S. Raghuram
Robert S. Haizmann
David R. Lowry
UOP
Des Plaines, Ill.
Willem J. Schiferli
Kuwait Petroleum Europoort BV
Europoort, The Netherlands
Early in 1990, Kuwait Petroleum Europoort BV (KPE), The Netherlands, successfully commissioned its "gasoline project."
The naphtha-processing units of KPE's gasoline project will significantly help the region meet the octane shortfall caused by the phaseout of lead from gasoline.
In 1985, KPE became the first company in Europe to market unleaded gasoline (under the Q8 brand). To meet the increasing demand for gasoline, particularly the unleaded grades, from the growing number of Q8 customers throughout Europe, KPE decided to install the most current naphtha-processing technology in its Europoort refinery.
KPE carried out extensive evaluations to identify the most cost-efficient options available for improving the octane number of naphtha. The evaluations, which included blending studies, compared the available technology options on the basis of investment cost, operating costs, maintenance requirements, operability, operational flexibility, worldwide experience, and refinery gasoline pool requirements.
As a result of its detailed studies, KPE selected the Penex C5/C6 isomerization process, the Molex separation and recycle process, and the continuous catalytic regeneration reforming process (CCR Platforming), all from UOP. The processes offered the best choices for KPE for the processing of its light and heavy naphtha streams.
KPE'S EUROPOORT REFINERY
Major processing units in the KPE refinery include the recently installed facilities of the gasoline project and a lube oil plant. At Europoort, UOP also designed and assisted in the commissioning of a full-range naphtha hydrotreater, a debutanizer, a dehexanizer, and a polybed pressure swing absorption (PSA) unit for hydrogen purification.
The KPE unit was the first to employ the Molex process for the separation and recycle of n-paraffins in its isomerization process. The isomerization and separation/recycle units have a design capacity of 6,260 b/sd, and the CCR reforming unit is designed to process 20,000 b/sd of fresh feed.
The CCR section, as well as the isomerization and separation/recycle units, were supplied as prefabricated modules. Modular construction minimizes costs and field erection time.
Although modular construction has been used by UOP on several CCR reforming and isomerization projects, the separation/recycle unit at KPE was the first of its kind to be supplied in modular form.
The major products of the KPE refinery include jet fuel, gas oil, gasoline, and lube oil. Gasoline derived from the isomerization, separation/recycle, and reforming processes is directed mainly to the European market.
A full-range naphtha is taken via an oxygen stripper to a hydrotreater. The naphtha hydrotreater is designed to process 25,300 b/sd of full-boiling-range naphtha.
Full-boiling-range naphtha from the crude distillation unit at KPE is taken to an existing hydrotreater with a capacity of 13,300 b/sd. The treated naphthas are then debutanized and dehexanized.
A predominantly C5-C6 stream is fed to the isomerization/separation/recycle units, and the remaining C7+ heavy naphtha is fed to the CCR reforming unit. Typical compositions of the light and heavy naphtha streams are shown in Table 1. A schematic of the KPE naphtha-processing units is shown in Fig. 1.
ISOM, AND SEPARATION PROCESSES
Light naphtha usually makes up about 10-15% of a typical refinery gasoline pool. At an average cost of 10-20/octane-bbl, light naphtha isomerization is a cost-effective means of increasing gasoline pool octane.
The research octane number (RON) clear of a light straight-run naphtha is about 66 to 72 depending on the crude source. Isomerization of the C5 and C6 n-paraffins to their more highly branched isomers, namely, isopentane, methyl pentanes, and dimethyl butanes, can increase the RON clear to about 83.
Reactor conversion and, therefore, the octane of the isomerate, is limited by equilibrium at a given reactor temperature. Further improvements in the octane can be achieved by separating the unconverted n-paraffins and recycling them to the isomerization section.
The isomerization process can be integrated with a sieve-based separation section to achieve n-paraffin recycle. The catalyst used in the isomerization process is highly active, and so the isomerization section can operate in the 115-150 C. range.
A close approach to equilibrium at these low temperatures allows the isomerization reactor effluent to reach desirable isoparaffin ratios. The liquid in the isomerization reactor effluent is separated from hydrogen-rich light gases.
Following stabilization, the liquid stream from the separator is fed to the separation/recycle unit. A schematic of the process is shown in Fig. 2.
The KPE unit was designed to recycle hydrogen back to the reactor. However, in the latest designs, the isomerization process does not use recycle hydrogen. Thus, the KPE isomerization unit utilizes once-through hydrogen technology.
The separation recycle process is one of UOP's separation processes which use solid adsorbents and operate in the liquid phase at relatively low temperatures. The adsorbent is contained in one vessel and in fixed beds separated by liquid distributors.
Feed and desorbent are introduced into the adsorbent chamber, and extract and raffinate streams are withdrawn from the chamber at different positions (Fig. 3).
A high-efficiency operation is obtained by a simulated continuous countercurrent movement of the solid adsorbent relative to the liquid flow. Such simulated movement is achieved without actual movement of the solid.
Instead, the positions at which feed and desorbent are introduced and the extract and raffinate are withdrawn are changed. The positions change one bed at a time in the same direction as the liquid flow.
This movement is effected by a stepwise rotation of a rotary valve, which serves to direct process flows into appropriate bed locations. Each valve step introduces feed and desorbent into the adsorbent chamber, one bed position lower than its current point, and correspondingly withdraws extract and raffinate from one bed lower.
The rotary valve and the pumparound liquid flow, which changes with the position of the rotary valve, is controlled by a UOP-supplied Monirex control system. This control package is integrated with a distributed control system.
The high-capacity adsorbent used in the separation/recycle process is selective to n-paraffins. Desorption of the n-paraffins from the sorbent is accomplished by the displacement of the adsorbed n-paraffins to the extract stream.
In the process, the desorbent is a blend of n-butane and i butane. Thus, both the extract and raffinate streams are withdrawn from the unit mixed with desorbent butanes.
These streams are taken to respective extract and raffinate fractionators to separate the feed components from the desorbent. The desorbent is recycled back to the process.
The extract stream, which is rich in n-paraffins, is recycled to the isomerization unit. The raffinate, which contains high-octane isoparaffins, is sent to gasoline blending.
The separation/recycle process is a simplified, low-cost variant of a similar process used for kerosine. Considerable design optimization of the process for use with light-naphtha-range feedstocks has resulted in a simple, low-cost option for use downstream of isomerization units.
The unit is simple to run because it has few operating variables. Mainly, adjustments to the liquid flow rates in the various zones within the adsorbent chamber are required.
At KPE, a calculation model was developed using the Honeywell TDC 3000 distributed control system. Current plans are to automate this process with direct input from on-line analyzers.
The operating utility costs associated with the separation/recycle unit are low because low-pressure steam is used for reboil in the extract and raffinate columns. The resulting design minimizes both capital and utility costs and thus makes the process ideal for the recovery and recycle of n-paraffins downstream of an isomerization unit.
ACTUAL OPERATIONS
The C5/C6 isomerization and separation/recycle units at KPE were brought onstream in March 1990. Operating results presented here were obtained during a period of operation soon after start-up.
The compositions of the feed and product streams in the isomerization/separation/recycle complex are shown in Table 2.
During the initial period of operation, the separator/recycle unit raffinate had an RON clear of 88.5 on a C5+ basis. The n-paraffin recoveries across the unit are about 96%, and the resulting concentration of non-normal compounds in the gasoline product from the unit is in excess of 99%.
The results of the feed-to-product octane upgrade are shown in Fig. 4. Combined isomerization and recycle has increased the RON clear of the light, straight-run naphtha by an average of 25 numbers.
Since start-up at KPE, variations in the modes of the upstream debutanizer and dehexanizer operation resulted in changes in feed composition to the isomerization/separation/recycle complex. But performance remained consistent, and octane improvements, reflecting the C5/C6 ratio of the feed, remained as predicted.
Including the separation/recycle unit with C5/C6 ISOMerization has proven to be a versatile and economical means of improving the octane value of light straightrun naphtha.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
