John T. WootenDense loading is preferable to sock loading in several instances. The advantages of dense loading include increased density, capacity, run length, reactor integrity, and product quality.
Catalyst Handling Service
Sock loading, as a result of its tendency to create void spaces, may not maximize a reactor's capacity. Sock loading, on the other hand, because it is more tolerant to particulate matter in the feed and distributes catalyst in a less dense state, may be preferable to dense loading in some situations. Sock loading often comes at a lower cost. For those refiners who do not require or cannot handle the increased capacities that dense loading allows, it is a viable option.
Maximum performance for catalytic processes will require that those involved with the catalyst handling are well trained and aware of the desired results expected by the catalyst manufacturer and the refiner. It is most important that discussions are conducted with the facility operations, engineering, and maintenance departments, and with a representative of the catalyst manufacturer.
Sock-loading methodPrior to the 1970s, the standard method for loading catalyst in a fixed bed reactor was sock loading.
In sock loading, a canvas tube conveys the catalyst from the reactor inlet manway to the bottom of the reactor catalyst bed. The sock is attached to a loading hopper or funnel at the reactor inlet, which discharges the catalyst through the sock upon the bed surface in a manner which prevents the individual cylinders from finding a stable, horizontal rest position. The cylinders stack in various horizontal and vertical positions.
The positioning of catalyst cylinders in random orientations encourages bridging of cylinders and void spaces between cylinders. During reactor operations, these bridges and void spaces tend to collapse. Bed density then increases as the bed depth shrinks.
Dense-loading methodsSince 1970, refiners, catalyst manufacturers, and catalyst-loading contractors have developed dense-loading devices that dramatically reduce void spaces and bridging. Dense loading can increase catalyst bed densities by as much as 17%.
Moreover, unlike sock loading, dense loading does not require personnel inside the reactor to distribute the catalyst evenly from the sock. Workers inside the reactor require breathing air and weight distribution shoes to prevent crushing of the catalyst underneath their weight.
Dense loading is accomplished by introducing the catalyst cylinders into the reactor in a manner that allows each cylinder to fall freely to the catalyst surface. Individual cylinders separately assume a horizontal rest position before being impinged by other cylinders. Under this regime, cylinders tend to pack horizontally, minimizing the possibility of bridging or creating void spaces.
The dense-loading technologies used today are largely similar in design and produce similar results. These dense-loading technologies mainly vary in the mode with which the catalyst is propelled from the loader. Some use air or nitrogen pressure as a propellant, and some use kinetic energy to move the catalyst from the loading apparatus.
Air-propelled vs. kinetic-propulsion systemsAir propelled and kinetic energy dense-loading systems have common catalyst-delivery systems, described as follows:
A hopper or funnel feeds catalyst particles into a vertical pipe which extends into the reactor. Catalyst passes down through the pipe and exits horizontally through an annular space or gap. The gap is varied in vertical width by adjusting the spacing between the loader pipe and flat deflector plate, or cone, attached to the bottom of the pipe.
The difference between air-propelled and a kinetic-propulsion systems is the manner in which the catalyst is distributed to the catalyst bed. Fig. 1 [113,980 bytes] shows an example of each method.
With kinetic energy, the catalyst is distributed by an air motor that rotates a particle distributor. Kinetic systems use propellers, rotating blades, or a series of rubber strips to distribute catalyst from the loader to the outer walls of the reactor. The loading rate and the horizontal distance of travel for the catalyst are controlled, in part, by rpm settings on the air motor.
In an air-propelled system, air is introduced into a sparger situated in the center of the loader pipe, above the deflector plate. Jets of air emerge from horizontal radial holes in the sparger, directed outward through the annular gap. The air pressure is 7-14 psig in a standard apparatus.
The RSI (Reactor Services International) Super COP is a modified version of the Atlantic Richfield Co. (ARCO) Catalyst Oriented Packing (COP) method for dense loading. Both methods use air to distribute the catalyst.
The Super COP uses two deflector plates, and the ARCO COP uses one. In the Super COP, the lower deflector plate feeds catalyst to the center of the reactor, while the upper plate feeds catalyst to the outer walls of the reactor.
Operation and troubleshootingDuring dense loading, it is important to minimize sudden surges of catalyst to the loading distributor. The loading personnel, therefore, must regulate the catalyst feed rate with the supply of catalyst.
To accomplish this, the catalyst feed hopper or funnel must be sized for the delivery speed and container size of the catalyst. In general, the delivery container and catalyst feed hopper should hold at least 28 cu ft of catalyst.
Foreign particulate, tape, tie straps, or plastic liners from drums or super sacks should be prevented from entering the dense loader. Foreign matter will block the catalyst distributor or redirect the catalyst unevenly, which causes a maldistribution of catalyst to the reactor bed. A 0.75 in. x 1.5 in. screen inserted into the catalyst feed hopper will prevent foreign particles from entering the loader.
Regardless of the dense-loading method selected, the catalyst feed pipe and distributor should be perpendicular and level at all times. An uneven distributor will produce a sloped bed. Even though the density calculation will be unchanged, it is thought that, with liquid-phase processes, feed rates follow the slope of the catalyst bed causing preferential or uneven conversion and reaction temperatures.
Improper settings of the catalyst feed rates, distributor air pressures, and distributor rpm settings will produce uneven catalyst distribution (Fig. 2 [134,889 bytes]). Uneven distribution usually manifests itself in a bed surface which has a central mound or a "doughnut" configuration. Higher air pressures, as with COP methods, will displace this mound outward, but if the air pressure is increased too much, the bed level will be higher at the reactor wall.
After 10-25% of the catalyst has been loaded, the loading personnel should inspect the bed to determine if it is firm and level. If the inspection shows that the bed surface is not level, it is necessary to level the bed and make adjustments to the dense loader. If the bed is not dense and solidly packed, the loading rate should be reduced.
Catalyst strength for dense loadingIndividual sprays of catalyst flow radially outward from the catalyst distributor. Some of these sprays impinge lightly on the reactor wall, but not with sufficient force to damage the catalyst significantly. Most of the catalyst falls onto the catalyst surface below, reaching a maximum, terminal velocity after falling 10-15 ft.
The horizontal velocity of most particles leaving the loader is sufficient only to carry them part way to the reactor wall. A minor portion of the particles has enough energy to go a bit further than the wall.
With air-propelled systems, the particles do not leave the loader at high velocities. Air used to propel the catalyst from the loader travels faster than the catalyst, reaches the reactor wall, and rebounds to serve as a cushion directing the catalyst downward or away from the reactor wall.
Catalyst manufacturers have arranged to have their catalyst tested for adequate strength. The primary test, called simply a drop test, tests the ability of the catalyst to withstand striking the catalyst-bed surface at vertical terminal velocity. This impact is far greater than that striking the reactor wall.
Catalysts strong enough to withstand the abrasion of falling between particles in an elongated sock have generally proven satisfactory for COP loading.
Capacity limitations for dense loadingThe achievement of maximum performance when the reactor is operating at maximum feed rate requires careful management of the reactor pressure drop.
Pressure drop problems can be avoided by selecting a catalyst with a shape that allows more free space in the bed or by using sock loading. With sock loading, however, the weight of catalyst charged to the reactor is limited. This limitation results in performance that is substantially less than can be achieved with dense loading of small diameter catalyst, which increases the amount of catalyst contacted by the feed to the reactor, and thus improves the overall effectiveness.
Initial pressure drop after start-up, with a new charge of catalyst, is a function of the size and shape of catalyst, the loading procedure, the feed rate, and the percent vaporization. Refiners using dense-loading techniques realize that initial pressure drop will be significantly greater than that experienced with sock loading. Pressure drops with dense loading can initially be greater by a factor of 1.5 to 2.5 than those with sock loading.
Because of more-uniform catalyst distribution and improved flow distribution, reactor-pressure drop is often lower with dense loaded catalyst later in the run.
To accommodate dense loading of the reactor catalyst, the capacities of the feed pump and recycle-gas compressor must be sufficient to overcome the increased pressure drop in the reactor. Some feeds may contain fouling substances such as rust scale or polymers, which if not removed by filtering, may contribute to a higher pressure drop and a shortened run time. If the feed pump and recycle gas capacities are not sufficient or if the feed contains fouling substances which cannot be filtered, dense-loading may not be an appropriate alternative for catalyst loading.
It should be noted, that even with sock-loaded beds, shrinkage in the bed depth during onstream operation may cause a significant increase in pressure drop and lead to damage of the internals in a fixed bed reactor. Shrinkage may also cause the feed to bypass the catalyst in a radial flow reactor.
Advantages of dense loadingIn existing vapor-only reactors or two-phase units at low conversions, dense-loading may:
- Increase capacity or run length, with no additional capital investment for reactors
- Permit operation at lower severity to up product quality and give higher yields
- Cut down on internal reactor damage due to catalyst slumping and elimination of hot spots or temperature gradients.
- Increase throughput or run length, at no increase in capital costs for reactors
- Permit use of less catalyst because of improved liquid-catalyst contacting
- Lead to production of higher product quality for a given reactor configuration.
The function of a redistributor is to redirect the flow of the feed gas to compensate for uneven loading. Redistributors include scale baskets and mechanical pieces welded to the reactor.
John T. Wooten is president of Catalyst Handling Service Co., which is a joint-venture company of Reactor Services International. He has spent 24 years in the catalyst-handling industry, managing projects in the U.S., Asia Pacific rim, and South America.
Wooten holds a bachelors degree from the University of Houston.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.