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 7, 2013

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 Process Economics

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

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 2:4

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 5 th ed.). Assume an average value. –Assume inlet and outlet temperatures for utility stream. Sample calculations given P&T p Equipment Sizing 3:4 Q: Provided by Aspen U: Found in P&T ΔT: Calculated after assuming temps.

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 Equipment Sizing 4:4 Gas phase catalytic reactions = Fast Order of magnitude would be: seconds range

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 Pinch Analysis 1:5 Supply temp Target temp ΔHΔH Integration of streams should be done using the HeatX model

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 Pinch Analysis 2:5 TsTs TtTt Q CP = Q/  T

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 CP L, CP V and  H vap Pinch Analysis 3:5

Approximation Pinch Analysis 4:5 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. T s T t Q T s T b Q1Q1 T b T t Q2Q2 Q3Q3 Can be specified in Aspen as having an outlet vapor fraction of Can be specified in Aspen as having an outlet liquid fraction of Specify outlet temperature at T t

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. Pinch Analysis 5:5

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

Next Lecture – March 19 –Process risk assessment and inherently safe design Other Business