GB2181666A

GB2181666A - Treatment of gases

Info

Publication number: GB2181666A
Application number: GB08525632A
Authority: GB
Inventors: Malcolm Ronald Creed; Peter John Parsons; Philip Sydney Redding; Roger Wyatt
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1985-10-17
Filing date: 1985-10-17
Publication date: 1987-04-29
Anticipated expiration: 2005-10-17
Also published as: GB8525632D0; GB2181666B

Abstract

Regeneration times for molecular sieve based adsorption beds used for pre-purifying, for example natural gas to be liquefied, can be significantly reduced by incorporating a silica gel layer upstream of the molecular sieve. The temperature at which both water and carbon dioxide is substantially completely desorbed is greater than 175 DEG C but less than 200 DEG C.

Description

SPECIFICATION Treatment of gases This invention relates to gas purification, more part icularlyto the purification of natural gas.

Adsorption processes, widely used in the natural gas supply and transmission industries, have applic ationforthepurification of feed gas to natural gas liquefaction plants.

Adsorption is the process by which particular molecules are transferred under certain conditions of temperature and pressure from a fluid medium to the surface of a solid adsorbent material. These materials are characterised by a very large surface area per unit of volume, which can give them a selective and useful adsorption capacity. The adsorption process is reversible when the conditions of tem perature and/or pressure are changed so that the material can be regenerated.

In practical application the process utilisestwo or more fixed beds of adsorbent operated cyclically, one bed adsorbing while the others are in various stages of regeneration.

Adsorption processes have been used in the natural gas supply and transmission industriesfora considerable number ofyears and adsorbents such as activated carbon, alumina and silica gel have been particularly important in gas drying and hydrocarbon recovery applications. The development of synthetic zeolites (molecular sieves) over the last25 years has extended their applications in gas processing to purification prior to liquefaction, hydrocarbon separation, sulphur removal, and drying.

Molecular sieves are used on natural gas liquefaction (LNG) plants to remove both carbon dioxide (CO2) and watervapourfrom the natural gas feed thereby preventing blockages occuring in the heat exchangers in the liquefaction stage.

In the United Kingdom, the early supplies of natural gas came from the Southern Basin of the North Sea where the average concentration of carbon dioxide is about 0.15%. The later supplies from the Northern fields have up to five times as much and thus existing gas purification plants have had to be uprated to accomodatethis increase.

The most widely used configuration for gas purification is a two-tower system as depicted in Figure 1 of the accompanying drawings. In this system one tower is on-line whilst the other is being regenerated, for example, by heating to drive off the adsorbed carbon dioxide and water.

Atypical form of operation is as follows:- WhilsttowerA is adsorbing, tower B is being regenerated; when breakthrough of tower A is approached the gas is switched to the regenerated tower. This provides a continuous supply of purified gas with adsorption carried out at approximately 25 C with downflowing gas. In the regeneration step the saturated adsorbent is heated using a hot upflow- ing sidestream of the purified gas to displace the COp and water.

During the initial phase of regeneration the gas is heated to approximately 3000C. To reduce theregeneration time and save fuel heating is stopped before all of the bed has reached the maximum tem erature. The flow of regeneration gas is continuous and transfers heat from the hot section of the bed to the unheated upper section, finally cooling the whole bed to approximately 25 C. Typically, temperatures of approximately 2500C are obtained atthetop ofthe bed using this "thermal pulse" technique.

The effects of using feed gases having high carbon dioxide levels means that the towers have to be regenerated more frequently. Thus for a conventional two-tower system the decrease in on-line time must be matched by a consequential decrease in regeneration if either processing of raw gas is not to be halted orthe costs of building and operating additional towers is to be avoided.

The present invention seeks to provide a process and means whereby these economic disadvantages can be avoided.

Thus the present invention provides a process for the removal of carbon dioxide and waterfrom gases by adsorption wherein said gas is passed through a bed of a molecular sieve and regenerated by passing a heated gastherethrough characterised in that said molecular sieve bed is immediately preceded by a bed of silica gel andthatthetemperature of re- generator gas exiting the beds is greaterthan 1 75'C but less than 200 C.

We have observed that only the top few centimeters of, say, a seven metre deep molecular sieve bed contain the water removed from the gas, e.g. natural gas,tobeliquified.The remainder of the bed contains the carbon dioxide. Hitherto it has been necessaryto heat all ofthe bed to a temperature of at least 2500C to remove both the water and the carbon dioxide. Our researches indicate that temperatures ofthe order of 1750C are all that are required to desorb the carbon dioxide. It is only that part of the sieve holding the water which requires the higher temperatures for desorption.Further work has shown that water saturated silica gel can be regenerated at significantly lowertemperaturesthan water-saturated molecular sieve. Thus by placing a layer of silica gel on top of the bed to absorb water first, the molecular sieve material is employed only for adsorption of the carbon dioxide and the time taken to heat the beds to the regeneration tempera- tures of less than 200 C is considerably shortened compared with conventional practice. There is a consequential saving in energy costs even if regenerative gas is heated initiallyto a highertemperature.

The present invention will be illustrated by referenceto Figures 2 and 3 ofthe accompanying drawings and bythefollowingexample.

In a two-tower system, for example as shown in Figure 1,thetop250mmofthe7metrebedswithin towers A and B were replaced with silica gel. The original plantdesignwasforan0.15%v/vloadingof carbon dioxide in a natural gas throughput of 16530 m3h r-l. The towers were designed to be regenerated at a gas throughput of 3850 m3hr#1.

Upon breakthrough of the adsorbate in the exit gas from thetower,the towerwas switched to the regeneration mode.

Figure2 shows the typical regeneration profiles for non-silica gel-containing beds and Figure 3 shows the regeneration profiles after modification in accordance with the invention.

As will be seen from a comparison of both Figures 2 and 3, the tests showed that it was possible to reducethe regeneration time byatotal of 44 minutesto 131 minutes and still regenerate a fully COP saturated bed. The regeneration profiles for the initial and modified cycles confirmed that CO2 desorption was not significantly affected by the shorter regeneration time. This modification has resulted in the unit being able to handle gas with a COP concentration of 0.65% v/v. The regeneration time was further reduced by increasing the regeneration gas flowrate. A 20% increase in flowrate enabled regeneration to be completed in 111 minutes and the plant can acceptgas containing up to 0.76% v/v CO2. In addition to allowing higher CO2 levels, the reduction in regeneration time gives a saving in fuel costs.

Following the cycle modifications the plant has operated successfullyfor3 months with CO2 levels between 0.5% v/v and 0.6% v/v with a short term peak of 0.72% v/v. Thus the combination ofthe addition of the layer of silica gel, increasing the regeneration gas flowrateandcycletimetuning has almost doubled the quantity of COP in the gas that the plant ca n handle.This has been achieved foraverysmall capital cost which has been offset by the reduction in operating costs.

Claims

1. A process for the removal of carbon dioxide and water from gases by adsorption wherein said gas is passed through a bed of a molecular sieve and regenerated by passing a heated gas through said bed, characterised in that said molecular sieve bed is immediately preceded bya bed of silica gel and that the temperature of the regenerating gas exiting the beds is g reatertha n 175 C but less than 200 C.

2. A process as claimed in claim 1 wherein said gas is natural gas.

3. A process as claimed in claim 1 or claim 2 substantially as hereinbefore described with reference to the accompanying drawings.