Department of Energy and Process Technology

Examination paper for TEP4215 Energy and Process

Academic contact duringexamination: Truls Gundersen

Phone: 916 817 26

Examination date: Friday 24 May 2013

Examination time (from-to): 09:00 – 13:00

Permitted examination support material: D: Specified, simple calculator allowed. No printed or handwritten documents allowed.

Otherinformation: None

Language: English (US)

Number of pages:4 (including this front page)

Number of pages enclosed: 0

Checked by:

______

DateSignature

Task 1 (50%)

In order to reduce energy consumption in a process, one would like to heat integrate four process streams. In the table below, Ts andTtare the supply and target temperatures of the streams, while mCp is the product of mass flow (in kg/s) and specific heat capacity (in kJ/kgºC):

StreamTs(C)Tt(C)mCp(kW/C)

H13506035

H238019015

C111034040

C222040030

FG800450var

CW2030var

Utilities are available in sufficient amounts in the form of hot Flue Gas (FG) that can be cooled from 800 to 450C and Cooling Water (CW) that can be heated from 20 to 30C. Assume that Tmin = 10C gives a reasonable trade-off between operating cost and investment cost. For FG and CW the entry ”var” means that these are variables and should be calculated.

a)Show that minimum external heating (in the form of flue gas) and minimum external cooling (in the form of cooling water) with maximum heat recovery for the specified value of Tmin isrespectivelyQH,min = 3750 kW andQC,min = 2150 kW. Where is the process Pinch (at what temperatures), and which stream is “causing” the Pinch?

b)What is the minimum number of heat exchangers (units), Umin,MER, if maximum heat recovery is to be achieved, and what is the corresponding minimum value for number of units, Umin, if some heat recovery is attempted but not necessarily maximum heat recovery?

c)Design a heat exchanger network for maximum heat recovery (QH,min andQC,min) while at the same time trying to minimize the number of units. Provide arguments for your choices in the design process. Check the network against established performance targets from sub-tasks (a) and (b) and explain possible deviations from these values.

d)Consider options to simplify the network that was developed under sub-task (c) in such a way (reduce the number of units and/or the number of stream splits) that it also provides options for reduced annual cost. Provide arguments for your suggested solutions (emphasize the strategy used), and quantify possible increases in energy consumption if the specification related to minimum driving forces is maintained (Tmin = 10C).

Task 2 (30%)

The figure below shows the flow of thermal energy (heat) as a function of temperature for a process plant. This so-called Heat Cascade shows temperature intervals with temperatures for hot process streams on the left hand side and cold process streams on the right hand side. The resulting heat balance (positive numbers mean heat surplus) is given for each temperature interval on the basis of heat supplied by the hot streams and heat required by the cold streams in the various intervals (these process streams are not shown in the figure).

The figure also suggests in a symbolic way that a heat pump can operate between a cold and a hot “reservoir” in order to lift thermal energy (heat) by supplying mechanical energy (electricity). Assume that the heat pump (with a single chemical component as working fluid) is operating as an ideal Carnot engine with effect (or heat) factor (COP) as indicated in the figure.

Assume there are 3 steam levels for external heating, HP steam at 240C, MP steam at 180C and LP steam at 140C (these temperatures are the condensation temperatures for the 3 steam levels. For external cooling, Cooling Water (CW) is available at 10C and can be heated to 20C. Assume also that the process contains 2 distillation columns that could be attempted heat integrated to save energy. One of the columns (A) has a reboiler temperature of 100C and condenser temperature of 90C, while the duty in the reboiler and the condenser is 1000 kW. The other column (B) has a reboiler temperature of 150C and a condenser temperature of 130C, while the duty in the reboiler and the condenser is 800 kW. For the entire task, one should assume that the minimum allowed temperature difference for heat transfer isTmin = 20C.

a)Calculate minimum external heating and cooling for the part of the process that is represented by the heat cascade in the figure above (the so-called “background” process). What is the process Pinch temperature for hot and cold streams?

b)Sketch (draw) the process heat surplus diagram (Grand Composite Curve).

c)On the basis of information about the background process, external utilities, distillation columns (A and B) as well as the heat pump, establish an energy system that reduces the total energy consumption to a minimum.Explain how the background process, the distillations columns and the heat pump can be combined (integrated) in such a way that the total need for steam and cooling water is reduced to a minimum. Provide the consumption (in kW) of the different utility types, and quantify the savings compared to not integrating the columns and the heat pump. If the heat pump is used, all data for the heat pump should be provided, such as heat effect (duty) in the evaporator and the condenser, COP and power consumption.

Task 3 (20%)

The sub-tasks below are formulated as statements and should only be answered by “TRUE”, “FALSE” or “PASS”. It is emphasized that explanations do not account (positive or negative) when evaluating the answers. This is meant to be a pure “multiple choice” task. A correct answer gives 20 points; a wrong answer subtracts 10 points, while “PASS” gives zero points.

a)”When separating a multi-component mixture into pure component products by distillation columns, the number of possible separation sequences is 4 when the number of components is 5 (i.e. N-1, where N is the number of chemical components)”.

b)In order to assure sufficient driving forces in heat exchangers (so-called Pinch exchangers) operating close to (above or below) the Pinch, the so-called mCp rules and topology rules are used that in generalized form for above and below can be formulated as:

”It is always possible to split a number of hot and cold streams in such a way that both the mCprules and the topology rules can be fulfilled when designing heat exchanger networks”.

c)”When splitting streams, the number of units (heat exchangers) will increase”.

d)”In modification projects (retrofit) for heat exchanger networks, the sum of (i) heat transfer across the Pinch in process/process exchangers, (ii) use of external heating below Pinch, and (iii)use of external cooling above Pinch will be exactly equal to the difference between existing energy consumption and minimum energy consumption for a specified value for Tmin (or HRAT – Heat Recovery Approach Temperature)”.

e)”Both supply and target temperatures for process streams are potential Pinch candidates”.

Best Wishes !!

Trondheim, 15.05.2013

Truls Gundersen

Side 1 av 4