Conservação da energia em processos de destilação

Conservation of energy in distillation processes

Distillation is still one of the most used industrial separation processes, despite all the developments in the separation processes business. As we know, distillation is a very versatile process and easy to design. On the other hand, distillation is a process, in which a lot of energy is spent, what might be a problem due to the recent increase of energy costs.

In the United States, for example, the department of energy says that there are more than 40.000 distillation columns, and that they consume about 40% of the total amount of energy used to operate plants in the refining and bulk chemical industries. Hence, improving the energy efficiency of this operation unit is important to achieve energy savings.

In the distillation process, heat is supplied at a feed heater and reboiler, and then cooled at an overhead condenser. In the conventional distillation process, the supplied heat at the reboiler is discarded in the overhead condenser.

Reducing the energy consumption of distillation columns is complicated. These columns come in many configurations with different operating objectives. This leads to distinct dynamic behaviors and different operational degrees of freedom, which requires specialized control configurations to optimize energy usage. In addition, some columns have numerous limits and restrictions on their operation, making it harder to optimize control.

The operation of distillation columns typically involves a tradeoff between energy usage and product recovery, and setting the proper target involves evaluating the relative economic value of these two factors. However calculating the operational targets can be tough, since distillations is a nonlinear process and regular product-valuation patterns add more nonlinearity to the economic objective function.

It is known that most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty), therefore the reflux ratio is an important factor to be taken care of in order not to spend or lose too much energy.

 WHITE, Douglas C. ‘’Optimize Energy Use in Distillation’’, (which can be found in the references), discusses about the topic in a case study. Briefly, the study shows that as the reflux is increased, the cost of energy for the separation increases almost linearly. As result, the amount of heavy component in top product decreases and the amount of bottom product increases. Per contra, this happens not linearly, so even if you keep increasing the reflux ratio, the compositions will decrease just a little bit.

The difference between the value of the product and the total cost of the feed and energy- called operating margin- for different energy prices show that the optimum reflux ratio depends on the price of energy. If the energy price is high, the optimum reflux ratio is the minimum value that is still able to keep the specification of the top product. If the energy price is low, the optimum reflux ratio might be unlimited. It may be more profitable to operate the column at impurity levels less than the limit.

Pressure is also a main factor about conservation of energy. Despite the knowledge that decreasing the operating pressure of light-hydrocarbon distillation columns usually reduces the consumption of energy, many of these columns are very often operated in higher pressures than the minimum ones.

The minimum required operating pressure is normally based on the heat-transfer medium used in the condenser- the maximum condenser operating pressure is estimated based on the medium, and from this, the pressure required for the target degree of condensation of the desired components can be calculated. It is essential that at the pressure chosen, the temperature of the reboiler-heating medium is sufficiently different from the temperature from the bottoms material; otherwise, there will not be an effective heat transfer.

On the other hand, for most of columns in actual operation, the minimum pressure limits depend and vary with feed rate, time of the day and weather conditions. Therefore, a closer study is necessary to measure all the variables.

The study shows that just a 7% reduction in the average operating pressure can already reduce considerably the energy costs. In addition, choosing a condenser medium with a lower temperature should also save a good amount of energy.

Per contra, changing the pressure requires simultaneously changing the bottoms temperature setpoint appropriately to hold the product compositions at their targets, which is complicated to do manually. Consequently, it requires advanced composition control on the column.

For packed columns, pressure changes can affect the mass-transfer coefficients. That needs to be reviewed carefully as part of the energy reduction evaluation.

Changing the pressure may also result in other impacts, such as the amount of reboiler heating medium needed and the hydraulic profile of the plant. In the case of partial condensation, pressure control can interact with the overhead receiver level. Also lowering the pressure could get the column closer to flooding. When all these effects are real, it may become a reason for not making pressure changes at all.