GB2121816A - Treatment of hydrocarbon feedstocks - Google Patents

Abstract

Naphtha distillate 2 from the processing of crude oil is prepared for use in a hydrocarbon conversion process, especially for the production of aromatic hydrocarbons, by fractionating it, preferably at a temperature in the range 100 to 160 DEG C and a pressure in the range 10 to 20 p.s.i.g., in a single distillation stage in which an overhead 3 comprising lighter reject components and a bottoms 4 comprising heavier reject components are removed from the product sidestream 5 comprising C6, C7 & C8 aromatic hydrocarbon precursors. The process achieves better separation of light and heavy ends and also makes considerable savings in energy compared with the usual two- stage process. <IMAGE>

Description

SPECIFICATION Treatment of hydrocarbon feedstocks The present invention relates to the treatment of feedstocks used for the production of hydrocarbons and in particuiar relates to the treatment of naphtha.

There are now many plants in operation throughout the world forthe production of hydrocarbons. A numberof different processes are in use and these include, for example, various types of cracking processes and reforming processes. The particular type of hydrocarbon conversion process used varies from place to place depending to some extent on the product spectrum required and on the type of feedstock available. Thus, for example, in the U.S.A.

there are many plants producing ethylene using ethane as a feedstock and relatively few plants producing ethylene by steam cracking of a feedstock such as naphtha. On the other hand, in Europe and Japan the position is reversed, many of the ethylene plants there using a steam cracking process.

The feedstocks used in hydrocarbon conversion plants are very varied in their composition and properties. Feedstocks are frequently referred to by generic terms such as gas oil or naphtha butthe use of these appently definitive terms does not mean that examples ofthefeedstockconcernedwill have identical compositions and properties. For example, there are several different kinds of gas oil as well as different kinds of naphtha.

Over the last 30 to 40 years, naphtha fractions have been widely used asfeedstocks in hydrocarbon production processes. They have been used in reforming processes forthe production of high-quality motor gasoline blending components as well as in reforming processesforthe large scale production of aromatic hydrocarbons, in particularforthe production of benzene, toluene and xylene. Another use for naphtha fractions has been in steam cracking processesforthe production of olefinic hydrocarbons such as ethylene, propylene and butadiene.

Today, most naphthas are petroleum distillates resulting from the processing of crude oil, the naphtha distillate generally having a boiling point higherthan thatofgasoline but lower than that of kerosine, say in the range 35 to 175"C. Naphthas generally contain amounts of paraffinic, naphthenic and aromatic materials. Thus, naphthas of high naphthenes and aromatic content are preferred feedstocks for catalytic reform ing. Atypical "paraffinic naphtha" may have a combined total content of naphthenes and aromatics of only 25to 30%. In a "moderately naphthenic naphtha", this total may rise to say 50 to 60% while "highly naphthenic naphthas" may have a total aromatics and naphthenes content of as much as 80%.

Crude oils from the Middle East, North Africa and the North Sea tend to yield naphthas having a total content of naphthenes and aromatics in the range 30 to 55% while some West African and Far Eastern sources of crude oil yield naphthas having an aromatic + naphthenes content of about 64to 75%.

It is usually necessary to treat naphthas before use in hydrocarbon conversion processes so asto remove amounts of unwanted components. If allowed to remain in the naphtha, such components might have adverse effects on the hydrocarbon conversion pro cess, forexample by reducing thethroughput in downstream units of the process plant, for example extractors. Thus moderately or highly naphthenic naphthas, suitable for use in catalytic reforming, may contain quantities of undesirable components, for example light paraffins and heavy Cs) ) hydrocarbons.

It is usual practice, therefore, to remove as much of these unwanted components as possible by prefractionating the naphtha in two distillation stages. In the fi rst stage, naphtha is fed at a bout 1 25"C to a distillation column operating at about 35 p.s.i.g. The lighter components including paraffins, are removed overhead at about 85 to 9000 while the remainder of the feed is removed as bottoms to a second distillation column. The feed to the second column is at an intermediate point and the column is operated at a lower pressure than that of the first colomn typically about 15 p.s.i.g. From the second column, benzene, toluene and xylene precursors are removed overhead and a heavy reject stream (Cg hydrocarbons and less volatile material) is removed as bottoms.

We have now surprisingly found that naphtha prefraction can be carried out more simply.

According to the present invention a process for fractionating naphtha to yield a naphtha suitable for use in hydrocarbon conversion comprises subjecting a naphtha to a single distillation stage from which is removed an overhead product comprising lighter reject components, a bottoms product comprising heavier reject components and a product sidestream comprising C, C7 and C8 aromatic hydrocarbon precursors. The product sidestream is ready for immediate use in a catalytic reforming process.

Preferably, the naththa fractionation process is operated at relatively low pressure, for example 10 to 20 p.s.i.g., more preferably at a pressure in the range 12 to 15 p.s.i.g. The fractionation is preferably carried out at a feed temperature in the range 100 to 160"C. A preliminary investigation by the Applicants has shown that, within this range, a temperature of the order of 140"C is likely to be the optimum fractionation temperature at the most preferred pressure of operation.

In the one-stage distillation process of this invention the process must be operated so that there is sufficient heat in the naphtha feed and reboiler liquid so as to give a practical liquid flow between the liquid product sidestream take-off point and the feed inlet point.

Those skilled in this artwill befamiliarwiththe need forthis type of operation in other distillation processes and will be familiar also with methods in which a liquid flow between these two points can be ensured. One such method, for example, involves taking off a vaporous side-steam at a point just below the side-stream product take-off point, heat exchanging the vaporous sidestream with the incoming naphtha feed stream, and recycling the vaporous sidestream, as liquid, to a point joust above the sidestream product take-off poin' The Applicnts have found that the process of this invention allows the production of a more acceptable naphtha productthan the conventional two-stage process and that using available waste low grade heat to preheat the feed there is a considerable saving in primaryfuel usage. In the process of this invention a higher reflux ratio is used atthetop ofthesingle distillation column than is used in the two-stage conventional process and this enables a better separation of the lighter component from the feed.Therefore, compared with the two-stage process, the naphtha product from the process of this invention contains less of the lighter reject components and probably less ofthe heavier reject components also.

As a result there is a decreased loss of aromatics and aromatic precursors in the overhead lightercomponents streams and probably in the heavier reject components also.

On a greenfield site the process ofthis invention allows considerable capital savings to be made in comparison with the conventional two-stage process.

Only one reboiler and one condenser are required and there is a saving of in the number of still plates needed. In addition, forthe same total energy input as in a two-stage process, the light ends content of the reformer feed is significantly reduced.

On an existing aromatics complex there is considerable scope for providing additional low grade pre-heat to the feed to the one-stage process of this invention.

The Applicants estimate that if the one stage process ofthe invention is operated at 13 p.s.i.g. and sufficient lowgrade heat is available to pre-heatthefeedtothe process to 12500, then there will be a saving in primary fuel of at least 30% and a lower concentration of light ends in the reformerfeed compared to the conventionaltwo-stage process. If the low grade heat available is sufficientto preheat the naphtha feed to 140"C then the saving in primaryfuel rises to at least 40% and again there are fewer light ends in the reformer feed.

One embodiment of the present invention is hereinafterfurther described with reference to the accompanying drawing which is a schematic diagram of a distillation column.

A distillation column 1 for naphtha fractionation has an inletforfeedstockalong line 2, and outlet3for overhead lighter reject material, an outlet 4for bottoms heavier reject material and a side-stream outlet 5, abovethe inlet2,fornaphtha product.

In typical use, a feed of 100 units/hr of naphtha material was preheated to about 1 25 C and fed along inlet 2 to the column 1 which was operated at a pressure of about 13 p.s.i.g. Overhead, about 25 units/hour unwanted lighter material was removed through outlet3while about 15 unitslhr unwanted heavier material was removed through outlet 4. About 60 units/hr of C6, C7 and Cs aromatic hydrocarbon precursor material was removed as a liquid sides tream through outlet 5 and was fed immediately, withoutfurthertreatment, to a catalytic reforming process.

In a comparison with a conventional two-stage distillation process using similar amounts of feedstock under similar conditions to the above, it was found that in the process hereinbefore described the product stream through outlet 5 contained 3 to 4% less light-ends and otto 2% less heavy ends than the product stream obtained in the two-stage process.

These percentages, although apparentlysmall, means that on a large scale plant considerable monetary savings can be made annually.

Claims (6)

1. Aprocessforfractionating naphtha to yield a naphtha suitable for use in a hydrocarbon conversion process comprising subjecting a naphtha to a single distillation from which is removed an overhead product comprising lighter reject components, a bottoms product comprising heavier reject components and a productsidestream comprising Cg, C7 and C8 aromatic hydrocarbon precursors.

2. A process as claimed in claim 1 in which the operating pressure is in the range 10 to 20 p.s.i.g.

3. A process as claimed in claim 1 or 2 in which the operating pressure is in the range 1 2to 15 p.s.i.g.

4. A process as claimed in any one of the preceding claims in which the fractionation is carried out at a feed temperature in the range 100 to 1 60"C.

5. A process for fractionating naphtha substantially as hereinbefore described with reference to the accompanying drawing.

6. Naphthasuitableforuseina hydrocarbon conversion process whenever prepared bya process as claimed in any one of claims 1 to 5.