Heat Integration of Crude Organic Distillation Unit

Manjusha Padole

Abstract

While oil prices continue to climb, energy conservation remains the prime concern for many process industries. The challenge every process engineer faced is to seek the answer to questions related to their process energy pattern. Distillation column are of great importance in process analysis as they are the most common and the most energy intensive separation systems and hence it is the first separation system to be analyzed specifically from a pinch view point. In this paper heat integration of crude organic distillation unit has been done using pinch technology. Pinch Technology involves composite curves, problem table algorithm and heat exchanger network design. Using targeting procedures, hot and cold utility reduction occurs. With this design, cost estimation has been done using heat exchanger cost equation. Although the results found can be used for development of new projects, as heuristics rules, the application has been limited due to lack of understanding of the subject

Figures (10)

Figure-1 Existing flow sheet for Crude Distillation Unit Measurements of cooling water flow and temperature drop indicate that the heat load on the following overheads cooler is 1.8 MW, and since the temperature drop is 60°C this gives a CP of 30kW/K. The current level of heat recovery in these two exchangers is 1640kW . The overall heat transfer coefficients U for the exchangers are then calculated to be 0.2 and 0.125kW/m’K, respectively. The basic relationship of U to the film heat transfer coefficients h (ignoring fouling and wall resistance) is:

Heat and Mass balance for organic process Here question marks denote unknown values and bracketed values were found by back calculation

Table-4

Composite curves for hot and cold fluid for actual and shifted temperature are drawn using data shown in table-4 and 5. Composite curves for actual temperature are shown in figure 2(a) and for shifted temperature are shown in figure 2(b). The grand composite curve (GCC) shown in figure-3 is another tool, also used in pinch analysis. This grand composite curve is plotted using cascade figure-5 that combines hot and cold composite curves in a single curve, also through the sum of their heat content in each temperature level. For zero value of the enthalpy (horizontal axis), the temperature of that point coincides with the pinch point. Using GCC, it is easier to observe that, in the temperature level above the pinch, the process just needs hot utility, whereas below the pinch the demand is for cold utility. In case of many utilities (multiple level utilities), it is possible to choose one of them, based on the closer temperature level to the demand, minimizing the heat transfer irreversibility. GCC also shows the use of steam turbine which is able to generate power using the degradation from high to low steam pressure. From this GCC, location of the pinch has been identified. At temperature 113°C pinch temperature occurs at which there is no heat transfer. Composite curve shows the same value as in problem table algorithm. It gives simple framework for numerical analysis. The cold utility target - Hot utility target should equal the bottom line of the infeasible heat cascade which is 3535.35kW. These calculations provide useful cross-checks that the stream data and heat cascades have been evaluated correctly e

Problem Table Algorithm (Temperature intervals and heat loads for above process) Table-6

The grid is much easier to draw than a flow sheet, especially as heat exchangers can be placed in any order without redrawing the stream system. Also, the grid represents the countercurrent nature of the heat exchange, making it easier to check exchanger temperature feasibility. Finally, the pinch is easily represented in the grid, whereas it cannot be represented on the flow sheet. For drawing a grid diagram, above the pinch, ¥CPhotr S VY CPooia below the pinch, 'CP,o¢ = Vi'CPooig. Heat exchanger network design for given process is shown in figure-6. From above grid diagram, heat exchanger, heater, cooler summery is given in table-7 and 8. From grid diagram according to rules, we need total 4 heat exchangers, 2 heaters and 2 coolers. Initially when two heat exchangers have been used heat recovery was around In these existing organic distillation processes, hot and cold utility requirement decreases but number of exchanger increases as shown in following grid figure-6.

Cost Estimation The final information is to be collected is the cost of heating and cooling and the capital cost of new heat exchangers'’. In this process, the hours worked per year are also needed i.e. here the figure is 5000. Heating is provided in the coal fired furnace whose mean temperature is approximately 400°C. The fuel costs $72/ton. Gross calorific value is 28.8 GJ/ton. Gross efficiency is 75%. Useful heat delivered cost is $3.33/GJ or $12 /MW. Cooling is much cheaper, cooling water is recirculated to a cooling tower which works in the range 25-35°C. Now exchanger cost is given by the following formula,