Pinch Technology – Best tool for energy targeting

Pinch technology or pinch analysis is a method used to target energy in chemical process by calculating thermodynamically feasible target. It was developed by teams led by Professor Bodo Linnhoff at University of Manchester (UK) in 1980’s. als In France, Jean-Paul Gourlia also contributed to popularize this approach with several reference papers. The pinch analysis is now has been extended beyond energy application and are similar methodology is being adopted for water pinch analysis and hydrogen analysis.

What is pinch Technology

A methodology for reducing energy consumption of processes by calculating thermodynamically feasible energy targets (or minimum energy consumption) and achieving them by optimizing heat recovery systems, energy supply methods and process operating conditions. Pinch analysis are also known as Process integration, Heat integration, Energy integration and Pinch analysis.

Lets understand with one process pinch with one simple case as shown below.

For eg Let us assume Case-1 is a simple process where heating and cooling is involved, total heat requirement is 200 KW out of which heating requirement is 140 KW while cooling requirement is 60 KW. It was asked to three different process engineer to reduce heat requirement and all of them came with proposal as case-2, case-3 and case-4

Let us review all the cases and try to find the best case, Based on all the four cases Case-3 and case-4 consumes lowest cost of operation while case-3 has lower cost of installation in comparison with case-4. Therefore best case-4 can be considered as the best case among below 4 cases.

Now If someone come with another case as where energy can be further reduced with lower consumption, then case-3 will not be the best case and organization will suffer with the wrong design of heat exchanger network. To solve this problem Pinch analysis needs to be introduced. Pinch technology method is based on thermodynamic principles and allows to determine the best heat exchangers network and utility system.

What is Pinch in pinch technology?

Pinch is the temperature point which divides thermodynamic process in two different range of temperature. GenerThis point is the closest approach between the hot and cold composite curves is the pinch point with a hot stream pinch temperature and a cold stream pinch temperature.

A heat exchanger network obtained using the pinch design method is a network where no heat is transferred from a hot stream whose temperature is above the pinch to a cold stream whose temperature is below the pinch.

Hot and cold composite curve

For finding the mininmum energy requirement in pinch technology we need to form hot composite curve and cold composite cure as shown below.

For finding hot composite curve we need to find temperature range and enthalpy of process stream which are having heat and which need to be cooled down for eg in the below graph (G1) of temperature vs enthalpy, there is two streams need to be cooled from 180 to 80 Deg C and 130 to 40 Deg C. Hot composite curve is prepared with considering all hot steams as single stream. The common line between 130 deg C to 80 Deg C are combined with correction in the slop of curve. This single line of hot stream is known as hot composit curve.

G1 -Hot Composite curve

Similarly For finding cold composite curve we need to find temperature range and enthalpy of process stream which are cold and where heat is required for eg in the below graph of temperature vs enthalpy, there is two streams need to be heated from 30 to 120 Deg C and 60 to 100 Deg C. Cold composite curve is prepared with considering all cold steams as single stream. The common line between 60 deg C to 100 Deg C are combined with correction in the slop of curve. This single line of cold stream is known as hot composite curve.

G2 Cold Composite Curve

after finding separate hot and cold composite curve separately, composite curve is prepared as per (G3). This composite curve is very important. Pinch point is identified based on composite curve along with minimum energy requirement. as shown in graph G3 Q Cmin is the minimum cold utility requirement and Q Hmin is the minimum hot utility requirement.

G3 Composite curve

No heat transfer is allowed across pinch point in pinch process design. Area above pinch point can be considered as heat sink while area below pinch point can be considered as heat source. Delta T min temperature is the used to estimate reasonable size of equipment. Lower the Delta T min higher the the cost of equipment, Typically for oil refinery Delta Tmin is in the range of 20-30 Deg C and for chemical it is in the range of 10-20 Deg C.

Pinch Principle

For pinch process design following point needs to be taken care

  • Heat must not be transferred across the pinch.
  • There must be no external cooling above the pinch.
  • There must be no external heating below the pinch

Targeting Multiple Utilities

As the utility cost depend on the temperature at which utility is required, for economical design the composite curve can be divided in further section with two different Delta T min as shown in graph G4

G4 Composite curve with Multiple Utilities

Grand Composite Curve

After identification of pinch point, Delta Tmin, Minimum heating and cooling requirement, It is necessary to prepare Grand composite curve as shown in graph G5, Utility are placed according to grand composite curve. The grand composite curve displays the net heat-flow characteristics of a process versus its temperature. This allows us to quickly identify regions where heating and cooling utilities are required. These utilities include heating utilities like high- and low-pressure steam. Cooling utilities like cooling water, chilled water or chilled brine

G5 Grand composite curve

Procedure to Construct GCC

  • Increase the cold composite temperature by ½ DTmin and decrease the hot composite temperature by ½ Dtmin.
  • As a result of this temperature shift, the composite curve touch each other at the pinch. ( Refer graph G5 (b) )
  • The curve are called the “shifted composite curve”.
  • The GCC is then constructed from the enthalpy rate differences between the shifted composite curve at different temperatures. ( Refer graph G5 (c) )
  • The GCC provided the same overall energy target as the composite curves.

Steps of energy target setting

  • Data Extraction from the flow sheet- flow rates, specific heats, supply and target temperatures of the streams.
  • Make a problem table algorithm for a chosen value of DTmin.
  • Plot hot and cold composite curves.
  • Identify the pinch temperatures from the hot and cold composite curves.
  • Read maximum hot and cold utility requirement.
  • Plot a grand composite curve.

Area target setting

  • Additional data required is the heat transfer coefficient of each stream.
  • Plot balanced hot and cold streams(by including the utility steams)
  • Divide into enthalpy intervals.
  • Determine the cold stream and hot stream temperatures for each enthalpy interval.
  • Calculate area in each interval.
  • Sum area overall the intervals to get the area target.

Units target setting

  • Minimum no of units , Umin = Ns-1 (where no of streams including the utility stream.)
  • No of units for maximum heat recovery / max energy recovery.

Umer = (Na-1) + (Nb-1)

Where, Na = no of stream above pinch,

              Nb = no of steam below pinch.

Actual no of units  Uactual  >=  Umer

Super targeting – ( Targeting for Total Annual Cost)

TAC = OC + CC X Af

Where TAC = Total Annual cost,, OC  =Operating cost of the network (Utility cost), CC= Capital cost of  the network

Af = annual cost factor = (1+r)t/t, Where r = rate of interest %, t = expected plant life, year

Continuous targeting is done with differnt T min based on the operational and capital cost.

Summary :

  • The pinch design method is a powerful  tool that helps in setting up of minimum energy, units, shell and cost targets before actual network design.
  • The method can be used for a grass root design as well as for debottlenecking and retrofit  designs

For online pinch tool, Please click here

Below video will help in further understanding of pinch Technology

For Information on heat pipes click here, For Mechanical vapor recompression system click here

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