Caustic Treating of MTBE and TAME Feedstocks

Reprinted from Hydrocarbon Technology International 1994

Ralph Maple, Merichem Company (USA)

The enactment of the US Clean Air Act of 1990 has posed great challenges for refiners worldwide. As a consequence, there has been a rush to produce oxygenates such as MTBE and TAME from isobutylene and methanol. In order to meet the stringent petroleum specifications and to protect process catalysts, MTBE and TAME feedstocks must be pretreated to remove sulphur and other contaminants in an economical manner that preserves product margins.

 

Reformulation of gasoline has dramatically expanded the market for two gasoline blending components: MTBE and TAME. MTBE is produced by reacting isobutylene with methanol while TAME is produced by reacting amylenes with methanol. The demand for these prime gasoline blending components is growing quickly. Since MTBE and TAME feedstocks are required to meet total sulphur specifications to protect process reaction catalysts, these gasoline blending components provide interesting opportunities for caustic treating processes.

MTBE isobutylene feedstock can be diverted from alkylation feed or produced by dehydrogenating isobutane. Isobutane can be separated from normal butane by fractionation or produced by isomerizing normal butane. Figure 1 illustrates these MTBE feed choices and where the accompanying caustic treaters might be employed. These various feedstocks must meet the typical specifications in Table I for optimum results.

Most refineries use either an HF or a sulphuric acid alkylation unit to alkylated mixed butylenes or mixed propylene's and butylenes. Since sulphur is detrimental to the alkylation process, a caustic treating system is in place at most refineries to extract the easily extracted methyl and ethyl mercaptans present in the mixed olefinic LPG stream. No other sulphur compounds, other than trace carbonyl sulphide (COS) or backextracted disulphides from inefficient regenerative caustic treaters, are present in alkylation butylenes feed. To conserve caustic, a caustic regenerator is almost always employed. A typical Merichem FIBER-FILM™ Contactor butylenes caustic treating system is shown in Figure 2. It uses the following chemistry:

 

 

The RSSR (disulphides) are removed from the regenerated caustic by gravity separation and/or solvent washed from the regenerated caustic with a naphtha solvent stream. A FIBER-FILM™ Contactor contacting solvent wash step is often used in high sulphur cases to achieve high disulphide removal efficiencies. HF alkylation feeds normally have a 10 maximum wppm total sulphur specification whereas sulphuric acid alkylation feeds have a 20 wppm maximum total sulphur specification. Both of these specifications meet the typical MTBE feed total sulphur specification of 20 wppm maximum.

Carbonyl sulphide is found in the propane/propylene fractions, not the butane/butylenes. Therefore the typical 1 wppm maximum carbonyl sulphide specification usually is met after the depropaniser. However, any backextracted disulphides will concentrate in the bottom of the depropaniser. Because the MTBE feed specification calls for a fairly easily achieved maximum of 20 wppm maximum total sulphur, most existing alkylation feed caustic treaters are adequate without revamp as long as concentrating the disulphide content in the depropaniser bottoms can be tolerated. However, a caustic treating revamp may be required to meet the typical mercaptan sulphur specification of no more than 5 wppm mercaptan sulphur. If carbonyl sulphide happens to be present because of poor fractionation, it can be removed down to the 1 wppm maximum specification via a THIOLEX prewashing with a MEA/caustic mixture. The MEA acts as a hydrolysis catalyst to convert carbonyl sulphide to carbon dioxide and hydrogen sulphide which are then removed by the caustic as follows:

 

 

Since metals are detrimental in all the paths to MTBE shown in Figure 1, a nondispersive FIBER-FILM™ Contactor is sometimes used for the mercaptan extractor to ensure that the sodium content of the product does not exceed 1 wppm. A water wash can be provided by the company for meeting ppb sodium specifications. However, MTBE licensors normally provide a water wash as the first processing stage of their process to remove basic compounds such as ammonia, acetylnitrile, and sodium below the MTBE feed specifications. Therefore the company's MTBE feed caustic treating systems do not normally include a water wash.

Since MTBE licensors use their own water wash as the first processing step in the MTBE process, that system will be required to meet the no free water specification. However, a no free water specification is met by the FIBER-FILM™ Contactor technologies when measured at treating temperature, before cooling

The 20 wppm maximum oxygen content specification typically imposed by MTBE technology is met by the company's technologies. Since air is injected only into the caustic at the bottom of the oxidizer tower and the excess is released off the top of the same tower, no air, except at minor solubility level, is present in the regenerated caustic returned to the extractor. Therefore if all the caustic soluble oxygen re-entered the butylenes in the extractor, the butylenes would contain far less than the 20 wppm. maximum specification.

Table 2.

Total Sulphur, wppm max.	20
Mercaptan Sulphur, wppm max.	5
Hydrogen Sulphide, wppm max.	1
Carbonyl Sulphide, wppm max.	1
Disulphides, wppm max.	20
Basic Compounds, wppm as
Ammonia	1
Oxygen, wppm max.	20
Water	No free
Metals (Fe,Pb,As,Na), wppb max.	30
Carbon Monoxide. wppm max.	5

TAME

The amylene feed to a TAME unit is a depentaniser overhead product. The depentaniser feed is dehumanized catalytic cracked gasoline. Figure 3 shows the fractionation scheme employed to produce TAME feedstock. The typical TAME feed specifications are listed in Table 2.

 

Caustic treating (sweetening) of dehumanized catalytic cracked gasoline is practiced at almost every refinery in the world. These sweeteners remove all the hydrogen sulphide and convert most of the mercaptans to disulphides. A sweet catalytic cracked gasoline will contain less than ten wppm mercaptan sulphur. Since disulphides boil at high temperatures, they go bottoms in the depentaniser along with the heavier mercaptans that are not sweetened. Light mercaptans such as methyl, ethyl, propyl, are very easily sweetened whereas heavier mercaptans are not. The boiling points of amylenes and the primary sulphur compounds involved are listed in Table 3.

The MERICAT caustic sweetening process shown in Figure 4 illustrates the most frequently used technology to sweeten catalytic cracked gasoline. Air is injected into the gasoline ahead of the treater to provide oxygen for the following reactions which occur almost simultaneously in the contactor section of the sweetener:

 

 

The sweetened dehumanized catalytic cracked gasoline will contain soluble oxygen and nitrogen since some excess air is used in most sweeteners. The depentaniser overhead will therefore have to be degassed to meet the 20 wppm maximum oxygen TAME feed specification. This degassing step is required regardless of the mercaptan content of the untreated dehumanized catalytic cracked gasoline because even a minor amount of air injection at the sweetener will cause the TAME feed to be off-spec regarding oxygen content.

Since carbonyl sulphide is not present in dehumanized catalytic cracked gasoline, it is not a problem to be addressed in TAME feed preparation. Any sodium entrainment from the sweetener will go bottoms in the depentaniser and therefore not reach the TAME unit. Since sodium can cause operational and maintenance problems for fractionators, MERICAT treaters utilize the FIBER-FILM™ Contactor to ensure that sodium does not exceed 1wppm.

Since the sweetened dehumanized catalytic cracked gasoline must pass through the depentaniser reboiler, careful consideration must be given to the reboiler temperature to prevent disulphide decomposition to lighter sulphur compounds which might go overhead to the TAME unit. Refiners have had no problems in this regard as long as the reboiler temperature does not exceed 270°F.

In some instances the refiner may have an inadequate existing sweetener from a mercaptan conversion and/or caustic entrainment standpoint. In this case, the existing sweetener can be switched from depentaniser feed sweetening to depentaniser bottoms sweetening which will unload the treater and greatly improve its performance. If this switch is made, the depentaniser overhead TAME feed will need a new treater. This new treater would be an extractive system such as a THIOLEX/REGEN as discussed in the MTBE section above.

A properly designed caustic treating system should easily meet the total sulphur specification of TAME feed since the majority of the sulphur compounds present are methyl, ethyl, and propyl mercaptan which are readily extracted by caustic if an efficient mass transfer device is employed. Since air is injected only into the caustic at the bottom of the oxidizer tower and the excess is released from the regenerated caustic off the top of the same tower, no air, except minor solubility level, returns to the mercaptan extractor. The solubility of oxygen in caustic is so low that the 20 wppm maximum oxygen TAME feed specification is easily met. The sodium entrainment from the THIOLEX will be no more than 1 wppm which will be removed in the TAME feed water wash supplied by the TAME licensor.

 

Capital and operating costs

The caustic treatment of MTBE and TAME feedstocks requires a relatively low capital investment, particularly when compared to the overall investment required for an MTBE or TAME facility. Merichem recently licensed a modularized 1000 metric ton/day amylenes caustic treating system that cost approximately $2 million.

The operating costs for these processes are also quite low as shown in the summary of annual operating cost factors in Table 4, for the same 1000 metric ton/day amylenes treating system.

In conclusion the caustic treatment of MTBE and TAME feedstocks is an effective and economical method to protect the downstream catalysts without resorting to hydrotreating or other very costly alternatives. The caustic treatment design and technology employed are critical factors that ensure system reliability.

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Ralph Maple is the manager of sales and marketing for the process technology division of Merichern Company and has 25 years' experience in various positions with the company. He holds a Bachelor of Chemical Engineering degree from Vanderbilt University, Nashville, Tennessee