CHAPTER 3- CRITICAL ANALYSIS AND DISCUSSION
3.3PROCESS FLOW DIAGRAM
Figure 3.3.1 shows the process flow diagram for the oil and gas industry.
In oil and gas industry, there are several steps in order to treat the wastewater. The wastewater will be going through the primary oil/water separation, secondary oil/water separation and tertiary treatment (if necessary). After that wastewater undergoing the secondary oil/water separation, it will send to the equalization system. Then, the wastewater is then routed to the biological system and tertiary treatment (if necessary) prior to discharge.
PROCESS WASTEWATER PRE-TREATMENT
A desalter is a process unit in an oil refinery that removes salt from the crude oil (Francis S. Manning & Richard E. Thompson, 1995). The sewage from the desalter is one of the causes of operating problems in wastewater treatment. This is because the concentration of benzene and other volatile organic compounds (VOCs) that vaporize in the sewers lead to excessive emissions as well as odour problems in the refinery. Thus, we need to manage the desalter properly.
The desalter oil/water separation need to be installed. This is because there are some advantages of installing it such as save space (suitable for limited capacity of a primary oil/water separator) and save cost. Also, one of the methods to handle the increase of the amount of solids that get discharged during mud washing of the desalter. The sewage of the desalter is sent to a floating roof tank. The oil is skimmed off and sent to a refinery slops and the water phase is sent to the wastewater plant (Thompson P, 2998).
The effluent will passed through the screening when it enters the plant. Screening is a unit operation that removes suspended large/gross solids from wastewater stream. These may include natural and man-made trash such as rocks. Screening device is a series of bars welded with approximately one inch spacing between the bars. The purpose of this device is to trap any large particles in order to avoid shock-loading.
Figure 3.4.1 shows the mechanical racks as the screening device
Then, the effluent will pass into the aeration tank. The function of the aerated grit chamber is to protect the pump and other mechanical devices, to avoid a reduction in flow capacity and clogging. The effluent will be decomposed by aerobic bacteria and other organisms in the presence of air. In a properly operating plant, these micro-organisms will form a dark brown mass called “activated sludge” (Lofti G, 2016). The incoming effluent will mix with the activated sludge and blown through diffusers at the bottom of the tank.
Figure 3.4.2 shows the aeration grit chamber in a spriral mixing pattern with the heavy, inert particles is removed by centrifugal action and friction against the tank walls.
After that, the strippers (steam/natural gas) is used in order to control the emissions of benzene and other volatiles. Proper pre-treatment and equalization need to be utilized to avoid the plug of the internals of the stripper because of the high content of the oil and solids in the desalter effluent.
Light organic suspended solid remains in wastewater after pre-treatment. Primary treatment is the preparation for the biological treatments. We can use gravity in a sedimentation tank (round or rectangular) or float in order to remove the light organic. The raw sludge is removed by the mechanical scrapers and pumps, while the floating material which is grease and oil will rise to the surface of the tank. It can be collected by a surface-skimming system.
API separators contain oil and oil-bearing sludge. It uses the difference in specific gravity to allow the heavier material to settle below lighter liquids. Hydrocarbons that float on the surface are skimmed off, while the sludge that settles to the bottom is removed periodically (Lofti G, 2016). Besides, the underflow baffle plates are used to prevent the oil from escaping into the outlet section.
Figure 3.4.3 shows the API separator.
SECONDARY OIL/WATER SEPARATION
From the primary oil/water separation, the sewage is sent to induced air flotation (IAF) unit for further oil and fine solids removal. Air is induced by a rotor-disperser mechanism in the IAF unit. The spinning rotor that acts as a pump forced the fluid through the disperser openings and create a vacuum in the stand pipe (Thompson, 1998). Thompson (1998) said the liquid moves through a series of cells before leaving the unit and the float passes over the weir on one or both sides of the unit.
Figure 3.4.4 shows the induced air floation (IAF) unit
Flow equalization is not a treatment process. It is used to minimize fluctuations in WW characteristics in order to prevent optimum conditions for the subsequent process (Francis S, 1995). It is used to dampen the variation in flow and strength of WW (BOD loading) in order to prevent shock loading to biological systems. It is also used in order to provide continuous feed to biological systems. There are three suitable locations of equalization system which is upstream of the API separator, upstream of the IAF or downstream of IAF. We choose to put the equalization tank at downstream of IAF in order to protect the downstream equipment which is biological system.
Francis, S. Manning and Richard, E. Thompson 1995, Oilfield Processing, Volume 2: Crude oil. Pennwell Books.
Thompson 1998, Energy and Environmental Profile of the U.S. Petroleum Refining Industry, US DOE.
Lofti, G. Mabrouk, E. and Kamel, Z. 2016, Applied Petrochemical Research, Volume 6, Issue 2, pp 155-162. Available from: https://link.springer.com/article/10.1007/s13203-015-0143-9