Activated carbon passes tests for acid-gas cleanup

Lewis G. Harruff, Stephen J. Bushkuhl
Saudi Arabian Oil Co.
Dhahran, Saudi Arabia

Use of activated carbon to remove hydrocarbon contaminants from the acid-gas feed to Claus sulfur-recovery units has been successfully pilot tested in Saudi Arabia.

Pilot plant results are discussed here along with issues involved in scale-up to commercial size.

Heavy hydrocarbons, particularly benzene, toluene, and xylene (BTX) have been linked to coke formation and catalyst deactivation in Claus converters. This deactivation results in reduced sulfur recovery and increased sulfur emissions from these plants.

This effect is especially evident in split-flow Claus plants which bypass some of the acid-gas feed stream around the initial combustion step because of a low hydrogen sulfide concentration.

This clean-up process was proven to be capable of removing 95% of the BTX and other C₆+s from acid gas over a wide range of actual plant conditions.

Following the adsorption step, the activated carbon was easily regenerated by use of low-pressure steam.

A post-regeneration drying step using plant fuel gas also proved beneficial.

Feed contaminants

Like many gas processors, Saudi Aramco uses Claus sulfur-recovery units to convert H₂S removed during gas-sweetening operations into elemental sulfur. These sulfur-recovery units use the modified Claus process: a thermal conversion stage followed by two or more catalytic conversion stages (Fig. 1 [26534 bytes]).

This process can attain 97-98% recovery of sulfur from feed gas when the catalyst is fresh.¹

In some sulfur-recovery units, it is necessary to bypass a portion of the feed stream around the thermal conversion step to maintain a stable flame in the reaction furnace. This bypass allows the Claus process to be used at low H₂S concentrations.

These plants are particularly sensitive to contaminants in the feed gas because some of the gas bypasses the combustion step.

Contaminants that are not destroyed in the thermal stage pass directly to the catalyst beds where they can react with the catalyst to deactivate it.

Contaminants such as heavy hydrocarbons in the feed stream can cause the Claus catalyst to deactivate. The rate of deactivation will depend in large part on the concentration of contaminants in the feed stream and the mode of operation.

Previous studies have shown that catalytic cracking of hydrocarbons can cause severe pore blockage in the catalyst, reducing its activity. This reduction lowers sulfur recovery by shifting the bulk of the reaction out of the first converter into downstream catalyst beds.²

Saudi Aramco has experienced severe catalyst deactivation in several sulfur-recovery units due, in large part, to hydrocarbon cracking.

In recent years, the heavy hydrocarbon content of the sulfur plant's acid-gas feed at these facilities has increased because of changes in sour-gas feed to these plants. The new sour-gas feeds have increased levels of aromatics.

A portion of these aromatics is co-absorbed with the H₂S and CO₂ in the amine-sweetening process resulting in higher concentrations of aromatics in the sulfur plant's feed.

The effects of the higher aromatic content are dramatic. Fig. 2 [17403 bytes] shows the activity loss in the first Claus catalyst bed with high aromatic feed vs. the activity loss for the same catalyst bed in a unit with low aromatic feed as measured by a drop in the reaction exotherm.

Both units are located at the same facility and, therefore, operated in a similar manner. Feed-gas streams differ in composition because of piping geometry and unit locations.

Because of these problems, an investigation was launched to determine ways to reduce the aromatic content of the feed to the sulfur-recovery units.

This investigation considered a variety of methods: changing amines in the gas-sweetening process; refrigerating the sour gas before sweetening to reduce heavy hydrocarbon content; adding an acid-gas enrichment system to enrich the H₂S feed, and adding an absorption tower utilizing lean oil to absorb the heavy hydrocarbons from the acid-gas stream.

All of these methods required substantial investments, and most of them were unable to reduce the concentration of aromatics to acceptable levels.

Activated carbon was investigated as a part of this study because of its reputation for removal of low-level

concentrations of volatile organic compounds (VOCs) from vapor streams. Activated carbon has proven especially effective for the removal of aromatic compounds.

Initial inquiries to various vendors indicated that removals of BTX in excess of 90% was possible at concentrations as low as 100 ppm.

Many investigators, however, indicated reservations about the severity of the service and the composition of the vapor stream. At that point, it became clear that further investigation, including pilot testing at an affected plant, would be necessary.

The following terms will be used throughout this discussion:

• Adsorption capacity-How much hydrocarbon a given amount of activated carbon can hold up to breakthrough of benzene.

• Adsorption efficiency-The percentage of hydrocarbon removed from the inlet-gas stream by the activated carbon before breakthrough of benzene.

Vapor-phase cleanup

Several parameters influence the efficiency of the adsorption of hydrocarbons by activated carbon and its capacity in vapor-phase applications.

Following are the most important general parameters to follow in vapor-phase adsorption design:

• Velocity through the carbon bed. Superficial velocities should be kept in the range of 1-1.7 fps.^3-5

• Contact time between the gas and the carbon. A minimum of 2-3 sec is recommended for vapor phase applications.⁴

• Bed depth. A normally used bed depth is 24-30 in.^3-6 This represents a balance between the length of the adsorption cycle and the pressure drop through the carbon bed. Deeper beds can be used as pressure drop and system requirements dictate.

• Relative humidity of the stream being treated. The relative humidity of the stream being treated should be kept to less than 40-50% to minimize capillary condensation of water in the activated carbon pores.⁶ Hydrocarbon adsorption efficiency drops off dramatically at a relative humidity greater than 50%.

• Temperature of the stream being treated. Higher temperatures result in a reduction in adsorption capacity.⁶ The loss in adsorption capacity is gradual as temperature rises.

• Type of carbon being used. Selection of carbon is dictated by a balance between adsorption efficiency and capacity and regenerability.⁷

This balance favors a carbon with a relatively high percentage of its pore volume in macropores (500 A in diameter) and mesopores (20-500 A) and lower amounts of micropores (<20 a).

coconut shell carbon, with a high volume percentage in micropores, has a large initial capacity but is hard toregenerate; the residual capacity after several cycles will be limited. wood or coal-based carbons can have lower percentages of micropores which will yield a moderate initial capacity with easier regeneration and good residual capacity.

For this application, there were several items that are not addressed by the above listing.

The unfortunate fact is that most of the research and applications for vapor-phase carbon adsorption has been done for air systems. Very little is in the open literature on other vapor-phase applications.

New empirical work was needed to reveal if this new application was viable and to verify design and operating

parameters. The questions that required resolution included:

• What effect do gases containing high concentrations of H₂S and CO₂ have on activated-carbon performance?

• Will appreciable amounts of sulfur form on the activated carbon?

• Will carbon have enough capacity at plant feed-gas temperatures to be economically viable?

• How will the feed-gas stream's water content affect the performance of activated carbon?

• Are there appreciable amounts of heavier hydrocarbons in the feed stream that would cause a premature reduction in carbon activity?

• Will the plant's operating conditions and fluctuations in gas composition have an adverse impact on carbon performance?

The magnitude of the uncertainties reflected in these questions required pilot testing, details of which follow.

Testing design

The test cell was designed to approximate full-scale operations to allow easy scale up of the results.

To that end, we used a vessel size of 6 in. ID x 30 in. long to hold the carbon bed (Fig. 3 [18413 bytes]). The vessel featured a support screen at the bottom, approximately 3 in. above the vessel base and a disengagement space at the top of the bed.

The vessel was designed to accommodate a carbon-bed depth of 24 in., a typical industrial bed depth.

The apparatus was suspended in a water bath which allowed for preheat of the inlet gas (to adjust gas relative humidity) and maintenance of a uniform bed temperature during adsorption operations. The water bath was designed to be easily drained to allow the higher temperature regeneration operations.

During normal adsorption operations (Fig. 3 [18413 bytes]), gas entered the water bath through twin 3/4-in. tubing coils. These coils connect to the bottom of the cell in the base of the water bath.

Gas left the top of the cell and passed through twin 3/4-in. tubing lines to twin rotameters where gas flow was measured.

Bed temperature was measured by a thermometer at the top of the cell. Bath temperature was measured by a thermometer in the base of the water bath.

Gas inlet and outlet pressures and outlet temperature were monitored with pressure gauges on the inlet and outlet lines and a thermometer located downstream of the rotameters.

During regeneration, the water bath was drained.

Steam entered the top of the cell through the 3/4-in. tubing. The normal gas inlet at the base of the cell was isolated during this time. The steam used for regeneration left through a second connection at the base of the cell which was connected to an external condenser (not shown).

Regeneration temperature was measured with the thermometer located in the top of the cell. Steam pressure was measured with a pressure gauge at the steam inlet to the cell. The steam rate was measured by collecting the condensate out of the condenser.

Plant fuel gas was used to dry the carbon after regeneration in some experiments. Before this operation, the water bath was filled and heated to 190 F. to preheat the fuel gas and aid the drying process. The drying gas was flowed in the same direction as the subsequent acid gas.

The feed-gas flow to the test cell was taken from the plant piping between the sulfur plant's feed knockout drum and the acid-gas preheater. At this point, the acid gas had been cooled and scrubbed with wash water and had all liquid water and hydrocarbon removed.

The gas is assumed to be completely saturated with water at this point in the process. Exhaust gas from the test cell was routed to a closed flare system.

Use of actual plant feed gas meant that there would be no control of the feed-gas' hydrocarbon composition, relative humidity, or temperature upstream of the test unit. Extensive analysis of the test results and close monitoring of temperatures and pressures were necessary to account for the effect of changes in these parameters.

The inherent disadvantage of lack of control of key parameters was more than offset by the experience gained by operating the test at actual, not simulated, plant conditions.

Beyond the numerical results captured, the experiment yielded valuable information about the reaction of the carbon unit to changes in overall unit operating conditions. This information gave additional confidence when it came time to scale-up the results for a commercial unit.

Accompanying boxes show the properties of the activated carbon used for the tests and the data taken every 15 min during each run.

In addition to the data collected on process conditions during the adsorption cycle, samples of the inlet gas were taken once every 2 hr with samples of the outlet gas taken hourly. These were analyzed for paraffins through C₇ and for benzene, toluene, ethyl-benzene, and xylenes (BETX) with a gas chromatograph in the plant lab specially configured to analyze for low levels of hydrocarbon in acid gas.

During regeneration, the hydrocarbon removed from the bed was skimmed from the regeneration water leaving the condenser. This hydrocarbon layer was held for later analysis.

An accurate material balance of the hydrocarbons leaving the bed was impossible because of skimming inefficiencies, vapor losses, and solubility of the hydrocarbon in the water phase.

The regeneration hydrocarbon liquid analysis was used to detect changes in liquid composition with changing operating parameters. It was also used to estimate the amount and composition of heavier hydrocarbons in the feed gas.

The process data for the adsorption phase and the feed and product gas analyses enabled a calculation of a material balance around the test cell. With a spreadsheet program, C₅₊ hydrocarbons were tracked for each adsorption cycle.

During the tests we used the benzene concentration in the inlet and outlet streams to determine the bed condition.

Breakthrough was defined as the point at which the benzene concentration in the outlet stream reached 50% or more of the benzene concentration in the inlet stream.

All tests were run to the same breakthrough point.

Virgin carbon was always preconditioned with several adsorption-regeneration-drying cycles before the test runs. During these conditioning runs, the capacity of the carbon would decrease as the "heel" of nonregenerable hydrocarbon built up.

The capacity leveled off within two to three cycles to a working capacity that was reproducible at a given set of conditions.

Test parameters, results

A series of trials was run using the apparatus and techniques already described to answers the questions posed earlier and to determine the process' sensitivity to various parameters.

In the course of the experiments, the process appeared stable and responsive over a wide variety of conditions. The following parameters were then tested:

• Effect of superficial gas velocity through the bed (0.45-1.25 fps).

• Effect of drying after regeneration before re-initiating the adsorption step.

• The effect of temperature on carbon performance (130-180 F.).

• The effect of regeneration temperature/steam pressure on carbon performance (10-55 psig).

During the studies, at least three trials were run at each set of conditions. If the results were inconsistent,

additional trials were run to achieve reproducibility.

Over 3 months, a total of 96 trials were conducted with the same charge of carbon. During this period, technicians analyzed more than 1,200 gas and liquid samples.

In addition, they analyzed several spent-carbon samples at different stages in the experiment to determine the impact of the gas on the carbon.

Normal vapor-phase design velocities, as stated earlier, range 1-1.7 fps. Over the course of the experiments, tests were run with velocities ranging 0.45-1.25 fps. In this range, there appeared little impact on the adsorption efficiency or the carbon capacity with changing velocities.

Velocity and contact time go hand in hand. In this range of velocities, the empty-bed contact time between the gas and carbon ranges 1.6-4.4 sec. For removal of aromatic compounds, a 2-sec contact time as reported in literature4 proved to be adequate.

Based on these experiments, a design parameter of 1 fps was adopted for this system.

Bed drying after regeneration

In activated-carbon systems, the presence of excessive amounts of water can reduce carbon efficiency and capacity.⁶

At high relative humidity, water can capillary condense in the small pores of the carbon at temperatures much higher than water's dew point. The presence of water in these small pores blocks them to gas flow and therefore hydrocarbon adsorption.

In these experiments, the carbon bed was dried with heated fuel gas flowing concurrently to the normal adsorption direction of flow. Tests were run at a variety of bed dryness levels ranging from fully dry to approximately 25% dry.

Test results show a dramatic increase in benzene-removal efficiency from 75% to 90% when the carbon bed is dried after regeneration. This efficiency increase is similar for partial drying as well as full drying.

The experimental results did not show a relation between carbon capacity and drying as long as most of the water was removed from the bed before benzene breakthrough by the normal drying action of the adsorption gas stream.

When the benzene mass transfer zone is in a dry portion of the carbon bed, benzene removal efficiency and carbon capacity remain high.

Adsorption efficiency and capacity are reduced when the benzene mass-transfer and the water mass-transfer zones overlap. Fig. 4 [16803 bytes] shows this effect clearly.

In this figure, the bottom curve shows the benzene-removal efficiency for a wet bed. The first points on that curve reflect a condition in which both the benzene and water mass-transfer zones overlap.

As the adsorption stage progresses, the water mass-transfer zone moves out of the benzene mass-transfer zone area, and the efficiency increases. The last points on the curve represent the benzene mass-transfer zone leaving the bed and breakthrough occurring.

Careful calculation of drying requirements is necessary to make sure that the benzene and water mass transfer zones do not overlap.

The speed with which the water mass-transfer zone moves is related to the feed-gas temperature, water dew point, and velocity. The speed with which the benzene mass-transfer zone moves is related to the benzene concentration in the feed gas, the carbon capacity at feed-gas temperature, and the gas velocity.

When the benzene mass-transfer zone overtakes the water mass-transfer zone, carbon efficiency and capacity are severely reduced.

A key to knowing the amount of water that must be removed after regeneration before the bed is ready for adsorption is knowing the amount of water the carbon will hold at regeneration conditions.

To determine this, technicians ran several tests with representative samples of virgin carbon at normal regeneration conditions. These tests indicate that the carbon used in these experiments can hold 70-80% of its weight in water after being treated with 50 psig steam.

Several other brands of activated carbon were tested with similar results.

Additional tests were conducted with samples of carbon that contained residual hydrocarbons from the pilot tests. These samples had been regenerated with 50 psig steam and dried with fuel gas after the final adsorption cycle.

The water capacity of these samples was reduced to about 43% by weight. It is apparent that the hydrocarbon heel on the used carbon reduces its capacity to hold water over virgin carbon.

Regeneration conditions

Literature sources indicate a broad range of acceptable regeneration conditions.^{3 4}

Temperatures between 250and 300 F. have typically been used for regeneration of activated carbon in vapor-phase service. Regeneration steam requirements generally range 0.3-1.0 lb steam/lb carbon. Steam at pressures 15-65 psig are normally used for periods ranging of 1-2 hr.

Given the broad range of conditions, an understanding of the limits was needed. Tests with steam pressures 10-55 psig were done at a regeneration time of 2 hr. Additional tests were done at 55-psig steam and 1-hr regeneration to determine the effect of changing regeneration duration on the performance of the carbon.

In these tests, the source of the steam was 65-psig saturated plant steam. The steam pressure was reduced before being introduced to the bed.

The carbon-bed temperature during each regeneration was approximately that of saturated steam at the pressure being used, indicating no superheat effects. Additionally, it should be noted that the steam controls for the pilot plant did not allow control of both pressure and flow for the steam.

The results, therefore, give a qualitative, not quantitative, feel for regeneration temperature effect. Steam rates

ranged 2-4 lb steam/lb carbon during the tests.

The test results show improvement in adsorption efficiency and carbon capacity with increasing steam pressure (Fig. 5 [20961 bytes]).

These test results show the most dramatic rise in carbon performance between the 10 and 20 psig regenerations. Smaller gains are made with further increases in pressure.

The only anomaly in the test results is for the 55-psig steam pressure. In this case, the steam rate was significantly lower than for the other cases because of limitations of the equipment described. The effect of steam rate was not further quantified.

Analysis of the results indicates that the trends observed here of increased adsorption efficiency and capacity with increased regeneration steam pressure (and by association, temperature) will translate to the commercial scale.

Actual steam usage per pound of carbon, however, will probably be less in a large, insulated, full-scale unit. This results from the considerable heat losses to atmosphere from the small, uninsulated vessel used in these tests.

Time was the second regeneration parameter studied during the pilot tests. These tests were done at regeneration times of 1 hr and 2 hr with 55 psig steam.

The results show little change in adsorption efficiency between the two regeneration times, but dramatic changes in carbon capacity were observed (Fig. 6 [13594 bytes]). It is apparent that increasing the regeneration time from 1 to 2 hr has a positive effect on the working capacity of the carbon in this service.

Additional tests were conducted at regeneration times of 4 and 6 hr. These tests indicated very little increase in efficiency or capacity over the 2-hr tests.

Test results

Based on the test results, basic design parameters for full-scale units were developed. These parameters included superficial velocity, contact time, bed depth, adsorption temperature, regeneration steam rates and temperatures, and drying requirements.

An additional critical parameter is carbon capacity. This depends largely on the type of carbon used, the adsorption temperature, and the inlet-gas composition. The design of the initial units installed in Saudi Arabia will be based on the capacity results from the pilot work.

An additional allowance of approximately 25% was included to cover fluctuations in inlet composition and other uncertainties. For future applications this allowance will need to be set according to the confidence one has in the design capacity data and the accuracy of inlet composition.

Basic flow

Based on the design parameters, a basic process flow can be developed (Fig. 7 [18779 bytes]). The final process will involve multiple vessels connected in parallel with some of the vessels in adsorption service and others in regeneration or standby.

The number of vessels finally selected will depend on an economic balance between equipment cost and operational flexibility and stability. More vessels will yield a more constant utility requirement for steam and drying gas and allow shorter cycle times but will require more equipment.

In contrast, fewer vessels will require higher utility peak rates, greater amounts of activated carbon, and allow

longer adsorption cycle times but will require less equipment and connections.

The economics must be evaluated case by case because the amount of carbon required and the vessel and piping sizes will have a large impact on which case is most cost effective.

System control

Good control of the adsorption/regeneration cycle is necessary to maximize the efficiency of the activated-carbon system.

Because of the nature of the service, there will be fluctuations in feed composition, temperature, relative humidity, and rate. Each of these parameters has an impact on the adsorption cycle time.

As in most process designs, the design point for our system represents the worst case conditions that the plant is expected to see on a regular basis. Because the system will operate at less severe conditions 90% of the time, a good control system is necessary to minimize utility usage and optimize cycle times. Minimal control can be used, however, where the utility costs are of less concern or unattended operation is anticipated.

• Minimal control. Basic control of the system can be achieved through use of fixed timers. These control the length of each cycle at a fixed duration set by the worst-case conditions anticipated.

  The timers would be tuned during initial start-up to reflect a typical operation. This type of control requires the

  minimum instrumentation.

  The drawbacks of this system are that it cannot respond to changing conditions to optimize the operation of the unit and does not account for changes in the carbon bed over time.

  This system works well when inlet contaminant concentrations and flows are fairly constant and the contaminant types and levels are well known. It is much less effective when there are uncertainties in the contaminant types and loadings and when temperature and relative humidity change frequently.

• Advanced control. An advanced control scheme will include timers as well as inputs from temperature sensors and analyzers.

  In this control scheme, the timers are set by information on the inlet concentration, flow, and temperature. The timers can be over-ridden by analyzers monitoring the outlet streams from each of the adsorbers.

  Breakthrough can be detected and cycle times monitored to allow an assessment of bed condition and adsorption effectiveness.

  Timers provide a backup for the instrumentation by allowing the process to default to standard timing should a fault in the instrumentation be detected. This system will use a programmable logic controller (PLC) or similar computer-based control system to execute the control of the adsorbers.

  Several different control strategies are possible with this control system.

Full-scale installation

Beyond design considerations are several items that must be addressed in the installation upstream of a new or existing sulfur plant.

When contemplating a design, one must consider pressure-drop implications, source of regeneration steam/heat, disposition of water from the regeneration, disposition of the hydrocarbon stream, source of cooling and heating for the feed and drying stream, and plot space availability.

Each of these will affect design of the unit.

Pressure drop is a prime concern in Claus sulfur-recovery units because the pressure of the feed is generally quite low, in the 10-15 psig range. Pressure drop can be minimized by selecting an activated carbon with a larger pellet size.

In this case, a 4-mm diameter extrudate was chosen. At a design velocity of 1 fps, this size of carbon will exert a pressure drop of about 1.2 in. of water/ft of bed. Judicious sizing of piping and support equipment should allow the system to be designed for a pressure drop of 1.5-2 psi.

At a design requirement of 2 lb steam/lb carbon for regeneration, approximately 2-10 lb (or more) of steam/lb of carbon will be required daily for regeneration. The amount required will depend on the amount of contaminants in the inlet stream and the carbon capacity at the process conditions.

These parameters will dictate the number of cycles per day required to achieve the necessary results. Almost any source of low-pressure steam that is essentially oxygen free will be sufficient to provide the regeneration heat.

If steam is unavailable, hot nitrogen or another inert gas may be able to be used to provide the necessary regeneration of the carbon.

The regeneration outlet stream will consist of water, liquid hydrocarbon, and some noncondensable gases. Because aromatics are fairly soluble in water, the handling of that water must be considered carefully. In a refinery or gas-processing plant, a waste-water stripper may be available to process this water.

If no such facility is available, a small waste-water handling system may need to be added to remove hydrocarbons from this stream before disposal is possible. Likewise, the liquid hydrocarbon must be handled carefully as it will contain large amounts of benzene.

The relative humidity of the feed stream must be adjusted before the adsorption can begin. This can be achieved by cooling the gas, separating out any liquids, then reheating the gas.

A gas must be at approximately 30-40 F. greater than its dew point to be above the capillary condensation point.

Because lower temperatures give higher carbon capacities, an evaluation of cooling costs vs. the cost of building a larger carbon unit must be conducted.

The unit is envisioned to be installed as an integral part of a sulfur-recovery unit. It can be located within the

amine treating unit, however, if required by plot limitations.

This list is meant to suggest the issues involved in the design of these units. Other issues will need to be addressed to suit the specific location and process considerations.

The pilot test determined the following:

• Activated carbon provides a cost-effective removal of more than 90% of C₆₊ hydrocarbons from acid gas with more than 95% removal of aromatic hydrocarbons.

• Activated carbon can be regenerated in place using low-pressure saturated steam.

Specifically, the questions raised earlier have been answered:

1. Gases containing high concentrations of H₂S and CO₂ appear to have no adverse effect on the adsorption of heavy hydrocarbons by activated carbon. Additionally, the use of activated carbon had no appreciable effect on the H₂S content of the gas.

2. No detectable sulfur formed on the activated carbon during testing.

3. The carbon tested had ample capacity at plant feed gas temperatures to be able to remove contaminants economically.

4. Testing established the effect of water on adsorption. Post-regeneration carbon drying and feed-gas preheat are required to overcome the effects of water on the adsorption process.

5. The small amounts of heavier hydrocarbons adsorbed by the carbon caused no detectable decline in carbon capacity over the course of the testing.

6. The process proved quite robust to changes in gas composition and operating conditions. Pilot testing on actual plant feed allowed a more realistic assessment of process capabilities.

Activated carbon properties

• Ca rbon source: Extruded vapor-phase activated carbon
• Carbon type: Bituminous coal-based
• Size: 4-mm pellet
• Surface area (minimum): 1,100 sq m/g
• Pore volume: 0.7 cc/g
• CTC: 65 wt %
• Bulk density: 25 lb/cu ft
• Bed depth: 24 in.
• Bed weight: 10.4 lb

Data every 15 min*

Adsorption cycle

Car bon-bed temperature (at bed outlet)
Water-bath temperature
Rotameter reading (gas flow rate)
Inlet-gas pressure
Outlet-gas pressure
Aci d-gas water saturation temperature (at the acid-gas scrubber)
Inlet-gas temperature
Outlet-gas temperature

Regeneration cycle

Carbon-bed temperature
Steam inlet pressure
Steam rate (twice per cycle)

Drying cycle

Carbon-bed temperature
Water-bath temperature
Rotameter reading (gas flow rate)
Outlet-gas temperature
Outlet-gas pressure

Acknowledgments

The authors wish to acknowledge Ramzi Abukhadra, Aramco Services Co. project engineer, whose idea led to this work. The work presented here represents an effort by Saudi Aramco's lab research and development center, process and control systems department, Shedgum gas plant, and, in later stages, the southern area projects design and construction department.

References

1. Paskall, H.G., "Capability of the Modified-Claus Process," A Final Report to the Department of Energy & Natural Resources of the Province of Alberta, Western Research, 1979, p. 111

2. Goodboy, K.P., et al., "Sulfur and carbon deposition on Claus catalyst examined," OGJ, Nov. 4, 1985, p. 89.

3. Shah, G.C., "Improve Activated Carbon Adsorber Operations," Hydrocarbon Processing, November 1992, pp. 61-63.

4. "Carbon Ad sorp tion/Emission Control-Benefits & Considerations," Vic Environmental Systems, Minneapolis, 1988.

5. Treybal, Robert E., Mass Transfer Operations, McGraw Hill Book Co., 1980, pp. 625-28.

6. Graham, J.R., et al., "Recover VOC's Using Activated Carbon," Chemical Engineering, February 1993, pp. 6-12.

7. Correspondence with carbon vendors.

Based on a presentation to the 75th Annual GPA Convention, Mar. 11-13, Denver.

The Authors