Case Study #2
Could a Clarifier tank reduce coke fines in the cutting water?
Would the Closed Coke Slurry System be an alternative?
A Coker operator in the US cut the coke in an open pit and separates the coke fines from the cutting water via a maze system before it is send to a surge tank. The operator planned adding a clarifier/ settling tank to reduce the amount of coke fines in the cutting water that erodes the cutting tools and the jet pump impellers. He also considered a cycle time reduction in the near future.
TRIPLAN was asked to study the overall situation to assess the performance of a clarifier system and to compare this with the CCS System.
Principles of Fines Separation:
1. Settling is separation by gravity
The greater the difference in density between a fluid and solid parts the better – and faster – the separation. Unfortunate for the Coker operation is that the density differences in the system ‘water - fines’ are very small. Hence, the degree of fines removal is largely a function of residence time. Time that is scarce as this may force the operator to sacrifice the quality of the cutting water as the next drum cutting is due.
The time constrains are compounded as the gravity effect to clean the water starts only than when the coke cutting operation is finished and the black water stream from the maze resides, e.g. the turbulence in the settling tank calm down.
Additives supports agglomeration
To agglomerate the fines to larger and heavier particles frequently chemicals are added manipulating the surface forces of each fines element to clog. With larger/heavier particles, the relative settling velocity increases and separation efficiency improves. Again, time remains the governing factor for an effective separation.
Floating coke foam a problem
Coke foam floating on the water surface is another problem that cannot solved with the gravity method. This relatively large amount of scum, a serious disturbance within Coker operation, is rather stable and apparently unimpressed from the settling efforts in the water tank underneath. Usually, an elaborate floating skimming system with mechanical devices is required in the settling tank to remove the foam. Pumps discharge the foam to any coke pile in the pit or the foam receives treatment in another handling system.
2. Dewatering Bin is fines removal by filtration
During coke cutting crushed coke lumps and water are pumped as slurry into the Dewatering Bin. Instantaneously all water separates from the coke by flowing through the buildup coke bed and drains immediately through screens at the bin wall and the slotted cone down to the clean water pit. In this way, almost all fines and foam are trapped in the coke bed and are withheld automatically. The Dewatering Bin with the self-filtration feature has a recovery rate of more than 95% fines.
The drained water in the clean water pit is visually clear and is immediately usable as Cutting water again. The sludge is pumped to the slurry pit to mix with the batch from the next cutting operation.
Evaluation of the methods:
The self-filtration method via the Dewatering Bin removes the fines at the point of origin, whereas a clarifier works on the far end of the problem chain. The settling effect in a Clarifier starts only after the cutting operation is finished and the turbulence in the tank has ceased. If fines removal by gravity is the chosen method, settling tanks are a necessity but does provide neither environmental improvements nor any economical advantages. Sole settling jeopardizes either cycle time or constitutes a constant cost factor and revenue loss area for Coker outage time and repair work in the sludge handling and jet pump system.
The filtration principle in the Dewatering Bin commences with coke cutting when the crushed coke forms the bed and finalizes with drum ream out. The subsequent residence time in the bin removes the free water content down to commercially accepted levels. The load out operation that follows transports the coarse coke material direct to operators existing loading facilities. Schedule reserve, residence time is maintained as each coke drum has a dedicated dewatering bin. The Closed Coke Slurry system has a unit reliability of more than 98%. Various environmental and economical advantages can be identified, namely cutting and crushing in-line with instant separation of water from coke, sensible fines handling, no hosing down/cleaning pit floor and maze, no steam plume with fines entrainment to atmosphere and resulting water losses, no releases of volatile organic material etc.
TRIPLAN could demonstrate to the operator that a clarifier does not provide the expected improvements due to the system inherent drawbacks and physical deficiencies as a function of time.
Besides cutting water quality and environmental advantages the Closed Coke Slurry system delivers, TRIPLAN together with the Coker operator identified also several cost reduction and performance improvement areas which provides in summary an attractive pay out time for the investment.
(Details of the economical evaluation not disclosed)