Questions Asked in Production Engineer Interview:
Surface Facilities Questions & Answers:
Q: Why Water is unwanted in crude oil?
A: We have to reduce the water & salt content because there are many bad effects for the existence of water and salt in crude oil, they are:
1. Salt Sedimentation in heat exchangers and furnaces, the thing that reduces their efficiency, such sediments reduce the pipe diameter, reduce the heat transfer area, thus their diameter will be reduced causing severe damage to these pipes. any damage in these pipes will need repair and maintenance and a great lost in production rates.
2. Forming hydrochloric acid: it is formed because of the decomposition of chloride salts and their reaction with water, this acid is very corrosive, and may corrode pipes and equipment.
3. increasing the energy consumption: water existence needs more power, for example; water needs 8 times the power needed to heat oil.
4. multiple effects: reduce the pipe and equipment capacity used in oil treatment.
Read also Wet Crude Treatment Plant
Q: How the water is mixed with crude oil?
A: water is spread in oil in 2 ways:
1. Free water: which is spread in the form of big particles of water, they are separated easily when settling.
2. Emulsion water: in which waters is surrounded by a strong film because of emulsification factors, this kind is not separated easily.
Q: what are the components of Wet Crude Treatment facility?
A: any wet crude treatment plant consists of the following:
1. wet crude tank: the main storage of wet crude.
2. main pumps: pumps wet crude to the treatment trains.
3. Demulsifier System: consists of a tank and two pumps.
4. Indirect heater: heats wet crude to reduce its viscosity.
5. Dehydrator.
6. Desalter.
7. Wash Water system: consists of a tank and two pumps.
8. Produced Water system: collects and treats the produced water from the process.
9. Heat Exchangers: they are of two types: water/ water and oil/oil.
10. Auxiliary systems: such as Instrument Air system and Fire – Fighting system.
Q: what is Emulsion?
A: a heterogeneous system that consists of at least one immiscible liquid dispersed in another liquid in the form of droplets.
Q: what are Emulsifiers or emulsifying agents?
A: they are substances that cause the formation of interfacial films around the dispersed oil droplets, preventing the coalescence process.
Q: How to form a stable emulsion between two liquids?
A: there are three conditions to form a stable emulsion between two liquids:
1. should be immiscible liquids.
2. a sufficient power to obtain this dispersion.
3. the existence of an emulsifying agent.
Read also Optimizing Wet Crude Treatment
Q: what a demulsifier mean?
A: high molecular weight polymers used to equalize the effect of emulsifying agents and to reduce the surface tension of water droplet.
Q: what are the emulsion treatment steps?
A:
1. breaking the emulsion: means the breaking of the film surrounding the water droplet.
2. the coalescence of water droplets after breaking the emulsion forming big droplets ,easy to settle, this process is made by the aid of electrostatic field.
3. Settling of the big droplets to the lower part of the vessel.
Q: what does Stock’s Law mean?
A: it is the law used to calculate the downward velocity of water droplet in oil ,this is Stock’s law:
reducing oil viscosity is essential in wet crude treatment, this is achieved by heating the wet crude oil by different means such as: heat exchangers, direct or indirect heaters, because viscosity is inversely proportional with the downward velocity of water droplets.
Q: what is wash water? and how it is used?
A: using fresh water is useful to reduce the concentration of the salt solution, this thing will help in reducing the salt content in crude oil, the mixing of crude oil and wash water occurs to get a good mixing between them.
using too much wash water leads to excessive current or a short circuit between the grades, may create paths for current leak , decrease the retention time and may cause water carryover with oil outlet.
using less amount of wash water, will reduce water washing, leading to high salt content in the oil outlet.
Q: how to organize the wash water ratio?
A: wash water ratio is organized by a control valve called “Ratio Valve”, all you have to do is to give a setpoint, this valve receives a signal from crude oil and wash water flowmeters, then closes or opens the wash water valve to control to achieve the required setpoint.
Q: what is the best wash water ratio?
A: the best wash water ratio is (3 – 5)% of the treated oil.
Q: How to calculate associated water?
A: Associated water is calculated as follows:
Associated Water = Produced Water – Wash Water
Q: what is the pump type used for chemical injection, and why?
A: reciprocating pumps is the better option for chemicals injected into ant stream, the main features of this kind of pumps which make it the most preferable type : low flowrate and high pressure, high pressure is required to help the chemical to be injected in a high-pressure stream.
Q: what is the best points of demulsifier injection?
A: the best points are:
– in the main pump suction to help better mixing with wet crude oil.
– before the mixing valve.
– 50 to 60 meters before the dehydrator.
Q: what is the best retention time for wet crude treatment?
A: according to the practical numbers, 10 to 30 minutes is the best retention time for wet crude treatment process, heavy crude needs longer time, to reduce their viscosity first.
Q: How to use electricity in wet crude treatment?
A: there are electric transformers are installed over dehydrators and desalters, these transformers are connected to electrical grids made of carbon steel, this system helps the separation of water by subjecting the emulsion to a high voltage electric field, water droplets becomes charged when water in oil emulsion passes through these grids, so these droplets are attracted and repulsed with each other according to their electrical charge, this will help them collide, form bigger droplet then settle.
electricity weakens the water droplet film, stretching it longitudinally and tangentially because of reducing the surface tension.
Q: how wet crude is mixed with wash water and demulsifier?
A: wet crude, wash water and demulsifier are mixed together in “Mixing Valve” works by maintaining a pressure difference through it, the best setpoint for ΔP through this valve is “10 – 15” psig.
Q: what does “High water content in the desalted crude” means?
A: there are many reasons for this:
– using too much wash water, Remedy: reduce wash water.
– high water content in the inlet wet crude, Remedy: increase demulsifier rate.
Q: what does “High salt content in the desalted crude” means?
A: there are many reasons for this:
– high salt content in the inlet wet crude oil or low wash water ratio, Remedy: increase wash water ratio.
– high inlet wet crude flowrate, Remedy: reduce the flowrate of the wet crude facility.
– insufficient mixing between wet crude and wash water, Remedy: increase ΔP through the mixing valve.