The crude oil feedstock is separated into valuable petroleum products in the Crude Distillation Unit (CDU). The crude oil distillation column is the key processing equipment that must be operated under optimized and stable conditions to provide on-spec products. For successful troubleshooting operating staff must have sufficient knowledge and operating experience in the crude oil distillation process. The process engineers must have analytical skills to diagnose the problem and implement suitable actions to bring back normal the crude distillation process. For further understanding please also view my previous blog ” Working of Crude Oil Distillation Column“.
In this blog, I have tried to list down the major operating problems and their solutions, related to crude oil distillation towers. The below figure shows the general arrangement of the crude oil distillation column.
Following are some problems faced by process operating staff with the crude oil distillation column;
1. Crude Distillation Column Flooding
Crude oil distillation tower flooding occurs at vapor loads greater than the maximum capacity of the tower and high levels at tower trays. Due to flooding, the interaction between vapor and liquid is so strong that stable counter-current operation is no longer possible. Pressure drop and liquid holdup increase drastically, and the distillation column efficiency decreases sharply. Flooding results in poor petroleum products quality.
The symptoms of flooding are 1. very high-pressure drop, that restricts the vapors to move up, 2. unstable levels and indicators, 3. temperature drop or low-temperature gradient across the flooded section of the tower, 4. change in products specifications or poor product quality, 5. One section (e.g. diesel) is flooding while the downstream section is drying out.
As flooding is confirmed by the above-mentioned conditions, the column becomes less capable of handling higher vapor rates and high feed rates. Control actions to troubleshoot the flooding problem includes; reducing the liquid rates to the tower, reflux rates, stripping steam to the tower and crude heater outlet temperature should also be reduced according to the unit feed rate. Further, if flooding in the specific section is observed then the product draw-out rate can be increased.
2. High Levels in Distillation Column
In the distillation column, the levels of the tower bottom and draw-off trays are maintained constant by adjusting the product rate. Distillation towers are equipped with level indicators, that guide the control room operators to adjust the conditions accordingly. A too high level will cause the liquid to rise to the trays section of the tower and behave like flooding in the column showing the symptoms of low-temperature gradient, high-pressure drops, and poor fractionation efficiency.
The stripping steam should be dry before it is put in service because water will rapidly vaporize creating a pressure surge and level fluctuations. Further, reduce the feed rate and increase the product draw-out rate from the tower. Physically verify the levels at the level glasses of where the problem is observed. Similarly, the rise in diesel and kerosene accumulator trays is also adjusted by controlling the flows of hot, and cold refluxes and product draw-out rate. To control the high-level problem reduce the crude heater outlet temperature or increase the product draw-out rate.
3. Dry Trays
The dry trays scenario is the reverse of flooding because in flooding the trays are filled with liquid while dry trays are empty of liquid. This situation may arise when the product withdrawn rate is higher than the quantity of internal reflux and the feed rate is lower in the column. In this case, the trays below the draw-off point run dry, and as a result, no fractionation takes place. At the same time, there is no sufficient liquid present to maintain the levels in the stripper draw-off trays and side-cut strippers. The tower or section of the tower with dry trays will have a low-pressure drop and low-temperature gradient.
To control the dry trays problem, it is necessary to either lower the product withdrawal rate or increase the internal reflux to the column. Moreover, the internal reflux can be increased by increasing the crude heater outlet temperature or feed rate to the tower. In addition, the reflux rate can be increased by reducing the rate of the next lightest product draw-off rate. The dry trays, caused by superheated vapors, increase the reflux rate to the tower and also reduce the feed inlet temperature.
4. Trapped Water
The trapped water causes poor fractionation and restricts oil and vapors contact resulting in poor product quality as well. Low temperature at top trays or tower top shows the presence of water. In addition, water evaporation may cause pressure increases in the tower. Trapped water can be removed gradually by increasing the top temperature or reducing the naphtha reflux flow. Water in the tower may also come due to the poor crude oil desalting process as explained below.
Further, the stripping steam to the column is condensed at tower overhead condensers and removed from the overhead receiver in the form of sour water. If too much water is coming with the reflux it will upset the crude column. It will accumulate on the upper trays causing the column to flood. The remedial action is to reduce the reflux rate, increase the tower top temperature and let the water evaporate. Tower pressure rise can be controlled by increasing the condensation at the tower overhead system and increasing the capacity of the compressor. Crude tower, inlet temperature, and stripping steam can also be reduced to decrease the vapor load on the tower, hence pressure will be normalized.
5. Water Carryover from Desalter
Water carryover due to the poor desalting process will cause an immediate temperature drop at the tower inlet and bottom. The reason for water carryover from desalter can be the low temperature of the crude feed. Maintaining the Desalter inlet temperature of 125~130 °C will improve the desalting efficiency. Water contains salts that cause corrosion in the column overhead system. Ca & Mg salts will move to the tower top and convert to HCl. The first indication of water carryover is the pressure rise of the pre-flash vessel.
The solution to this problem lies with the efficient operation of crude Desalter. Reduce the water level in the Desalter up to normal Increase the flow of corrosion inhibitor and neutralization agent at the tower top. Further effects of water carryover are energy loss and poor efficiency of the tower, which will badly affect the product specifications as well.
6. High Salt Content in Desalted Crude
The salt content in crude oil feed greater the 1 PTB will have adverse effects on the crude column and downstream sections in terms of corrosion and fouling of equipment. This might be due to low water injection rate, too low or too high temperature of crude feedstock, low dosing rate of Demulsifier agent, and low-pressure drop across the crude oil washing water mixing valve, at the Crude Oil Desalter.
7. Low-End Point of Products
This shows that products are lighter and needed to be improved in their distillation point. This can be achieved, by increasing the temperature of the tower feed and tower top, reducing the reflux rate, and increasing the product draw-out rate.
8. Change in Crude Feed Composition
An abnormal change in the crude oil composition may upset the operation of the crude distillation unit. For smooth operation, keep in mind the behaviour of each crude and adjust the ratio of crudes according to the plant configuration and capacity limitations.
a. Effects of Lighter Crude Oils
Very light crude will produce a high flow of lighter ends, that may overload the gas and naphtha sections. Further, lighter crude will make lighter the kerosene and diesel products as well. With the introduction of lighter crude, the pressure of the pre-flash vessel will be increased and the vapor load in the crude distillation tower will be increased. Flow rates of lighter products Kerosene and naphtha will be increased.
To control the high vapors load due to light crude oil, reduce the tower inlet temperature and flow of stripping steam to the tower. Increase the flow rates of hot refluxes, to maintain the specifications of the products. Increase, the draw-off rate of lighter products. Increase, the tower temperature by reducing the reflux to make the products heavier.
b. Effects of Heavy Crude Oils
Heavy crude oil will have the opposite effects as light crude and will produce more bottom thus overloading the vacuum distillation unit. Increase the heater outlet temperature and stripping steam flow to the tower. Decrease the hot flow rates of hot refluxes to maintain the product specifications. Decrease the tower top temperature by increasing reflux to make the products lighter.
9. Heavy Color of Diesel Product
If the colour of the diesel product is becoming darker, it means that heavy hydrocarbon vapours are coming with diesel. Diesel colour can be improved by increasing the hot reflux, which will condense the heavy vapours and send them back to the bottom. Further, the tower the top temperature can be decreased by increasing top reflux which will cause lighter ends to come back to the tower. Moreover, heater outlet temperature can also be deceased to reduce hot vapours to rise in the tower and make the product lighter.
10. Controlling Flash Points of Kerosene and Diesel Products
The low flash point of the Diesel or Kerosene products can be improved by increasing the stripping steam to side strippers or making them heavier. Products can be made heavier, by increasing the heater outlet temperature reducing the reflux rates and increasing the stripping steam rate.
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