University of Petroleum & Energy Studies, DehradunChemical Engineering Department

Home Work Assignment 1

Submitted By:-RAJAT WADHWANISap ID- 500023794Roll No. R820212027Branch- APE-GAS

Q1. Describe the steps of General Design Procedure of Heat Exchangers. [10] Ans1:Step.1-Obtain the required thermo-physical properties of the fluid at the caloric temperature or arithmetic mean temperature.Step.2- Perform energy balance and find out the heat duty Q of the heat exchanger.Step.3- Assume a reasonable value of heat transfer coefficient Uo.they can be assumed from the book with respect to process hot and cold fluids.Step.4- Determine the tentative no. of tube and shell passes, also determine the LMTD and the correction factor Ft.Step.5- Determine the heat transfer area required. A= (Q)/ (LMTD.Uo.Ft).Step.6- Select tube material, decide the tube diameter and tube length. Calculate the no. Of tubes required for the heat transfer area. N=A/ (3.14*d*L).Step.7- Decide the type of shell and tube exchanger, select the tube pitch and calculate the inside shell diameter that can calculate the desired no. of tubes.Step.8- Select the type of baffle its size spacing and number.Step.9- Determine the tube side film heat transfer coefficient using the suitable form of seider-tate equation in laminar and turbulent flow regimes.

Q2. A kettle re-boiler is used to partially vaporize the process liquid using steam as the heating fluid. Compare and contrast kettle with thermo-siphon re-boiler. [10]

Ans.2

In a kettle re-boiler, the column bottom liquid flows to the kettle where direct-fired, steam- or oil heated tubes partially vaporize it. The vapor generated returns to the column bottom via a vapor return line. The remaining liquid is withdrawn from the re-boiler, usually via an overflow weir and sump arrangement. Because the vapor is directly generated by boiling on the surfaces of heated tubes, a kettle re-boiler is a very close approximation to a theoretical stage.

The circulation in a thermo-syphon re-boiler is driven by the density difference between the entering liquid and the two-phase mixture that recirculates back to the tower sump (an everyday example is the action in a coffee percolator). The recirculating flow may be driven purely thermally, or it may require use of a pump on the tower bottoms if the natural liquid head is insufficient to drive the flow at the desired rate. The entering liquid is partially vaporized, but all of the resulting vapor liquid mixture is returned to the tower sump via the return line. The returning vapor-liquid mixture is separated in the sump. The vapor flows upward, while the liquid joins the flow from the bottom tray or packing support plate and returns for another pass through the re-boiler. The product liquid is drawn directly from the tower sump (or the liquid line joining sump to re-boiler). It is not drawn from the re-boiler itself.

Figure compares the PFDs for thermo-syphon and kettle re-boilers. In the thermo-syphon setup, the lean solvent stream is drawn from the liquid line returning the combined liquid flows (column bottoms plus liquid portion of the 2-phase re-boiled return Stream 2) rather than from the re-boiler itself.

Kettle re-boilers are very simple and reliable. They may requirepumpingof the column bottoms liquid into the kettle, or there may be sufficientliquid headto deliver the liquid into the re-boiler. In this re-boiler type, steam flows through the tube bundle and exits as condensate. The liquid from the bottom of the tower, commonly called thebottoms, flows through the shell side. There is a retaining wall or overflowweirseparating the tube bundle from the re-boiler section where the residual re-boiled liquid (called the bottoms product) is withdrawn, so that the tube bundle is kept covered with liquid and reduce the amount of low-boiling compounds in the bottoms product.

Thermo-syphonre-boilers do not require pumping of the column bottoms liquid into the re-boiler. Natural circulation is obtained by using the densitydifference between the re-boiler inlet column bottoms liquid and the re-boiler outlet liquid-vapor mixture to provide sufficient liquid head to deliver the tower bottoms into the re-boiler. Thermo-syphon re-boilers (also known as calandrias) are more complex than kettle re-boilers and require more attention from the plant operators. There are many types of thermo-syphon re-boilers including vertical, horizontal, once-through or recirculating.

The thermo-siphon re-boiler has larger pressure drop and bigger amount of boiled fluid. Whereas a kettle re-boiler has small pressure drop. Kettle type re-boiler prefers vacuum distillation whereas. Thermo-siphon re-boiler does not prefer vacuum distillation. Thermo-siphon is preferable for atmospheric distillation unit whereas, Kettle is not preferred for atmospheric distillation unit. Thermo-siphon re-boiler is not preferred where absolute pressure difference is less than .3 bars. If viscosity of bottoms is very high then thermo-siphon re-boiler cannot be used. For corrosive and viscous bottoms kettle type re-boiler is preferred. If vaporization rate divided by mass flow rate term is higher than 0.8 then kettle type re-boilers are preferred.

Q3. Describe the construction and working of plate type heat exchangers. [10] Ans.3Aplate heat exchangeris a type ofheat exchanger that uses metal plates to transferheatbetween twofluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much largersurface areabecause the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperaturechange. Plate heat exchangers are now common and very smallbrazedversions are used in the hot-water sections of millions ofcombination boilers. The high heat transfer efficiency for such a small physical size has increased the domestic hot water (DHW) flow rate of combination boilers. The small plate heat exchanger has made a great impact in domestic heating and hot-water. Larger commercial versions usegasketsbetween the plates, whereas smaller versions tend to be brazed.The concept behind a heat exchanger is the use of pipes or other containment vessels to heat or cool one fluid by transferring heat between it and another fluid. In most cases, the exchanger consists of a coiled pipe containing one fluid that passes through a chamber containing another fluid. The walls of the pipe are usually made ofmetal or another substance with a highthermal conductivity, to facilitate the interchange, whereas the outer casing of the larger chamber is made of aplasticor coated withthermal insulation, to discourage heat from escaping from the exchanger.The plate heat exchanger consists of a frame, which consists of a head, follower, column, carrying bar, guiding bar, and a number of clamping bolts. In between head and follower a varying number of pressed plates are clamped together.

Each plate is supplied with a gasket, so that the plates form a closed system of parallel flow channels, through which the media flow alternatively at every second interval.

The Plate Stack - MediaThe gaskets are glued on the plates, securing tightness between media and the atmosphere. Between the different media there are double gaskets, which have intermediate drain areas, meaning that mixing of the two media is impossible.Every second plate in the stack has to turn 180, so that the plates form a closed system of parallel flow channels, through which the media flow alternatively at every second interval.

Gasket Construction - New GasketsThe first plate after the head and intermediate frames is mounted with a gasket in all the gasket grooves. These gaskets are cut out of ordinary gaskets.

Glue Less or Glued GasketsSome plate heat exchangers are delivered with glue less "Sonder Snap" gaskets. In these units it is possible to change the gaskets without using glue. However, the gasket on the first plate after the head and intermediate frame should be glued on in the pattern displayed above

Q4. Design the lube oil cooler, to handle 450L/min of lube oil from 65C to 45C. Cooling water temperatures are 45C and 39C respectively. The process fluid properties are given as following: Kinematic Viscosity () of lube oil = 45.5 cSt @ 40 C Density () of lube oil = 869 Kg/m3 Specific Heat (Cp) of lube oil = 2.14 KJ/Kg C Thermal Conductivity (k) of lube oil = 0.13 W/m C @ 40 C Assume 4-1 HEX with L=6 ft., 18 BWG 5/8 in OD tubes. Assumed value of U0 is 400 W/m2 C. Calculate U for first trial calculation. Show full steps. [10]

Ans.4

Q5. Describe the major steps in Pinch Technology Calculations. [10]

Ans.5

Heat transferred from hot stream should be equal to heat transferred to the cold stream (neglecting heat losses). Heat can only be transferred from a hot stream to cold stream. The temperature of cold stream will always be less than the hot stream along the heat exchanger. Pinch analysis is a methodology for minimising energy consumption of chemical processes by calculating thermodynamically feasible energy targets (or minimum energy consumption) and achieving them by optimising heat recovery systems, energy supply methods and process operating conditions. It is also known as process integration, heat integration, energy integration or pinch technology. The process data is represented as a set of energy flows, or streams, as a function of heat load (kW) against temperature. These data are combined for all the streams in the plant to give composite curves, one for all hot streams (releasing heat) and one for all cold streams (requiring heat). The point of closest approach between the hot and cold composite curves is the pinch point (or just pinch) with a hot stream pinch temperature and a cold stream pinch temperature. This is where the design is most constrained. Hence, by finding this point and starting the design there, the energy targets can be achieved using heat exchangers to recover heat between hot and cold streams in two separate systems, one for temperatures above pinch temperatures and one for temperatures below pinch temperatures. In practice, during the pinch analysis of an existing design, often cross-pinch exchanges of heat are found between a hot stream with its temperature above the pinch and a cold stream below the pinch. Removal of those exchangers by alternative matching makes the process reach its energy target.