Criterion Catalyst Co. L.P. and ABB Lummus Crest Inc. have entered into a joint licensing agreement whereby Lummus' commercially demonstrated Arosat process is combined with Criterion's synergetic catalyst system for low-pressure diesel aromatics saturation.
The result is the SynSat process, short for synergetic saturation. Criterion defines a synergetic system as one in which the special combination of two catalysts, in a stacked bed-type configuration, work together such that the products of the reaction over the first catalyst enhance the performance of the second catalyst.
The combination of catalyst types used varies for differing applications, but generally consists of NiMo, CoMo, and NiW catalysts.
The SynSat process can be applied to new units as well as existing. The technology permits conversion of diesel aromatics at low pressure.
Unit revamp opportunities, afforded by its low-pressure operation, are also possible, according to Gary L. Hamilton, ABB Lummus Crest, and Arthur J. Suchanek, Criterion. They described the new process at the National Petroleum Refiners Association annual meeting in San Antonio, Mar. 17-19.
AROMATICS REDUCTION
Regulations for future diesel fuels are under considerable scrutiny. Although recent legislation allows a maximum of 35 vol % aromatics, most see this as a hold pattern, recognizing that lower aromatics diesel fuels will be part of the not too distant future.
Many feel that California's present regulations for aromatics are too strict and that final specifications will be similar to those for European diesel fuels: 45-50 cetane index and 20-25% aromatics.
There are four ways generally accepted as means of reducing aromatics in diesel boiling range feedstocks, according to Criterion. Each has its own merits and is aimed at specific processing problems defined by the refinery diesel balance, processing limitations, and product quality targets.
The four routes are:
- High pressure, moderate liquid hourly space velocity (LHSV), single-stage saturation.
This route can net aromatics reduction of greater than 50%-generally to an aromatics level of 1 0% or less in the product. The unit would be in the 1,500+ psig total pressure and 1.0 LHSV range, using alumina-based catalyst systems. Feed aromatics can vary considerably, but are generally higher than 50%.
- High pressure, moderate LHSV hydrocracking.
Like the high-pressure aromatic saturation approach, the feed would generally be highly aromatic, with a large percentage of cycle oil or coker distillates. The process could be single or two-stage, using either alumina, silicaalumina, or zeolite catalyst systems to produce naphtha, jet, and diesel.
The total pressure would be in the 1,500-2,000 psig range.
Many units worldwide upgrade cycle oil and coker oils in this manner.
- Moderate to high pressure, two-stage saturation.
Noble metal saturation catalysts require nearly complete removal of heteroatoms via severe hydrotreating with an alumina-based catalyst system in the first stage, and gas separation before the second stage. There are processes available like this, but the pressure level is generally set by the first stage hydrotreatment.
This type of two-stage processing is generally in the 1,200-1,500 psig range, but considerable work is being done to reduce the pressure requirements.
- Low pressure, low LHSV, single-stage saturation.
This approach, the primary one for SynSat, can achieve moderate reductions in aromatics (40-50%) and can be readily adapted to existing hydrotreaters as a low-cost means of reducing diesel aromatics. The total pressure can be as low as 700 psig and LHSV varied to net the required product, but generally 0.5 or less.
PROCESS APPLICATION
The SynSat process can be used as an add-on to an existing hydrotreater, as a new moderate-pressure unit, or as a high-pressure unit, if the refining scheme dictates this approach. Almost any feedstock can be processed to the 10-20% aromatics range with SynSat.
PROCESS DESCRIPTION
The SynSat reactor design utilizes both cocurrent and countercurrent liquid/vapor contacting in separate catalyst beds. The design is geared to optimize both hydrogen partial pressure and reactor temperature profiles. A simplified schematic illustrating the process flow is provided in Fig. 1.
Fresh feed, makeup hydrogen, and recycle gas are combined and heated to reaction temperature. This mixed-phase feed enters the top catalyst section of the reactor where it is cocurrently contacted at controlled temperature for maximum reaction rate. The reaction heat is removed from the bed by further vaporization of the reaction mix and by the use of gas quenching.
As the reaction mix leaves the top catalyst section, the vapors and unreacted hydrogen are withdrawn from the reactor. The vapor leaving the reactor is used to preheat reactor feed and liquid recycle before being condensed, cooled, and flashed in the high-pressure flash drum to produce both liquid and vapor recycles and high-pressure purge gas.
The liquid leaving the top catalyst section is combined with liquid from the HP flash drum before entering the bottom catalyst section. In the bottom catalyst section, the final aromatics reduction is effected, employing countercurrent contacting of the liquid with fresh makeup hydrogen. This gas is introduced into the bottom catalyst section at a relatively low temperature to help maintain the temperature profile of this section. The descending hydrocarbon liquid is exposed to increasing hydrogen partial pressure as it passes downward through the reactor. This, in addition to the lower temperatures in the bottom catalyst section, favors a higher percentage aromatics conversion.
The vapor from the bottom catalyst section joins the vapor from the top catalyst section and exits the reactor. The net liquid product leaves the bottom catalyst section and is used to preheat liquid recycle and feed before being sent to product stripping. Options for product stripping include reboiled or steam stripping.
LOW-PRESSURE SYNSAT
To illustrate the capability and economics of SynSat, the feed designated "B" (Table 1) is considered in two systems-one a retrofit and one new-to reduce aromatics by 40-50%, relative to the 20% aromatics range.
The existing hydrotreater may actually be a unit newly designed or recently modified to produce 500 ppm sulfur diesel.
This unit probably would be 700 psig or higher pressure and 2 or less LHSV.
Retrofit is a viable possibility, or the unit can be conceived with SynSat in mind. In this way, the refiner will be ready when tighter diesel specs are set.
The required SynSat LHSV will be about 0.5. Assume that the existing unit is 700-900 psig and 1.0 LHSV. Retrofit to SynSat will require additional reactor volume, possible compressor modifications, and possible hydrogen purification. Fig. 2 illustrates what this unit would look like.
The estimated investment for retrofit of an existing 20,000 b/sd unit is $11-13 million, or $550-650/b/sd of throughput. If two or more SynSat reactors are necessary, the investment would be increased by the appropriate multiple to reach the required catalyst volume.
The estimated investment for a new 20,000 b/sd SynSat unit is $29-33 million, or $1,450-1,650/b/sd of throughput. These investments are total installed inside battery limit costs.
HIGH-PRESSURE SYNSAT
There are feedstocks and refinery situations that will require the processing of diesel boiling range materials that are high in aromatic content or must be reduced to less than 10% aromatics in the product. These situations will dictate higher-pressure processing.
An estimated investment for a 20,000 b/sd unit charging Feed B and requiring 10% aromatics product is $45-50 million, or $2,250-2,500/b/sd. This is 45-50% higher than for a new low-pressure SynSat unit.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
