MERCAPTAN REMOVAL RATE EXCEEDS 99% AT CANADIAN GAS PLANT

Bill Judd
Union Carbide Chemicals & Plastics Canada Inc.
Calgary

Installation of a Selexol solvent unit at Pembina Resources Ltd.'s Diamond Valley gas plant has been effective in polishing mercaptans and other sulfur-bearing compounds from a variable gas stream.

The actual removal rate exceeds 99%, and an absolute treated gas target of

START-UP PROBLEMS

The Pembina Resources plant at Diamond Valley (near Turner Valley, Alta.) was started up in June 1985. It was built to replace the historic Turner Valley gas plant which operated from 1925 until its decommissioning in 1985.

The plant processes sour gas and liquids streams from nearby wells. These streams contain mercaptans, which are ultimately concentrated in the regenerated gases from molecular sieve beds which treat propane and butane products.

The molecular sieve absorbs some of the propane or butane along with the mercaptans, so that during regeneration of the bed, a considerable slug of hydrocarbon is released into the regenerated gas.

Controlled disposal of this gas through the fuel gas proved unsatisfactory because of the difficulty of maintaining complete combustion in the burners.

In November 1990, a Selexol unit was installed to polish mercaptans and H,S from the molecular sieve-regenerated gas.

The composition of the gas varies considerably - mercaptan spikes have been measured at greater than 10 mole %.

Sweetening the regenerated gas made it possible to recycle the C1, C2, and C, components from this stream to the plant feed without causing a buildup of sulfur products inside the system.

At the same time, the volume of regenerated gas being directed to the fuel-gas system was reduced enough to ensure consistent and complete combustion.

GAS TREATING

Selexol solvent is a regenerable, physical solvent capable of Detective or combined removal of such compounds as CO2, H2S, COS, CS2, H2O, and mercaptans from a variety of natural and synthesis-gas streams.

Its attributes - low regeneration energy (regeneration mostly by pressure let-down), chemical and thermal stability (inert even with oxygen), along with characteristics of low corrosion, inherent nonfoaming, and low solvent losses - make it versatile and cost-effective for gas treating.

Physical solvent processes depend on differences in solubility in order selectively, to absorb one or several components from a gas stream.

In the case of natural gas containing, acid gases and mercaptans, the solvent must have a strong affinity for the acid gases and mercaptans and very low solubility for methane and ethane.

Selexol's separation of solubilities for acid gases and mercaptans makes it effective in a wide range of gas-treating applications. Table 1 lists the relative solubility of various gases in Selexol.

The advantageous solubility, for CO, COS, H,S, mercaptan, and CS, makes the process attractive for gas treating. Water is also readily absorbed, opening the possibility, of simultaneous sweetening and dehydration.

Co-absorption of hydrocarbons, particularly heavier ones (C3+), requires extra attention in evaluation and design.

Selexol's effectiveness in gas treating can be characterized as follows:

Total sulfur removed to less than 16 ppm
CO2 retained or removed to levels less than 2,000 ppm
Hydrocarbon dewpoint, as desired
Water dewpoint, as desired.

As with other absorption processes, higher pressures, lower temperatures, more stages of contact, and lower lean loadings enhance effectiveness and efficiency.

Generally speaking, typical gas-processing conditions are suitable for Selexol: pressures of 300-1,500 psig, temperatures of 0-120 F., absorber towers of 20-30 trays.

Because the relative solubilities of target gases (for example, CO2, H2S, mercaptan, H2O) can be so different, however, each case must be evaluated separately.

A typical process schematic has many similarities to the more familiar regenerable chemical solvent processes.

After inlet separation and filtration, the feed gas enters a counter-current trayed or packed absorber where contaminants are absorbed. The rich solvent then flows to a recycle flash drum.

There the pressure is reduced to an intermediate level so that less soluble co-absorbed gases (for example, methane or ethane) are selectively flashed from the solvent and recompressed into the feed strean. Further pressure reduction accomplishes the bulk of the solvent regeneration.

This flash regeneration stage can be one or more separate steps depending on the system needs and the nature of the off-gases.

Some applications will also use a stripper column for counter-current contact with a stripping gas to reach particularly low lean loadings. Because of the chemical stability of Selexol, the stripping gas can be air.

The lean solvent is then pumped through a cooler and recycled to the absorber. Occasionally, heat is applied to assist the regeneration portion of the process. With the addition of an external heat source, lean-rich heat exchange and off-gas condenser with accumulator may be recommended.

MERCAPTANS

At the Diamond Valley gas plant, sour gas and liquid streams are separated, stabilized, sweetened, and fractionated to produce sulfur, natural gas, ethane, propane, butane, and condensate (C5+; Fig. 1).

Mercaptans in the feed are concentrated in the propane and butane product streams and removed in molecularsieve units. The propane and butane have separate molecular-sieve units, each having two beds to allow for one in service and one on regeneration.

The gas evolving from the beds during regeneration contains a mixture of hydrocarbon and trace sulfur compounds-mostly methyl and ethyl mercaptan with smaller amounts of propyl and butyl mercaptans as well as H2S and COS.

It was originally intended 0 that this sour regeneration gas would be incinerated as part of the fuel gas. Unexpected volumes and swings in composition, however, made this approach unmanageable with respect to smooth operation of the burners using the gas.

It was found that the regeneration cycles produced waves of composition-first a slug of hydrocarbon (either propane or butane depending on the unit being regenerated) followed by a spike of mercaptan. As a result, ensuring complete combustion was impossible.

That's where Selexol comes in. Its high solubility of mercaptans, H,S, COS, and other sulfur compounds, relative to propane and butane (methyl mercaptan being approximately 10 times more soluble than butane) allowed for the design of a process to reduce significantly the hydrocarbon slug in the cycle. This, in turn, helped to smooth out the operation of burners using this fuel gas.

SOLVENT UNIT

In November 1990, the new unit was installed with capacity for up to 600 Mscfd of feed gas at 400 psig and 100 F. lean solvent temperature. Table 2 outlines the nominal design composition as well as the expected total sulfur peak.

The target was to sweeten the molecular-sieve regeneration gas to less than 100 ppm total sulfur. As a result of this process, this mixture of methane, propane, and butane could be recovered by recycling to the plant inlet, without contributing to a buildup of sulfur products in the system.

The unit schematic describes the basic layout (Fig. 2).

Sweetening is achieved in a 25-tray contactor, with the gas and lean Selexol cooled to approximately 100 F. with aerial coolers and solvent flow of 5 gpm.

Solvent regeneration is accomplished in a single stage which incorporates pressure reduction, heat, and strip ping gas. Flashing to 10 psig provides most of the regeneration.

To assist, approximately 5 wt % of the solvent is maintained as water and vaporized in the reboiler for use as stripping steam in the trayed regenerator tower. The heat added to attain the boiling point of 288 F. and generate steam, also aids the regeneration process.

To conserve water and solvent, a reflux condenser and three-phase separator operate at 90 F. and 12 psig.

OPERATION

In practice, the total sulfur routinely has spikes which exceed 1 mole %, with occasional combined mercaptan readings (by Gastec) of greater than 10 mole %.

From the outset, mercaptans have been controlled. Total sulfur reading in the treated gas is consistently

Compared with inlet concentrations of 10%, Selexol's removal rate exceeds 99% (Fig. 3).

The unit runs smoothly, with a constant flow of solvent able to ride out the spikes of sulfur compounds.

The most significant struggle at the start-up was related to the co-absorption of hydrocarbon.

Without a separate flash for these components (mostly propane and butane), sufficient amounts were carrying through to Selexol regeneration to cause continued problems for combustion of the waste gas.

After consideration of various operational factors for limiting hydrocarbon pickup, reducing solvent flow was found to be the most effective. The unit was designed to run at 12 gpm. Instead, it is now run at a constant rate of 5 gpm.

This has optimized the system by forcing minimum Hydrocarbon pick-up (both propane and butane being less soluble than the mercaptans), while maintaining excellent mercaptan removal.

Even with some hydrocarbon absorption, foaming has not been a problem. After more than 1 year of service, there is no evidence of corrosion.

The initial location of the solvent pump on the hot lean side encountered problems with seals. Relocating to the downstream side of the lean-rich heat exchanger reduced the pump operating temperature and relieved the problem.

Readily available materials such as Teflon and EPCM are compatible, while some others (for example, most rubbers and neoprene) are not recommended.

In the process of dealing with the early pump leaks, operators learned about the cleaning and paint-removing properties of the Selexol, as well as its tendency for irritating bare skin.

As part of a strict plant safety and environmental sensitivity program, the plant's vapor-recovery system includes the solvent drain tank. Because of the high degree of mercaptan solubility in the solvent, some mercaptans were found making their way into the drain tank.