PROCESS VARIABLES

This purpose of this chapter is to explain the main variables in Unit control.

The variables described are: CDU feed rate, fired heater outlet temperature, main fractionator overhead temperature, pumparounds duty and stabilizer bottom temperature.

3.3.1. CDU feed rate

The unit is fed by the crude charge pumps in unit 60. The crude oil flows through the cold preheat train and enters into the desalter. In order to keep constant the desalter pressure, the crude oil flow at the crude charge pumps is set to the same flow as the measure at the fired heaters (011-FIC-069), by the controllers 011-FY-092 and 011-HY-002.

If the pressure in the desalter increases the inlet flow from crude charge pumps will be reduced.

3.3.2. Fired heater outlet temperature

This variable is very important, due to the fact that the column overflash is highly dependent of this temperature. If an increase in duty is required, the air flow is increased before the fuel rate is increased. On the other hand, if a decrease in heater duty is required, fuel flow rate is firstly reduced and secondly air flow rate is reduced. This control scheme ensures that the fired heater is always operating with an air excess, to ensure a good combustion and avoid risk of after burning in the stack.

3.3.3. Main fractionator overhead temperature

The overhead vapour temperature is controlled by the amount of heat removed from the top pumparound circuit. This temperature is important because the overhead product naphtha end point can be adjusted by changing the set point. If the temperature is higher than the set point, the top reflux duty will increase. As a result the overhead product end point and quantity will be reduced. On the other hand, if the temperature is lower than its set point, the overhead product end point is lower than it should be and the control system will decrease the top pumparound duty. So, if an increase in the overhead vapour product end point and quantity is desired, the top pumparound temperature set point will be increased.

3.3.4. Main fractionator pressure.

The column operating pressure depends on the pressure controlled at the main fractionator accumulator drum D-1103. The operating pressure of a distillation column is related with the relative volatility of the components to be separated in the different streams. Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. As pressure increases the relative volatility approaches to one. This means that the vapour pressure characteristics of the components are more similar, and therefore they are more difficult to be separated.

In addition, if the remaining variables keep constant, an increase in the column pressure will decrease the overflash and therefore the internal column flow and the separation achieve will be lower.

3.3.5. Pumparounds.

Pumparounds must be well controlled to obtain specific products. Pumparound duty control is essential to keep internal liquid and vapour flow inside the column. The control is acting on the recycled product flow to the column, and on the total pumparound duty (through duty controller).

Pumparounds are critical to maintain the optimum vapour/liquid rates in the fractionation of the column. Inadequate pumparound heat removal and poor pumparound distribution will result in higher column pressure, poorer fractionation and an increase in the flash zone pressure. Excessive pumparound heat removal may result in difficulties in maintaining some end point and may cause internal flooding.

3.3.6. Specification Adjustement.

The principal specifications to which the products from the fractionator can be adjusted are the boiling point ranges and the flash point. In general, if the fractionation is good, the end boiling point of a side-cut equals the initial-point of the next heavier product plus a gap. A gap is the result of a good fractionation; however overlap is more normal and will increase as fractionation efficiency decrease. Product withdrawal flow rate are adjusted to provide the required end points for the products and are generally set up from the top of the column working down.

As a general rule, increasing the drawoff rate will raise the end point of the side stream cut whilst decreasing the drawoff rate will lower its end point. An indication of the end point is the draw off temperature of the stream leaving the column. Increasing the product withdrawn, increases the draw off temperature and the product end point

The flash point of LGO, and HGO products can be modified by adjusting the flow of stripping steam via, FIC-017 and FIC-019 respectively (to increase the flashpoint the stripping steam has to be increased). However, the stripping steam can only be increased to the point where it starts to interfere with the end point of the next higher product. If a further increase in the flashpoint is required, other changes must be made, allowing an increase of the end point of the next higher product.

Kerosene flash point can be adjusted by the kerosene stripper reboiler E-1110 duty via 011-UIC-031 or, in case reboiler E-1110 is out of service, with steam flow via 011-FIC-015.

The specific gravity is directly related to initial and final product boiling point, so changing the product’s yields is required to achieve this specification.

The freeze point is related to chemical composition of the product. As final boiling point increases, the paraffin content in the product increases too, so the freeze point obtained will be higher.

If residue entrainment occurs HGO can get black colour. This means that the washing section (between HGO draw off tray and flash zone) doesn’t have enough liquid flow. It is necessary to increase the overflash or reducing the HGO draw off rate.

3.3.7. Stripping Steam in the Main fractionator column.

Increasing this flow rate, it is possible to decrease the content of lighter fractions still present in the residue, increasing flash point and the initial boiling point of the residue. The stripping steam flow, controlled by 011-FV-012 will affect temperature of the flash zone and amount of overflash.

3.3.8. Overflash

Overflash is that portion of the feed which is vaporized in addition to the overhead and sidestream products. The overflash condenses on the wash section plates and returns to the flash zone and bottoms stripping section. Its purpose is to prevent coke deposition in the wash section plates and preventing entrainment of residue up into HGO section, which would result in off spec HGO.

The CDU is designed for a minimum of 5% overflash.

3.3.9. Stabilizer temperature

The stabilizer bottom temperature has to be controlled in order to maintain the C4 -specification in this stream. This control is a cascade loop in which the stabilizer bottom temperature is controlled by the high pressure steam flow through the desuperheater (DS-1101). If the bottom temperature decreases, the amount of high pressure steam will increase, and therefore the reboiler duty, in order to raise the temperature again.

The temperature in the top section at constant pressure defines the quality of the LPG product; this temperature is controlled on E-1122 (auto variable pitch controller). Settling this control at a lower temperature will decrease LPG C5 content and will increase the tower reflux. Increasing this temperature will increase quantity and C5 content in the LPG and will reduce the reflux rate.