due to the design of the package used, a counter current flow stream was required to be input into the pressure swing adsorption column as well as the gaseous material itself. Therefore, careful selection of the second incoming stream was required in order to minimise the effect that it could have on the bio-product mixture. The second incoming stream to the adsorption column was chosen to be an inert stream of nitrogen for two separate reasons. The first reason was that nitrogen gas has inert properties meaning that it wouldn’t affect the composition of the biogas in any way at all when flowing past it in the adsorption column. The second reason for selecting nitrogen as opposed to other inert substances was that nitrogen can be obtained through the least additional cost to the process. Therefore, there are both economical and unaffected composition benefits to this unit selection.
After the removal of the hydrogen sulphide from the gaseous stream a further adsorption column was used in order to remove the final component from the biogas, carbon dioxide. This adsorption column was based upon similar principles to the previous column whereby the changing pressure allows the undesired component to be adsorbed onto the adsorbent and thus a gas stream of different composition would leave the top of the column. In this case the carbon dioxide would be adsorbed and the gas stream of methane only composition would leave the top of the column. As with the previous case an additional inlet stream was required to be specified for the adsorption column and therefore the most suitable choice of input material was an inert gas. For the same reasons as the first column, nitrogen gas was used in order to minimise interaction between the streams and reduce process costs, however, there was an additional reason being that only one inventory of inert gas would be required and there would not be the additional safety considerations and costs of storing two different inert gases in two different pressure vessels.
Two cooling heat exchangers were setup before and after the adsorption column just to reach the desired temperature.
After the second adsorption column the gaseous product stream would only contain the pure methane gas and therefore would be almost ready for sale as a commercial product. Before this can occur since the methane gas stream was heated to a higher temperature previously, it requires cooling before being directed to the national grid. The cooling unit would allow the methane gas to reach the appropriate temperature for further processes such as pipeline transfer or compressing.
References:
- Karimi, M., 2009. Simulator Comparison. s.l., NTNU.
- prosim.net, 2019. ProSimPlus. s.l., s.n.