Computational Methods in Chemical Engineering Computational Methods in Chemical Engineering
Spring Semester 2022This is a computational Method in a Chemical Engineering assignment.
Spring Semester 2022
CBEN200 Spring 2022 - 1 - January 6, 2022 Homework #8 For each problem, include a (1) PDF version of your output with your final answers clearly indicated and (2) a copy of your electronic file(s) used for your solutions. Combine all PDF files into a single file. You are to use Aspen Plus for all problems. Problem #1 (30 points) After graduation you’ve joined an energy efficiency team. One your first project you find yourself working with a team of mechanical engineers on a building HVAC system. As the only chemical engineer, you are assigned to designing the system and worrying about the overall process considerations while the others draw things in SolidWorks, or whatever it is that they do. You’ve decided to use the refrigerant R-410A, which is a mixture of two refrigerants: R-32 (difluoromethane) and R-125 (pentafluoroethane), at 50 wt% of each component. Use the SRK thermodynamic method for the calculations. For the office building in question, you expect that the system will need to handle 100 m³/min of air at peak operation. You expect the warm air leaving the building and entering your system at 27°C; after cooling the air should be returned to the building at 16°C. For an air-based cooler such as this, a 15°C approach temperature is the best you can expect (i.e., the cold refrigerant should be 15°C colder than the coldest temperature on the air side). Heat from the compressed refrigerant is rejected to the surrounding atmospheric air, which may be as hot as 42°C on a hot summer day. This heat exchanger for this will be using an air-cooled exchanger (see the figure to the right1). There are four devices used in this refrigeration cycle (as depicted in the process flow diagram1 to the right) A. High-pressure refrigerant has been condensed to a liquid. This liquid is routed to an Expansion Valve where the fluid expands adiabatically to a lower pressure. B. This produces a very cold, mixed liquid- vapor stream of refrigerant that is routed to an Evaporator (sometimes called a Process Chiller) heat is extracted from the hot building air and taken up by the refrigerant. 1 GPSA Engineering Data Book, SI units, 14th ed. CBEN200 Spring 2022 - 2 - January 6, 2022 On the building-air-side of this heat exchanger the air gives up heat, gets colder, and is returned to the building. On the refrigerant-side the refrigerant takes up this heat and boils, creating vapor. C. This cold vapor is routed to a Compressor that pressurizes the refrigerant vapor. The high-pressure gas will also be much warmer. This high-pressure vapor is routed to a Condenser… D. Heat is extracted from the hot refrigerant, causing it to condense; this condensed liquid is now back at step A and is routed to the Expansion Valve. The heat from the refrigerant is exhausted to outside air. Determine the following. a) What should the temperature (in °C) of the refrigerant entering the evaporator be, assuming a 15°C approach? b) Create a stream in your Aspen Plus simulation with the composition of the refrigerant. Using the temperature specified in (a), find the pressure (in bar) of the stream as a saturated vapor. This will be the pressure specified as the discharge of the Expansion Valve (and will also be the pressure of the vapor mix exiting the Evaporator). c) What should the temperature (in °C) of the refrigerant leaving the condenser be, assuming a 15°C approach on the air-inlet/refrigerant-exit? d) Create a stream in your Aspen Plus simulation with the composition of the refrigerant. Using the temperature specified in (c), find the pressure (in bar) of the stream as a saturated liquid. This will be the pressure of the discharge of the Compressor (and also the liquid leaving the Condenser). e) Build a flowsheet for this system in Aspen Plus, including all work, refrigerant, and air streams. Assuming/using the following, and using the NRTL-RK thermodynamic method, • A low refrigerant system pressure from (b) • A high refrigerant system pressure from (d) • Isentropic compressor function at 75% efficiency • Saturated liquid refrigerant leaving the Condenser • Saturated vapor refrigerant leaving the Evaporator • Countercurrent operation of heat exchangers • No pressure losses in the heat exchangers • All air streams to be at 101 kPa absolute pressure When modeling the Evaporator and the Condenser you can choose whether to use a single HeatX for each exchanger or a pair of Heater blocks (connected with a Q stream). Include an image of the complete flowsheet in your submission. Use the Modify Stream Results to show the Temperature, Pressure, Vapor Fraction, and Mass Flow Rate values on the flowsheet. f) Find the circulation rate, (in kg/sec) of refrigerant required to deliver this cooling service. CBEN200 Spring 2022 - 3 - January 6, 2022 g) You are uncertain that the 75% specified isentropic efficiency (SEFF) for the compressor has been accurately quoted. Perform a Sensitivity analysis to determine the effect of the compressor’s isentropic efficiency (from 55% to 90% by 1% increments) on its indicated power (IND-POWER). Display a plot with your submission (use your preferred plotting software).