Intensifying Heat/Mass Transfer and Plant Design Layout
Process intensification in heat and mass transfer is achieved particularly in chemical systems by optimizing molecular energy in gases and liquids involved in the reaction and heat transfer processes. This process focuses on key different components that play specific roles in mass and heat transfer in these systems. These include methods such as absorption, mist disengagement, filtration, dust disengagement, electrolysis, and distillation among others.
The buoyancy of fluids is used for mass transfer intensification and bears a general relationship defined by the equation, Δρg. These variables affect the interfacial shear characteristic of liquid droplets directly influencing the flow of heat in a system. On the other hand, electrical bulbs, waxes, and similar sources of heat and light depend on variables such as the diameter of the electric valve and the diameter of wax globules (Kearney, 1999). The behavior of these devices may be inferior leading to poor heat transfer and larger heat losses.
On the other hand, the intensification of heat transfer relies on various mechanisms involved in the process. This intensification correlates with the nature of the fluid stream and the method through which heat is transferred within the system. Generally, when fluid turbulence is high, higher shear forces result, influencing values of heat transfer coefficients. This is well illustrated in the following expressions. These values can be manipulated to optimize mass and heat transfer.
- Nu = 1 Re0.8 Pr0.33 3.1
- Sh = 2 Re0.8 Sc0.33 3.2
Nu = Nusselt number; Re = Reynolds number; Pr = Prandtl number; Sh = Sherwood number. The film transfer coefficient can be obtained by integrating these items in the calculation process. In these expressions, d is the diameter of the fluid and v is the propagation velocity.
Increasing the pumping velocities may lay severe penalties on the piping system and other system components. That may adversely affect process intensification since every element incorporated in the process need to withstand all operating conditions. On the other side, the benefits of mass transfer intensification can be realized by compounding smaller diameter pipes, manipulating the geometry of the internal surface of the pipe and the geometry of the surface in contact with the fluid. Enhanced heat transfer within systems comes with a number of benefits.
Heat transfer intensifications, therefore, are in two categories. The passive category focuses on the external powers of systems while the active category emphasizes rotational effects. Passive techniques incorporate the host equipment enhancement techniques. These techniques include coating surfaces, surface catalysis, and porous structures, among others (Kearney, 1999).
Passive enactments are characterized by the coatings that are applied to surfaces to promote transformations of fluids to liquid or steam. In addition to that, it has been demonstrated that dropwise condensation of fluids comes with positive benefits overall. It sustains endothermic processes within these systems and leads to optimum heat intensifications. In addition to that, coating surfaces also intensify the heat and mass transfer and make them act as catalysts.
By integrating these techniques into the process, heat and mass transfer are intensified. Despite these advantages, catalytic surfaces suffer from the disadvantage of a short lifespan. Another passive mechanism of process intensification is surface modification. Surfaces can either be fully or partially penetrated in the welding and drilling process. These processes allow the formation of protruding features that impact heat transfer in the process. This approach optimizes heat transfer on surfaces by using protruding microns that form on the surface of the material.
Extended surfaces intensify heat transfer by making use of the passive heat transfer technique. Extended surfaces include motor vehicle rotors and radiator fans. Other examples are rotary absorption heat pumps. Heat transfer can be intensified in this process by designing and redesigning the surface that comes into contact with the fluid or mass in the transfer process.
On the other hand, certain mechanisms may be used to enhance mass transfers. These include mixing, rotation, and vibrations. A rotating system can use an enhanced method to transfer mass from one point to another. When applied accelerations are higher, the buoyancy of fluid also varies accordingly. That reduces the film size of a fluid increasing a turbine’s output into the system. That reduces the heat transfer area, increases the evaporation duty, and reduces compressor power in the process. In addition to that, heat-actuated systems draw a range of benefits by optimizing heat and mass transfer. Among them is the reduction of heat transfer when operating at a fixed temperature, the capacity to exchange higher amounts of heat is achieved, reducing the log mean temperature difference is the process.
Mixing relies on the viscosity of the liquid component in the intensification process. Mixing influences fluid paths and intensifies mass transfer in a system. On the other hand, vibrations influence to a large extent the intensity of mass transfers in a system. Vibrations remove the fouling effect of specific fluids while intensifying mass transfer.
Other intensification methods include electrical intensification techniques. One specific advantage of this method is that heat is generated directly from the working surface and the material itself. This approach breaks down barriers oftoeat transfer. Variables that influence this approach include the dielectric and dissipation constants of these materials. Attempts to catalyze microwaves with micro-reactors have failed in the short term.
The design or layout of a plant is influenced by the concept of process intensification in the specific area of heat and mass transfer intensifications due to a host of benefits that are realized at industry levels. This affords radical changes in the design and layout of these plants (Plant Layout, n.d).
Hydrodynamic and kinetic reactions within mass and heat transfers systems afford different plant designs and layouts. Issues such as waste disposal and the choice of materials to be used in the construction define a plants’ layout design. This is to optimize process intensification. In addition to that, material properties used in the construction of these plants are influenced by PI. One example is the production of ammonia. Pressurized synthesis of ammonia has experienced drastic drops in pressure to a few hundred bars. It further simplifies plant design, plant maintenance, operation, and control mechanisms (Gupta, 1999).
Adams (2010) argues that PI influences design factors that determine the continuous operation of a plant and its lifespan inherently not evident in previous models. Different techniques are incorporated to determine the most efficient and cost-effective design. This is exemplified in chemical plants where semi-continuous and discrete processing techniques are used after an intensive evaluation.
Ways In Which Process Intensification Could Lead To Energy Savings/Reductions In CO2 Emissions
One approach is to reduce the viscosity of fluids such as crude oils using physical methods. When catalytic substances are used in the refining process, a significant reduction in the amount of the pollutant gas is reduced in the process. In addition to that, features incorporated in the design of a component influence to a large extent the intensification process that directly limits the amount of polluting gases released into the atmosphere.
A radical approach to the design and layout of a plant bears a strong relationship to compaction heat exchanger approaches among other examples. In this approach, various units are integrated together to work as a single unit in the process to optimize operating space and other aspects of plant design. Process intensification methods can span hardware equipment and software methods. Hardware methods include ultrasonic reactors, monolithic reactors, and several others. On the other hand, that includes static exchangers, compact heat mixers, among several others. Software methods include malfunctional reactors that include reverse flow reactors, reactive separations among others.
One approach to achieving energy savings is by incorporating spinning disc reactor technology. This technology is capable of producing films using centrifugal forces and the film is characterized by a thin layer that is highly sheared. This provides micromixing capabilities through the extremely high coefficients of convective heat transfers values. In addition to that, this environment provides higher and faster reaction kinetics.
Another approach is external magnetic fields. These fields possess a body of forces that can be applied to electrically nonconducting substances or materials and fluids. This approach influences a two-phase flow, particularly in trickle flow reactors.
Another method is the use of ultrasound technology. Ultrasound stimulates and enhances mass transfer in systems and has the possibility of creating very high temperatures in a system. This process is also referred to as Cavitation. In addition to that, process intensification can be achieved in chemical processes commonly referred to as sonochemistry.
Molecular simulations in areas such as quantum physics, molecular dynamics, and Monte Carlo simulations bear strong relationships with approaches of improving energy savings (Keil, 2007).
Different strategies are used to reduce CO2 emissions and lead to energy savings. One of these areas where this is applied is in petroleum processing refineries. This is one of the sectors that produce large amounts of CO2 for the environment. However, this sector employs conventional refining methods and is in dire need of being reengineered. One key characteristic that is an impediment to process intensification in this field is the fouling concept.
Thus to be successful in this field, process restrictions need to be removed to enhance system and process efficiency in mass and heat transfer within these systems. In this scope, large furnaces are used in the process to capture and stimulate endothermic reactions through hydrocarbon pyrolysis. The heat generated from these furnaces is continuously removed from the gases through a labyrinth of pipes and piping systems. Adiabatic reactions occur that relea in the range of 900 degrees centigrade in se temperature the process gases. In addition to that, the furnace incorporates a gas turbine cycle on a catalytic surface. This is achieved through various methods.
One of them is pressurized combustion. This method extracts high-grade heat while lower-grade heat is utilized in running other processes. CO2 produced from oils refineries results from the continuous removal of coke that is deposited on catalytic surfaces of the refinery impeding efficiency in heat and mass transfer. To effectively handle this problem, the concept of bulk chemicals plays a fundamental role in determining energy savings in the production process.
One such approach is incorporating the concept of local production that embraces a radical shift in the manufacturing process. Various elements in the manufacturing process need to be removed or manipulated to achieve the desired energy savings. Other factors incorporated in the manufacturing process need to be identified and removed or manipulated in the process intensification strategy. One such approach is an intensive approach in reforming methane. This is an endothermic process that consumes heat as an input causing methane to break into its constituency gases. The reaction is here represented to provide an insight into the chemical process, CH4 + H2O → C0 + 3H2.
In addition to that, the reaction is two-way. Energy is conserved in the process by the use of catalytic pellets in the reaction process that can be gainfully collected and applied elsewhere. However, heat is released by methane gas and a system is incorporated that ensures efficient utilization of the released heat. The catalytic surface also retains some combustion heat that can be safely removed and taken away. In addition to that, combustion heat recovered through this way and other means are combined to generate power for other uses.
Another way process intensification is used to achieve energy savings and lead to reductions in CO2 emissions is upstream hydrocarbon production strategies and transportation methods. These methods focus on physical processes that lead to energy efficiency and reduced inventories. One approach is to manufacture chemicals at the site where they are needed as inputs to a process. This has been the case with chemical production in plants found in Norway. This plant focuses on the separation of ammonia and oxygen with the resulting chemical from the process being Nitrogen oxide. This process is environmentally friendly since no harmful products are produced in the process (Wöhlk & Hofmann, 1975).
How Process Intensification May Help Units in the Carbon Capture Process
The above figure illustrates a carbon capture process in a typical Bituminous CO2 Capture Power Plant in a block diagram. The plant incorporates an absorption column, a stripper column, reboiler, a storage and handling amine system, disposable spent amine, stripper column, and flue gas cooler, each with a specific role. Based on the principles of process intensification principles it its incumbent for the plant design to incorporate process intensification methods in the carbon capture and mass/heat transfer processes (Synowiec, 1967).
This process begins with problem identification and the objective functions and variables. These include plant location as one of the functions and pant design. In addition to that, variables such as optimal performance of the plant, costs associated when integrating PI into the process. In addition to that, energy wastes in the process, system efficiency in capturing carbon, costs, and flexibility of the carbon capture cycle bear a binary relationship in making an optimal decision.
As previously mentioned, plant location, design layout, proximity to new and upcoming plans, and piping systems play a critical role in enhancing process intensification in the carbon capture process. Other variables to consider when evaluating and analyzing the situation are health-related issues, safety in handling chemical substances such as amine, and transportation of CO2 to various sites.
The process Intensification strategy demands that the design of a power plant or any processing plant be integrated to minimize space requirements while reaping the benefits associated with the distance factor. Compaction plays a terminal role the here. In addition to that, each step of this process has measures to identify bottlenecks based on the requisite PI principles. Every point that is identified in the problem is directed to sub-problems and compared with PI options. A variety of PI options are evaluated to identify the most feasible option to supplement the identified problem gap (Matusewitsch & Baranov, 1967).
To efficiently integrate PI and compact the design to obtain optimal solutions for specific requirements to be met, the following elements have to be incorporated in the above process, with some exceptions.
Sufficient space has to be made available to utilize carbon capture at different locations. These locations will accommodate additional carbon capture components in addition to establishing a balance within the plant to utilize cooling water and heat dissipated from elements in this process with the auxiliary power distributions for the capture equipment integrated into the system. Process intensification elements are located at various strategic points to ensure optimal use of space, mass transfer, and efficiency in capturing heat. The DeNOX equipment should incorporate pre-combustion and post-combustion PI measures to ensure systems efficiency in the capture of gases such as NO.
On the other hand, the steam turbine generator and related functional components (PF) play the role of combustion-based intensification elements where a water solvent should be used in the process to optimize heat absorption and transfer. To ensure the systems generates heat in the generation of amine substances process, steam pressures are reduced in the process to minimal levels of almost 3.8 bara. Low-grade heat, as discussed elsewhere in this document is tapped at the opportune moment during the retrofit cycle. Thus the penalty from the capture of carbon dioxide is minimized in the process by exploiting this process efficiently in water-steam-condensation. Therefore, it is worth noting that the above plant should incorporate the following elements during the capture of low-grade heat.
The water-steam cycle should be provisioned with a bypass mechanism to enable the capture of heat with a sufficient number of input water heaters. Process integration minimizes the area covered by flowing steam and condensed water, thus enhancing heat and mass transfers (Ding, Chen, Wang, Yang, He, Yang, Lee, Zhang, Huo, 2007).
The overall cooling effect of compressed CO2 and flue gas cooler is improved by the use of the latter two approaches despite the limitations associated with inefficient space. After capture retrofit, the LP section of the steam will experience significant reductions in the flow of steam during the extraction of amine scrubber with specific emphasis on PI principles (Larson, 1978). Two feasible alternatives gibe two feasible solutions. However, the option of emphasizing PI principles will provide a cost-effective and efficient solution. The options include condenser pressure after the capture, a strategy that reduces the flow effect on cooling water, and the next option being condenser pressure with the reduced mass flow.
Other areas to be considered for the integration of PI into the above system include the air system that operates with compressed air, electrical systems, and control and instrumentations.
The new design that addresses all issues covered above incorporating the PI principles is hereunder illustrated, with various redesign factors.
Steps Involved in the PI Methodology in the production of Neu5Ac
The following is a practical approach addressing the process intensification framework. A two-pronged approach to the framework affords general and specific performance evaluations to the candidate processes. The hierarch enjoys a stepwise approach with the lower and higher levels of varying complexities. The steps are largely detailed in the diagram illustrated below. It is a bio based reaction process.
The first step in the methodology is to identify feasible and infeasible options and remove infeasible elements at early stages. A detailed analysis of subsequent steps continuously enables their removal to remain with the most feasible ones. Therefore, the methodology put into use structured knowledge and the principles of process intensification.
The onset of the methodology is to accept input from an existing process. Then the objective function (OF) defines the problem function specific to PI synthesis. This is characterized by two optimization variables, that is X and Y. Y is a binary decision while X is the design factor. In addition to that, the product is defined by the measurements, d as the product parameter and ß as the equipment parametric value.
The basis for identifying and selecting the objective function is based on a variety of associated costs that should be minimized in the process to optimize system outputs. Variables such as cost, volume, complexity, capital, costs, wastes, energy consumption among others are factored in the process.
Each step in the process has measures to identify bottlenecks and requisite principles for PI. Every point that is identified in the problem is directed to sub-problems and compared with PI options. A variety of PI options are evaluated to identify the most feasible option to supplement the identified problem or gap. A superstructure, as illustrated below is developed to address the gap and enable PI based on a generic model of the superstructure as illustrated below.
At every point, process models that are inefficient and that do not point to Pi are removed while analysis subsequent steps. At step four, a structural and logical approach in the synthesis of the problems and PI sub-problems and the constraints placed in the process are identified and redundant alternatives are delinked from the system. Then at step five, operational constraints are identified and feasible solutions relevant to PI are addressed at once. In the process, feasible variables are determined in the process and incorporated to achieve process efficiency.
The sixth step commences once the optimum solution for PI is reached, further steps in refining the solutions commence. That includes setting standards or benchmarks for PI operations against which they are evaluated to achieve operational efficiency as the seventh step.
The eighth step involves identifying PI options that provide optimal solutions to wastes incurred in the operational execution of the steps. Each of the selected options is rigorously tested thorough experimentation and rigorously validated against established benchmarks. This step is also achieved through simulations and experiments. This rigorous testing and validations, which appear at the 9th stage lays limited weight on resources, time, and financial expenditure.
The tenth step involves combining sub-solutions of the sub-problems with special emphasis on PI benchmarks. These solutions are integrated and subsequently tested and evaluated against the old model for some time to evaluate the benefits accruing thereof. Once testes have been conducted on the new system, the 11th step follows where the new model is implemented organization-wise to reap the benefits of PI. That is followed by the 12th step which intensely involves post evaluations of the system and other improvements that may be identified in the process.
Practical application of process intensification strategies cannot afford to be ignored and the following analysis details the impact of process intensification in diverse areas of application.
The following case study illustrates these elements very well. This is on the production of Neu5Ac. This is an important pharmaceutical substance particularly in the field of cancer.
The onset of PI is problem identification. Problem identification incorporates the element of identifying the objective function as discussed earlier. In this case the objective function is defined in the following relation: MaxOF = yield = f (Y, X, q). The objective function is chosen to optimize the efficiency of the system outputs. Variables, logical rules and structural mechanisms are some of the metrics for evaluating PI in this process. The process model is influenced by a binary approach to decision making. Other parameters include identifying solutions and solvents that are mixed at various points to enable manipulation of the outcomes with specific emphasis on PI. Limitations may span operational constraints, among other parameters.
The yield is defined by the difference between the output product and the input product as a function of the substrate. That takes one to step two of the process.
In this step, properties such as enzyme activities and related chemical information is collected and carefully documented. Then an evaluation of the current process is done and parametric contributions to process inefficiencies identified. These parameters include waste per unit weight of the materials involved in the process, shortfalls in downstream product processing, and a synthesis of the route taken in the production process. Intensive consultations are then done on PI problems and sub-problems, based on the knowledge base of biological processes and the alternative strategies that can be followed to implement PI. This is followed by the in situ product removal that takes the data and the removed product in the system to step two of the sub problem. That is in reference to figure 1 of the model.
Then a generic model that constitutes process intensification elements is developed. It is ten synthesized, evaluated, and validated against experimental data and significance tests are performed to evaluate the degree of deviations from a predetermined value within an acceptable limit of 5%.
The superstructure provides a method of fixing binary decisions as discussed above with emphasis on the value of Y, with every viable solution of the generic model to produce Neu5Ac evaluated. Different steps are involved at this point. These include reactions, enrichments, and other down-streaming reactions. Alternatives such as boiling, evaporations and others are considered at this stage.
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