The oil and gas industry

Because it produces energy that fuels practically all other businesses and is used for domestic purposes, the oil and gas industry is likely one of the most important sectors of the world economy. With its discovery in the nineteenth century, oil has been a primary fuel, with the dependence on the material observed in the generation of energy for powering the rapid industrial revolution that was the foundation of many countries' economic prosperity around the world (Speight, 2014). Despite increased environmental concerns such as carbon dioxide emissions from the use of oil and gas fuels, the resources are still critical elements of the current energy matrix from powering cars to lighting and heating homes as well as for driving machines.

Oil and gas extraction entails four primary stages of exploration, well development, production, and abandonment of the site. The survey entails the search for rock deposits that contain oil and natural gas deposits. The exploration phase involves the geophysical prospecting as exploratory drilling. After the completion of the survey, well development ensues in which wells are constructed in the hydrocarbon existing fields. The production phase is when the hydrocarbons are extracted and separated from substances such as gases, liquids, and others.

Due to the complex nature of the oil production processes, process control systems can be seen as essential components of the oil and gas industry. The necessity of the control system is to eliminate the failures that may arise and hence leading to the catastrophic incident or unplanned downtime. The application of the advancement process control (APC) in the production of oil and gas is essential in ensuring consistency, safety, and cost-effective operations.

Overall Process Description

The oil and gas industry comprises of three parts: the downstream, midstream and upstream sectors (Speight, 2014). Each of the components focuses on a critical aspect of the oil and gas production. The upstream industry deals with the exploration and production of hydrocarbon while the upstream sector entails the refining and marketing the various hydrocarbon products. The third sector is the midstream component that involves the processing, storage, and marketing of the commodities like the liquefied gas, natural gas and other elements that are produced from the crude oil processing. This case study focuses on the process control of production in the refining stage of the production of oil and gas.

The refining process aims at providing a wide range of defined products according to the required specifications. Refineries use a distillation column to separate the crude oil into fractions with the relative quantities seen to be directly dependent on the crude used. It is thus necessary to obtain a feedstock that allows the production of the required number and quality of end products. The success of the refinery depends on the ability to accept any the crude that is available for use. The primary operations entailed in the refining process include product distribution terminals that are utilised for dispensing the products to bulk customers.

The application of advanced process control (APC) for refinery units include the availability of sophisticated interactive problems. Furthermore, the group is made up of more constraints than variables that require adjustments like the qualities, alarm and process limits. The operations are also challenging to optimise manually mainly due to the changing constraints with the operators tending to be conservative. The complicated nature of the economics with the changing operational requirements also necessitate that an APC is enforced to allow proper control of the operations.

The process starts with the heating of the incoming crude to its boiling point. The heated feedstock then enters the fractional distillation column where it is separated into several fractions based on the definite boiling points of the components that are being separated. The separation is also done based on the weights of the portions. The column is of a reflux type in which hot rising vapours heat the colder condensed fluids and hence allowing proper heat exchange due to the temperature differences of the substances that are being moved along the columns. As the hot vapours heat the condensed fluids, a reverse process occurs in which the gases with the heat in turns become cold (Devold, 2013). Clear thermal zones from which products can be drained are therefore created due to the creation of the refluxing process that occurs in the reflux distillation column. Both the continuous and vacuum distillation can be used to separate the fractions to avoid the overheating of the raw feedstock to over 3700C. The danger of overheating is noted in the possibility of thermal cracking as well as the excessive coke that may lead to the obstruction of the vessels and pipes.

A sidecut stripper is also used in addition to the primary column to improve the separation of the fraction. The sidecut is also known as the fractions that are emerging from the sides and not from the bottom of the columns. Such proportions include gasoline, diesel, and kerosene. The fractions are made of a mixture of alkanes and aromatics as well as other hydrocarbons, and hence there is no linear and uniform relationship between in the rise of the carbon number and the boiling point and density. On the other hand products like naphtha are considered as an overhead product as it is obtained from the top of the central column and not from the sides. The residue is the primary bottom product.

After the distillation stage, the second process is the cracking and reforming. Cracking is the process of breaking long-chain hydrocarbons into a range of low-chain compounds that are volatile. The cracking operations are conducted in the presence of a catalyst and are essential for increasing the amount of the alternative volatile hydrocarbons. Apart from catalytic cracking, the long chain hydrocarbons can also be broken down into smaller components using heat (thermal) and steam. Thermal cracking entails the breaking down of the large molecules at high temperatures while steam may also be mixed with the hydrocarbon vapours to obtain the volatile compounds from the larger substances. Reforming or aromatisation entails the conversion of open chain (aliphatic) hydrocarbon into aromatic compounds with the same number of carbons as the parent molecule in the presence of catalysts. Some of the types of the reactions involved in the reforming and aromatisation include cyclisation, dehydrogenation, isomerisation, and cyclisation. Some of the catalysts that are used in the reforming and aromatisation process may entail platinum, nickel, or palladium. However, it can be observed that the platinum is the best catalyst and hence the process is sometimes referred to as platforming. Reforming is utilised in the refinery units to enable the production of aromatic compounds like xylene, benzene, and toluene that are essential industrial chemicals (Degnan, 2015). The reforming process is also necessary for increasing the octane number of straight-run gasoline through the enlarging the proportion of the aromatic hydrocarbons in the fuel. The reforming and cracking process is used to produce more gasoline. Even though the products of refining differ from one company to another, it can be observed that most refinery units are engineered to provide as much gasoline as possible due to the high demand of the product in the transportation sector. However it can be noted that the refineries are also focusing on the production of other products that are used as fuels or petrochemicals.

Process Flow Diagram

The figure below represents a typical process flow diagram for the crude oil refinery.

Figure 1: Typical Process Flow Diagram for Crude Oil Refinery

Process Flow Diagram Description

The crude oil feedstock is passed through the furnace where it is heated to boiling after which the heated mixture is taken into the first distillation column where atmospheric distillation takes place through a fractionating chamber (Abdel-Aal, Aggour, & Fahim, 2015). The fractionating compartment is made up perforated trays that allow for the vapours to condense at each stage and after that flow into the storage tanks. The preheating at the first furnace is limited to 3500C to prevent thermal cracking as already stated. In the atmospheric distillation chamber, the extraction is done close to 1.2–1.5 atm. The deposit leaving the atmospheric distillation column is referred to the as long residue and is passed through vacuum distillation process in which the process is conducted at a pressure that is slightly less than the atmospheric pressure. The vacuum distillation is necessary for upgrading the crude oil used. The difference between the vacuum and atmospheric stages is noted in the sense that the vacuum chamber is made up of a system of packed beds that are used for condensation as compared to trays in the atmospheric distillation. Vacuum distillation helps in the production of gases.

The existence of cracking units is noted due as a requirement of meeting the various product demands (Abdel-Aal, Aggour, & Fahim, 2015). Heavy gas oil is thus thermally or catalytically cracked in the distinct chambers. Cracking done as a secondary process and can, therefore, be observed to be made possible by the existence of additional components that were not part of the conventional refineries. The cracking phase leads to products like gasoline, gas oil, and gases such as butane and propane as well as a slurry that is given out as the residue of the process.

Process Control Philosophy

The process control philosophy in oil and gas refining is based on the processes that enable the distribution of the feedstock across the plant, pre-heating of the crude oil, temperature and pressure maintenance, monitoring of the flow rate, and collection of the fractions based on the weight and boiling points (Howes et al. 2014). On the same note, it is essential to monitor aspects of the unit operations like mixing and refluxing of the vapour streams. The monitoring and control are enabled through the use of interfaces on which the operators may manually or automatically observe the various process parameters like temperature, flow rate, pressure, and others. In automatic control systems, the parameters are monitored using a computer system that in most cases is tailor-made software that can detect the various changes in the plant and make corrective measures if necessary (Degnan, 2015). For instance, in advanced process control of oil refineries, graphic user interfaces (GUIs) are enabled through the use of computer communication systems to allow effective monitoring and hence appropriate adjustments of the process parameters. The following parts are therefore playing a significant role in the process control philosophy in a refinery.

Pumps. Are essential in pumping the feedstock through the piping system. The flow of the feed and the products are enhanced by the presence of the different pumps that perform various functions. The first pump propels the feed into the fractional distillation chamber. The second pump forces the residue from the atmospheric distillation chamber into the vacuum distillation unit. Further pumps exist to move hot products around the plant.

Motor operated valves. The valve is to serve in controlling the flow of the crude oil into the distillation columns (Chaudhuri, 2016). Apart from the flow valves, the mixing valves aid in the uniform dispersion of the crude oil to allow homogenous evaporation of the different components in the feedstock. On the same note, mixing is conducted to allow uniform heat transfer and exchange in the distillation chambers.

A crude oil tank. The tank is fitted with a floating roof to minimize evaporation losses and risk of explosion and fire due to the elevated ambient temperature.

Temperature controls. Temperature control interfaces exist in the system especially in the preheating stage to maintain the temperature below 4000C to avoid the thermal cracking of the crude. However, the temperatures are increased in the thermal cracking chambers to between 4600C and 6000C to aid the processes.

Pressure Controls. Pressure monitoring is essential for both the vacuum and atmospheric distillation (Chaudhuri, 2016). In an atmospheric distillation column, the pressure is maintained at 1-2 atm. On the other hand, pressure levels are monitored in the vacuum distillation chamber to maintain relatively low-pressure conditions that are less than the atmospheric pressure. Vacuum distillation enables further refining of the residues from the atmospheric distillation column.

Desalter. The desalter is another equipment that is used as part of the process control operations in the crude oil refinery. The unit is primarily applied to control the quality of the crude oil feedstock by removing the inorganic salts such as sodium chloride. Removal of inorganic salts is conducted immediately after the pre-heating of the feedstock has been accomplished.

Catalytic reformers and cracking units. The existence of catalytic reformers is to enable the process of reforming to occur at the right conditions to maximise the operation efficiency. It is necessary to control the cracking and reforming operations to enable the production of the required products in their appropriate quantities and qualities.

Piping and Instrumentation Diagram (P&ID)

The P&ID shows the presence of pumps, valves, pipes, and other equipment that are useful in the effective operation of the refinery (Chaudhuri, 2016). The piping system enables the flow of the products and feedstock in the units while the instrumentation is essential in performing of functions like monitoring and control of the processes. The piping and instrumentation diagram is therefore essential in demonstrating how the pieces of the equipment are connected to obtain efficient operation and control of the refining processes.

Figure 2: Piping and Instrumentation Diagram

HAZOP for the Oil and Gas Refinery

Oil and gas industry may be considered as one of the most hazardous sectors given the nature of the products that can be observed to be highly flammable and can easily explode. Furthermore, the products are highly volatile and can easily be lost into the atmosphere. Saturation of the evaporated compounds into the atmosphere is one of the major causes of fire and explosion in most refineries around the world. Undesirable outcomes may also result due to the nature of the facility (Howes et al. 2014). The condition of the piping system is observed to a major determinant of the process safety. Hazardous events result when incidences like leaking are reported. Leaking in the pipes may result from burst due to corrosion. Hazard & Operability Study (HAZOP) is noted as an essential tool for identifying factors that cause undesirable outcomes in a plant. In the oil and gas sector the time taken to undertake HAZOP depends on whether facility is new or an existing piping and instrumentation diagram (P&ID). The table below represents the HAZOP for the oil refinery P&ID.

Hazard

Unwanted Event

Preventative Control

Evaporative loss of products

Risk of explosion and fire and reduced amount of the products

Proper securing of the tanks and valves

Faulty valves

The uncontrolled flow of substances and hence leading to unnecessary mixing of the products.

Routine and dedicated maintenance of the valves to ensure that they are in the right conditions.

Presence of ignition sources

Ignition sources of open flame and friction are likely to cause a fire.

Elimination of the possible sources of ignition

Leaking Pipes

Loss of products through leaks and spills and may lead to fire

Replacement of the worn-out pipes.

Higher than average pressures and temperatures

High pressures may lead to the bursting of the process equipment especially over the limit that cannot be withstood by the unit.

High temperatures not only lead to unrequired thermal cracking may also lead to an explosion as the pressure builds in the enclosed sections of the plant.

Proper control and monitoring to maintain the required levels of the two parameters.

Latest Development in Process Control in Oil and Gas Refinery

The conventional control system in the oil and gas refineries is known as the Distributed Control System (DCS) that is used is to provide monitoring of the processes and equipment functionality. Recent studies like that of Morsi & El-Din (2014) have pointed out at some developments in the oil and gas refinery like the Supervisory Control and Data Acquisition (SCADA)/Programmable Logic Control (PLC). SCADA/PLC is mostly applied for control in small industries like irrigation, water treatment, and electric power stations and is seen to be finding applications in the oil refinery sector. In oil refining, SCADA/PLC is made up of four units that include crude oil storage, pretreatment of crude oil, distillation chamber and product storage or dispatch section. The system is further based on the identification of the products of the refining of petroleum as liquefied petroleum gas (LPG), naphtha, gasoline, kerosene, and diesel (Morsi & El-Din 2014). The component is therefore developed to optimise the production of the listed substances. The Programmable Logic Controller (PLC) is connected to the SCADA and is used as the primary means of monitoring the boiler operations by observing the parameters like temperature, flow control and pressure in the system.

SCADA is further proposed as the most useful tool for process automation of refineries that allows the elimination of human errors since the monitoring of the plant is automated. As SCADA monitors the facility, PLC is used as an internal storage for instructions that are useful for the implementation of functions like logic, sequencing, counting, arithmetic, and timing to control the various forms of machine processes like monitoring. SCADA is further noted as comprising of a graphic user interfaces (GUI) that are designed to monitor the oil and gas refinery. Wireless telemetry is identified as another system that is increasingly becoming useful in the control of the processes in the oil refineries. Plants are also being installed with online process control and monitoring software that enables the integrated management of quality of the operations from any point of the plant. The online monitoring systems are also essential for the curbing the dangers and hazardous aspects of the process like high temperature and pressure as well as corrosive conditions that may lead to unwanted outcomes on the operators or the equipment.

The latest developments in the process control are made due to the realized benefits of improved systems to monitor the operations (Howes et al. 2014). Some of the benefits associated with the modern process control tools include the improved product purity, proper management of the process constraints, increased efficiency, and maximum profit attainment. Furthermore, effective process control also serves to reduce the potential hazards that are associated with the oil and gas refinery operations.

References

Abdel-Aal, H. K., Aggour, M. A., & Fahim, M. A. (2015). Petroleum and gas field processing. CRC Press.

Chaudhuri, U. R. (2016). Fundamentals of petroleum and petrochemical engineering. CRC Press.

Degnan, T. F. (2015). Chemical reaction engineering challenges in the refining and petrochemical industries—the decade ahead. Current Opinion in Chemical Engineering, 9, 75-82.

Devold, H. (2013). Oil and gas production handbook: an introduction to oil and gas production. Lulu. com.

Howes, S., Le Pore, J., Mohler, I., & Bolf, N. (2014). Implementing Advanced Process Control for Refineries and Chemical Plants. Goriva i maziva, 53(2), 97-119.

Morsi, I., & El-Din, L. M. (2014). SCADA system for oil refinery control. Measurement, 47, 5-13.

Speight, J. G. (2014). The chemistry and technology of petroleum. CRC press.