Miscellaneous CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 17 – Equipment Sizing, Economics and Pinch Analysis March 17, 2013

Process Economics General Rule of Thumb Based on discussions with industrial design engineers Each major piece of equipment results in approximately $1,000,000 in capital investment (including installation, engineering time, instrumentation etc. etc.) Thus for a plant with a reactor and two distillation columns, the typical price will be around $3,000,000 Remember, that this is “order of magnitude” estimation

Equipment Sizing 1:4 Distillation Columns Required data is column diameter and height Height is found from tray spacing specification Remember that the first and last stage are the condenser and reboiler, respectively.

Equipment Sizing 2:4 Distillation Columns (Continued) If identified diameter is beyond the curves found in P&T, assume (for pricing purposes) that the separation is performed in NC columns in parallel all with the same specifications. The number of columns required should be based on equal distribution of the cross-sectional area NOT the diameter:

Equipment Sizing 3:4 Heat Exchangers Required information is usually the heat transfer area, apart from furnaces and fired heaters, which may be estimated based on the duty. Overall heat transfer coefficients can be found in P&T (p. 663 in 5th ed.). Assume an average value. Assume inlet and outlet temperatures for utility stream. Sample calculations given P&T p. 693. Q: Provided by Aspen U: Found in P&T ΔT: Calculated after assuming temps.

Equipment Sizing 4:4 Reactors and Vessels Required information is usually the reactor volume along with the amount of catalyst. Both require detailed kinetic expressions Can also be determined from residence time When pricing in P&T remember the rule of thumb for major pieces of equipment, i.e. the reactor will not cost $50,000 if the distillation columns cost $1,000,000 Gas phase catalytic reactions = Fast Order of magnitude would be: seconds range

Integration of streams should be done using the HeatX model Pinch Analysis 1:5 Units to Include Heaters, coolers and reactors Aspen will provide supply and target temperatures as well as enthalpy change for each hot and cold stream Remember that a hot stream in pinch terminology will actually be TWO streams in Aspen Integration of streams should be done using the HeatX model ΔH Target temp Supply temp

Pinch Analysis 2:5 Identifying the CP values (No phase change) The heat capacity (CP) of a stream that does NOT undergo any phase changes is readily calculated from the information provided by Aspen Q CP = Q/DT Ts Tt

Pinch Analysis 3:5 Identifying the CP values (phase change) Dashed line represents approximation if the duty of the exchanger is averaged over the temperature range. Quality of approximation depends on the relative values of CPL, CPV and DHvap

Pinch Analysis 4:5 Approximation For mixtures with very different boiling points of the constituents the outlet temperatures in exchangers 1 and 2 will be quite different, but it will still give a better approximation than just an average over the entire temperature range. Q Liquid @ Ts Vapor @ Tt Can be specified in Aspen as having an outlet liquid fraction of 0.001 Can be specified in Aspen as having an outlet vapor fraction of 0.001 Specify outlet temperature at Tt Q1 Q2 Q3 Liquid @ Ts Liquid @ Tb Vapor @ Tb Vapor @ Tt

Pinch Analysis 5:5 Units NOT to Include Distillation columns and flash drums Due to dynamic implications, distillation columns should not be directly integrated with the utility system Evaluate all columns together in tabular form with duties and temperature levels If there is a condenser with a temperature higher than a reboiler in another column, then those two are candidates for integration.

General Comment ASPEN Help File References for equipment such as scrubbers/absorbers

Other Business Q&A Session with Consultant – March 21 Bob Kline will participate via videoconference Next Lecture – March 26 Process risk assessment and inherently safe design