GB2282082A - Method for improving the efficiency of cryogenic-membrane seperation hybrids - Google Patents

Abstract

A cryogenic - membrane hybrid system comprises a cryomembrane, in which the membrane is used at low temperatures. <IMAGE>

Description

METHOD FOR IMPROVING THE EFFICIENCY OF CRYOGENIC MEMBRANE SEPARATION HYBRIDS Field of The Invention The present invention relates to improvements of hybrids using cryogenic separations and membranes together. More particularly, the invention relates to an improvement utilizing cryomembranes which can operate at subambient temperatures ( < 0 0C).

BACKGROUND OF THE INVENTION Generally speaking, hybrids of the type referred to herein comprise cryogenic apparatus, such as cryogenic distillation column, coupled with a membrane device. Separation of the components of the gas stream treated in the hybrid is achieved by the combined use of membranes and cryogenic temperatures, the system being constructed according to the components involved.

U.S. Patent No. 4,595,405 teaches the use of N2/O2 separation membranes with air separation cryogenic distillation columns, to achieve greater nitrogen recoveries and energy efficiencies. In one example the membrane is used upstream of the cryogenic column, and in another case the membrane receives its feed from one or more of the treated streams from the cryogenic column. According to this cited patent one or more of the membrane product streams is recycled to the cryogenic column.

One of the uses of membranes in membrane/cryogenic hybrids is as a pretreatment device to remove potentially freezing products or impurities (water or carbon dioxide) before cooling the feed to cryogenic temperatures (US 5,116,396). Another use of membranes in front of a cold box is to remove to the permeate side the noncondensibles, so that the condensible concentration of the reject gas is higher and the cryogenic unit can be operated at higher temperatures (US 5,082,481).

However, of particular interest in the context of the present invention are those hybrids in which the membrane part of the hybrid receives its feed from the cryogenic unit. This is the case where the membrane permeable noncondensible (e.g. H2) part of the feed is a lower concentration component, and the cryogenic unit is used to remove condensibles (e.g. light hydrocarbons), so that the vapor overhead is enriched in the permeable component and thus enables higher fluxes and permeate purities of the membrane. Also of interest is the case where the membrane is coupled to a cryogenic column to upgrade the product content of an intermediate stream, such as described, e.g., in US 4,595,405.

In all of these cases the treated stream from the cold box must be reheated to ambient temperature to operate the membranes at economic fluxes. Usually, the recovery of these membranes per pass is limited by flux and purity considerations, so that one or more of the membrane product streams has to be recycled. In US patents 4,654,063 and 4,595,405 it is the retentate stream which is recycled, although US patent 4,595,405 allows for recycling the oxygen rich permeate stream as well where it is regarded as a product.

It can be shown that such recycling of process streams between room and cryogenic temperatures introduces additional thermodynamic inefficiencies. Every time a mole of material needs to be cooled and subsequently reheated as part of the process, the minimum net work input required is given by the difference between the work needed to cool that mole of material, less the work that can be recovered on reheating the same material. If the heating and cooling were done reversibly against an infinitessimally small temperature difference , no net work would be required. However heat exchangers are operated with a finite temperature difference to drive the heat transfer. The larger the temperature difference between hot and cold streams the greater the heat transfer rate and the smaller the required heat exchange area.

However, the thermodynamic inefficiency of the heat exchanger increases as the temperature difference increases.

The work lost per unit of material passing through the heat exchanger can be calculated by completing a cycle around the heat exchanger with a reversible heat pump with a heat sink/source at the surrounding temperature To (see Atkinson, T.D., Gas Separation & Purification. 1, 85-96, 1987). For a countercurrent heat exchanger operated between temperatures T1 and T2 with a given constant temperature difference oT between the hot and cold streams, the same flowrate and the same material in both streams, the lost work is given by: dXHE = Cp*TO[ln(T2/Tl) - ln((T2-TY(T1-8T))) where dXHE denotes the change in exergy from running a heat exchanger with a finite temperature difference. For a fluid of gaseous nitrogen, with To=300 "K, T2=273 "K, and T1=173 "K and a hot/cold stream temperature difference of 3 "K, the change in exergy would be -2.05 kJ/kg of nitrogen cycled through the exchanger, i.e. 2.05 kJ lost work/kg nitrogen cycled. For this reason heat exchangers in cryogenic service are run with smaller temperature differences (e.g. 30C) than those used at ambient or higher service.

Therefore every mole of material recycled between the cold box and the membrane and back again has an associated exergy loss. In addition, the products of the membrane unit are often compressed to provide the final product stream or before recycling to the cold box. Such compression work is proportional to the temperature of the gas at the inlet whether the compression is done adiabatically or isothermally.

This is given by the following relations: (Wcomp)ad = (RT1/X) x ((P2e1) (Wcomp)isotherm = RT1 x Ln(P2/P1) wherein: # = Cp - Cv Cp For example the extra work needed to isothermally compress a gas between two pressures at ambient temperature, To, as opposed to at the cryogenic temperature, Tc, is given by: [W(To)W(Tc)isotherm =Rx(ToTc)xLn(P2/P1) For example if a gas stream is isothermally compressed from 1 bara to 6 bara at room temperature instead of at -115 C where it is produced, the additional energy required is given by: [W(To)W(Tc)i isotherm = 2086 kJ/kmol Similarly if the same gas were adiabatically compressed between the same two pressures and assuming =0.29 (diatomic molecule), the additional energy required would be: [W(To)-W(TC)]ad = 2735 kJ/lçmol Therefore the running of present state of the art cryogenic membrane hybrids with the membrane operating at ambient temperature suffer a double energy penalty.

SUMMARY OF THE INVENTION It is a purpose of the present invention to provide an improved processes which utilizes the membrane unit at the cryogenic temperatures at which the treated stream exits the cryogenic unit as feed for the membrane unit.

It is another object of the invention to provide such a process which is energy efficient, which needs compressors and heat exchangers of reduced sizes, and which leads to lower capital costs on these units.

In one aspect, the invention is directed to a cryogenic - membrane hybrid system comprising a cryomembrane, wherein the membrane is used at low temperatures.

In another aspect, the invention is directed to a process using a cryomembrane in a cryogenic - membrane hybrid system, wherein the same split as the section of column is done, thereby saving a recycle stream through the heat exchangers.

The invention further encompasses the use of cryomembranes in cryogenic - membrane hybrid processes.

The term "cryomembrane", as used herein, is meant to indicate membranes which can operate at temperatures considerably less than ambient, preferably below -30 C.

Cryogenic membranes suitable for use with the invention are for example, but not only, those described in copending Israeli Patent Application No. 105442, filed April 19, 1993, the specification of which is incorporated herein by reference. These cryogenic membranes are molecular sieve membrane of any suitable type, e.g., carbon molecular sieve membranes (CMSM) or glass molecular sieve membranes (GMSM). The CMSM membranes, for instance, can be prepared according to the methods described in USP 4,685,940 and GB 2,207,666.

These membranes as well understood by the skilled persons and, therefore, are not described herein in detail, for the sake of brevity.

As described in the abovementioned copending Israeli patent application No. 105442, the molecular sieve cryomembrane allows operation at very low temperatures with the possibility of operating with practical fluxes and very high selectivities (e.g. 02/N2 > 15, O2/A > 15, H2/CH4 > 300).

Such membranes can be used to obtain additional efficiencies and size reductions in applications where membranes are already used in hybrid systems with cryogenic processes.

Brief Description of Tlle Drawings - Fig. I is a schematic illustration of a generalized hybrid system; - Fig. 2 is a simplified scheme of the system of Fig. 1; - Fig. 3a shows the flow sheet for the process described in U.S.

Patent No. 4,654,063; - Fig. 3b is a modification of the system of Fig. 3a, according to one preferred embodiment of the invention; - Fig. 4a shows the simplified diagram of the base case flow sheet of Example 1 of US 4,595,405; - Fig. 4b shows a modified process scheme, according to a preferred embodiment of the invention.

Detailed Descrintion of Preferred Embodiments Referring to Fig. 1, the hybrid system contains one or more heat exchangers (HEI..HEN), one or more cryogenic phase separation or fractionation columns (D1..DN), one or more membrane separators (MI..MN) with optionally one or more expansion valves (VI..VK), and/or expansion turbines (T1 .. TJ) for achieving refrigeration. The product (permeate and/or retentate) is compressed in a compressor (Cl.. CN) to be brought up to the process pressure at which it is needed or for storage in pressurized gas cylinders. It is to be noted that the membrane units are operated at or near the temperatures of the treated streams coming out of the phase separation colunms and not at ambient temperature.

For simplicity of description the process will be described as in Fig. 2 containing one heat exchanger and one phase separator/fractionator with one membrane separation device, whereas in actuality each of these units could be a train of several such devices with a multitude of interconnections between them as necessary for the efficiency of the process. The feed stream (1) is already compressed to some initial process pressure Pl and temperature To, and is after pretreatment to remove any freezable impurities. It is passed through the heat exchanger (HE) and refrigeration devices until it is partially liquefied, and this stream (2) is fed to the phase separation/fractionation column (D).

At least one fractionated stream (4) from this column is returned to the train of heat exchangers and separation column. Another at least one stream (3) at temperature Tl from the separation column (D1) is optionally fed to a compressor (CF) and this pressurized feed (5) to the membrane separator (M). The permeate stream (6) at pressure P2 is optionally fed to a compressor (CP) as stream (8), and recovered as product (9) at pressure P3, or fed back to the fractionator/separator D as stream (10), or is passed back up the heat exchanger network to give a waste stream or is fed back as a stream. The retentate stream (7) is passed back into the separation column (D) as stream (12) or into the heat exchanger network as stream (11). From this column another product stream (4) is obtained and passed back up through the heat exchange network to obtain a product stream (13) at temperature To.

If the option discussed is where the permeate is recompressed as a product (9) and the retentate is recycled to the separation column (stream 12), then work (exergy) saved in not opeíáting the membrane M1 at ambient temperature is given by: Wsaved = N6*R(T0-T1 * ((P3/P2) - I) - (N12*MW*dXHE + Wg(cool)) saved compressor work saved heat pump work where Ni are the number of moles in stream i , MW is the average molecular weight and dX is the exergy change per unit mass if stream 12 was cycled through the heat exchanger instead of being fed to the separator. Wg(cool) is the additional work needed to cool streams since the cooling potential of stream 9 is now not exploited in the heat exchanger. While this last term reduces the amount of work saved, the overall work balance leads to a net work savings as illustrated in the attached examples.

The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative examples.

Example 1 The feed from a hydrocracker effluent gas containing 25% hydrogen is treated in a cryogenic membrane hybrid as in US patent 4,654,063. The components are as is found in Table I. The stream 20 in Table II gives the composition and conditions of the feed stream, 20. Figure 3a shows the flow sheet for the process as described in US patent 4,654,063. The exchanger HEl cools the feed to the point where C4 and higher hydrocarbons are removed. The low temperature point of this exchanger is -45 OC. The phases are separated in column D1. The overhead (43) is further cooled in exchanger HE2 to -170 OC to remove all of the C2-C3 and most of the C1 fractions as liquid. The noncondensible overhead is heated to room temperature as stream 24 and passed through the membrane. The permeate (28) which is the recovered hydrogen product is then compressed by the compressor (CP) from 6.89 bara to 34.9 bara to return as process gas. The retentate (30) is cooled again thus cycling it through a heating cooling cycle with the attendant extra work. The permeate need not be cooled again but is rather compressed.

In Fig. 3b, the molecular sieve cryomembrane unit capable of giving similar selectivity and permeability as the room temperature unit is moved to the exit from HEl and the entrance to HE2, so that it is at -450C. It could be moved to -1700C as well provided the compressor would operate at this low temperature. The compressor is operated on the permeate stream (28) adiabatically, and assuming T = 0.29 as for diatomic gases, the exit temperature from the compressor will be 68"C which is acceptable for the process gas. The compositions and process conditions of the feed (20), permeate (28) and retentate (30) streams are given in Table II. Allowing for the lower compression temperature, the fact that the retentate is now not recycled through HEl and that the permeate cooling power is not exploited, a summary of the work saved by using the present invention (Fig. 3b) over the prior art arrangement (Fig.

3a) is reported in Table III. As can be seen there is a net work input savings of 109.7 KW. This is a 13% reduction in energy use compared to the hybrid reported in US 4,654,0632. In addition a total of 630 kmol/hr (a 25% reduction in molar flow) are not recycled through HE1 allowing a smaller exchanger to be used.

Table I

No. Component Name BP ( C) L(KJ/mol) 1 H2 Hydrogen -253.00 0.892 2 N2 Nitrogen -196.15 5.571 3 CH4 Methane -161.50 8.160 4 C2H6 Ethane -88.60 14.682 5 H2S Hydrogen Sulf. -60.30 18.696 6 C3H8 Propane -42.10 18.726 7 C4H10 Butane -11.00 22.318 8 C5H12 Pentane 36.11 25.739 Table U

Stream No. 20 28 30 36 Mole Fraction (%) X1 25.0 96.5 36.3 96.5 X2 5.0 2.8 55.2 2.8 X3 33.0 0.7 8.5 0.7 X4 27.0 X5 6.5 X6 2.0 X7 1.0 X8 0.5 TOTAL MOL% 1 1 1 1 G(Kg-Mol/Hr) 1931.818 492.7 141.4 492.7 P(bara) 19.31034 6.9 18.8 34.5 T( C) 38 45 45 68.0 Table III

Stream No. 28 Saved Heat Pump Work -61.3 2.9 Saved Compression Work 168.0 Total Saved Work/STREAM 106.8 2.9 Total Saved Work: 109.7 All work quantities in kW.

Examnle 2 Use of low temperature ci-vomembrane with a liquid nitrogen generator The hybrid scheme is that taught in Example 1 of US 4,595,405, utilizing a single column cryogenic air unit in tandem with a room temperature membrane. The simplified diagram of the base case flow sheet is shown in Fig. 4a. It is essentially composed of: pretreatment unit 401, a main refrigeration unit 402, , composed of compressors and heat-exchangers, a main heat-exchanger 403 with counter current cooling done by cooled process streams, an additional expander 404 for obtaining further cooling/refrigeration, a cryogenic nitrogen enriching column 405, equipped with a vaporizer/condenser 406 at the top, to condense pure nitrogen overhead, a phase separator 407; a compressor 408 for further N2 liquefaction, a sub-cooler 409 for the bottoms, a compressor 410, for the membrane feed and the membrane unit. The process scheme in simplified form is as follows. Air feed (100) is compressed and precooled in 401 and fed, combined along with process return streams (114 and 108) into stream (103), and fed to the initial refrigeration unit 402 for compression and cooling, and from there two compressed streams (104 and 105) are fed to the main heat exchanger 403 and cooled against process return streams (108, 111). The stream (104) is further cooled and conducted against the return streams to form liquid feed stream (106), while the stream (105) is partially cooled to produce stream (107) which is then fed to an expander 404 to provide further cooling and to generate a cold stream (108) which is partly fed to the column 405 and partly returned to the heat exchanger 403 as a cold stream to cool and condense the incoming streams. In column 405 the feed is distilled in an enriching section leaving an oxygen rich bottom liquid stream (109) which is withdrawn, passed through a subcooler 409 and fed to the vaporizer condenser 406 to condense the pure nitrogen product. The reboiled bottom fluid vapor is passed from the vaporizer/condenser 406 back to stream (110) to the subcooler, to be heated by the bottom liquid (109), and this oxygen-rich vapor leaves the subcooler at a warmer temperature as stream (111), and is passed back through the main exchanger 403, till it reaches ambient temperature (-3000K), where it is fed as stream (112) to the feed compressor 410 and the pressurized oxygen-rich stream (113) is fed to the membrane unit 411 whose membrane selectively permeates oxygen. The permeate is circulated as a cooled gas as do enriched oxygen byproducts (115), and the retentate with reduced oxygen content (stream 114) can now be returned to the feed line to be cooled along with the feed.

The rest of the process flow sheet is not relevant to the discussion of the present invention, except to indicate that the final product streams are streams (117) and (118) which carry the pure liquid N2 and gaseous N2, respectively.

The process scheme according to a preferred embodiment of the invention is shown in Fig. 4b. The description of Fig. 4b is as in Fig. 4a, referring to the same numbering, and description which applies to both figures is not repeated, for the sake of brevity. The modified scheme is chiefly distinguished by the membrane feed compressor 410 and membrane unit 411 being fed directly from the outlet stream (111) from the subcooler 409. The membrane unit can be one of the molecular sieve cryomembranes referred to above, which shows 02 permeabilities at the low temperature, of -200 L/m2-hr bar, and O2/N2 selectivities of 20 - 40.

Alternatively, the membrane unit can be a more open cryomembrane which shows O2 permeability greater than 500 L/m2-hr bar and O2/N2 selectivity of 3.

The retentate (114) of the membrane unit 411 is returned to the appropriate plate of the distillation column 405. The O2 waste stream (115) is passed through the main heat exchanger to provide cooling of incoming streams and then released as waste or as byproduct enriched oxygen. The pressure, temperature and the molar flows of the relevant streams are given in Table IV. The work saving obtained when comparing the base case and the cryomembrane in high selectivity mode [oc (02/N2) = 20J is 240 KW. Thus, a substantial saving is obtained by preventing lost work in the compressor 410 and the main heat exchanger 403.

Table W BASE CASE PRESENT INVENTION

Stream No. 100 106 117 112 113 114 100 106 117 112 113 114 T ( C) 29 -165 493 32 32 32 29 -165 -193 465 -140.9 -140.9 P(IPa) 101.3 431 137.9 345 690 652 101.3 431 137.9 345 690 662 Flowrate 151 136A 100 223.6 223.6 1732 151 136.4 100 223. 223.6 187.1 (Kgmolehr) Nitrogen Recovery: 83.7% 83.7%

Claims (6)

1. CLAIMS: 1. A cryogenic - membrane hybrid system comprising a cryomembrane, wherein the membrane is used at low temperatures.
2. 2. A process using a cryomembrane in a cryogenic - membrane hybrid system, wherein the same split as the section of column is done, thereby saving a recycle stream through the heat exchangers.
3. 3. Use of cryomembranes in cryogenic - membrane hybrid processes.
4. 4. A process using a cryomembrane in a cryogenic - membrane hybrid system, wherein the membrane is operated at reduced temperature thereby reducing equipment size and energy consumption.
5. 5. A cryogenic - membrane hybrid system comprising a cryomembrane, essentially as described and illustrated.
6. 6. A process using a cryomembrane in a cryogenic - membrane hybrid system, essentially as described and illustrated.