John D. Elliott
Foster Wheeler Corp.
Clinton, N.J.
Among refineries with delayed cokers, there is now major incentive to maximize coker fresh feed throughput while producing maximum possible liquid yields.
There are design features and operating considerations that can increase liquid yields.
Maximizing coker throughput is typically a necessity in overall refinery operations because coker capacity to process the bottom of the barrel can be a bottleneck to the entire refinery crude throughput.
The incentive for achieving the maximum liquid yield from a coker is obvious: clean distillates, even cracked distillates, are valuable while fuel grade coke is not. Another way to look at a maximum liquid yield coker operation is as a minimum coke yield operation.
The unfortunate but inescapable fact is that coke, along with gas and light distillates, is formed at the expense of heavy coker gas oil (HCGO) yield. In fact, for each 100 metric ton of HCGO converted, 20-50 metric tons of coke may be produced, depending on properties and operations.
In a modern, complex refinery HCGO can be upgraded to valuable light distillate transportation fuels by means of hydrotreating plus fluid catalytic cracking or hydrocracking with net coke makes less than 10%. The economic incentive to maximize the yield of liquid products in the coker can be significant when considered on an overall refinery basis.
There are, however, situations in which operating strategies aimed at maximum liquid yield are not valid. These may include operations producing a relatively high value anode grade coke or where downstream catalytic conversion of HCGO is not available. However, in our experience, the economics always require maximizing liquid yields after the governing constraint is satisfied.
VARIABLES AFFECTING LIQUID YIELDS
Three operating control variables in a delayed coker govern the product quality and yields for a given feedstock. These variables are heater outlet temperature, coke drum pressure, and the ratio of recycle to fresh feed.
TEMPERATURE
At constant pressure and recycle, the coke yield decreases as the drum temperature is increased. Part of the heavy hydrocarbon, which at lower temperature remains in the drum to be converted to coke, is flashed off at the higher temperature. However, there is only a narrow range over which the temperature can be adjusted. Because delayed coking is an endothermic reaction, the heater outlet temperature is the control point for adjusting the coking reaction temperature.
In actual practice, if the temperature is too low, the coking reaction does not proceed far enough, resulting in the formation of pitch or high VCM (volatile combustible matter) coke. Above a specific temperature, the coke formed will be excessively hard and difficult to remove from the drum with the hydraulic decoking equipment. In addition, at higher temperatures the possibility of prematurely coking the heater tubes and/or the transfer line increases. Controlling VCM content of the coke is important to maximizing liquid yields-coke VCM translates to lost liquid product. Current practice is to target fuel coke VCM contents of 810 wt %.
PRESSURE
A decrease in pressure has the effect of vaporizing more of the heavy hydrocarbon liquid that would otherwise be trapped in the coke drum and converted to coke and light hydrocarbons (Table 1). The strong economic incentives for producing maximum liquid yields in cokers where the feedstock produces fuel grade coke drive the operation to the lowest possible coke drum pressure.
Fig. 1 illustrates the incentive. As a result, these types of cokers are currently being designed to operate with coke drum pressures of 15 psig (1.055 kg /CM2g).
RECYCLE RATIO
Recycle ratio has the same general effect as pressure on coker product distribution, i.e., as recycle is increased the coke and gas yields increase while the pentane and heavier liquid yield decreases. In practice, recycle is used primarily to control the end point of the coker gas oil.
The same economics that are forcing coker operations to lower operating pressures are at work on recycle ratios. In general, refiners operate at as low a recycle ratio as product quality and unit operations permit. Operations below 5% recycle are common and many new designs incorporate "ultra low-recycle coking" technology if downstream conversion facilities can accept the higher endpoint, metals content, and carbon residue of the heavy coker gas oil product.
DESIGNING FOR LOW PRESSURE OPERATION
Following are some design considerations for low pressure operation.
PRESSURE BALANCE
The design for low pressure coker operation requires careful attention to the coker pressure balance. The control point for the coker pressure is the fractionator overhead accumulator/compressor suction (Fig. 2). In a typical design specified for a 15 psig coke drum operating pressure, the coker fractionator overhead accumulator is designed with a pressure of 2 psig. The overall pressure balance is:
Fractionator overhead accumulator: 2.0 psig (0.14 kg /CM2g)
Fractionator overhead circuit: 4.5 psi (0.32 kg /CM2)
Fractionator: 3.5 psi (0.25 kg/CM2)
Coke drum overhead: 5.0 psi (0.35 kg/cM2)
These pressures add up to the coke drum pressure of 15.0 (1.06 kg/cM2g).
COMPRESSOR
One of the concerns in specifying equipment for this type of design is the compressor. Maximum allowable compressor discharge temperature for a coker gas compressor is about 300-315 F. (149-157 C.). Cokers designed to operate at a conservative 25 psig (1.76 kg /CM2g) coke drum pressure have compressors specified with relative compression ratios and hence outlet temperatures below 15 psig coker operations.
Coke drums operating at pressures lower than 15 psig may require three stages of compression depending on the required discharge pressure.
FRACTIONATOR
Fractionator sizing and specification of fractionator internals is affected by low pressure design. Compared to a higher pressure operation, the increase in vapor volume to be handled requires a cross sectional area increase inversely proportional to the square root of the decrease in absolute pressure. Thus, a 12 ft (3,650 mm) diameter column required for a 25 psig coker operation need only be specified as 13 ft (3,950 mm) diameter for a 15 psig operation.
Fractionator internals are traditionally specified as valve trays. This generally continues to be satisfactory on new designs for 15 psig cokers. However, for revamp designs, a careful evaluation of all components often leads to recommendations to replace pumparound trays with packed beds both for increased capacity and to reduce coke drum operating pressure.
Piping in the fractionator overhead circuit needs to be made larger to permit operation at lower coke drum pressures.
DRUM OVERHEAD PIPING
Designing for low pressure coking requires a close look at coke drum overhead piping circuits. One of the larger pressure drops that the designer has control over is that imposed by the valves. Traditional wedge plug valves are supplied with a 70% port opening and are not available in sizes larger than 20 in. diameter.
There are some larger units that are designed with 24 in. coke drum overhead piping that has to be necked down to 20 in. diameter. In addition, the flow is routed through three 70% port opening 20 in. diameter wedge plug valves in series resulting in a significant pressure drop.
To eliminate this line resistance, current designs call for low pressure drop valving. Low pressure drop valving can be alternatively specified as high temperature ball valves or 90% port opening wedge plug valves. There has been a recent trend to retrofit ball valves to existing coker overhead lines and these valve manufacturers are developing long experience listings.
Another area of concern in coke drum overhead piping pressure drop is the pressure drop imposed by coke accumulations. Coking of the overhead lines is mitigated by the injection of a quench oil (more on this later). However, high unit charge rates (in excess of design) can result in marginally high coke drum velocities which in turn accelerate the coking of the overhead lines.
If the fouled vapor lines are not regularly cleaned, the operating pressure in the coke drums can increase by as much as 10-15 psi (0.7-1.1 kg /CM2). In response, some refiners are considering projects to retrofit individual overhead lines from individual pairs of coke drums to the fractionator.
In this manner they expect to lower overall average coke drum pressure and realize more economic yields. The separate lines will allow the maintenance staff to hydroblast the coke from the lines without shutting down the entire coker.
INJECTION STEAM
Foster Wheeler has recommended the use of increased quantities of heater injection steam to lower the coke drum effective pressure and vapor phase cracking. This operation has been performed on several Foster Wheeler cokers and has enhanced liquid yields.
FRACTIONATOR SPRAYS
Another way to reduce coke make is to use the minimum possible recycle ratio. In practice, there are difficulties in running low recycle rates. A fractionator wash stream of HCGO internal reflux which induces a recycle stream, is required to control the HCGO endpoint, decrease HCGO carbon residue, and to reject entrained coke particles from the HCGO.
It is difficult to wash coke drum vapors effectively at low wash rates. Also at low wash rates, trays have a great tendency to coke, increasing the fractionator pressure drop.
A large number of U.S. refiners operate with zero fractionator wash rates for the greater part of the coking cycle. These "zero recycle" cokers operate at true recycle ratios of 2-4% for the greater part of the operating cycle, depending upon the amount of coke drum overhead quench used.
In response to the need effectively to operate and control ultra-low recycle cokers maintaining recycle ratios less than 5%, a number of cokers have been retrofitted with wash zone sprays. Foster Wheeler's design uses a spray header in place of wash zone trays. Shed baffles are located well below this spray chamber, requiring heat and mass transfer to take place in the spray zone without benefit of packing or internals. This design permits controlled wash oil stream turndown rates as low as 25% of design. In actual operation, controlled recycle ratios as low as 2% have been achieved.
OVERHEAD QUENCH CONTROL
In an ultra-low recycle coking, a significant portion of the recycle is generated by the quenching of the coke drum vapors to retard line coking. In a 5% recycle design, nearly half of the duty necessary to condense the recycle may be contributed by the overhead quench. A few refiners have found that overhead line coking can be controlled effectively by removing insulation from the line in place of using quench oil. Unfortunately, this also prevents the refiner from exercising a very fine control over the amount of recycle induced in the overhead line. To achieve this end, refiners desiring to operate Ultra-low recycle cokers are using sophisticated instrumentation to regulate the flow of quench oil to well insulated overhead lines. A recent Foster Wheeler design incorporating a delta temperature control loop is shown in the attached sketch (Fig. 3).
COKER FRACTIONATOR
The proper design of the bottom section of the coker fractionator can be very important in achieving low recycle operation. We have seen a few cokers where the location of the fresh feed outlet nozzle is above the liquid level in the bottom of the fractionator. Because the fresh feed enters the tower at a temperature below its bubble point, the splashing liquid absorbs and condenses the coke drum vapor which enters the tower in the same zone effectively eliminating the refiner's ability to control the recycle at a low ratio. In these cases, simply retrofitting an internal pipe to direct the fresh feed below the liquid level will restore recycle control.
The liquid in the bottom of the fractionator is usually not in equilibrium with the vapor space above it. To achieve ultra-low recycle it may be necessary to thermally isolate the liquid pool. This is accomplished by means of a heat shield-a donut shaped baffle which partially segregates the hot vapor and cold liquid while allowing recycle liquid to mix with the fresh feed.
In the event of upset foamovers, high carryovers of coke fines can plug the fractionator bottoms strainer. Foster Wheeler's design for the coker strainer is a tall slotted standpipe design that will permit the operator to raise the fractionator liquid level while maintaining the effectiveness of the strainer.
This type of operation also raises the cold fresh feed level towards the inlet of the coke drum vapor inlet jeopardizing control of low recycle ratios. The use of an external fractionator bottoms strainer on a pumped circulating stream provides a means of withdrawing the coke fines and restoring the level to normal.
INSTRUMENTATION
Instrument purges on the heater inlet and at the bottom of the fractionator as well as external pump seal requirements need to be considered when designing an ultra-low recycle coker. Generally it is acceptable to use cleanouts and intermittent instrument purges.
OPERATING FOR LIQUID YIELDS
In addition to design features, there are also operating considerations for maximum liquid yields.
PRESSURE BALANCE
Maintaining a pressure balance in a maximum liquid yield coker requires frequent inspection of the coke drum overhead lines for fouling. As mentioned earlier, cokers that are being pushed with respect to capacity tend to have increased overhead line coking. This situation is aggravated when the refiner is forced to low coke drum outages (i.e., high coke levels). Under these circumstances it is not uncommon for refiners to require overhead line cleaning every 4-6 weeks. Current techniques of hydroblast cleaning by outside contractors allow this to be accomplished without shutting down the unit. This is possible since the major coke accumulations are near the coke drum outlets and the fouling beyond the switch deck block valves is usually relatively minor.
RECYCLE CONTROL
Maintaining control of an ultra-low recycle operation requires understanding the mechanisms of recycle generation. Many cokers are provided with an alternate fresh feed nozzle to inject a portion of the fresh feed over the recycle shed baffles. This design provides flexibility for alternate moderate and high recycle operations. However, in a low recycle operation, use of this feature will effectively eliminate the control of unit recycle and product quality.
Minimizing coker recycle requires careful control of fractionator HCGO wash oil and coke drum overhead line quench oil (usually HCGO). Quench oil should be controlled so that the overhead line temperature difference between coke drum outlet and the switch deck is about 15-25 F. (8 to 14 C.). Following a coke drum switch, the quench oil can be shut down until the temperature of vapors from the drum just switched into reaches a stable maximum. This allows the operators time to realign the quench oil to the new drum and maximizes hydrocarbon vapor product to the fractionator during a period that it is normally unstable.
To achieve an ultra-low recycle operation, the fractionator wash oil is controlled to the lowest possible rate consistent with acceptable HCGO quality. Some refiners even block in the wash oil, effectively operating a dry wash zone and producing high metals and carbon residue in the HCGO product. In these types of operations the refiners frequently need to turn on the wash oil at coke drum switch to maintain a cleaner HCGO quality during the period of fractionator upset.
Maintaining recycle control implies the ability to measure recycle. This is usually done by summing the output of the heater inlet pass controllers and dividing through by the fresh feed meter rate. Because these meters are orifice plates, their inherent inaccuracy relative to a targeted ultra-low recycle percentage makes the procedure questionable. Thus, the refiner has the option of retrofitting expensive, highly accurate meter runs or keeping track of operations by indirect means. Our recommendations for the latter include performing analyses of the HCGO product (ASTM D1160, carbon residue, metals) and comparing these to operating conditions (coke drum conditions, overhead temperature differences, and wash oil plus quench oil rates).
HEATER OUTLET CONTROL
Loss of recycle control during coke drum warmup and coke drum switch can result from loss of heat input to the fractionator. Some refiners have found it helpful to increase heater outlet temperatures by 5 F. (3 C.) or more during the latter stages of the warmup to compensate. The increased temperature is maintained for 112 to 1 hr following the switch as a means of mitigating fractionator upsets and reducing the time required to bring the coke drum to final operating temperature.
This procedure has the additional advantage of reducing the VCM content of the top layer of coke in the drum that is produced just prior to the switch.
The increased temperature also reduces the foaming tendency at the end of the coking cycle and may allow the refiner to reduce the silicone antifoam usage.
FEEDSTOCK VARIATIONS
Cutting deeper in the vacuum unit and feeding the coker a higher true boiling point (TBP) cut point residue will increase overall refinery liquid product yields. This is because the coker recycle TBP cut point is between 900 and 950F. (482-510 C.) in a maximum liquid yield operating mode as opposed to achievable vacuum bottoms cut points of 1,100 F. (593 C.) or higher. If fed to the coker, the bulk of the 950-1,100 F. cut point vacuum gas oil will be converted to coke and lighter products.
Foster Wheeler's ASCOT process takes this approach one step further. Through the use of an upstream solvent deasphalting section, the effective cutpoint of the coker feed is increased, resulting in dramatic decreases in overall coke yields.
SHORT CYCLE OPERATIONS
Most fuel grade cokers are operated on short cycles (i.e., coking cycles less than 24 hr).
In these cases it is usually necessary to increase heater outlet temperatures by 5-10 F. (3-6 C.) to maintain coke product VCM values. Otherwise, the resultant increase in average coke VCM content will be detrimental to the maximum liquid yield operation.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
