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Technical focus: MHX Exchanger

06/09/2010
Among the unit operations available in ProSimPlus (steady state process simulation and optimization software), the MHX Exchanger enables to calculate the heat exchange between several cold and warm fluids with a given pinch temperature, when looking for a low pinch temperature (of a few degrees) for example.


Technical focus: MHX Exchanger



MHX Exchanger


Such exchanges are common in liquefied natural gas (LNG) treatment processes, industrial gas (oxygen, nitrogen, argon, helium, etc…) treatment processes, air gas and hydrocarbons separation.
When designing or analyzing such processes, the MHX Exchanger module will be used for:
  - Verifying the feasibility of an existing exchanger (rating mode): properties of all outlet streams are known and an enthalpy balance is made in order to calculate the duty required from the utilities to conduct the thermal exchange.
  - Calculate an exchanger: unspecified stream temperatures are calculated in order to satisfy the thermal balance on the exchanger. If several streams are not specified, their respective output temperatures will be the same.
  - Run energetic integration studies (simultaneous flow of several streams in the module).

The global parameters of the heat exchanger (global exchange coefficient (UA factor), pinch and logarithmic mean temperature difference (LMTD)) are estimated from the heat integration calculation on the exchanger. This calculation will be impacted by streams enthalpy curves (representation of the fluid temperature with respect to the duty exchanged by this fluid all along the device).
The geometry of the exchanger is not taken into account.
Each outlet stream is defined by its thermal and pressure specification and its mass composition. The flow rate of the fluids remains unchanged. The module calculates the enthalpy and the pressure of the outlet stream as well as the enthalpy and the pressure of intermediary points between inlet and outlet of the stream in order to build the enthalpy curves. The number of intermediary points is defined by the user.

Exchanger specification
As the geometry of the device is not taken into account, specifying the exchanger comes down to defining the flow path and the characteristics of each fluid. Each fluid is identified by an inlet stream and an outlet stream, selected among the streams connected to the module on the flowsheet.
Specific parameters are associated to the stream, in particular the outlet temperature, the difference between the temperature of the inlet and outlet streams and the temperature of a reference inlet stream (same fluid or another fluid), the fluid pinch or the global pinch, its bubble or its dew temperature, the duty exchanged by the fluid and the molar vapor ratio at the outlet.

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Stream selection
 
  Stream specification
 

It is also possible to specify the pressure of the fluid by defining the pressure drop, the outlet pressure, the dew or the bubble pressure. Additionally the calculation mode of the pressure profile along the exchanger must be defined. This parameter impacts the fluid enthalpy curve and the energetic integration of the device.

Lastly, different options are available to define the decomposition of the fluid enthalpy curve which represents the temperature of this fluid with respect to the heat exchanged along the device. In order to produce a better representation of enthalpy curves in non-linear parts (i.e. evaporation parts), a number of decomposition intervals points corresponding to phase changes can be specified. Other options are available to fine tune the MHX Exchanger calculation. In particular, it is possible to define a heat leak (cold side) and a heat loss (hot side) on the exchanger, located at the inlet of the heat exchanger or spread proportionally to the duty exchanged. It is also possible to take into account utilities in the global heat transfer coefficient calculation.

Results
At convergence, the available results are:
     Thermal state of each stream (cold or hot);
     Outlet temperature of each fluid;
     Streams pressure drops (difference between inlet stream pressure (input data) and the outlet stream pressure (calculated));
     Heat duty exchanged by the streams (difference between the enthalpy flow of the outlet stream (calculated) and that of the inlet stream (input data));
     Heat duty difference between hot and cold streams;
     Cold and hot utilities quantities;
     Exchanger global UA (unless hot and cold composite curves are crossing);
     Exchanger global LMTD (unless hot and cold composite curves are crossing);
     Pinch value (minimal spread between hot composite curve and cold composite curve).


Additionally, the following profiles are generated by the module, in graphical and numerical format:
     Hot and cold composite curves temperature / cumulated enthalpy T=f(Q) and Q=f(T);
     Temperature difference between composite curves along the exchanger deltaT=f(Q);
     Evolution of the UA factor along the exchanger UA=f(Q);
     Hot and cold composite curve DTML=f(Q);
     For each stream, the composite curve of a fluid with its opposite composite curve T=f(Q) and Q=f(T);
     Enthalpy curves of the different streams and hot and cold composite curves T=f(Q) and Q=f(T).


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Hot and cold composite curve